Nitrogen (N) cycling by soil microbes can be estimated by quantifying the abundance of microbial functional genes (MFG) involved in N-transformation processes. In agro-ecosystems, biochars are regularly applied for increasing soil fertility and stability. In turn, it has been shown that biochar amendment can alter soil N cycling by altering MFG abundance and richness. However, the general patterns and mechanisms of how biochar amendment modifies N-cycling gene abundance have not been synthesized to date. Here, we addressed this knowledge gap by performing a meta-analysis of existing literatures up to 2019. We included five main marker genes involved in N cycling: nifH, amoA, nirK, nirS and nosZ. We found that biochar addition significantly increased the abundance of ammonia-oxidizing archaea (AOA), nirK, nirS and nosZ by an average of 25.3%, 32.0%, 14.6% and 17.0%, respectively. Particularly, biochar amendment increased the abundances of most N-cycling genes when soil pH changed from very acidic (pH < 5) to acidic (pH: 5.5-6.5). Experimental conditions, cover plants, biochar pyrolysis temperature and fertilizer application were also important factors regulating the response of most N-cycling genes to biochar amendment. Moreover, soil pH significantly correlated with ammonia-oxidizing bacteria (AOB) abundance, while we found that most genes involved in nitrification and denitrification were not significantly correlated with each other across studies. Our results contribute to developing quantitative models of microbially-mediated N-transforming processes in response to biochar addition, and stimulate research on how to use biochar amendment for reducing reactive N gas emissions and enhancing N bioavailability to crop plants in agro-ecosystems.