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.
The information on genotypic responses to the seasonal variation in the starch content, starch yield, and starch quality of cassava is limited. The objective of this research was to investigate the seasonal variation of starch characteristics of three cassava genotypes grown under irrigation. The experiment was conducted at four planting dates (20 April, 30 June, 5 October, and 15 December 2015). Three cassava genotypes (CMR38-125-77, Kasetsart 50, and Rayong 11) were evaluated in a randomized complete block design with four replications and the plants were harvested at 12 months. The planting date contributed the largest portion of the total variation in the starch content, starch yield, and starch granule size. The amylose content variability was heavily influenced by genotype. Cassava planted on 5 October or 15 December had greater starch content, starch yield, and starch granule in most genotypes. This was likely due to a higher temperature and solar radiation during the 3–9 months post-planting. CMR38-125-77 showed a consistently high starch content, starch yield, and high amylose content for most planting dates except for the starch yield on 20 April, of which Rayong 11 was the best. These findings will be useful for choosing suitable cassava genotypes for different growing seasons and for facilitating breeding efforts for high starch-yielding and high-quality cassava starch in the future.
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