The objective of this study was to test whether biochar amendment could offset the effects of nitrogen (N) deposition on soil microbial biomass carbon (MBC) and enzyme activity. We applied N (low and high rates) and biochar (low and high rates) individually, and in combination (all permutations) to the soil of a Torreya grandis cv. “Merrillii” orchard as a 13-month field experiment during 2015-2016 in Zhejiang Province, China. MBC significantly increased in the low N treatment but decreased in the high N treatment (P<0.05). MBC significantly decreased in both biochar-only treatments, and this effect became stronger as biochar amendment rates increased. Biochar amendment amplified the positive effects of low N treatment on MBC and mitigated the negative effects of high N treatment. Catalase, cellulase, and urease activities significantly increased with N addition. Cellulase, nitrite reductase, and urease activities significantly increased in the biochar treatments. The positive effects of low N addition on catalase were amplified in the low biochar amendment treatment, but the positive effects on cellulase decreased significantly. The effects of biochar amendment on MBC and enzyme activities of soil that receives atmospheric N deposition were regulated by the rates of both biochar amendment and N deposition, and varied with enzyme type.
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