Long-term effect of chemical fertilizer, straw, and manure on labile organic matter fractions in a paddy soil

To assess the effect of long-term fertilization on labile organic matter fractions, we analyzed the C and N mineralization and C and N content in soil, particulate organic matter (POM), light fraction organic matter (LFOM), and microbial biomass. Results showed that fertilizer N decreased or did not affect the C and N amounts in soil fractions, except N mineralization and soil total N. The C and N amounts in soil and its fractions increased with the application of fertilizer PK and rice straw. Generally, there was no significant difference between fertilizer PK and rice straw. Furthermore, application of manure was most effective in maintaining soil organic matter and labile organic matter fractions. Soils treated with manure alone had the highest microbial biomass C and C and N mineralization. A significant correlation was observed between the C content and N content in soil, POM, LFOM, microbial biomass, or the readily mineralized organic matter. The amounts of POM-N, LFOM-N, POM-C, and LFOM-C closely correlated with soil organic C or total N content. Microbial biomass N was closely related to the amounts of POM-N, LFOM-N, POM-C, and LFOM-C, while microbial biomass C was closely related to the amounts of POM-N, POM-C, and soil total N. These results suggested that microbial biomass C and N closely correlated with POM rather than SOM. Carbon mineralization was closely related to the amounts of POM-N, POM-C, microbial biomass C, and soil organic C, but no significant correlation was detected between N mineralization with C or N amounts in soil and its fractions.