Effects of Different Manures in Combination with Fulvic Acid on the Abundance of N-Cycling Functional Genes in Greenhouse Soils

To investigate the effects of different manures in combination with fulvic acid on the abundance of N-cycling functional genes in greenhouse soils, Chinese cabbage was planted for three growing seasons. A total of six treatments—pig manure (P), pig manure + fulvic acid (PH), chicken manure (C), chicken manure + fulvic acid (CH), sheep manure (S), sheep manure + fulvic acid (SH) and no fertilization (CK)—were set up. The abundance of 13 soil N-cycling functional genes (gdhA, amoA-1, amoA-2, amoB, narG, nirK-1, nirK-2, nirK-3, nirS-1, nirS-2, nirS-3, nosZ and nifH) were investigated after the harvest of the third growing season using a gene chip approach. The results showed that fertilization treatments increased the abundance of most N-cycling functional genes in the soil, such as nitrification genes amoA-2 and amoB as well as denitrification genes narG, nirK-1, nirS-1 and nirS-2, with the stronger influence of sheep and pig manure than chicken manure. Fortunately, the additional fulvic acid reduced the increasing effect resulting from pig, chicken and sheep manure application. The abundance of functional genes for nitrogen cycling in soil was positively correlated with the content of soil organic matter, available phosphorus and NO3−-N, and negatively correlated with electrical conductivity. Overall, fertilization treatments increased soil nitrification and denitrification genes abundance, with a risk of increasing soil nitrogen loss, but the supplementary fulvic acid could limit the increase. In this study, it was concluded that the sheep manure (31.3 t/ha) + fulvic acid (7.5 kg/ha) treatment was more powerful in regulating the abundance of N-cycling functional genes in soil.