Potential effects of laccase on the formation and accumulation of Fe-OM complexes in coastal saline paddy soil under straw and nitrogen fertilization

The bindings of organic matter (OM) with iron (Fe-OM complexes) are important for soil carbon (C) sequestration. Our objective was to evaluate the role of laccase in the formation of Fe-OM complexes in a coastal saline paddy soil. The experiment included two factors: (1) two nitrogen (N) levels, 255 (N1) and 400 (N2) kg N ha−1; (2) three straw levels: 0 (S0), 4500 (S1) and 9000 (S2) kg C ha−1. The results showed that the combined application of straw and N fertilizer increased the content of Fe-bound OC and laccase activity, both of which were the highest in N1S1 treatment, which was 7.48% and 0.58 μmol g−1 h−1, respectively. The content of Fe-bound OC was positively correlated with laccase activity, Feam content and soil moisture content, but negatively correlated with pH. Both Fe-bound OC and laccase activity were positively correlated with aliphatic functional groups and hydroxyl or amidogen, while both were negatively correlated with aromatic functional groups. Therefore, laccase might promote the formation of Fe-bound OC by changing the chemical structure of OC. In addition, the combined application of straw and N fertilizer could change laccase activity by changing microbial community structure (e.g. Chloroflexi, Ascomycetes and Zygomycota).