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.
Groundnut as a pre-rice crop is usually harvested 1-2 months before rice transplanting, during which much of legume residue N released could be lost. Our objectives were to investigate the effect of mixing groundnut residues (GN, 5 Mg ha À1 ) with rice straw (RS) in different proportions on: (i) regulating N dynamics, (ii) potential microbial interactions during decomposition, and (iii) associated nitrous oxide and methane emissions at weekly intervals during the lag phase until rice transplanting (i, ii) or harvest (iii). Decomposition was fastest in groundnut residues (64% N lost) with a negative interaction for N loss when mixed 1:1 with rice straw. Adding groundnut residues increased mineral N initially, while added rice straw led to initial microbial N immobilization. Mineral N in mixed residue treatments was significantly greatest at the beginning of rice transplanting. Soil microbial N and apparent efficiency were higher, while absolute and relative microbial C were often lowest in groundnut and mixed treatments. Microbial C:N ratio increased with increasing proportion of added rice straw. N 2 O losses were largest in the groundnut treatment (12.2 mg N 2 O-N m À2 day À1 ) in the first week after residue incorporation and reduced by adding rice straw. N 2 O-N emissions till rice harvest amounted to 0.73 g N 2 O-N m À2 in the groundnut treatment. CH 4 emissions were largest in mixed treatments (e.g. 155.9 g CH 4 m À2 , 1:1 treatment). Mixing residues resulted in a significant interaction in that observed gaseous losses were greater than predicted from a purely additive effect. It appears possible to regulate N dynamics by mixing rice straw with groundnut residues; however, at a trade-off of increased CH 4 emissions.
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