Effects of combined application of animal slurry and mineral fertiliser on rice yield and soil nitrogen cycle microbes

This paper studied the response of rice yield and soil nitrogen (N) cycling microorganisms to the combined application of animal slurry and mineral fertiliser. A pot experiment was conducted on typical yellow-brown paddy soil. There were five treatments: (1) CK - no N fertilisation; (2) S0 - mineral fertilisation; (3) S30 - 30% slurry with 70% mineral fertilisation; (4) S60 - 60% slurry with 40% mineral fertilisation; and (5) S100 - slurry application. Rice yield, yield components, and soil properties were investigated at harvest. The abundance of soil N cycle functional genes abundance was quantified via quantitative real-time PCR. The rice yield reached a high level when the proportion of slurry used to replace mineral fertiliser was 30-50%. The yield in response to mineral fertiliser (S0) was equivalent to that in response to no N fertilisation since the formation of effective panicles was inhibited. With the slurry replacement ratio increase, the available phosphorus and potassium contents in the soil improved, but the nitrate content decreased. Considering the entire soil N cycle, nitrogen-fixing microbes (nifH), ammonia-oxidising archaea (AOA amoA) and nitrite-reducing microbes (nirS and nirK) had greater abundances, reaching 108 copies. Compared with those in the S0 treatment, the abundances of most N cycle functional genes in the S30 treatment, except for napA, significantly increased from 31.2% to 100.9%, and the increase in the abundance of nirS and nosZ in the S100 treatment reached 4 times, which was obviously greater than that of the other genes. Correlation analysis revealed that high soil pH promoted N fixation and nitrification, while NH4+-N had the opposite effect on N fixation and nitrification, and available phosphorus and potassium actively influenced denitrification. These results showed that a 30-50% slurry application ratio was recommended for rice, which was beneficial for maintaining high yields and high abundances of soil N cycle functional genes.