The effects of rice (<i>Oryza sativa</i> L. ssp. japonica) husk biochar on nitrogen dynamics during the decomposition of hairy vetch in two soils under high-soil moisture condition

<p>Legumes, including hairy vetch (<i>Vicia villosa</i> Roth), are widely used as green manures. They fix nitrogen (N) and provide the N to other crops when they decompose, and thus are considered alternatives for chemical N fertilizers. However, N-rich plant residues, including hairy vetch, are also sources of soil nitrous oxide (N<sub>2</sub>O) emissions, a greenhouse gas. On one hand, rice (<i>Oryza sativa</i> L. ssp. japonica) husk biochar is widely used as a soil conditioner in Japan and has been reported as a tool to mitigate soil N<sub>2</sub>O emissions. We conducted a soil core incubation experiment (1.5 months) to compare the N<sub>2</sub>O emissions during the decomposition of surface-applied hairy vetch (0.8 kg dried hairy vetch m<sup>−2</sup> soil) under semi-saturated soil moisture conditions (~100% water-filled pore space (WFPS)), using two soil types, namely Andosol and Fluvisol. Throughout the incubation period, the use of biochar suppressed soil NH<sub>4</sub><sup>+</sup>-N concentrations in Andosol, whereas the effect of biochar on NH<sub>4</sub><sup>+</sup>-N was not clear in Fluvisol. Biochar increased the nitrate (NO<sub>3</sub><sup>−</sup>-N) levels both in Andosol and Fluvisol, suggesting a negative influence on denitrification and/or a positive influence on nitrification. Biochar application did not influence the cumulative N<sub>2</sub>O emissions. Our study suggests that rice husk biochar is not a good option to mitigate N<sub>2</sub>O emissions during the decomposition of surface-applied hairy vetch, although this study was performed under laboratory conditions without plants. However, the trends of the inorganic-N concentration changes followed by the addition of hairy vetch and biochar were markedly different between the two soil types. Thus, factors behind the differences need to be further studied.</p>