This study aimed to investigate the effects of different levels of soil- and plant-associated bacterial diversity on the rates of litter decomposition, and bacterial community dynamics during its early phases. We performed an incubation experiment where soil bacterial diversity (but not abundance) was manipulated by autoclaving and reinoculation. Natural or autoclaved maize leaves were applied to the soils and incubated for 6 weeks. Bacterial diversity was assessed before and during litter decomposition using 16S rRNA gene metabarcoding. We found a positive correlation between litter decomposition rates and soil bacterial diversity. The soil with the highest bacterial diversity was dominated by oligotrophic bacteria including Acidobacteria, Nitrospiraceae, and Gaiellaceae, and its community composition did not change during the incubation. In the less diverse soils, those taxa were absent but were replaced by copiotrophic bacteria, such as Caulobacteraceae and Beijerinckiaceae, until the end of the incubation period. SourceTracker analysis revealed that litter-associated bacteria, such as Beijerinckiaceae, only became part of the bacterial communities in the less diverse soils. This suggests a pivotal role of oligotrophic bacteria during the early phases of litter decomposition and the predominance of copiotrophic bacteria at low diversity.

Soil nitrous oxide emissions markedly increase after a short waterlogging, and they are strongly correlated with denitrification gene mRNA levels within the 0-

Agricultural soil is often subjected to waterlogging after heavy rainfalls, resulting in sharp and explosive increases in the emission of nitrous oxide (N₂O), an important greenhouse gas primarily released from agricultural soil ecosystems. Previous studies on waterlogged soil examined the abundance of denitrifiers but not the composition of denitrifier communities in soil. Also, the PCR primers used in those studies could only detect partial groups of denitrifiers. Here, we performed pyrosequencing analyses with the aid of recently developed PCR primers exhibiting high coverage for three denitrification genes, nirK, nirS, and nosZ to examine the effect of short-term waterlogging on denitrifier communities in soil. We found that microbial communities harboring denitrification genes in the top 5 cm of soil distributed according to soil depth, water-soluble carbon, and nitrate nitrogen. Short-term waterlogging scarcely affected abundance, richness, or the alpha-diversities of microbial communities harboring nirK, nirS, and nosZ genes, but significantly affected their composition, particularly in microbial communities at soil depths of 0 to 1 cm. Our results indicated that the composition of denitrifying microbial communities but not the abundance of denitrifiers in soil was responsive to short-term waterlogging of an agricultural soil ecosystem.

In this review, the current knowledge of nitrous oxide (N₂O) emissions from soybean (Glycine max (L.) Merr.)ecosystems, particularly on postharvest N₂O emissions, is summarized and controlling factors of postharvest N₂O emissions from soybean ecosystems are discussed. A new biological method to mitigate N₂O emission is also presented. The latest (2006) guidelines of the Intergovernmental Panel on Climate Change (IPCC) concluded that N₂O emissions derived directly from biological nitrogen (N) fixation were not significant in legume crop ecosystems. The default N₂O emission factor from biological N fixation was revised to zero, whereas the default N₂O emission factor for the decomposition of legume crop residues (postharvest N₂O emissions) was the same as that for nonlegume crop residues (1%). From previously measured field data, the percentage of N in soybean residue emitted as N₂O after harvest was calculated to determine emission factors. Values ranged from 0.0–10.0% (average 1.3% ± 2.7%, median: 0.2%), indicating relatively low emission factors. The average emission factor calculated for N₂O emissions from N in soybean residues was slightly higher than the default emission factor for N in crop residue specified in the guidelines. However, the volume of field-measured postharvest N₂O emissions in soybean ecosystems is low compared with data for N₂O emissions during the crop growing season. Previous field-measured data demonstrated that N₂O emissions often peaked sharply immediately after the harvest. However, values for N₂O fluxes in the currently available field data were determined on a weekly or monthly basis. This large time gap between samplings may have resulted in underestimation of postharvest N₂O emissions in soybean ecosystems. More detailed field studies on postharvest N₂O emissions from soybean ecosystems are needed. Nodule decomposition is the major source of postharvest N₂O emissions in soybean ecosystems. A new microbiological method was recently developed to mitigate postharvest N₂O emissions from soybean ecosystems utilizing N-fixing bacteria with increased N₂O-reducing activity (increased expression of nosZ). This approach may potentially be applied to other leguminous crops and non-legume ecosystems. Ongoing research on the relationships between soil N₂O emissions and nosZ may reveal a new method for N₂O mitigation in the future.