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.

Rice yields in soils of low soil organic matter may benefit from preceding leguminous green manure crops. A pre-rice crop experiment, including groundnut (Arachis hypogaea), mungbean (Vigna radiata), Sesbania (Sesbania rostrata), and a mixture of Sesbania and multipurpose cowpea (Vigna unguiculata) was conducted on a characteristic sandy soil of North East Thailand. The Sesbania-cowpea intercrop gave a similar total plant biomass as the Sesbania green manure alone (7 t ha-I ) but with the advantage to yield an edible product. The direct economic yield of cowpea was 1.3 t ha-I green beans and greater than that achieved with groundnut or mungbean. The Sesbania-cowpea combination also proved to enhance rice yields by 0.8 t ha-I . The benefits in rice production were similar to the Sesbania green manure alone but surpassed the yields with the other grain crops or urea fertilizer of 30-60 kg N ha-I . Sesbania dry matter production increased with increasing planting density. The resulting variation in plant quality, e.g. lignin, however, was low. Rice responses to treatments were more related to the total residue N yields than to changes in plant quality.Apart from mungbean (25%) the pre-rice leguminous crops were able to obtain a considerable (>39%) proportion of their N from N z fixation. The green manure Sesbania however fixed a larger proportion (79-89%) of its N than the grain crops (25-62%). This led not only to high amounts of N z fixed by Sesbania but together with a N harvest index of zero yielded a large systems N benefit. With grain legumes this benefit was moderated by the N export in harvestable products. In the case of mungbean this may even result in effective soil N mining. Residue N use efficiency varied between 19-29% and was similar to that obtained from a single application of chemical N fertilizer (17-28%).For the farmer the Sesbania-cowpea intercrop option seems thus the most promising one not only regarding rice yield benefits but also in terms of soil fertility enhancement and generation of edible products.

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In situ produced plant residues contain a mixture of different plant components of varying quality. To assess synergistic or antagonistic interactions occurring during the decomposition and mineralization of such mixtures, individual plant parts (stems, leaves, leaf litter and roots) or the mixture of stems, leaves and leaf litter of the agroforestry species pigeonpea (Cajanus cajan) or of crop residues of peanut (Arachis hypogaea) or of the weed hairy indigo (Indigofera hirsuta) were incubated in pots for 19 weeks. Periodically, remaining plant residues were sieved out (>2 mm), weighed and N content as well as soil mineral N determined. Above-and below-ground residues of peanut decomposed fastest and showed the largest N release in agreement with their high N concentration and low-acid detergent fibre (ADF) : N ratio. Hairy indigo was hypothesized to be of lower quality than pigeonpea because of its high-polyphenol content. However, it decomposed faster than pigeonpea, largely because of the higher N and lower lignin concentration of its components. Ranking of individual plant components for N mineralization resulted in the following pattern, leaves > leaf litter > roots > stems. In mixtures of the different plant components a similar species order in decomposition was obtained, e.g. peanut > hairy indigo > pigeonpea. The amount of N released from the mixture was dominated by stem material that comprised 46-61% of the mixture. The interactions in mixtures were relatively small for peanut (generally high-quality components) as well as for pigeonpea (low proportion of high-quality components, i.e. N rich leaf material). However, a positive interaction occurred during later stages of N mineralization in the mixture of hairy indigo as it had a significant proportion of N rich components and absence of highly reactive polyphenols. Thus, for plants with low to intermediate chemical quality attributes, manipulating plant composition (e.g. by varying harvest age, affecting stem and leaf proportions) will be important to obtain significant interactions during decomposition when its components are mixed.

To reduce greenhouse gas emissions farmers are being encouraged not to burn sugarcane residues. An experiment was set up in NE Thailand, where sugarcane residues of the last ratoon crop were either burned, surface mulched or incorporated and subsequently the field left fallow or planted to groundnut or soybean. The objectives of the current experiment were to evaluate the residual effects of these treatments during the following new sugarcane crop on (i) microbial and mineral N dynamics, (ii) performance of sugarcane and (iii) effectiveness of recycled legume residues compared to mineral N fertilizer on N use efficiencies, 15 N recovery in the system and in soil particle size and density fractions (using 15 N labelled legume residues and fertilizer). The millable cane and sugar yield were positively affected by sugarcane residue mulching and incorporation compared to burning suggesting microbial remobilization of previously immobilized N. Residual effects of legumes increased sugarcane tillering and yield (127 and 116 Mg ha -1 for groundnut and soybean, respectively) compared to the fallow treatment without N fertilizer (112 Mg ha -1 ). Soybean residues of higher C:N ratio (33:1) and lignin content (13%) compared to groundnut residues (21:1 C:N, 5% lignin) decomposed slower and improved N synchrony with cane N demand. This led to a better conservation of residue N in the system with proportionally less 15 N losses (15-17%) compared to the large losses from groundnut residues (50-57%) or from mineral N fertilizer (50-63%). 15 N recoveries in soil were larger from residues (41-80%) than from fertilizer (30%) at final harvest. Recycled legume residues were able to substitute basal fertilizer N application but not topdressing after 6 months.