Peer review

Purpose: Application of exogenous carbon (C) and nitrification inhibitors are common measures to regulate soil nitrogen cycling and mitigate greenhouse gas emissions. However, we lack knowledge on their direct effects on denitrification processes. Methods: A 15 N anaerobic microcosm experiment was conducted to investigate the effects of exogenous C and/or nitrification inhibitors on denitrification rate, N2O/(N2O + N2) product stoichiometry ratio, and nirS- and nirK-type denitrifiers in a calcareous soil. Exogenous C were wheat straw and wheat straw–derived biochar, and nitrification inhibitors were chemical nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) and biological nitrification inhibitor methyl 3-(4-hydroxyphenyl) propionate (MHPP). Results: Wheat straw, composite with much of liable C, significantly increased denitrification rate (3.96–4.19 times higher than control), N2O emission, and N2O/(N2O + N2) ratio (p < 0.05) by upregulating nirK and nirS genes of denitrifiers, and changing the community structure of nirS-type denitrifiers. By contrast, biochar, composite with much of recalcitrant organic C, increased the N2O/(N2O + N2) ratio but did not stimulate the denitrification rate (P > 0.05). One percent (w/w applied-N) DMPP, 1% MHPP, and 5% MHPP exhibited minor effects on denitrification rate, while 10% MHPP increased the denitrification rate by 1.32 times relative to control (p < 0.05). Wheat straw biochar applied with two nitrification inhibitors reshaped the community structure of nirK- and nirS-type denitrifiers, decreasing the denitrification rate compared with control, and the N2O/(N2O + N2) ratio compared with application of biochar alone (p < 0.05). Conclusions: The direct effects of exogenous C on denitrification and denitrification processes were dependent on the type of exogenous C applied. High dosage of MHPP (> 10% w/w) increased both denitrification rate and N2O/(N2O + N2) ratio.

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