Effects of substituting biogas slurry for chemical nitrogen fertilizer on soil organic nitrogen and bacterial communities in coastal paddy fields

Objectives We studied the effects of biogas slurry on soil organic nitrogen (SON) mineralization in paddy fields and its relationship with the change in soil microbial community.

Methods The field trials were conducted on Jiangsu coastal rice fields in 2017. Four levels of biogas slurry N application were set with constant total nitrogen input (225 kg/hm²) at 0%, 33%, 66% and 100% (BS0, BS33, BS66, BS100). After 3 years of continuous treatment (2019), soil samples were collected to determine the mineralization characteristics of SON. The content of total hydrolyzable N, non-hydrolyzable N, ammonium N, amino acid N, amino sugar N, unknown N in SON and the structure of bacterial composition were analyzed.

Results BS66 treatment recorded higher soil potential mineralizable organic N (N₀) (39.7%) than BS0 treatment. Both amino acid N and non-acid hydrolysis N contents increased with biogas slurry application (P<0.05). BS66 treatment had a higher increase in amino acid N (39.2%) and and BS100 had a higher increase in non-acid hydrolysis N (73.9%) than BS0 treatment. The substitution of fertilizers with biogas slurry increased the relative abundance of Chloroflexi and Actinobacteria, and decreased the relative abundance of Nitrospirae (P<0.05). The bacterial genera exhibited a varying response to substituting fertilizers with biogas slurry. As the ratio of biogas slurry to chemical fertilizers increased, the relative abundance of Subgroup_6_unclassified increased and Subgroup_17_unclassified decreased. The relative abundance of KD4-96_unclassified decreased under BS33 treatment and increased under BS66 treatment and BS100 treatment; Subgroup_7_unclassified had a relative abundance ≥1% due to the replacement of chemical fertilizer with biogas slurry. The results of RDA showed that soil N₀ and Thermodesulfovibrionia_unclassified had a significant negative correlation (P<0.01), non-acid hydrolysis nitrogen had a positive correlation with Subgroup_6_unclassified (P<0.05), and a significant negative correlation with Betaproteobacteria_unclassified (P<0.05). Amino acid nitrogen and Proteobacteria_unclassified, amino sugar nitrogen and KD4-96_unclassified were all positively correlated (P<0.05). The unknown nitrogen and Desuslfarculaceae_unclassified were significantly negatively correlated (P<0.05).

Conclusions The substitution of biogas slurry for chemical fertilizers changed the structure of the soil microbial community, increased soil nitrogen storage capacity, improved soil nitrogen mineralization potential, and enhanced soil nitrogen supply capacity.