Effects of rice husk biochar and soil moisture on the accumulation of organic and inorganic nitrogen and nitrous oxide emissions during the decomposition of hairy vetch (Vicia villosa) mulch

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ABSTRACT Hairy vetch (Vicia villosa Roth ssp. dasycarpa) mulch is widely used as green manure. This mulch decomposes in the soil and produces nitrous oxide (N2O) emissions. Although rice husk charcoal can be used as a soil conditioner to reduce N2O emissions, the effects of charcoal on the decomposition of vetch mulch and consequent N2O emissions have not been well studied. We conducted an incubation experiment to examine how the decomposition of vetch mulch, charcoal amendments and two soil moisture regimes (field capacity and at a slightly drier moisture regime) affect the mineralisation of N during decomposition. We applied high-performance size-exclusion chromatography and chemiluminescent N detection to quantify soluble organic-N compounds of various molecular sizes. Changes over time in inorganic-N, soluble organic-N fractions and N2O fluxes were measured in soils . We found that vetch mulch induced the accumulation of small-molecular-size soluble organic-N compounds in the early phase during decomposition; however, in wet soils with charcoal, the small-molecular-weight N fraction was depleted earlier than in wet soils without charcoal, suggesting more rapid mineralisation in wet soils. Charcoal in wet soils resulted in earlier and sharper N2O peak emissions and did not reduce cumulative N2O emissions in vetch mulch treatments, suggesting that previously reported N2O emission reductions due to charcoal do not occur in the soils . The rates of N2O emission did not correlate with the concentrations of soluble organic N and inorganic N, indicating that other factors control the N2O emission rates during the decomposition of vetch mulch. Our results indicated that the incorporation of charcoal does not reduce N2O emissions in soils where vetch mulch is applied. The depletion of nitrate-N in wet soils with charcoal demonstrates that adding charcoal can reduce nitrate-N leaching from soils.