K fertilizer alleviates NO emissions by regulating the abundance of nitrifying and denitrifying microbial communities in the soil-plant system.

Potassium (K) fertilizer additions can result in high crop yields of good quality and low nitrogen (N) loss; however, the interaction between K and N fertilizer and its effect on NO emissions and associated microbes remain unclear. We investigated this in a pot experiment with six fertilizer treatments involving K and two sources of N, using agricultural soil from the suburbs of Wuhan, central China. The aim was to determine the effects of the interaction between K and different forms of N on the NO flux and the abundance of nitrifying and denitrifying microbial communities, using static chamber-gas chromatography and high-throughput sequencing methods. Compared with no fertilizer control (CK), the addition of nitrate fertilizer (NN) or ammonia fertilizer (AN) or K fertilizer significantly increased NO emissions. However, the combined application (NNK) of K and NN significantly reduced the average NO emissions by 28.3%, while the combined application (ANK) of K and AN increased NO emissions by 22.7%. The abundance of nitrifying genes amoA in ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) changed in response to N and/or K fertilization, but the denitrifying genes narG, nirK and norl were strongly correlated with NO emissions. This suggests that N or K fertilizer and their interaction affect NO emissions mainly by altering the abundance of functional genes of denitrifying microbes in the soil-plant system. The genera Paracoccus, Rubrivivax and Geobacter as well as Streptomyces and Hyphomicrobium play an important role in NO emissions during denitrification with the combined application of N and K.