Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Data from: Rapid plastic breeding response to rain matches peak prey abundance in a tropical savannah bird
    Hidalgo Aranzamendi, Nataly ; Hall, Michelle L. ; Kingma, Sjouke ; Pol, Martijn van de; Peters, Anne - \ 2019
    Dryad
    trophic interactions - annual cycle - tropics - phenology - Timing of reproduction - Phenotypic Plasticity - avian life-history - unpredictable environment - Malurus coronatus
    1. Changes in climate are shifting the timing of life cycle events in the natural world. Compared to northern-temperate areas, these effects are relatively poorly understood in tropical and southern regions, where there is limited information on how timing of breeding and food availability are affected by climatic factors, and where patterns of breeding activity are more unpredictable within and between years. 2. Combining a new statistical modelling approach with 5 years of continuous individual-based monitoring of a monsoonal tropical insectivorous bird, we quantified (i) the proximate climatic drivers at two trophic levels: timing of breeding and abundance of arthropod prey; (ii) the effect of climate variation on reproductive output and (iii) the role of individual plasticity. 3. Rainfall was identified as the main determinant of phenology at both trophic levels. Throughout the year, likelihood of egg laying increased very rapidly in response to even small amounts of rain during the preceding 0-3 weeks. Adult body mass and male sperm storage also increased rapidly after rain, suggesting high breeding preparedness. Additionally, females were flexible, since they were more likely to nest if their previous attempt was longer ago and unsuccessful. Arthropod abundance also increased after rainfall, but more slowly, with a peak around 10 weeks. Therefore, the peak food availability coincided with the presence of dependent fledglings. 4. Fitness benefits of nesting after more rain appeared to be linked to offspring quantity rather than quality: nest attempts following higher rainfall produced larger clutches, but showed no improvement in nestling mass or relative fledging success. The response of clutch size to rainfall was plastic, since repeated sampling showed that individual females laid larger clutches after more rain, possibly mediated by improved body mass. 5. Rapid, individually flexible breeding in response to rainfall and slower increase in arthropod abundance also as a response to rainfall, might buffer insectivorous species living in tropical seasonal environments from climate-change induced phenological trophic mismatches.
    Mapping Geospatial Processes Affecting the Environmental Fate of Agricultural Pesticides in Africa
    Hendriks, Chantal M.J. ; Gibson, Harry S. ; Trett, Anna ; Python, André ; Weiss, Daniel J. ; Vrieling, Anton ; Coleman, Michael ; Gething, Peter W. ; Hancock, Penny A. ; Moyes, Catherine L. - \ 2019
    International Journal of Environmental Research and Public Health 16 (2019)19. - ISSN 1660-4601
    artificial compound - crop protection - environmental data - insecticide residue - satellite data - tropics

    The application of agricultural pesticides in Africa can have negative effects on human health and the environment. The aim of this study was to identify African environments that are vulnerable to the accumulation of pesticides by mapping geospatial processes affecting pesticide fate. The study modelled processes associated with the environmental fate of agricultural pesticides using publicly available geospatial datasets. Key geospatial processes affecting the environmental fate of agricultural pesticides were selected after a review of pesticide fate models and maps for leaching, surface runoff, sedimentation, soil storage and filtering capacity, and volatilization were created. The potential and limitations of these maps are discussed. We then compiled a database of studies that measured pesticide residues in Africa. The database contains 10,076 observations, but only a limited number of observations remained when a standard dataset for one compound was extracted for validation. Despite the need for more in-situ data on pesticide residues and application, this study provides a first spatial overview of key processes affecting pesticide fate that can be used to identify areas potentially vulnerable to pesticide accumulation.

    Rapid plastic breeding response to rain matches peak prey abundance in a tropical savanna bird
    Hidalgo Aranzamendi, Nataly ; Hall, Michelle L. ; Kingma, Sjouke A. ; Pol, Martijn van de; Peters, Anne - \ 2019
    Journal of Animal Ecology 88 (2019)11. - ISSN 0021-8790 - p. 1799 - 1811.
    annual cycle - avian life-history - phenology - phenotypic plasticity - timing of reproduction - trophic interactions - tropics - unpredictable environment

    Changes in climate are shifting the timing of life cycle events in the natural world. Compared to northern temperate areas, these effects are relatively poorly understood in tropical and southern regions, where there is limited information on how timing of breeding and food availability are affected by climatic factors, and where patterns of breeding activity are more unpredictable within and between years. Combining a new statistical modelling approach with 5 years of continuous individual-based monitoring of a monsoonal tropical insectivorous bird, we quantified (a) the proximate climatic drivers at two trophic levels: timing of breeding and abundance of arthropod prey; (b) the effect of climate variation on reproductive output and (c) the role of individual plasticity. Rainfall was identified as the main determinant of phenology at both trophic levels. Throughout the year, likelihood of egg laying increased very rapidly in response to even small amounts of rain during the preceding 0–3 weeks. Adult body mass and male sperm storage also increased rapidly after rain, suggesting high breeding preparedness. Additionally, females were flexible, since they were more likely to nest whether their previous attempt was longer ago and unsuccessful. Arthropod abundance also increased after rainfall, but more slowly, with a peak around 10 weeks. Therefore, the peak food availability coincided with the presence of dependent fledglings. Fitness benefits of nesting after more rain appeared to be linked to offspring quantity rather than quality: nest attempts following higher rainfall produced larger clutches, but showed no improvement in nestling mass or relative fledging success. The response of clutch size to rainfall was plastic, since repeated sampling showed that individual females laid larger clutches after more rain, possibly mediated by improved body mass. Rapid, individually flexible breeding in response to rainfall and slower increase in arthropod abundance also as a response to rainfall, might buffer insectivorous species living in tropical seasonal environments from climate change-induced phenological trophic mismatches.

    Data underlying “Multi-Scale monitoring and modelling of the Kapuas River Delta”
    Kästner, K. ; Vermeulen, B. ; Hoitink, A.J.F. ; Geertsema, T.J. - \ 2019
    hydrodynamics - morphodynamics - sand-bedded rivers - sediment transport - tropics
    unprocessed raw data: - water level (pressure gauge) - velocity (boat mounted VADCP and HADCP) - turbulence (bottom mounted coupled VADCP) - bathymetry (single beam) - side scan sonar - bed material grain size - water samples (suspended sediment concentration) - particle size (lisst casts) - salinity (gauged)
    Homoeostatic maintenance of nonstructural carbohydrates during the 2015–2016 El Niño drought across a tropical forest precipitation gradient
    Dickman, Lee Turin ; McDowell, Nate G. ; Grossiord, Charlotte ; Collins, Adam D. ; Wolfe, Brett T. ; Detto, Matteo ; Wright, S.J. ; Medina-Vega, José A. ; Goodsman, Devin ; Rogers, Alistair ; Serbin, Shawn P. ; Wu, Jin ; Ely, Kim S. ; Michaletz, Sean T. ; Xu, Chonggang ; Kueppers, Lara ; Chambers, Jeffrey Q. - \ 2019
    Plant, Cell & Environment 42 (2019)5. - ISSN 0140-7791 - p. 1705 - 1714.
    climate - ENSO - NSC - Panama - storage - sugars - tropics - vegetation

    Nonstructural carbohydrates (NSCs) are essential for maintenance of plant metabolism and may be sensitive to short- and long-term climatic variation. NSC variation in moist tropical forests has rarely been studied, so regulation of NSCs in these systems is poorly understood. We measured foliar and branch NSC content in 23 tree species at three sites located across a large precipitation gradient in Panama during the 2015–2016 El Niño to examine how short- and long-term climatic variation impact carbohydrate dynamics. There was no significant difference in total NSCs as the drought progressed (leaf P = 0.32, branch P = 0.30) nor across the rainfall gradient (leaf P = 0.91, branch P = 0.96). Foliar soluble sugars decreased while starch increased over the duration of the dry period, suggesting greater partitioning of NSCs to storage than metabolism or transport as drought progressed. There was a large variation across species at all sites, but total foliar NSCs were positively correlated with leaf mass per area, whereas branch sugars were positively related to leaf temperature and negatively correlated with daily photosynthesis and wood density. The NSC homoeostasis across a wide range of conditions suggests that NSCs are an allocation priority in moist tropical forests.

    Changes in soil organic carbon stocks after conversion from forest to oil palm plantations in Malaysian Borneo
    Rahman, Niharika ; Neergaard, Andreas de; Magid, Jakob ; Ven, Gerrie W.J. van de; Giller, Ken E. ; Bruun, Thilde Bech - \ 2018
    Environmental Research Letters 13 (2018)10. - ISSN 1748-9318
    deforestation - land use change - oil palm - organic residue management - SOC stocks - tropics

    The continuous rise in the global demand for palm oil has resulted in large-scale expansion of industrial oil palm plantations - largely at the expense of primary and secondary forests. The potentially negative environmental impacts of these conversions have given rise to closer scrutiny. However, empirical data on the effects of conversion of forests to industrial oil palm plantations on soil organic carbon (SOC) stocks is scarce and patchy. We evaluated the changes in SOC stocks after conversion of tropical forest into oil palm plantations over the first and second rotation period in Sarawak, Malaysian Borneo. Soil samples were collected from three age classes of oil palm plantations converted from forest (49, 39 and 29 years ago respectively) with three replicate sites and four adjacent primary forest sites as reference. In each site under oil palm, the three management zones, namely weeded circle (WC), frond stacks (FS), and between palm (BP), were sampled separately. All soil samples were collected from five soil layers (0-5, 5-15, 15-30, 30-50 and 50-70 cm). Samples were analysed for SOC concentration, soil bulk density, pH and soil texture. Results showed SOC stocks declined by 42%, 24% and 18% after 29, 39 and 49 years of conversion respectively. Significant differences in SOC stocks were found among different management zones in the oil palm plantations, and the trend was similar for all age classes: FS > WC > BP, demonstrating the necessity of considering within-plantation variability when assessing soil C stocks. The largest differences between SOC stocks of the reference forest and converted plantations were found in the topsoil (0-15 cm depth) but differences were also found in the subsoil (>30 cm). Our results will contribute towards future modelling and life cycle accounting to calculate the carbon debt from the conversion of forest to oil palm plantations.

    Community ecology of Neotropical ticks, hosts, and associated pathogens
    Esser, Helen J. - \ 2017
    Wageningen University. Promotor(en): H.H.T. Prins; F.J.J.M. Bongers, co-promotor(en): P.A. Jansen. - Wageningen : Wageningen University - ISBN 9789463436908 - 200
    metastigmata - host specificity - host parasite relationships - biodiversity - species diversity - pathogens - size - community ecology - tickborne diseases - panama - tropics - hosts - metastigmata - gastheerspecificiteit - gastheer parasiet relaties - biodiversiteit - soortendiversiteit - pathogenen - grootte - gemeenschapsecologie - ziekten overgebracht door teken - panama - tropen - gastheren (dieren, mensen, planten)

    The ongoing loss of global biodiversity is unprecedented in both magnitude and pace, raising urgent questions as to how this loss will affect ecosystem functioning and human well-being. Control of infectious diseases has been proposed as an important ecosystem service that is likely to be affected by biodiversity loss. A negative relationship between biodiversity and disease risk could offer a win-win situation for nature conservation and human health. However, the generality of this relationship remains the subject of contentious debate. The aim of this thesis was to contribute to a better understanding of the interactions between ticks and their vertebrate hosts in a biodiversity hotspot, and how loss of biodiversity affects these interactions and ultimately, tick-borne disease risk. My study was unique in that I simultaneously considered and directly assessed broader communities of Neotropical wildlife, ticks, and tick-borne pathogens across an anthropogenic disturbance gradient.

    Determining whether and how biodiversity loss affects tick-borne disease risk in tropical forests requires a thorough understanding of tick-host associations, which are a function of tick-host specificity as well as host biological and ecological traits. In chapter 2, I therefore quantified the degree to which adult ticks are host-specific in my study region: Panama. Using quantitative network analyses and phylogenetic tools with null model comparisons, I found that the adult life stages of most tick species were specific to a limited number of host species that were phylogenetically closely related. In Chapter 4 I showed that species assemblages of adult ticks became increasingly diverse on larger-bodied host species, indicating that adult ticks in Panama tend to select for large reproduction hosts.

    In contrast to adult ticks, understanding the ecological interactions between immature ticks and their hosts in the tropics has long been hampered by a lack of morphological identification keys. Therefore, in Chapter 3, I describe the development of a DNA barcode reference library for the molecular identification of larvae and nymphs. This reference library was highly effective in species-level identification of immature ticks collected from birds (Chapter 3) and small mammals (Chapter 4 and 6). Several avian ecological traits were positively associated with tick parasitism, but the potential role of wild birds in tick-borne disease transmission seems to be limited in Panama. Immature ticks did not show any specificity to particular bird species or avian ecological traits (Chapter 3), and species assemblages of immatures ticks were equally diverse across a large number of host taxa (Chapter 4). This suggests that larvae and nymphs may feed more opportunistically than their adult counterparts.

    High host specificity in adult ticks implies high susceptibility to tick-host coextinction, even if immature ticks feed opportunistically. In chapter 5, I tested this hypothesis by surveying tick and vertebrate host communities across a forest fragmentation gradient. Forest fragments consisted of previously connected islands and peninsulas in the Panama Canal and ranged 1000-fold in size. Abundance and species richness of ticks was positively related to that of wildlife, which in turn was related to the size of the forest fragment. Specialist tick species were only present in fragments where their specific reproduction hosts were captured by camera traps. Further, less diverse tick communities were dominated by a generalist tick species. These results indicate that loss of wildlife had cascading effects on tick communities through local host-parasite coextinction.

    In Chapter 6, I studied how communities of wildlife, ticks, and tick-borne microbes changed along a more ‘typical’ disturbance gradient, in which forest fragments were embedded in an agricultural and sub-urban landscape, rather than surrounded by water. I found that wildlife community disassembly either diluted, amplified, or had no effect on infection prevalence in ticks, depending on the pathogen and degree of disturbance. However, hyperabundance of medium- to large-sized frugivores and herbivores (important reproduction hosts for adult ticks) in sites that lacked apex predators was related to exponential increases in tick density, negating any effect of reduced pathogen prevalence. Moreover, high tick species richness in these sites was related to high microbial and pathogen richness. High parasite diversity is thus a source of infectious diseases. When medium- to large-sized frugivores and herbivores also disappeared, densities of infected ticks declined, suggesting a non-linear relationship between biodiversity loss and tick-borne disease risk, in which initial loss of apex predators increases disease risk, but further loss of species decreases disease risk again.

    In this thesis, I have quantified host-feeding relationships of adult and immature Neotropical ticks, many of which (in the case of larvae and nymphs) were largely unknown. I have shown that adult ticks tend to be highly host-specific, particularly to larger-bodied vertebrates, whereas immature ticks appear to have broader host-use patterns. I found that ticks are susceptible to local host-tick coextirpation, and that the relationship between biodiversity loss and tick-borne disease risk is non-linear. My results emphasize the importance of directly assessing host community composition and suggest that the presence of specific (reproduction) hosts are a more important factor than species richness per se for tick population and tick-borne disease dynamics.

    Perspectieven voor kastuinbouw als duurzaamheidssprong in tropen : Low-tech kas in Rwanda leerproject voor bedrijfsleven
    Elings, Anne - \ 2016
    horticulture - protected cultivation - tropics - rwanda - research projects - vegetables - tomatoes - food supply - agricultural education

    Het Nederlands bedrijfsleven en kennisinstellingen zijn betrokken bij het kastuinbouwproject Smart Horticulture Rwanda. Anne Elings van Wageningen UR Glastuinbouw hoopt op kansen voor de tuinbouwtoeleveranciers.

    The contribution of sustainable trade to the conservation of natural capital
    Oorschot, M. van; Wentink, Carsten ; Kok, Marcel ; Beukering, P. ; Kuik, O. ; Drunen, M. van; Berg, J. van den; Ingram, V.J. ; Judge, L.O. ; Arets, E.J.M.M. ; Veneklaas, E.J. - \ 2016
    The Hague : PBL: Netherlands Environmental Assessment Agency (PBL publication 1700) - 96
    certification - sustainability - cost benefit analysis - resource conservation - natural resources - tropics - ecosystem services - biobased economy - certificering - duurzaamheid (sustainability) - kosten-batenanalyse - hulpbronnenbehoud - natuurlijke hulpbronnen - tropen - ecosysteemdiensten - biobased economy
    PBL Netherlands Environmental Assessment Agency has conducted a study into the potential impact of certified sustainable production on natural capital and the related ecosystem goods and services. Forests are a well-known example of natural capital; they are valuable to society, among other things because they store large amounts of carbon. The performed cost-benefit analyses show that certified resource production has several societal benefits, such as reductions in environmental pollution, soil erosion and health damage. However, for resource producers, the financial returns of more sustainable production methods are often limited. The uneven distribution of costs and benefits over public and private actors forms a barrier to any further scale up of sustainable production. Thus, there is a need for additional solutions, besides certifying trade to help conserve ecosystems elsewhere in the world.
    Economic trade-offs of biomass use in crop-livestock systems: Exploring more sustainable options in semi-arid Zimbabwe
    Homann Kee, S. ; Valbuena Vargas, D.F. ; Masikati, P. ; Descheemaeker, K.K.E. ; Nyamangara, J. ; Claessens, L.F.G. ; Erenstein, O. ; Rooyen, A.F. van; Nkomboni, D. - \ 2015
    Agricultural Systems 134 (2015). - ISSN 0308-521X - p. 48 - 60.
    conservation agriculture - smallholder farmers - intensification - productivity - challenges - strategies - countries - benefits - tropics - africa
    In complex mixed crop-livestock systems with limited resources and biomass scarcity, crop residues play an important but increasingly contested role. This paper focuses on farming systems in the semi-arid areas of Zimbabwe, where biomass production is limited and farmers integrate crop and livestock activities. Conservation Agriculture (CA) is promoted to intensify crop production, emphasizing the retention of surface mulch with crop residues (CR). This paper quantifies the associated potential economic tradeoffs and profitability of using residues for soil amendment or as livestock feed, and explores alternative biomass production options. We draw on household surveys, stakeholder feedback, crop, livestock and economic modeling tools. We use the Trade-Off Analysis Model for Multi Dimensional Impact Assessment (TOA-MD) to compare different CR use scenarios at community level and for different farm types: particularly the current base system (cattle grazing of maize residues) and sustainable intensification alternatives based on a CA option (mulching using maize residues ± inorganic fertilizer) and a maize– mucuna (Mucuna pruriens) rotation. Our results indicate that a maize–mucuna rotation can reduce trade-offs between CR uses for feed and mulch, providing locally available organic soil enhancement, supplementary feed and a potential source of income. Conservation Agriculture without fertilizer application and at non-subsidized fertilizer prices is not financially viable; whereas with subsidized fertilizer it can benefit half the farm population. The poverty effects of all considered alternative biomass options are however limited; they do not raise income sufficiently to lift farmers out of poverty. Further research is needed to establish the competitiveness of alternative biomass enhancing technologies and the socio-economic processes that can facilitate sustainable intensification of mixed crop-livestock systems, particularly in semi-arid environments.
    Integrating ecosystem services into the tropical commodity value chain : cocoa, soy and palm oil
    Berg, J. van den; Ingram, V.J. ; Judge, L.O. ; Arets, E.J.M.M. - \ 2014
    Wageningen : Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu) (WOt-technical report 6) - 101
    ecosysteemdiensten - theobroma cacao - cacao - glycine soja - palmoliën - waardeketenanalyse - basisproducten - duurzaamheid (sustainability) - tropen - innovaties - ecosystem services - theobroma cacao - cocoa - glycine soja - palm oils - value chain analysis - commodities - sustainability - tropics - innovations
    This technical report explores the governance options available to the Dutch government to promote the sustainable use and maintenance of ecosystem services in tropical commodity value chains with Dutch links. It examines how ecosystem services can be given a more explicit role in public and market mechanisms, using the cocoa, soy, palm oil and timber chains as case studies. The document presents a discourse analysis of the way Dutch policies and practice address ecosystem services, updating the report of a forerunner study on the timber chain (Van den Berg et al. 2013). The discourse analysis indicates that the term ecosystem services still lacks a clear definition in Dutch policy, with ecosystem services largely being seen as an economic issue, which can be solved by market drive, voluntary and multi-actor value chain based solutions. The report presents results of a detailed examination of specific cases of innovation in sustainability initiatives and payments for ecosystem services projects in the cocoa chain, the Round Table for Responsible Soy (RTRS) and the Round Table for Sustainable Palm Oil (RSPO). Lessons learnt from the analysis of these cases include the need to simplify what is meant by ecosystem services – for example using the term natural capital - to make it more appealing and intuitive, particularly for business. More evidence is needed on the impact of certification and how it maintains or enhances ecosystem services. Internationally agreed impact indicators are also recommended. The array of available certification schemes could be harmonised. A mix of policy instruments appears to offer more scope for the government, using market based ‘carrots’ and incentive-based ‘sticks’ (such as tax incentives and pilot projects) to stimulate new partnerships and initiatives. Challenges include giving ecosystem services an explicit role in policysupported innovations, and engaging with all value chain stakeholders, particularly community and consumer organisations.
    Constraints and opportunities for tree diversity management along the forest transition curve to achieve multifunctional agriculture
    Ordonez, J.C. ; Luedeling, E. ; Kindt, R. ; Tata, H.L. ; Harja, D. ; Jamnadass, R. ; Noordwijk, M. van - \ 2014
    Current Opinion in Environmental Sustainability 6 (2014). - ISSN 1877-3435 - p. 54 - 60.
    ecosystem services - functional diversity - biodiversity - agroforestry - tropics - systems
    On-farm tree diversity patterns result from a social-ecological process shaped by different actors. Farmer preferences, tree-site matching, seed dispersal, tree domestication and delivery via nurseries all play important roles in forming these patterns. As part of a wider interest in tree cover transition curves that link agroforestation stages of landscapes to a preceding deforestation process, we here focus on ‘tree diversity transition curves’ i. as a conceptual framework to understand current processes and how shifts in drivers affect tree diversity and ii. to help identify constraints and opportunities for interventions. We provide some examples of current research efforts and make suggestions for databases and analyzes that are required to improve our understanding of tree diversity transitions. We explore drivers, consequences and entry points for tree diversity management to achieve multifunctional agriculture.
    Biochar increases plant-available water in a sandy loam soil under an aerobic rice crop system
    Melo Carvalho, M.T. de; Holanda Nunes Maia, A. de; Madari, B.E. ; Bastiaans, L. ; Oort, P.A.J. van; Heinemann, A.B. ; Soler da Silva, M.A. ; Petter, F.A. ; Marimon-Junior, B.H. ; Meinke, H.B. - \ 2014
    Solid Earth 5 (2014). - ISSN 1869-9510 - p. 939 - 952.
    upland rice - hydraulic conductivity - management - productivity - temperatures - fertility - retention - tropics - model
    The main objective of this study was to assess the impact of biochar rate (0, 8, 16 and 32 Mg ha-1) on the water retention capacity (WRC) of a sandy loam Dystric Plinthosol. The applied biochar was a by-product of slow pyrolysis (~450 °C) of eucalyptus wood, milled to pass through a 2000 µm sieve that resulted in a material with an intrinsic porosity =10 µm and a specific surface area of ~3.2 m2 g-1. The biochar was incorporated into the top 15 cm of the soil under an aerobic rice system. Our study focused on both the effects on WRC and rice yields 2 and 3 years after its application. Undisturbed soil samples were collected from 16 plots in two soil layers (5–10 and 15–20 cm). Soil water retention curves were modelled using a nonlinear mixed model which appropriately accounts for uncertainties inherent of spatial variability and repeated measurements taken within a specific soil sample. We found an increase in plant-available water in the upper soil layer proportional to the rate of biochar, with about 0.8% for each Mg ha-1 biochar amendment 2 and 3 years after its application. The impact of biochar on soil WRC was most likely related to an effect in overall porosity of the sandy loam soil, which was evident from an increase in saturated soil moisture and macro porosity with 0.5 and 1.6% for each Mg ha-1 of biochar applied, respectively. The increment in soil WRC did not translate into an increase in rice yield, essentially because in both seasons the amount of rainfall during the critical period for rice production exceeded 650 mm. The use of biochar as a soil amendment can be a worthy strategy to guarantee yield stability under short-term water-limited conditions. Our findings raise the importance of assessing the feasibility of very high application rates of biochar and the inclusion of a detailed analysis of its physical and chemical properties as part of future investigations.
    Environmental and physiological drivers of tree growth : a pan-tropical study of stable isotopes in tree rings
    Sleen, J.P. van der - \ 2014
    Wageningen University. Promotor(en): Pieter Zuidema; Frans Bongers; Niels Anten. - Wageningen : Wageningen University - ISBN 9789461739544 - 174
    bomen - groei - plantenfysiologie - jaarringen - milieueffect - isotopen - tropische bossen - tropen - trees - growth - plant physiology - growth rings - environmental impact - isotopes - tropical forests - tropics

    Forests in the wet tropics harbour an incredible biodiversity, provide many ecosystem services and regulate climatic conditions on regional scales. Tropical forests are also a major component of the global carbon cycle, storing 25% of the total terrestrial carbon and accounting for a third of net primary production. This means that changes in forest structure and forest cover in the wet tropics will not only affect biodiversity and ecosystem services, but also have implications for the global carbon cycle and – as a result – may speed up or slow down global warming. Deforestation rates are still high in the tropics and have profoundly affected the extent of forests in many countries. Additionally, there are indications that undisturbed and pristine tropical forests are changing. The most notable changes found by the monitoring of permanent forest plots are an increase of tree growth and forest biomass per unit of surface area over the last decades. If this is indeed the case, it would entail that the world’s tropical forests are potentially absorbing a significant fraction of human caused CO2emissions and as such are mitigating global warming. However, increased tree growth and forest biomass have not been found in all studies. Furthermore, it is unknown whether the observed changes in intact forests are part of a long-term change, or merely reflect decadal scale fluctuations. These uncertainties lead to an ongoing debate on whether tree growth and forest biomass have increased in tropical forests and – if so – to what extent. In addition, there is also a scientific discussion on the factor(s) that could underlie the potential changes in tree growth and forest biomass. Possibly, they are caused by an internal driver, like the lasting effect of large scale disturbances in the past, or by external drivers. Possible external factors affecting tropical forest dynamics are (1) climate change (temperature and precipitation), (2) increased nutrient depositions and (3) increased atmospheric CO2concentration.

    In this thesis, I investigated the environmental changes that could have formed the basis for changes in tropical tree growth. I used two relatively new tools in tropical forest ecology: tree-ring measurements and stable isotope analyses. Tree-ring widths were measured to obtain long-term information on tree growth. Stable isotopes in the wood of tree rings were analysed to provide information on the environmental and physiological drivers of tree growth changes. This thesis is part of a larger project on the long-term changes in intact forests in the wet tropics (the TroFoClim project, led by Pieter Zuidema) and also includes the PhD theses of Mart Vlam and Peter Groenendijk. In this project, ~1400 trees of 15 species were examined that were collected in three forest sites distributed across the tropics (in Bolivia, Cameroon and Thailand).

    For the assessment of long-term changes in growth and stable isotopes, it is important to understand shorter term fluctuations due to forest dynamics (i.e. gap formation), because these interfere with changes on a longer temporal scale. The formation of a gap in a closed canopy forest, after the death of a tree, can cause considerable environmental changes in the surrounding area, e.g. in light, nutrient and water availability. This can strongly affect the growth rates of the remaining trees. However, in most studies the environmental drivers of changes in tree growth after gap formation are not considered. In CHAPTER 2 I measured carbon isotope discrimination (Δ13C) in annual growth rings of Peltogyne cf.heterophylla, from a moist forest in North-eastern Bolivia, and evaluated the environmental drivers of growth responses after gap formation. Growth and Δ13C was compared between the seven years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10m) to single tree-fall gaps; the other half grew more than 40 m away from gaps (control trees). I found that increased growth was mainly associated with decreased Δ13C suggesting that this response was driven by increased light availability and not by improved water availability. Interestingly, most small trees did not show a growth stimulation after gap formation. This result was hypothesized to be caused by an increased drought stress. However, the measurement of Δ13C showed that increased water stress is unlikely the cause for the absence of increased growth, but rather suggested that light conditions had not improved after gap formation. These results show that combining growth rates with changes in Δ13Cis a valuable tool to better understand the causes of temporal variation in tree growth.

    An important potential driver of long-term changes in tree growth is climate change, e.g. global warming and altered annual precipitation. To understand the effect of climate change on tree growth, the availability of reliable data on historical climate is off course crucial. For the study areas in Bolivia and Thailand, previous studies have investigated the occurrence of temporal trends in temperature and precipitation. For the study area in Cameroon however, as well as for West and Central Africa in general, the availability of instrumental climate data is very restricted. This limits the possibility to relate past climatic variation to changes in tree growth and calls for proxies that allow reconstruction of past climatic conditions. In CHAPTER 3 I assessed the potential use of stable isotopes of oxygen (δ18O) in tree rings as a tool for the reconstruction of precipitation in tropical Africa. I measured δ18O in tree rings of five large Entandrophragmautiletreesfrom North-western Cameroon. A significant negative correlation was found between annual tree-ring δ18O values (averaged over the five individuals) and annual precipitation amount during 1930-2009 in large areas of West and Central Africa. I also found tree-ring δ18O to track sea surface temperatures (SST) in the Gulf of Guinea (1930-2009). These two results are related because rainfall variabilityin West and Central Africa is profoundly influenced by the SST of the tropical AtlanticOcean. Thus a high SST in the Gulf of Guinea is associated with high precipitation over large parts of West and Central Africa and recorded in tree rings by a relatively low δ18O value. On the other hand, dry years when SST is low, are recorded by relatively high tree-ring δ18O values. I also found a significant long-term increase of tree-ring δ18O values. This trend seems to be caused by lowered precipitation from 1970 to 1990 (the Sahel drought period). From 1860 to 1970, no significant long-term trend was observed in tree-ring δ18O values, suggesting no substantial change in precipitation amount occurred over this period.

    Another potential driver of altered tree growth and biomass in intact tropical forests is the increase of anthropogenic nutrient depositions, especially nitrogen. The deposition of nitrogen has likely risen due to an increased industrialization and use of artificial N fertilizers in most tropical countries. Nitrogen can stimulate plant growth, as is well known from the positive effect of N fertilizer application on crop yields. Previous studies have shown that the stable isotope ratio of nitrogen (δ15N) increased during the last decennia in the wood of trees from Brazil and Thailand as well as in tree leaves from Panama. This increased δ15N has been interpreted as a signal that tropical nitrogen cycles have become more ‘open’ and ‘leaky’ during the last decades in response to increased anthropogenic nitrogen depositions. The underlying mechanism is that high rates of nitrogen deposition and high ambient nitrogen availability lead to an increased nitrification. This process can cause a gradual 15N-enrichment of soil nitrogen. In CHAPTER 4 I analysed changes in tree-ring δ15N values of 400 trees of six species from the three study sites. In the trees from Cameroon no long-term change of tree-ring δ15N values was found (1850-2005), even though NH3and NOxemissions seem to have increased strongly around the study area since 1970. Possibly, the very high precipitation at that site causes the local nitrogen cycle to be already very ‘leaky’, limiting the effect of additional nitrogen input on the δ15N signature of soil nitrogen. Alternatively, nitrogen input in this forest might be much lower than reconstructed NH3and NOxemissions suggest. For the study site in Bolivia, no significant change of tree-ring δ15N values was found (1875-2005), which is in line with the expected result for areas with a low anthropogenic nitrogen input. I found a marginally significant increased of δ15N values since 1950 in trees from Thailand, which confirms previous observations. This points to an effect of increased anthropogenic nitrogen deposition, which could have stimulated photosynthetic rates, if indeed nitrogen was limiting tree growth.

    The most often hypothesized factor to cause a long-term increase of tree growth is the rise of atmospheric CO2. Since the onset of the industrial revolution (~1850) global atmospheric CO2concentration has increased by 40%. Elevated CO2can directly affect plants by increasing the activity, as well as the efficiency, of the CO2fixing enzyme rubisco and thereby increase photosynthetic rates. Potentially more important in plant communities subjected to periods of limited water availability (like a dry season) is that elevated CO2 can cause a reduction of stomatal conductance, which lowers evapo-transpiration and hence reduces water losses.This increases water-use efficiency (i.e. the amount of carbon gained through photosynthesis divided by the amount of water loss through transpiration) and might allow plants to extend their growth season and/or increase their photosynthetic activity during the hottest hours of the day when water-stress might be severe. Elevated atmospheric CO2is thus a very likely candidate to have stimulated tropical tree growth (also referred to as CO2fertilization), provided at least that plant growth was either carbon or water limited. In CHAPTER 5 I tested the CO2fertilization hypothesis by analysing growth-ring data of 1100 trees from the three study sites. The measurement of tree-ring widths allowed an assessment of historical growth rates, whereas stable carbon isotopes (δ13C) in the wood of the trees were used to obtain an estimate of the CO2concentration in the intercellular spaces in leaves (Ci) and of water-use efficiency (intrinsic water-use efficiency; iWUE). I used a sampling method that controls for ontogenetic (i.e. size developmental) changes in growth and δ13C. With this method, trees were compared across a fixed diameter (i.e. same ontogenetic stage). I chose two diameters: 8 cm (referring to small understorey trees) and 27 cm (referring to larger canopy trees). A mixed-effect model revealeda highly significant and exponential increase of Ciat each of the three sites, and in both understorey and canopy trees. Over the last 150 years Ciincreased by 43% and 53% for understorey and canopy trees respectively. Yet, the rate of increase in Ciwas consistently lower than that of atmospheric CO2. This ‘active’ response to elevated atmospheric CO2resulted in a significant and large increase of iWUE. Over the last 150 years, iWUE increased by 30% and 35% for understorey and canopy trees.A long-term increase of iWUE indicates either a proportional increase of net photosynthesis and/or a decrease of stomatal conductance and thus transpiration, both of which could have stimulated biomass growth. However, I found no increase of tree growth over the last 150 years in any of the sites. Although there are several possible explanations for these findings, I argue that it is most likely that tropical tree growth is generally not limited by water and carbon, but by a persistent nutrient limitation (e.g. of phosphates) and that this has prevented tropical trees to use the extra CO2for an acceleration of growth.

    In this thesis I have studied environmental and physiological drivers of tree growth changes. I found evidence of decreased precipitation over the last decades at the study site in Cameroon (CHAPTER 3), a changed nitrogen cycle at the study site in Thailand (CHAPTER 4) and an overall change in the physiology of all tree species studied (increased iWUE; CHAPTER 5). One of the main findings of this thesis is however, that these changes have not led to a net change of tree growth over the last 150 years (CHAPTER 5). This is an important finding that could have two major implications. Firstly, the absence of a long-term growth stimulation suggests that the increase of iWUE is mainly driven by a reduced stomatal conductance, which likely leads to a reduced evaporative water loss. If trees across the tropics are reducing evapo-transpiration, this will change affect hydrological cycles, e.g. leading to a lower humidity, higher air temperatures and a reduced precipitation. Secondly, the absence of a growth stimulation over the last 150 years suggests that the carbon sink capacity of tropical forests is currently overestimated, e.g. by Dynamic Global Vegetation Models, which assume strong CO2fertilization effects and as such a high capacity of tropical forests to mitigate global warming. I anticipate that the planned Free Air Concentration Enrichment (FACE) experiments in the tropics will shed light on the reasons why increased CO2does not stimulate the growth rates of tropical trees. Furthermore, I argue that combining tree-ring measurements and stable isotope analyses together with permanent plot research is the most promising way to increase our understanding of the changes in tropical forests.

    Interest grows for Dutch mid-tech and low-tech greenhouse technology : A greenhouse to suit all tropical conditions (interview with Anne Elings)
    Kierkels, T. ; Elings, A. - \ 2014
    In Greenhouses : the international magazine for greenhouse growers 3 (2014)2. - ISSN 2215-0633 - p. 28 - 29.
    glastuinbouw - kassen - bouwtechnologie - bouwconstructie - teelt onder bescherming - tropen - ontwerp - maleisië - materialen - aangepaste technologie - greenhouse horticulture - greenhouses - construction technology - building construction - protected cultivation - tropics - design - malaysia - materials - appropriate technology
    The Netherlands sets the standard for high-tech greenhouses worldwide. But increasingly suppliers are looking too at possibilities within the mid-tech and even the low-tech market segments. The Dutch government is supporting demonstration projects, for example in Mexico, East Africa and Malaysia. Technically it’s all going well.
    Resource use efficiency and farm productivity gaps of smallholder dairy farming in North-west Michoacán, Mexico
    Cortez Arriola, J. ; Groot, J.C.J. ; Amendola Massiotti, R.D. ; Scholberg, J.M.S. ; Mariscal Aguayo, D.V. ; Tittonell, P.A. ; Rossing, W.A.H. - \ 2014
    Agricultural Systems 126 (2014). - ISSN 0308-521X - p. 15 - 24.
    recent trends - nitrogen - performance - volatilization - tropics - systems - losses - yield
    Smallholder dairy farms that intensify production risk resource degradation and increased dependence on external feeds and fertilizers due to lack of knowledge and appropriate technology, which undermines farm productivity and profitability. Here we analyze underlying causes at farm level of such process through an integrated analysis at the farm scale by assessing current resource use efficiency for grazing-based dairy farming systems representative of NW Michoacán, Mexico. Whole-farm yield gaps were quantified by comparing current farms to virtual reference farms that have the same farm surface area but improved farm management. Productivity of reference farms was calculated by assuming best crop production practices (as observed within the set of case study farms) and improved herd management. Three family-based (FB) and three semi-specialized (SS) dairy systems spanning three levels of intensification in terms of density of livestock units (LU): extensive (E, 1.2 LU ha-1) were monitored during one year (rainy and dry seasons) to assess productivity and resource use efficiencies. Milk production was generally low and variable (2.2–4.3 Mg milk cow-1 lactation-1, and 0.6–5.8 Mg ha-1) due to high incidence of mastitis, a large fraction of non-productive animals in the herd and inefficient reproduction management. During the dry season, grazing areas provided insufficient metabolizable energy, and milk production was sustained through increased use of concentrates (from 310 g kg-1 DMI in rainy season to 454 g kg-1 DMI-1 in dry season of the herd) and conserved forage. All farms had positive nitrogen, phosphorus and potassium balances, averaging 75 ± 16, 15 ± 6, and 19 ± 6 kg ha-1, respectively. Nutrients in animal excreta were mostly not recycled on the farms but lost to the environment, and nutrient surpluses increased with livestock density. The reference farms exhibited an attainable milk yield of 2.7 Mg ha-1 on the basis of full feed self-supply, and 4.2 Mg ha-1 when the crude protein limitation in the ration was lifted. Compared to the reference farm actual milk yields were on average 78.4% lower on FB farms and 57.9% lower on SS farms. The underlying causes of the farm yield gap differed between farms and were due to sub-optimal areas of forage maize, low forage and forage maize productivity and deficient herd management. We conclude that the farm yield gap analysis was effective in identifying the major shortcomings in management of the dairy farming systems and enabled formulation of change avenues for farm reconfiguration focusing on combined improvements in crop, feed and herd management and recycling of nutrients through manure management.
    An institutional analysis of deforestation processes in protected areas: The case of the transboundary Mt. Elgon, Uganda and Kenya
    Petursson, J.G. ; Vedeld, P. ; Sassen, M. - \ 2013
    Forest Policy and Economics 26 (2013). - ISSN 1389-9341 - p. 22 - 33.
    mount-elgon - forest - conservation - management - biodiversity - livelihoods - tropics - parks
    Protected areas (PAs) are a country's key strategy to conserve and manage forest resources. In sub-Saharan Africa, the effectiveness and efficiency of PA institutions in delivering sustainable outcomes is debated, however, and deforestation has not been avoided within such formal regimes. This paper analyzes the processes that led to deforestation within the PAs on the transboundary Mt. Elgon, Uganda-Kenya, employing institutional theory. Landsat satellite imagery helped identify and quantify forest loss over time. The study showed how, since 1973, about a third of all forests within the PAs on Elgon have been cleared in successive processes. Within formal protected area regimes, complex political and institutional factors drive forest loss. We argue, therefore, that policies to counter deforestation using a PA model have to be considered and understood against the broader background of these factors, originating both inside and outside the PA regimes. (C) 2012 Elsevier B.V. All rights reserved.
    Nederland krijgt belangstelling voor mid tech en low tech kassenbouw (interview met Anne Elings)
    Kierkels, T. ; Elings, A. - \ 2013
    Onder Glas 10 (2013)10. - p. 34 - 35.
    glastuinbouw - kassen - bouwtechnologie - bouwconstructie - teelt onder bescherming - tropen - ontwerp - maleisië - materialen - aangepaste technologie - greenhouse horticulture - greenhouses - construction technology - building construction - protected cultivation - tropics - design - malaysia - materials - appropriate technology
    Nederland zet de toon in high tech kassenbouw over de hele wereld. Maar in toenemende mate kijken de toeleveringsbedrijven ook naar mogelijkheden binnen het mid tech en zelfs het low tech segment. De Nederlandse overheid ondersteunt demonstratieprojecten in bijvoorbeeld Mexico, Oost-Afrika en Maleisië. Technisch lukt het allemaal goed.
    Runoff, discharge and flood occurrence in a poorly gauged tropical basin : the Mahakam River, Kalimantan
    Hidayat, H. - \ 2013
    Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Ton Hoitink. - S.l. : s.n. - ISBN 9789461737434 - 114
    oppervlakkige afvoer - afvoer - overstromingen - monitoring - tropen - modellen - rivieren - cartografie - voorspelling - kalimantan - indonesië - runoff - discharge - floods - monitoring - tropics - models - rivers - mapping - prediction - kalimantan - indonesia

    Tidal rivers and lowland wetlands present a transition region where the interests of hydrologists and physical oceanographers overlap. Physical oceanographers tend to simplify river hydrology, by often assuming a constant river discharge when studying estuarine dynamics. Hydrologists, in turn, generally ignore the direct or indirect effects of tides in water level and discharge records. This thesis aims to improve methods to monitor, model and predict discharge dynamics in tidal rivers and lowland wetlands, by focussing on the central and lower reaches of the River Mahakam (East Kalimantan, Indonesia), and the surrounding lakes area. The 980-km long river drains an area of about 77100 km2 between 2°N - 1°S and 113°E - 118°E. Due to its very mild bottom slope, a significant tidal influence occurs in this river. The middle reach of the river is located in a subsiding basin, parts of which are below mean sealevel, featuring peat swamps and about thirty lakes connected to the river via tie channels.

    Upstream of the lakes area, at about 300 km from the river mouth, an acoustic Doppler current profiler (H-ADCP) has been horizontally deployed at a station near the city of Melak (Chapter 2). The H-ADCP profiles of velocity are converted to discharge adopting a new calibration methodology. The obtained time-series of discharge show the tidal signal is clearly visible during low flow conditions. Besides tidal signatures, the discharge series show influences by variable backwater effects from the lakes, tributaries and floodplain ponds. The discharge rate at the station exceeds 3250 m3s-1 with a hysteretic behaviour. For a specific river stage, the discharge range can be as high as 2000 m3s-1. Analysis of alternative types of rating curves shows this is far beyond what can be explained from kinematic wave dynamics. Apart from backwater effects, the large variation of discharge for a specified river stage can be explained by river-tide interaction, impacting discharge variation especially in the fortnightly frequency band.

    A second H-ADCP station has been setup in the lower reach of the Mahakam, near the city of Samarinda, where the tidal discharge amplitude generally exceeds the discharge related to runoff (Chapter 3). Conventional rating curve techniques are inappropriate to model river discharge at this tidally influenced station. As an alternative, an artificial neural network (ANN) model is developed to investigate the degree to which tidal river discharge at Samarinda station can be predicted from an array of level gauge measurements along the tidal river, and from tidal level predictions at sea. The ANN-based model produces a good discharge estimation, as established from a consistent performance during both the training and the validation periods, showing the discharges can be predicted from water levels only, once that a trained model is available. The ANN models perform well in predicting discharges up to two days in advance.

    Chapter 4 addresses the role of backwater effects and tidal influences on discharge time-series used to calibrate a rainfall-runoff model. The HBV rainfall-runoff model is implemented for the Mahakam sub-catchment upstream of Melak (25700 km2). In a first approach, the model is calibrated using a discharge series derived from the H-ADCP measurements from Melak station. In a second approach, discharge estimates derived from a rating curve are used to calibrate the model. Adopting the first approach, a comparatively low model efficiency is obtained, which is attributed to the backwater and tidal effects that are not captured in the model. The second approach produces a relatively higher model efficiency, since the rating curve filters the backwater effects out of the discharge series. Seasonal variation of terms in the water balance is not affected by the choice for one of the two calibration strategies, which shows that backwaters do not have a systematic seasonal effect on the river discharge.

    To allow for investigation of the causes of backwater effects, satellite radar remote sensing is employed to monitor water levels in wetlands (Chapter 5). A series of Phased Array L-band Synthetic Aperture Radar (PALSAR) images is used to observe the dynamics of the Mahakam River floodplain. To analyze radar backscatter behavior for different land cover types, several regions of interest are selected, based on land cover classes. Medium shrub, high shrub, fern/grass, and degraded forest are found to be sensitive to flooding, whereas peat forest, riverine forest and tree plantation backscatter signatures only slightly change with flood inundation. An analysis of the relationship between radar backscatter and water levels is carried out. For lakes and shrub covered peatland, for which the range of water level variation is high, a good water level-backscatter correlation is obtained. In peat forest covered peatland, subject to a small range of water level variation, water level-backscatter correlations are poor, limiting the ability to obtain a floodplain-wide water surface topography from the radar images.

    Chapter 6 continues to investigate the degree in which satellite radar remote sensing can serve to distinguish between dry areas and wetlands, which is a difficult task in densely vegetated areas such as peat domes. Flood extent and flood occurrence information are successfully extracted from a series of radar images of the middle Mahakam lowland area. A fully inundated region is easily recognized from a dark signature on radar images. Open water flood occurrence is mapped using a threshold value taken from radar backscatter of the permanently inundated areas. Radar backscatter intensity analysis of the vegetated floodplain area reveals consistently higher backscatter values, indicating flood inundation under forest canopy. Those observations are used to establish thresholds for flood occurrence mapping in the vegetated area. An all-encompassing flood occurrence map is obtained by combining the flood occurrence maps for areas with and without vegetation.

    Chapter 7 synthesizes the findings from the previous chapters. It is concluded that the backwater effects and subtle tidal influences may prevent the option to predict river discharge using rating curves, which can best be interpreted as a stage-runoff relationship. H-ADCPs offer a promising alternative to monitor river discharge. For a tidal river, an ANN model can be used as a tool for data gap filling in an H-ADCP based discharge series, or even to derive discharge estimates solely from water levels and water level predictions. Discharge can be predicted several time-steps ahead, allowing water managers to take measures based on forecasts. The stage-runoff relationship derived from a continuous series of H-ADCP based discharge estimates may be expected to be much more accurate than a similar rating curve derived from a small number of boat surveys. The flood occurrence map derived from PALSAR images can offer a detailed insight into the hydroperiod, the period in which a soil area is waterlogged, and flood extent of the lowland area, illustrating the added value of radar remote sensing to wetland hydrological studies. In future work, radar-based floodplain observations may serve to calibrate hydrodynamic models simulating the processes of flooding and emptying of the lakes area.

    Soil heterogeneity and soil fertility gradients in smallholder agricultural systems of the east african highlands
    Tittonell, P.A. ; Muriuki, A. ; Klapwijk, C.J. ; Shepherd, K.D. ; Coe, R. ; Vanlauwe, B. - \ 2013
    Soil Science Society of America Journal 77 (2013)2. - ISSN 0361-5995 - p. 525 - 538.
    western kenya - resource-allocation - organic-matter - management - maize - variability - quality - productivity - indicators - tropics
    Heterogeneity in soil fertility in these smallholder systems is caused by both inherent soil-landscape and human-induced variability across farms differing in resources and practices. Interventions to address the problem of poor soil fertility in Africa must be designed to target such diversity and spatially heterogeneity. Data on soil management and soil fertility from six districts in Kenya and Uganda were gathered to understand the determinants of soil heterogeneity within farms. Analysis of the variance of soil fertility indicators across 250 randomly selected farms (i.e., 2607 fields), using a mixed model that considered site, sampling frame, farm type, and field as random terms, revealed that the variation in soil organic C (6.5–27.7 g kg-1), total N (0.6–3.0 g kg-1), and available P (0.9–27 mg kg-1) was mostly related to differences in the inherent properties of the soils across sites (50 to 60% of total variance). Exchangeable K+ (0.1–1.1 cmol(+) kg-1), Ca2+ (1.5–14.5 cmol(+) kg-1), Mg2+ (0.6–3.7 cmol(+) kg-1), and pH (5.1–6.9) exhibited larger residual variability associated with field-to-field differences within farms (30 to 50%). Soil fertility indicators decreased significantly with increasing distance from the homesteads. When this variable was included in the model, the unexplained residual variances—associated with soil heterogeneity within farms—were 38% for soil C; 32% for total N; 49% for available P; 56, 49, and 38% for exchangeable K+, Ca2+ and Mg2+, respectively; and 49% for the pH. In allocating nutrient resources, farmers prioritized fields they perceived as most fertile, reinforcing soil heterogeneity. Categorization of fields within a farm with respect to distance from the homestead, and soil fertility classes as perceived by farmers, were identified as entry points to target soil fertility recommendations to easily recognizable, distinct entities.
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