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Organic matter management is believed to solve many of the chemical and physical problems of coarse-textured, low fertility soils of Northeast Thailand. We tested the influence of different plant residues available in this area on soil C and N dynamics in upland (Oxic Paleustult) and lowland (Aerie Paleaquult) soils. Residues included groundnut (upland) or Sesbania rostrata stover (lowland), rice straw, Tamarindus indica and Dipterocarpus tuberculatus leaves applied at 10 t ha-1 (dry matter). For the former three residues additional application rates of 20 t ha-1 were included as well as a mixture (50:50) of groundnut/Sesbania -rice straw treatment. Groundnut stover and Sesbania had C/N ratios <28: I and low lignin, and polyphenol contents whereas rice straw had the highest C/N ratio of 79: I. Dipterocarp and tamarind leaves were characterized by high lignin (> 17%) and polyphenol (>4.5%) contents. These latter residues, despite slow decomposition, apparently resulted in only moderate soil C «I mm) build-up after one year due to the fact that a large proportion of their residues remained in particulate form (> I mm). Thus the mixture of groundnut/Sesbania with straw was among those residue treatments that led to the highest soil C «I mm) buildup under both upland and lowland conditions.Groundnut stover under upland condition resulted in immediate net N mineralisation but also an early decline in soil mineral N presumably due to leaching. By mixing groundnut or Sesbania with rice straw with a high C/N ratio residue N mineralisation could be delayed and prolonged, improving potentially the synchrony ofN release and plant demand. Additions of dipterocarp and tamarind resulted in an initial N immobilization phase and net mineral N release remained low thereafter. Dynamics of microbial biomass N were closely related to N mineralisation and immobilization cycles in both upland and lowland experiments. Residue N concentration was the most significant factor controlling N release in both systems. While extractable polyphenols exhibited a significant influence on N release in upland conditions their effect was not evident in the lowland.

The relationship between biomass production and N2 fixation under drought‐stress conditions in peanut genotypes with different levels of drought resistance is not well understood. The objective of this study was to determine the effect of drought on biomass production and N2 fixation by evaluating the relative values of these two traits under well watered and water‐stress conditions. Twelve peanut genotypes were tested under field conditions in the dry seasons of 2003/2004 and 2004/2005 in north‐east Thailand. A split‐plot design with four replications was used. Main‐plot treatments were three water regimes [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW], and sub‐plot treatments were 12 peanut lines. Data were recorded on biomass production and N2 fixation under well watered and water‐stress conditions. Genotypic variations in biomass production and N2 fixation were found at all water regimes. Biomass production and N2 fixation decreased with increasing levels of drought stress. Genotypes did not significantly differ in reductions for biomass production, but did differ for reductions in N2 fixation. High biomass production under both mild and severe drought‐stress conditions was due largely to high potential biomass production under well‐watered conditions and, to a lesser extent, the ability to maintain high biomass production under drought‐stress conditions. High N2 fixation under drought stress also was due largely to high N2 fixation under well‐watered conditions with significant but lower contributions from the ability to maintain high nitrogen fixation under drought stress. N2 fixation at FC was not correlated with the reduction in N2 fixation at 2/3 AW and 1/3 AW. Positive relationships between N2 fixed and biomass production of the tested peanut genotypes were found at both levels of drought stress, and the relationship was stronger the more severe the drought stress. These results suggested that the ability to maintain high N2 fixation under drought stress could aid peanut genotypes in maintaining high yield under water‐limited conditions.

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Four cultivars of groundnut were grown in upland soil in Northeast Thailand to study the residual benefit of the stover to a subsequent maize crop. An N-balance estimate of the total residual N in the maize supplied by the groundnut was made. In addition three independent estimates were made of the residual benefits to maize when the groundnut stover was returned to the land and incorporated. The first estimate (Estimate 1) was an N-balance estimate. A dual labelling approach was used where ~SN-labelled stover was added to unlabelled microplots (Estimate 2) or unlabelled stover was added to 15N-labelled soil microplots (Estimate 3). The nodulating groundnut cultivars fixed between 59-64% of their nitrogen (as estimated by the 15N isotope dilution method using non-nodulating groundnut as a non-fixing reference) producing between 100 and 130 kg N ha 1 in their stover. Although the following maize crop suffered from drought stress, maize grain N and dry weights were up to 80% and 65% greater respectively in the plots where the stover was returned as compared with the plots where the stover was removed. These benefits were comparable with applications of 75 kg N ha-l nitrogen in the form of urea. The total residual N estimates of the contribution of the nodulated groundnut to the maize ranged from 16.4-27.5 kg N ha -1. Estimates of the residual N supplied by the stover and fallen leaves ranged from 11.9-21.3 kg N ha -~ using the N-balance method (Estimate 1), from 6.3-9.6 kg N ha -~ with the labelled stover method (Estimate 2) and from 0-11.4kg N ha -~ with the labelled soil method. There was closest agreement between the two ~SN based estimates suggesting that 'apparent added nitrogen interactions' in these soils may not be important and that N balance estimates can overestimate the residual N in crops following legumes, even in very poor soils. This work also indicates the considerable ability of local groundnut cultivars to fix atmospheric nitrogen and the potential benefits from returning and incorporating legume residues to the soil in the upland cropping systems of Northeast Thailand. The applicability of the 15N methodology used here and possible reasons for the discrepancies between estimates 1, 2 and 3 are discussed.

Nitrogen fixation in groundnut and soyabean and the residual benefits of incorporated legume stover to subsequent rice crops were estimated in farmers' fields using 15N-isotope methods. Three field experiments were conducted, two which examined Nz-fixation in groundnut by 15N-isotope dilution using a non-nodulating groundnut as a reference crop and one in which N2-fixation in two soyabean genotypes was compared using maize as the non-fixing reference crop. Groundnut fixed 72-77% of its N amounting to 150--200 kg N ha-1 in 106-119 days and soyabean derived 66-68% of its N from N2-fixation which amounted to 108-152 kg N ha-1 under similar conditions. When legume stover was returned to the soil, there was a net contribution of N from N2-fixing varieties of groundnut in all cases ranging from 13-100 kg N ha -1, whilst due to the high % N harvest index in soyabean (87-88%) there was a net removal of N of 37-46 kg N ha-1. In all cases if the legume stover was removed there was a net removal of N in the legume crop which ranged between 54 and 74 kg N ha-1 in N2-fixing varieties of groundnut and from 58 to 73 kg N ha-l in soyabean, whilst maize removed 66 kg N ha-1 if its stover was returned and 101 kg N ha-J when the stover was removed.Growth of rice was improved in all cases where groundnut stover was returned resulting in increases in grain yield of 12-26% and increases in total dry matter production of 26-31%. Soyabean residues gave no increases in rice grain yield but increased total dry matter production by 12-20%. Rice accumulated more N in all cases where legume stover was returned to the soil, and N yields were larger in all cases after the N2-fixing legumes than after the non-fixing reference crops. N difference estimates of the total residual N benefits from the N2-fixing legumes ranged from 11-19 kg N ha -t after groundnut and 15-16 kg N ha-1 after soyabean. The amounts of N estimated directly by application of 15N-labelled stover amounted to 7.2-20.5 kg N ha-l with groundnut which represented recovery of 8-22% of the N added in the stover. In soyabean only 3.0-5.8 kg N ha-l was estimated to be recovered by tSN-labelling which was 15-23% of the added N, whilst only 1.3 kg N ha-1 (4% of the N added) was recovered by rice from the maize stover. An indirect 15N-method based on addition of unlabelled stover to microplots where the soil had previously been labelled with 15N gave extremely variable and often negative estimates of residual N benefits. Estimates of residual N from the added stover made by N difference calculations did not correspond with the estimates by direct 15N-labelling in all cases and possible reasons for this are discussed.

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