Effects of C/N ratio variation in swine biogas slurry on soil dissolved organic matter: Content and fluorescence characteristics.

Nutrient-rich biogas slurry shows favorable prospects for application as an organic fertilizer in farmland. At the same time, due to differing sources and treatment methods, the C/N ratio of biogas slurry varies greatly. The effect of differences in C/N of biogas slurry on soil organic matter properties remains unclear. In this experiment, pig farm biogas slurry differing in C/N (3, 6, 8.84 and 12) was applied instead of fertilizer. Fluorescence spectroscopy combined with parallel factor analysis and principal component analysis were used to determine dissolved organic carbon (DOC) and fluorescent dissolved organic matter (FDOM) in soil dissolved organic matter (DOM). The experimental results showed that the DOC and FDOM contents of soil could be significantly increased at the initial stage of biogas slurry application. Compared with CK, on the 60th day, biogas slurry with a C/N of 12 exhibited the greatest improvement in DOC, FDOM as well as for Component 1, Component 2 and Component 3 contents in soil FDOM, 40.93%, 66.25%, 65.35%, 40.47%, and 78.42% respectively. However, compared with the 0th day, by the 60th day, biogas slurry with a C/N of 8.84 exhibited the greatest decrease in Component 4 content in soil FDOM, 74.68%. Biogas slurry with a C/N of 8.84 exerted the greatest promotion effect on corn growth, the utilization and transformation of Component 4 by plants and microorganisms in the soil. And it showed the strongest improvement in the degree of FDOM humification in the soil, the humification index increased from 4.16 ± 0.17 to 4.92 ± 0.58, compared with CK. This study provided new insights for the utilization of biogas slurry with respect to soil physical and chemical properties and maize plant growth.