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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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    Developing management packages for acid sulphate soils based on farmer and expert knowledge : field study in the Mekong Delta, Viet Nam
    Quang Tri, Le - \ 1996
    Agricultural University. Promotor(en): J. Bouma; V.T. Xuan. - S.l. : Le Quang Tri - ISBN 9789054855583 - 200
    zure gronden - kattekleigronden - landevaluatie - grondvermogen - bodemgeschiktheid - ruimtelijke ordening - landgebruik - zonering - expertsystemen - vietnam - kunstmatige intelligentie - acid soils - acid sulfate soils - land evaluation - land capability - soil suitability - physical planning - land use - zoning - expert systems - vietnam - artificial intelligence

    Effective interaction of farmers' expertise and expert knowledge has been a special point of attention for this study. The objectives of the study were to describe the process of interaction between farmers and experts in improving the use of acid sulphate soils and to point out difficulties encountered. Actual conditions for four major areas were described including variabilities. Four representative areas: Tan Thanh, Tri Ton, Phung Hiep, and Hong Dan in the Mekong Delta, Viet Nam were selected for this study. Physical conditions were defined in terms of soil properties, as reflected by soil classification, and hydrological conditions were defined by climate data including flooding. Methods for land improvement in different areas were first defined by describing and analyzing measures taken by farmers, and by next developing schemes for improvement, using expert knowledge. These schemes were expressed by decision trees as a part of expert systems. But many questions were left and a series of experiments was designed and executed to answer those questions that could not be answered by experts.

    Four study areas were selected. Tan Thanh and Tri Ton (in the Plain of Reeds and Long Xuyen Quadrangle, respectively) have acid sulphate soils with sulphuric horizons within 50 cm from the soil surface, and deep flooding (>100cm) during the rainy season. The Phung Hiep area with moderately acid sulphate soils is not deeply flooded and is located in the fresh water area of Ca Mau peninsula. The Hong Dan area contains mainly moderately and strongly acid sulphate soils with salt water intrusion during the dry season. The main agricultural problems in these study areas, where most farmers are poor, arise from the fact that most modem practices or new cropping patterns are applied incompletely and at unequal levels. Also unfavourable soil acidity coincides with high flooding depth or salt water intrusion, poor infrastructure for irrigation and drainage and shortage of capital for farm production. Furthermore, price fluctuations of farm products are very high, which makes economic production very difficult.

    A study at farmers' level shows that the history of fanners' land use, including their soil-water-crop management practices clearly indicates changes of land use over time and the important role of expert knowledge in initiating these changes. Developments also illustrate the positive effects of the creative interaction between fanners and experts in developing innovative management systems. Ten, four, sixteen and eleven actual land use types, including land and water practices, were described for the Tan Thanh, Tri Ton, Phung Hiep, and Hong Dan areas, respectively. Water-soil-crop management practices such as construction of canals and infrastructure for irrigation, construction of high raised beds for cultivation of upland crops, and construction of a ditch system for the prawn/shrimp-rice system were essential parts of these land use systems.

    Studies by experts paid more attention to the cultivation of crops, mainly rice, and the associated water management practices. Sixteen promising land use types were defined for a land suitability classification study based on present land use systems of four study areas. Double cropping of modem rice varieties is present in all study areas. Eight land qualities were identified for a farm-level land evaluation. Land quality "(potential) soil acidity" is important for all land use types selected. Land qualities: "flood hazard", "drought hazard", "salt water intrusion", and "fresh water availability" are important for double cropping of modem rice varieties. Land qualities: "potential for daily tidal flooding and drainage" are critical for raising of prawn/shrimp. For land evaluation at farm level, decision trees were developed and used as an decision support system. Decision trees were made for each promising land use type. Using decision trees as a decision support system for land evaluation study at the farm level was shown to be helpful to identify optimal management decisions. Conditions for improvement of suitability were identified and visualized in those decision trees, in terms of improved soil and water practices based on farmers' expertise and expert knowledge.

    While developing these schemes, it became clear that many unanswered questions on land use requirements and water management practices were encountered. These were formulated and field experiments were carried out to answer the questions. Some conclusions were drawn from these experiments: (1) For yam cultivation in acid sulphate soil areas with a deep flood in the wet season, application of a mulch layer on top of the raised bed resulted in a 46% higher yield as compared with urimulched plots; (2) the use of fresh water in an irrigation frequency of 20 days, when available during the dry period of the cropping season, increased yields of yam by 31%. (3) Rolled-carpet raised beds gave higher yields as compared with mixed-raised beds. (4) A relatively high yield of yarn can be obtained by application of N 120 -P 60 -K 60 as is already being practised by farmers in the area or N 80 -P 160 -K 100 . Higher fertilizer applications give no better yields. Staking yam vines is of limited importance and does not increase yarn production. A planting density of 40 cm. x 50 cm is proposed for yam cultivation. (5) For pineapple cultivation in areas with dry season salinity, supplementary irrigation in the early dry season had no effect on pineapple yield. (6) Production was higher on high raised beds as compared with traditional lowraised beds because excess water in the rainy season. Old raised beds gave significantly higher yields as compared with younger raised beds because of lower acidity due to longer leaching by rain. (7) A yield increase of pineapple of about 20% can be obtained by a combination of high P and K fertilization ( 10N-7g P 2 O 5 -10K 2 O/plant). And (8) application of the zero-tillage technique was profitable as it shortened the duration of double cropping of Winter-Spring modem rice followed by Summer-Autumn rice in areas with long duration flooding. These accumulated results provided quantitative data to characterize land qualities and land use requirements relating to fresh water availability, soil acidity, and flooding hazard . Also data for agronomic practices were obtained by experiments to be used for formulating optimal management packages.

    Under different physical conditions, optimal management packages on acid sulphate soils were finally formulated for the four study areas in. These optimal management packages were based on the farmers' practice, expert knowledge, results of land evaluation study at farm level and accumulated results obtained by field experiments. There are many options including various cultural, soil and water management practices for the different physical conditions encountered. Double modem rice cropping, and growing yam, pineapple, and sugarcane, are suitable in areas where fresh water is available during the dry season. In areas with salt water intrusion during the dry season, land use types incorporating shrimp are more profitable. Melaleuca spp. is most suitable for remote and unfavourable areas for crop growth as a means for reforestation in order to protect natural environments. These results provides a good start to obtain description of realistic and acceptable systems. In the future, these description can also be used to identify areas of additional research by measurement or by simulation modelling.

    Scenario studies on soil acidification at different spatial scales
    Vries, W. de; Kros, J. ; Groenenberg, J.E. ; Reinds, G.J. ; Salm, C. van der; Posch, M. - \ 1995
    In: Scenario studies for the rural environment : selected and edited proceedings of the symposium scenario studies for the rural environment, Wageningen, The Netherlands, 12 - 15 September 1994 / Schoute, J.T.H., Finke, P.A., Veeneklaas, F.R., - p. 169 - 188.
    zure gronden - kattekleigronden - neerslag - chemische eigenschappen - zuurgraad - zure regen - modellen - onderzoek - acid soils - acid sulfate soils - precipitation - chemical properties - acidity - acid rain - models - research
    Three dynamic soil acidification models have been developed for application on local, national and continental (European) scales, namely NUCSAM, RESAM and SMART. This paper gives an overview of results of various model validation and scenario studies for the effects of SOx, NOx and NHx deposition on soils. Furthermore, the various strong and weak points of the models are evaluated in terms of uncertainties in model predictions, the use of the models in acidification abatement policies, and the limitations and possibilities of using the models in other scenario studies, such as changes in land use, hydrology and heavy-metal deposition.
    Effects of acid deposition on Dutch forest ecosystems
    Vries, W. de; Leeters, E.E.J.M. ; Hendriks, C.M.A. - \ 1995
    Water Air and Soil Pollution 85 (1995)3. - ISSN 0049-6979 - p. 1063 - 1068.
    zure gronden - kattekleigronden - bodem - stikstof - bosschade - zure regen - nederland - acid soils - acid sulfate soils - soil - nitrogen - forest damage - acid rain - netherlands
    Effects of elevated sulphur and nitrogen deposition on the solution chemistry of Dutch forest soils are mainly manifested by increased aluminium concentrations, associated with increased concentrations of sulphate and nitrate. Critical aluminium/base cation ratios are often exceeded below 20 cm soil depth. Besides, elevated nitrogen deposition during the last decades has affected the forest nutrient status and caused large changes in vegetation. About half of the Dutch forests have absolute shortage of phosphorus and relative magnesium deficiences compared with foliar nitrogen contents. There is no evidence of a relationship between soil acidification and nutrient imbalances on the one hand and forest vitality on the other.
    Evaluation of water management strategies for acid sulphate soils using a simulation model: a case study in Indonesia
    Bronswijk, J.J.B. ; Groenenberg, J.E. ; Ritsema, C.J. ; Wijk, A.L.M. van; Nugroho, K. - \ 1995
    Agricultural Water Management 27 (1995)2. - ISSN 0378-3774 - p. 125 - 142.
    zure gronden - kattekleigronden - drainage - sloten - indonesië - modellen - onderzoek - acid soils - acid sulfate soils - drainage - ditches - indonesia - models - research
    Reclamation and drainage of potential acid sulphate soils results in acidification and release of toxic elements, hampering their use for agriculture. Ecologically valuable habitats located downstream of reclamation areas may be disrupted. Proper water management is essential to a sustainable reclamation of these soils. The Simulation Model for Acid Sulphate Soils (SMASS) has been developed to predict the effects of water management on soil and water quality. A case-study in the Barambai area in Indonesia, for which six water management strategies were evaluated, illustrates how SMASS can be applied to make a scientifically based evaluation of water management strategies.
    Uncertainties in long-term predictions of forest soil acidification due to neglecting seasonal variability
    Kros, J. ; Groenenberg, J.E. ; Vries, W. de; Salm, C. van der - \ 1995
    Water Air and Soil Pollution 79 (1995)1/4. - ISSN 0049-6979 - p. 353 - 375.
    bosschade - zure regen - zure gronden - kattekleigronden - neerslag - chemische eigenschappen - zuurgraad - modellen - onderzoek - forest damage - acid rain - acid soils - acid sulfate soils - precipitation - chemical properties - acidity - models - research
    Soil and soil solution response simulated with a site-scale soil acidification model (NUCSAM) was compared with results obtained by a regional soil acidification model (RESAM). Uncertainties due to neglecting seasonal variability in long-term predictions were investigated by comparing long-term RESAM and NUCSAM simulations. Although both the seasonal and the interannual variations in soil solution parameters were large, the trend in soil solution parameters of RESAM and NUCSAM corresponded quite well. Generally, it appeared that the uncertainty due to time resolution in long-term predictions was relatively small.
    Modelling the response of terrestrial ecosystems to acidification and desiccation scenarios
    Kros, J. ; Reinds, G.J. ; Vries, W. de; Latour, J.B. ; Bollen, M. - \ 1995
    Water Air and Soil Pollution 85 (1995)3. - ISSN 0049-6979 - p. 1101 - 1106.
    zure gronden - kattekleigronden - plantengemeenschappen - vegetatie - onderzoek - modellen - acid soils - acid sulfate soils - plant communities - vegetation - research - models
    In order to evaluate the effects of eutrophication, acidification and drought on site factors, the SMART2 model has been developed. For the Netherlands, combinations of acidification and seepage scenarios were evaluated with SMART2. The results are focused on pH, nitrogen availability and base saturation. Results showed that deposition reductions lead to a relatively fast improvement of the site factors, increase in pH and base saturation and decrease in nitrogen availability, whereas a reduction in groundwater abstractions by 25% has little or no effect on pH and nitrogen availability.
    Application of soil acidification models with different degrees of process description (SMART, RESAM and NUCSAM) on an intensively monitored spruce site
    Salm, C. van der; Kros, J. ; Groenenberg, J.E. ; Vries, W. de; Reinds, G.J. - \ 1995
    In: Solute modelling in catchment systems / Trudgill, S.T., - p. 327 - 346.
    zure gronden - kattekleigronden - bosbouw - neerslag - zure regen - zure depositie - chemische eigenschappen - zuurgraad - modellen - onderzoek - acid soils - acid sulfate soils - forestry - precipitation - acid rain - acid deposition - chemical properties - acidity - models - research
    A one-layer (SMART) and a multi-layer (RESAM) soil acidification model with temporal resolutions of one year and a multi-layer model with a resolution of one day (NUCSAM) were applied to an intensively monitored spruce site in Solling, Germany. The major aim was to study the influence of model simplifications and reduction of temporal and vertical resolutions on the simulation of soil solution concentrations. Results showed that all the models were able to simulate most of the concentrations reasonably during the examined period (1973-1989). However, the one-layer model, SMART, had difficulties in simulating strong changes in soil solution concentrations owing to a lower retardation in the soil system.
    Modelling phosphate sorption kinetics in acid soils.
    Freese, D. ; Riemsdijk, W.H. van; Zee, S.E.A.T.M. van der - \ 1995
    European Journal of Soil Science 46 (1995). - ISSN 1351-0754 - p. 239 - 245.
    bodem - fosfor - absorptie - adsorptie - zure gronden - kattekleigronden - modellen - onderzoek - soil - phosphorus - absorption - adsorption - acid soils - acid sulfate soils - models - research
    A world perspective on acid sulphate soils.
    Dent, D.L. ; Pons, L.J. - \ 1995
    Geoderma 67 (1995). - ISSN 0016-7061 - p. 263 - 276.
    zure gronden - kattekleigronden - statistiek - wereld - acid soils - acid sulfate soils - statistics - world
    SMASS : a simulation model of physical and chemical processes in acid sulphate soils : version 2.0
    Bronswijk, J.J.B. ; Bosch, H. van den; Nugroho, K. - \ 1994
    Wageningen : DLO Winand Staring Centre (Technical document / DLO Winand Staring Centre 21) - 161
    zure gronden - kattekleigronden - fysicochemische eigenschappen - bodemeigenschappen - bodemchemie - infiltratie - hydraulisch geleidingsvermogen - kwel - modellen - onderzoek - acid soils - acid sulfate soils - physicochemical properties - soil properties - soil chemistry - infiltration - hydraulic conductivity - seepage - models - research
    Soil salinity and acidity : spatial variabil[it]y and effects on rice production in West Africa's mangrove zone
    Sylla, M. - \ 1994
    Agricultural University. Promotor(en): N. van Breemen, co-promotor(en): L.O. Fresco. - S.l. : Sylla - ISBN 9789054852865 - 175
    zure gronden - kattekleigronden - bodem - natrium - verbetering - verzilting - rijst - oryza sativa - landevaluatie - grondvermogen - bodemgeschiktheid - west-afrika - geostatistiek - acid soils - acid sulfate soils - soil - sodium - improvement - salinization - rice - oryza sativa - land evaluation - land capability - soil suitability - west africa - geostatistics

    In the mangrove environment of West Africa, high spatial and temporal variability of soil constraints (salinity and acidity) to rice production is a problem for the transfer and adoption of new agronomic techniques, for land use planning, and for soil and water management. Recently, several National and International Agricultural Centers have initiated research programs to characterize environments where their newly developed technologies have to be applied. However, the mangrove agro- ecosystems in West Africa have not been characterized in a detailed way. Most of the soils in this environment are potential or actual saline acid sulfate soils. The spatial and temporal variability of soil salinity and acidity in these coastal lowlands results from complex interactions between climate, coastal morphology, river hydrology, vegetation, landform and tidal flooding. Diagnosing the occurrence of both potential and actual acid sulfate soils is the first step in land use planning for such areas. But to cope with the intricacies of these soils, understanding the processes of soil salinization and acidification at different scales should be formalized to properly characterize mangrove environments.

    The main objectives of this thesis were: 1) to give a comprehensive characterization framework for the West African mangrove environments with emphasis on the possibilities of and constraints for rice cultivation; 2) to determine the various causal factors for soil salinization and acidification; 3) to test whether temporal variability of soil chemistry is sufficient to provide a time window of minimum stress during the rice growing period; 4) to relate the response of rice to improved agronomic practices in specific environments and to provide a means to characterize specific rice growing locations, and 5) to test rice varietal responses to saline and acid sods under different agronomic practices and to relate yields and yield components to the nutrient contents in leaves, in order to diagnose physiological disorders.

    First, a multi-scale approach was developed involving a range from Macro to Micro level based on the pre-conditions of acid sulfate soil formation. The main factors for classification are climate and coastal morphology at Macro scale; hydrology, physiography and vegetation complexes at Meso level; and topography (catena), vegetation species, tidal flooding and sedimentation rate at Micro level. Information from previous process-based studies on acid sulfate soil formation and data from secondary sources were used. Different environments were then distinguished and their characteristics were summarized by ecological zone. Constraints to rice production and potentials for agricultural development were matched with environmental conservation issues.

    To determine the significance of the causal factors developed in the multiple scale approach, 12 sites were selected along 4 river basins in West Africa, vz. from north to south the Gambia, the Casamance (Senegal), the Geba (Guinea Bissau) and the Great Scarcies (Sierra Leone). Along each river basin 3 sites were selected based on distance from the river mouth. Within a site a strip of land perpendicular to the river was selected for intensive grid sampling (40 by 20 m). Soil samples were taken at each grid node during the dry season of 1991. The relation between causal factors and soil salinization and acidification was determined at Macro and Meso levels by nested ANOVA and yielded a classification of the study area in main ecoregions and sub-environments within watershed. At a detailed scale, geostatistics were applied and zones within catena were defined in terms of their main soil characteristics. A nested statistical approach and geostatistics were used complementarily to disentangle the complexity of the causes of soil salinization and acidification.

    Temporal variability was studied by monitoring soil solution chemistry at each main landscape unit within the catena. Since the production of rice critically depends on the lowering of salinity and acidity by natural flooding during rainy season, time windows during which soil limitations are minimal were defined and matched with rice varietal duration.

    The response of rice to different improvement techniques were tested by means of a network of trials in the 1991 and 1992 rainy seasons. The residual effects of lime and phosphate rock (applied in 1991) during 1992 was also evaluated. Lime dressing (2 t ha -1) was found effective whenever dissolved Ca and Mg in the soil were low, and had a clear residual effect in the year after application. Application of phosphate rock did not seem to be effective in general. For iron toxicity, the molar fraction of Fe and (Ca + Mg) in soil solution and in flag leaves were found to be more relevant for diagnosing physiological disorders than the absolute Fe content in the soil solution and in rice flag leaves at panicle initiation.

    In the 1993 rainy season, rice varietal behavior under different improvement techniques within the main soil limitations in the mangrove environment was tested. Differences in yield and yield components and element contents in flag leaves at panicle initiation were observed between varieties in saline and less saline acid soils. Multiple correlation between rice yields, yield components, element contents in flag leaves at panicle initiation was found to be an effective diagnostic tool for assessing physiological disorders.

    The approach used in this study provides a logical framework to describe mangrove environments. The multiple-scale can assist in identifying the information required to cater for the needs of various decision-makers and land use planners. It also provides a key to develop technology packages for intensified and sustainable use. It can be used for the extrapolation of site-specific information to geographically different areas, with similar characteristics.

    Time trends & mechanism of soil acidification
    Wesselink, L.G. - \ 1994
    Agricultural University. Promotor(en): N. van Breemen; J.J.M. van Grinsven. - S.l. : Wesselink - ISBN 9789054852896 - 129
    zure gronden - kattekleigronden - bosbouw - verzuring - bodem ph - bodemaciditeit - neerslag - chemische eigenschappen - zuurgraad - zure regen - acid soils - acid sulfate soils - forestry - acidification - soil ph - soil acidity - precipitation - chemical properties - acidity - acid rain

    The effects of acid atmospheric deposition on forest ecosystems have been studied intensively in the past two decades. Measurements of element budgets in forested ecosystems throughout the world have shown that acid deposition may deplete stores of exchangeable base cations in the soil, decrease the soil pH, increase rates of mineral weathering, and release potentially toxic Al into the soil solution. In summary, acid atmospheric deposition can strongly alter the chemical environment for living organisms.

    In this thesis the mechanisms that may control those phenomena are studied, with emphasis on soil chemical processes. Central in this study are long and comprehensive data sets from continuous measurements (monitoring) of element fluxes in forest ecosystems, specifically from the Solling experimental forest in Germany.

    Base Cations

    Using over 20 years of data on deposition and soil chemistry at Solling, chapter 2 discusses how long-term changes in atmospheric deposition of acid anions and base cations affect dissolved and exchangeable pools of base cations in the soil, through the process of cation exchange. Positive effects of declining SO 4 deposition in the 1980s, which potentially reduces leaching of base cations from the soil, were partially offset by (i) continued high concentrations of dissolved SO 4 in the soil and (ii) declining deposition of the base cations Ca and Mg. The latter appears to result from stricter controls on particle emissions from fuel combustion and industrial processes. Recent studies indicate that declining deposition of base cations is observed in other regions of Europe and North America as well. This is a matter of concern as forests may be sensitive to changes in base cation deposition which is, together with mineral weathering, the most important source of nutrients to forests on acidified soils.

    In chapter 3 we quantified rates of mineral weathering at Solling, as a source of nutrients and acid neutralization. Estimates of field weathering rates were obtained from, (i) long-term input- output mass balances, correcting for changes in stores of exchangeable base cations in the soil, and (ii) elemental analysis of the soil profile. At Solling, rates of supply of Ca, K and Na by weathering are much lower than by atmospheric deposition; for Mg the contributions are of similar magnitude. This emphasizes (i) the relevance of atmospheric deposition of base cations as a nutrient source to the Solling forests, and (ii) the concern with respect to its present decline. To investigate mechanisms of mineral weathering of Mg and K, a series of laboratory weathering experiments with soils from Solling were conducted.

    These focused on weathering of illite, the dominant mineral source of K and Mg. A major problem in quantifying rates of mineral weathering is the large discrepancy in rates obtained in laboratory studies and field estimates, with laboratory rates being generally one to three orders higher. In the laboratory, effects of pH, temperature, and soil solution composition on illite weathering were studied. Corrected for the effects of these factors, Mg and K release rates from illite measured in the laboratory were comparable (K) or a factor 2 to 4 higher (Mg) than rates inferred from field methods. It is concluded that field estimates of weathering rates, from elemental analysis of the soil profile or input-output flux balances, remain necessary to obtain reliable estimates of base cation weathering rates.

    Anions

    In a simplified way, anions in the soil solution can be regarded as carriers of base and acid cations, through charge balance constraints. Understanding the processes that determine dissolved concentrations of these components in the soil is therefore of prior importance. In chapter 4 we discussed the processes that govern the mobility of dissolved Cl, SO 4 and NO 3 in the Solling spruce soil over a period of 18 years. For this purpose a simulation model describing soil hydrology and a number of biogeochemical processes was introduced. Using throughfall water and concentrations as input to the model, concentrations of Cl in soil solutions were successfully simulated, a requirement to further study components that interact chemically or biologically with the soil.

    Sorption of SO 4 is of significance in the soils at Solling due to comparatively high contents of Al and Fe oxides. By contrast, in the mineralogically poor sandy soils in the Netherlands SO 4 often behaves chemically nearly conservative. The steep increase in dissolved SO 4 in the Solling spruce soil during the mid-1970s was modelled adequately using adsorption parameters reported in literature, and was explained by a steep adsorption isotherm and a state of near saturation of SO 4 sorption sites in the early 1970s, at the start of the monitoring program. Rates of desorption of SO 4 in the 1980s in response to declining SO 4 deposition, described by the same model, were less than those measured in the field. In the absence of evidence for alternative controls on dissolved SO 4 , like jurbanite (AlOHSO 4 dissolution, the mechanism of SO 4 desorption remains unclear.

    The model increased our understanding of the processes that control repeated 3-year cycles of high and low concentrations of N0 3 in deeper layers (90 cm) of the Solling spruce soil. Recently, there is increasing interest in N-dynamics in forest ecosystems, as N-saturation has been hypothesized as an additional cause of forest decline. In the Solling spruce soil, concentrations of N0 3 water leaving the soil profile increased when the N-flux into the mineral soil, from deposition and net mineralization, exceeded the N uptake capacity of the forest at a threshold value of around 4 kmol ha -1yr -1. At present, total deposition of N on the spruce forest amounts to 3.3 kmol ha -1yr -1and is close to this threshold value. This points to the critical role of N mineralization processes in the litter layer, where large stores of N have accumulated over the past 25 years. The annual release of N from the spruce litter layer was positively correlated with rainfall. This was apparent from large N releases from litter in relatively wet years which subsequently increased NO 3 concentrations at the 90 cm depth.

    Aluminum

    The main acid neutralizing process in acid forest soils under high acid loads is the release of AI to the soil solution. Soil acidification models generally describe the solubility of reactive soil Al by equilibrium or kinetic dissolution of Al-trihydroxide (Al(OH) 3 ), In chapters 5 and 6, we scrutinized the evidence for this reaction mechanism from extensive sets of field data, but found little support for the gibbsite model. By contrast, evidence points to AI binding to soil organic matter as a control of dissolved Al, even when organic matter contents are low. In chapter 6 a new, mechanistic, and yet simple model was presented that explained much of the spread in Al-solubility, as observed in 29 different soil layers from 18 forest soils. The model describes Al-solubility as a function of the degree to which humus binding sites are saturated with Al. It involves only two soil specific parameters, the organic C content of the soil and an estimate of the pool of organically bound soil Al, and may therefore be easily incorporated in current soil acidification models.

    The soils studied in this thesis generally receive high loads of acid deposition, and acidity is largely neutralized by release of Al in the upper 20 cm of the mineral soil. It has been suggested that in surface layers of these soils depletion of reactive soil Al may result within decades. The depletion hypothesis was put to test by measuring pools of organically bound soil Al in old (around 1980) and new (around 1993) samples from research sites at Hackfort and Gerritsfles in the Netherlands, and Solling in Germany. Indeed, significant decreases with time were found. Independently measured depletion of organically bound soil Al and declining Al solubility, as observed previously from 6 years of field monitoring at Hackfort in the 1980s, proved to be consistent with the humus-Al model presented in this thesis. Rates of depletion of organically bound soil Al at Hackfort, the Netherlands, during a period of about 13 years were lower than expected from long-term monitoring of Al solute fluxes. This suggests that pools other than organically bound Al, i.e. silicates and inorganic Al hydroxides, may contribute to Al mobilization as well. Literature indicates that release of Al from these pools may be kinetically constrained. The current discussion on whether Al activities in solutions of acid forest soils are controlled by kinetically constrained (time dependent) dissolution of inorganic Al hydroxides or by equilibrium binding to soil organic matter is, in my view, clarified when the kinetic and the equilibrium process are regarded as sequential; kinetic release supplying Al to the organic exchanger, which equilibrates with the soil solution.

    GENERAL CONCLUSIONS

    1. In the recent past, deposition of SO 4 , Ca and Mg on the German Solling forests has declined significantly as a result of reduced industrial emissions.
    2. Results from Solling indicate that soil de-acidification, i.e. an increase in the pool of exchangeable base cations, as a result of declining SO 4 deposition may be offset by i) continuous release of sorbed SO 4 from the soil and ii) declining deposition of base
    cations.
    3. Weathering rates of Ca, K and Na at Solling, Germany, are much lower than their supply by atmospheric deposition; for Mg the contributions of deposition and weathering are of similar magnitude.
    4. Illite is the main mineral source of Mg and K at Solling. Although coming from the same mineral source, the two cations are released by different mechanisms. Magnesium release is controlled by chemical kinetic processes and its release is highly
    pH dependent (R≈(H +) 0.8), whereas K is released from interlayers by diffusion, a process that showed independent of pH.
    6. A six-fold increase of dissolved SO 4 in the Solling spruce soil (90 cm) in the mid 1970s, and continuously high concentrations thereafter, is explained by 'peaking' SO 4 deposition loads in the mid-1970s that entered a mineral soil where the SO 4 sorption capacity was nearly saturated.
    6. Repeated 3-year cycles of high and low concentrations of NO 3 in deeper layers (90 cm) of the Solling spruce soil are related to excess release of N from the litter layer in comparatively wet years.
    7. Strong changes in pH and less strong changes in pAl (-log{Al 3+}) in response to varying acid anion concentrations, generally observed in acid forest soils, and the measured increased of CEC with time in the surface soil of the Solling spruce sites, support the hypothesis of Al solubility control by complexation to soil organic matter.
    8. Data from 29 soil layers of 18 forest soils, covering a range in soil pH, Al activities, and reactive soil Al pools indicate that Al solubility control by complexation to soil organic matter is a general phenomenon, even when organic C contents are low.
    9. Much of the variation in Al solubility observed in 29 different soil layers of 18 forest soils is explained by the organically bound Al: soil C ratio.
    10. In surface soils where stores of organically bound Al are low and release rates high depletion of reactive, organically bound, soil Al is significant on a time scale of 12 to 14 years only.
    11. Independently measured declines in (i) Al solubility in surface soils at Hackfort the Netherlands, from 6 years of monitoring of soil water chemistry and (ii) pools of organically bound soil Al, are consistent with the Al-humus equilibrium model
    proposed in this thesis.
    12. In acid forest soils where dissolved Al activities are controlled by equilibrium complexation to soil organic matter, kinetically constrained release of Al from inorganic Al pools may well contribute to acid neutralization.

    Effects of flooding on pH of rice-producing, acid sulfate soils in Indonesia.
    Konsten, C.J.M. ; Breemen, N. van; Suping, S. ; Bagus Aribawa, I. ; Groenenberg, J.E. - \ 1994
    Soil Science Society of America Journal 58 (1994). - ISSN 0361-5995 - p. 871 - 883.
    rijst - oryza sativa - zure gronden - kattekleigronden - landevaluatie - grondvermogen - bodemgeschiktheid - veldcapaciteit - waterverzadiging - indonesië - rice - oryza sativa - acid soils - acid sulfate soils - land evaluation - land capability - soil suitability - field capacity - waterlogging - indonesia
    Seasonal and spatial variation of heavy metal solubilities in a fallow, acidic loamy sand loam
    Castilho, P. del; Blaauw, D. ; Salomons, W. - \ 1993
    In: Integrated soil and sediment research : a basis for proper protection : selected proceedings of the first European conference on integrated research for soil and sediment protection and remediation (EUROSOL), held in Maastricht, 6 - 12 September 1992 / Eijsackers, H.J.P., Hamers, T., Kluwer (Soil & environment vol. 1) - p. 255 - 256.
    bodemoplossing - bodem - zware metalen - zure gronden - kattekleigronden - soil solution - soil - heavy metals - acid soils - acid sulfate soils
    Time series analysis of changes in the soil solution: evidence for approach to nitrogen saturation in Dutch forest soils.
    Stein, A. ; Breemen, N. van - \ 1993
    Agriculture, Ecosystems and Environment 47 (1993). - ISSN 0167-8809 - p. 147 - 158.
    zure gronden - kattekleigronden - neerslag - chemische eigenschappen - zuurgraad - zure regen - statistiek - waarschijnlijkheidsanalyse - wiskunde - acid soils - acid sulfate soils - precipitation - chemical properties - acidity - acid rain - statistics - probability analysis - mathematics
    Sulfidic materials in the Western Mekong Delta, Vietnam.
    Brinkman, R. ; Nguyen Bao Ve, ; Tran Kim Tinh, ; Phuoc Hau, Do; Mensvoort, M.E.F. van - \ 1993
    Catena 20 (1993). - ISSN 0341-8162 - p. 317 - 331.
    zure gronden - kattekleigronden - vietnam - acid soils - acid sulfate soils - vietnam
    Selected Papers of the Ho Chi Minh City Symposium on Acid sulphate soils. Vietnam, March 1992
    Dent, D.L. ; Mensvoort, M.E.F. van - \ 1993
    Wageningen : ILRI (ILRI Publication 53) - 425
    zure gronden - kattekleigronden - wereld - acid soils - acid sulfate soils - world
    Analysis of aluminium sensitivity in sorghum (Sorghum bicolor (L.) Moench) genotypes
    Tan, K. - \ 1993
    Agricultural University. Promotor(en): G.R. Findenegg; W.G. Keltjens. - S.l. : Tan - ISBN 9789054850946 - 155
    oogstschade - bodemverontreiniging - bodemgiftigheid - bodemzoutgehalte - Sorghum bicolor - toxische stoffen - chemie - indicatorplanten - toxinen - planten - bodem - aluminium - oogsttoename - oogstverliezen - opbrengsten - zure gronden - kattekleigronden - chemische factoren - crop damage - soil pollution - soil toxicity - soil salinity - Sorghum bicolor - toxic substances - chemistry - indicator plants - toxins - plants - soil - aluminium - yield increases - yield losses - yields - acid soils - acid sulfate soils - chemical factors

    Twelve genotypes of sorghum ( Sorghum bicolor (L.) Moench) differing in Al sensitivity were grown in an acid soil (with additions of lime or MgSO 4 ) and in nutrient solutions (with or without Al at constant pH) for periods between 14 and 35 days. The objective was the identification of the factors controlling dry matter yield of sorghum under different growth conditions. In both media Al was the major constraint, restricting growth in two independent ways: (1) by inducing Mg deficiency and (2) via damaging the roots ( i.e. by giving them a stubby and discolorated appearance and by reducing their specific root length, m g -1dry root). The sensitivities of the genotypes against Al-induced Mg deficiency and Al-induced root damage were not correlated. At moderate acidity (pH around 4.8), Mg deficiency dominantly limited growth whilst at a higher acidity (pH ~4.2) root damage overruled Mg deficiency in its negative effect on growth. At pH 4.8, addition of Mg improved growth by reducing the degree of Mg deficiency. At pH 4.2, Mg improved growth mainly by preventing the roots from Al- induced damage.

    Several external factors modified the AI sensitivity of the genotypes by strengthening or weakening the negative effects of Al on Mg nutrition and root development. At pH 4.2, Ca and NH 4 both counteracted AI-induced root damage but aggravated Al-induced Mg deficiency. The contrary was true for NO 3 . When the concentration of soluble Al was kept approximately constant at 15 μM, both Al-induced root damage and Mg deficiency were aggravated by acidity in the range pH 3.9-4.8. Aluminium toxicity in sorghum grown in nutrient solution was independent of P deficiency, although an increased P supply partly eliminated Al phytotoxicity.

    The results stress the importance of both Al and Mg ions and their interactions in determining growth response of sorghum and other cereals to acid soils.

    The transformation of potential into actual acid sulphate soils studied in column experiments
    Ritsema, C.J. ; Groenenberg, J.E. ; Bisdom, E.B.A. - \ 1992
    Geoderma 55 (1992)3/4. - ISSN 0016-7061 - p. 259 - 271.
    zure gronden - kattekleigronden - acid soils - acid sulfate soils
    The transformation of potential into actual acid sulphate soils was studied in a 450 day column experiment to determine the rate and extent of pyrite oxidation and accompanying buffering processes. One potential acid sulphate soil was derived from the lower section of a Typic Sulfaquept (Soil Survey Staff, 1987) or Thionic Fluvisol (FAO, 1988) near the village of Nieuwkoop, the Netherlands, and the other from beneath the acidified upper part of a Typic Sulfaquent (Soil Survey Staff, 1987) or Thionic Fluvisol (FAO, 1988) of the Pulau Petak region in South Kalimantan, Indonesia.

    Drainage of both the potential acid sulphate soils caused a strong acceleration in pyrite oxidation, i.e. approximately 10 g FeS2 per kg soil in the Dutch soil and more than 20 g in the Indonesian soil. In both columns, iron, released by the oxidation of pyrite, is immobilized instantaneously due to the precipitation of jarosite and/or iron hydroxides.

    The presence of carbonates prevented acidification of the soil solution upon pyrite oxidation in the Dutch soil, whereas severe acidification of the soil solution occurred in the Indonesian soil. The cationic composition of the adsorption complex is seriously affected in layers where pyrite oxidation occurred; main exchange reactions are Al3+ against Ca2+ and Mg2+ for the drained Dutch and drained Indonesian soils, respectively.

    The experimentally obtained actual acid sulphate soils show a high similarity with the actual acid sulphate soils found in the regions under consideration.
    Research on acid sulphate soils in the humid tropics : proposal phase 2
    Anonymous, - \ 1992
    Wageningen : LAWOO (Research on acid sulphate soils in the humid tropics )
    zure gronden - kattekleigronden - verbetering - bodem - niet-metallische elementen - onderzoek - zwavel - tropen - acid soils - acid sulfate soils - improvement - soil - nonmetallic elements - research - sulfur - tropics
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