Drivers of denitrification and nitrification in a dryland agroecosystem: The role of abiotic and biotic factors

Agricultural practices such as tillage and fertilization impact soil nitrogen (N) cycling processes, but how they alter the coupling between the activity, abundance and diversity of N-cycling microbes remains to be understood. Here, we used a fifteen-year trial in a dryland agroecosystem on the Loess Plateau of China (two tillage regimes crossed with six fertilization treatments) to understand how (de)nitrification potentials are determined by soil abiotic conditions and the abundances and compositions of the (de)nitrifier communities. We measured the abundances of bacterial (AOB) and archaeal (AOA) ammonia oxidizers and nirK- and nirS-nitrite reducers, their community compositions, potential nitrification (PNA) and denitrification (PDA), and soil abiotic conditions. PNA and PDA across the 12 treatments were positively correlated to AOB abundance and nirS abundance, respectively. Co-occurrence network analysis revealed the presence of dominant ecological modules of (de)nitrifiers sensitive to agricultural treatments, and more complex network under no-tilled than tilled conditions as well as under multiple fertilizers than unfertilized conditions. Path analysis and random forest analysis both showed that PNA was explained by AOB abundances and the relative abundance of one module of (de)nitrifiers driven by soil ammonium concentration, while PDA was most related to soil organic carbon concentration, pH and to a lesser extent nirS abundance. These findings demonstrate that, in agricultural soils, the potential of denitrification –a facultative activity for denitrifiers– is mainly predicted by abiotic conditions, while the potential of nitrification –an obligate activity for nitrifiers– is determined by biotic variables, here AOB abundances and a particular cluster of microbial populations.