Mitigation of CH4 and N2O emissions from a forage rice field fertilized with aerated liquid fraction of cattle slurry by optimizing water management and topdressing

Fertilization of aerated liquid fraction of cattle slurry (ALCS) to forage rice fields is a method to utilize surplus amount of cattle slurry and produce feed. However, ALCS application causes high emissions of CH4 and N2O from rice fields. In this study, we tested the abilities of several different field management strategies to mitigate CH4 and N2O emissions. The management strategies consisted of two different drainage patterns (midsummer drainage, C; drainage before topdressing, B) and two topdressing patterns (a stepwise increase in topdressing, I; a stepwise decrease in topdressing, D). We also analyzed changes in the archaeal community in each treatment during the experimental period to examine the relationship between CH4 emissions and methanogenic archaea. In each treatment, chemical fertilizer was applied as a basal dressing (50 kg N ha−1) and ALCS was applied three times as a topdressing (214-244 kg N ha−1). In CI, BI, CD, and BD, the cumulative emissions were 191, 158, 127, and 34.5 kg C ha−1 for CH4, respectively, and 0.52, 2.32, −0.18, and −0.21 kg N ha−1 for N2O, respectively. The abundance of Methanocella, a genus of methanogenic archaea, increased over time in CI and CD, but remained constant in BI and BD. This result suggested that drainage before topdressing effectively suppressed the growth of Methanocella. The lowest carbon dioxide equivalents (CO2 eq) value was in BD (75-84% lower than those in other treatments). These results indicate that the combination of drainage before topdressing and a stepwise decrease in topdressing is a simple, low-cost management strategy to mitigate greenhouse gas emissions from rice fields supplemented with ALCS.