Biochar and lime amendments promote soil nitrification and nitrogen use efficiency by differentially mediating ammonia-oxidizer community in an acidic soil

Lime and biochar have been proposed as effective ameliorants for increasing soil fertility and nitrogen (N) cycling by regulating the ammonia-oxidizer community. However, the biological mechanisms of lime- and biochar-induced changes in the ammonia-oxidizer community, soil nitrification, crop productivity, and N use efficiency (NUE) remain largely unknown. Here, we established a 9-year field experiment to explore the ammonia-oxidizing archaeal (AOA) and bacterial (AOB) communities and measure potential nitrification activity (PNA), crop productivity, and NUE in response to biochar and lime amendments. We found that soil pH was significantly increased under lime amendment more than under biochar amendment, while soil organic carbon, total nitrogen, and ammonia nitrogen followed the opposite trend. Lime and biochar amendments significantly increased the abundance and diversity of AOA and AOB, and substantially shaped the AOA and AOB community compositions. The co-occurrence networks analysis indicated that potential keystone taxa had negative relationships with connected members in the network, which exhibited significant correlations with the abundance and diversity of AOA and AOB communities, PNA, crop productivity, and NUE. Structural equation modelling indicated that biochar amendment may mediate potential keystone taxa to increase the abundance and diversity of AOB community, and exhibit a stronger impact on PNA, crop productivity, and NUE than lime amendment. Collectively, our results highlight the ecological importance of lime- and biochar-induced effects on the ammonia-oxidizer community and soil N cycling dynamics.