Organic-C quality as a key driver of microbial nitrogen immobilization in soil: A meta-analysis

Microbial nitrogen (N) immobilization in soil can be enhanced by increasing carbon (C) bioavailability, yet the response of microbial N immobilization to the addition of organic matter is uncertain. In the present study, we investigated the effect of organic matter quality on microbial N immobilization. The response ratio (ln R), i.e., the ratio of microbial N immobilization in organic-C amended soil to that in control treatments, was calculated using data from 51 published studies. Overall, the addition of organic-C increased significantly microbial N immobilization by 105% relative to unamended soil. The type of organic-C affected the response of microbial N immobilization to organic-C addition. Glucose (classified as a labile compound) was more effective in stimulating microbial N immobilization than cellobiose and cellulose (classified as intermediately decomposable compounds). Tannin and oxalic acid (classified as recalcitrant compounds) did not significantly affect microbial N immobilization. The C/N ratio of organic materials did not affect the response of microbial N immobilization in the short-term (<280 d); differences in the effect size (ln R+) were not significant among different C/N ratio groups. The effect of the form of inorganic-N (ammonium [NH4+] or nitrate [NO3-]) on the response of microbial N immobilization to organic-C addition was not significant, with a mean ln R+ of 0.838 (confidence interval [CI]: 0.590-1.099) for NH4+ and 1.642 (CI: 0.951-2.452) for NO3-. Moreover, the In R of microbial NH4+ immobilization showed a significant positive relationship (P < 0.01) with that of microbial NO3- immobilization. The In R+ for the experimental period decreased in the order of <30 d, 60-120 d, >= 120 d, and 30-60 d. The chemical quality of organic-C defined by three C pools (i.e., labile, intermediate, and recalcitrant) rather than the C/N ratio is a critical factor regulating the response of microbial N immobilization to organic-C addition.