Integrating Stand and Soil Properties to Understand Foliar Nutrient Dynamics during Forest Succession Following Slash-and-Burn Agriculture in the Bolivian Amazon
Broadbent, E.N. ; Zambrano, A.M.A. ; Asner, G.P. ; Soriano, M. ; Field, C.B. ; Souza, H.R. de; Pena Claros, M. ; Adams, R.I. ; Dirzo, R. ; Giles, L. - \ 2014
PLoS ONE 9 (2014)2. - ISSN 1932-6203 - 23 p.
carbon-isotope discrimination - tropical rain-forests - n-15 natural-abundance - northeastern costa-rica - below-ground carbon - land-use change - n-p ratios - secondary forest - organic-matter - brazilian amazon
Secondary forests cover large areas of the tropics and play an important role in the global carbon cycle. During secondary forest succession, simultaneous changes occur among stand structural attributes, soil properties, and species composition. Most studies classify tree species into categories based on their regeneration requirements. We use a high-resolution secondary forest chronosequence to assign trees to a continuous gradient in species successional status assigned according to their distribution across the chronosequence. Species successional status, not stand age or differences in stand structure or soil properties, was found to be the best predictor of leaf trait variation. Foliar d13C had a significant positive relationship with species successional status, indicating changes in foliar physiology related to growth and competitive strategy, but was not correlated with stand age, whereas soil d13C dynamics were largely constrained by plant species composition. Foliar d15N had a significant negative correlation with both stand age and species successional status, – most likely resulting from a large initial biomass-burning enrichment in soil 15N and 13C and not closure of the nitrogen cycle. Foliar %C was neither correlated with stand age nor species successional status but was found to display significant phylogenetic signal. Results from this study are relevant to understanding the dynamics of tree species growth and competition during forest succession and highlight possibilities of, and potentially confounding signals affecting, the utility of leaf traits to understand community and species dynamics during secondary forest succession.
Overlap in nitrogen sources and redistribution of nitrogen between trees and grasses in a semi-arid savanna
Priyadarshini, K.V.R. ; Prins, H.H.T. ; Bie, S. de; Heitkonig, I.M.A. ; Woodborne, S. ; Gort, G. ; Kirkman, K. ; Fry, B. ; Kroon, H. de - \ 2014
Oecologia 174 (2014)4. - ISSN 0029-8549 - p. 1107 - 1116.
n-15 natural-abundance - southern african savanna - root activity - patterns - water - availability - vegetation - physiology - isotopes - depth
A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semi-arid savanna. We also used 15N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (d15N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar d15N values were similar for all three tree species differing in the potential for N fixation through nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.
Decomposition and nutrient release in leaves of Atlantic Rainforest tree species used in agroforestry systems
Duarte, E.M.G. ; Cardoso, I.M. ; Stijnen, T. ; Mendonça, M.A.F.C. ; Coelho, M.S. ; Cantarutti, R.B. ; Kuyper, T.W. ; Villani, E.M.A. ; Mendonça, E.S. - \ 2013
Agroforestry Systems 87 (2013)4. - ISSN 0167-4366 - p. 835 - 847.
n-15 natural-abundance - biological nitrogen-fixation - tropical agroecosystems - plant residues - green manure - minas-gerais - soil - legumes - brazil - biodiversity
Aiming to support the use of native species from the Atlantic Rainforest in local agroforestry systems, we analysed chemical and biochemical components related to leaf decomposition of Inga subnuda, Senna macranthera, Erythrina verna, Luehea grandiflora, Zeyheria tuberculosa, Aegiphila sellowiana, and Persea americana. These tree species are native (except for P. americana) and commonly used in agroforestry systems in the Atlantic Rainforest. For the three first species (Fabaceae), we also analysed the remaining dry matter and released nutrients from leaves, using litter bags, and biological nitrogen fixation, using Bidens pilosa and Brachiariaplantaginea as references of non-N2-fixing plants. Leaves from I. subnuda, L. grandiflora, and P. americana had a lower decomposition rate than the other species, exhibiting negative correlations with lignin/N and (lignin+polyphenol)/N ratios. The percentages of remaining dry matter after 1 year were 69 % (I. subnuda), 26 % (S. macranthera) and 16 % (E. verna). Higher nutrient release was found in decreasing order from residues of E. verna, S. macranthera, and I. subnuda. The percentages of nitrogen fixation were 22.6 % (E. verna), 20.6 % (I. subnuda) and 16.6 % (S. macranthera). Diversification of tree species in agroforestry systems allows for input of diversified organic material and can contribute to maintaining and improving soil functions resulting in improvements of soil quality.
Productivity and residual benefits of grain legumes to sorghum under semi-arid conditions in southwestern Zimbabwe
Ncube, B. ; Twomlow, S.J. ; Wijk, M.T. van; Dimes, J.P. ; Giller, K.E. - \ 2007
Plant and Soil 299 (2007)1-2. - ISSN 0032-079X - p. 1 - 15.
biological nitrogen-fixation - groundnut vigna-subterranea - n-15 natural-abundance - smallholder farms - bambara groundnut - cropping systems - improved fallows - west-africa - soil - maize
The productivity and residual benefits of four grain legumes to sorghum (Sorghum bicolor) grown in rotation were measured under semi-arid conditions over three cropping seasons. Two varieties of each of the grain legumes; cowpea (Vigna unguiculata); groundnut (Arachis hypogaea); pigeon pea (Cajanus cajan); Bambara groundnut (Vigna subterranea), and sorghum were grown during the first season. The same experiment was implemented three times in different, but adjacent fields that had similar soil types. At the end of the season the original plots were split in two and residues were either removed or incorporated into the subplots. The following season sorghum was planted in all subplots. In 2002/03 (314 mm rainfall) cowpeas produced the largest dry grain yield (0.98 and 1.36 t ha¿1) among the legumes. During the wettest year (2003/04, 650 mm rainfall) groundnut had the highest yields (0.76 to 1.02 t ha¿1). In 2004/05 (301 mm rainfall) most legume yields were less than 0.5 t ha¿1, except for pigeon pea. Estimates of % N from N2-fixation from the legumes were 15¿50% (2002/03), 16¿61% (2003/04) and 29¿83% (2004/05). Soil water changes during the legume growth cycle were proportional to varietal differences in total legume biomass. Sorghum grain yield after legumes reached up to 1.62 t ha¿1 in 2003/04 compared with 0.42 t ha¿1 when following sorghum. In 2004/05, sorghum yields after legumes were also higher (up to 1.26 t ha¿1) than sorghum after sorghum. Incorporation of crop residues had no significant effect on sorghum yield. Beneficial effect of legumes on yields of the subsequent sorghum crop were more readily explained by improvements in soil nitrogen supply than by the small observed changes in soil water relations. Our results demonstrate clear potential benefits for increasing grain legume cultivation in semi-arid environments through the use of improved germplasm, which also gave substantial increases in subsequent sorghum productivity (up 200% in a wet season and 30¿100% in a dry season), compared with an unfertilized sorghum crop following sorghum.
Niche-based assessment of contributions of legumes to the nitrogen economy of Western Kenya smallholder farms
Ojiem, J.O. ; Vanlauwe, B. ; Ridder, N. de; Giller, K.E. - \ 2007
Plant and Soil 292 (2007)1-2. - ISSN 0032-079X - p. 119 - 135.
soil fertility management - n-15 natural-abundance - exploring diversity - improved fallows - africa - n-2-fixation - fixation - adoption - systems
Nitrogen (N) deficiency is a major constraint to the productivity of the African smallholder farming systems. Grain, green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N-2-fixation. The N-2-fixation of bean (Phaseolus vulgaris), soyabean (Glycine max), groundnut (Arachis hypogaea), Lima bean (Phaseolus lunatus), lablab (Lablab purpureus), velvet bean (Mucuna pruriens), crotalaria (Crotalaria ochroleuca), jackbean (Canavalia ensiformis), desmodium (Desmodium uncinatum), stylo (Stylosanthes guianensis) and siratro (Macroptilium atropurpureum) was assessed using the N-15 natural abundance method. The experiments were conducted at three sites in western Kenya, selected on an agro-ecological zone (AEZ) gradient defined by rainfall. On a relative scale, Museno represents high potential AEZ 1, Majengo medium potential AEZ 2 and Ndori low potential AEZ 3. Rainfall in the year of experimentation was highest in AEZ 2, followed by AEZ 1 and AEZ 3. Experimental fields were classified into high, medium and low fertility classes, to assess the influence of soil fertility on N-2-fixation performance. The legumes were planted with triple super phosphate (TSP) at 30 kg P ha(-1) stop, with an extra soyabean plot planted without TSP (soyabean-P), to assess response to P, and no artificial inoculation was done. Legume grain yield, shoot N accumulation, %N derived from N-2-fixation, N-2-fixation and net N inputs differed significantly (P <0.01) with rainfall and soil fertility. Mean grain yield ranged from 0.86 Mg ha(-1), in AEZ 2, to 0.30 Mg ha(-1), in AEZ 3, and from 0.78 Mg ha(-1), in the high fertility field, to 0.48 Mg ha(-1) stop, in the low fertility field. Shoot N accumulation ranged from a maximum of 486 kg N ha(-1) in AEZ 2, to a minimum of 10 kg N ha(-1) in AEZ 3. Based on shoot biomass estimates, the species fixed 25-90% of their N requirements in AEZ 2, 23-90% in AEZ 1, and 7-77% in AEZ 3. Mean N-2-fixation by green manure legumes ranged from 319 kg ha(-1)(velvet bean) in AEZ 2 to 29 kg ha(-1) (jackbean) in AEZ 3. For the forage legumes, mean N-2-fixation ranged from 97 kg N ha(-1) for desmodium in AEZ 2 to 39 kg N ha(-1) for siratro in AEZ 3, while for the grain legumes, the range was from 172 kg N ha(-1) for lablab in AEZ 1 to 3 kg N ha(-1) for soyabean-P in AEZ 3. Lablab and groundnut showed consistently greater N-2-fixation and net N inputs across agro-ecological and soil fertility gradients. The use of maize as reference crop resulted in lower N-2-fixation values than when broad-leaved weed plants were used. The results demonstrate differential contributions of the green manure, forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers' needs.
Biological nitrogen fixation is not a major contributor to the nitrogen demand of a commercially growth South African sugarcane cultivar
Hoefsloot, G. ; Termorshuizen, A.J. ; Watt, D.A. ; Cramer, M.D. - \ 2005
Plant and Soil 277 (2005)1-2. - ISSN 0032-079X - p. 85 - 96.
n-15 natural-abundance - gluconacetobacter-diazotrophicus - acetobacter-diazotrophicus - fixing bacterium - plant-growth - n-2-fixing bacteria - n-2 fixation - in-vitro - sp-nov. - cane
It has previously been reported that endophytic diazotrophic bacteria contribute significantly to the nitrogen budgets of some graminaceous species. In this study the contribution of biological nitrogen fixation to the N-budget of a South African sugarcane cultivar was evaluated using 15N natural abundance, acetylene reduction and 15N incorporation. Plants were also screened for the presence of endophytic diazotrophic bacteria using acetylene reduction and nifH-gene targeted PCR with the pure bacterial strains. 15N natural abundance studies on field-grown sugarcane indicated that the plants did not rely extensively on biological nitrogen fixation. Furthermore, no evidence was found for significant N2-fixation or nitrogenase activity in field-grown or glasshouse-grown plants using 15N incorporation measurements and acetylene reduction assays. Seven endophytic bacterial strains were isolated from glasshouse-grown and field-grown plants and cultured on N-free medium. The diazotrophic character of these seven strains could not be confirmed using acetylene reduction and PCR screening for nifH. Thus, although biological nitrogen fixation may occur in South African sugarcane varieties, the contribution of this N-source in the tested cultivar was not significant.
Nitrogen content and d15N signature of ombrotrophic Sphagnum plants in Europe: to what extent is the increasing atmospheric N deposition altering the N-status of nutrient-poor mires?
Bragazza, L. ; Limpens, J. ; Gerdol, R. ; Grosvernier, P. ; Hajèk, M. ; Hajkova, P. ; Lacumin, P. ; Kutnar, L. ; Rydin, H. ; Tahvanainen, T. - \ 2005
Global Change Biology 11 (2005)1. - ISSN 1354-1013 - p. 106 - 114.
n-15 natural-abundance - atmospheric deposition - isotope composition - n-15/n-14 ratios - vascular plants - wet deposition - bog vegetation - carbon-dioxide - nitrate - growth
Alteration of the global nitrogen (N) cycle because of human-enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. Because Sphagnum-dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, and 15N isotopic signature of Sphagnum plants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m2 yr1. Mean N concentration in Sphagnum capitula was about 6 mg g1 in less polluted mires and about 13 mg g1 in highly N-polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum. Sphagnum plants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation in Sphagnum tissues. The latter probably is related to a shift from N-limited conditions to limitation by other nutrients. The capacity of the Sphagnum layer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO3+NH4+). Sphagnum plants had 15N signatures ranging from about 8 to about 3. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating that 15N signature of Sphagnum plants can be used as an integrated measure of 15N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH3 (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower 15N signature compared with mires located in areas dominated by NOx emissions (i.e., mainly affected by industrial activities).
Woody legume fallow productivity, biological N2-fixation and residual benefits to two successive maize crops in Zimbabwe
Chikowo, R. ; Mapfumo, P. ; Nyamugafata, P. ; Giller, K.E. - \ 2004
Plant and Soil 262 (2004)1-2. - ISSN 0032-079X - p. 303 - 315.
n-15 natural-abundance - nitrogen-fixation - root development - soil fertility - cassia-siamea - shrub
Three woody legumes were planted as two-year 'improved fallows' to evaluate their residual nitrogen (N) effects on two subsequent maize crops under minimum and conventional tillage management. Maize monoculture and cowpea-maize-maize sequence treatments were included as controls. N-2-fixation was estimated using the N-15 natural abundance method to allow the N contribution from the fallows to be partitioned into N2-fixation and soil N capture. Acacia angustissima accumulated the largest amount of both below-ground and above-ground biomass during the 2-year growth period. Using Hyparrhenia grass as the reference plant, the proportion of N-2-fixed in litter was 56, 55, 84 and 58% for Acacia, Sesbania sesban, Cajanus cajan, and cowpea, respectively, resulting in inputs of biologically fixed N of 122, 84, 96 and 28 kg N ha(-1). Maize growth following the legumes for two subsequent cropping seasons was in most cases not directly related to the N inputs. The first year maize crop was severely infested with cutworms (Agrotis sp.) in the Sesbania and Acacia plots, while the second season maize was affected by drought particularly in the Acacia plots where there was high moisture demand by the re-growing trees. During the second season, N uptake by maize following Sesbania was significantly higher than the rest of the treatments. Conventional tillage resulted in better yields than minimum tillage across all treatments and in both seasons. It can be concluded that (1) improved legume fallow system results in net positive soil N balances, (2) despite large inputs of litter and continued recycling of N during the cropping phase, legume species that re-grow after cutting can result in depressed yields in drought seasons, and (3) tillage is beneficial even after improved legume fallows.
Maize productivity and mineral N dynamics following different soil fertility management practices on a depleted sandy soil in Zimbabwe
Chikowo, R. ; Mapfumo, P. ; Nyamugafata, P. ; Giller, K.E. - \ 2004
Agriculture, Ecosystems and Environment 102 (2004)2. - ISSN 0167-8809 - p. 119 - 131.
biological nitrogen-fixation - n-15 natural-abundance - agriculture - benefit - legumes - africa
There is a need for an improved understanding of nitrogen (N) dynamics in depleted sandy soils in southern Africa. A field experiment was conducted to evaluate the performance of different soil fertility improvement practices on a degraded granitic sandy soil in Zimbabwe. Legumes capable of accumulating large amounts of N through biological N2 fixation and subsoil N capture were tested against soybean/maize rotation, cattle manure fertilization and continuous maize (Zea mays L) with or without fertilizer. Soybean (Glycine max) accumulated 82 kg ha-1 N (seed+stover), while mucuna (Mucuna pruriens) produced 87 kg ha-1 N in its biomass. Soybean fixed 76% of its N, while mucuna fixed 96% of the accumulated N as estimated by the 15N natural abundance method. Although the following maize crop in the second season suffered from drought stress, maize N uptake was 14.8 kg ha-1 following soybean and 16.4 kg ha-1 following mucuna, compared with 5.2 kg ha-1 for the unfertilized maize and 25.6 kg ha-1 for the maize fertilized with N at 90 kg ha-1. Cajanus cajan and Crotalaria paulina added barely 10 kg ha-1 of N through their biomass and had no effect on N uptake by maize. Apparent recovery of the added N by maize was 47% for the fertilized maize, 36% for soybean, 12% for mucuna and 9% for cattle manure. There was very little partitioning of N into grain and uptake was mostly before the onset of the drought. Despite the large differences in added residue N, differences in soil mineral N were only evident up to 4 weeks after the beginning of the rains, after which mineral N concentrations became very small in all treatments due to leaching, rather than crop uptake. By the eighth week after crop emergence, maize root length density had increased to about 0.1 cm cm-3 at the 60–80 cm depth, the rapid increase apparently stimulated by the drought. It was concluded that mineral N available to maize from the residues tested falls short of what is required to sustain high maize yields. In these environments where biomass accumulation in many legumes is restricted by soil biophysical factors (poor nutrient concentrations, acidity, coarse texture), combinations of legume rotations and mineral N fertilization will remain the most viable option for sustainable agriculture.