Effects of 17-year fertilization on soil microbial biomass C and N and soluble organic C and N in loessial soil during maize growth

As labile organic pools, soluble organic matter and soil microbial biomass are sensitive to changes in soil management and therefore good indicators of soil quality. Effects of a 17-year long-term fertilization on soil microbial biomass C (SMBC) and N (SMBN), soluble organic C, and soluble organic N during the maize growing season were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwest China. The fertilization treatments included no fertilizer (CK), inorganic N, P, and K fertilizer (NPK), cattle manure plus NPK fertilizer (MNPK), and straw plus NPK fertilizer (SNPK). Our results showed that C storage in the 0–20 cm soil layer was 28% to 81% higher in the fertilized treatments compared to the unfertilized treatment. In the 0–10 cm soil layer, SMBC and SMBN in the three fertilized treatments were higher than in the unfertilized treatment on all sampling dates, while microbial biomass C and N in the 0−10 cm soil layers were the highest at grain filling. In the same soil layer, soil-soluble organic C generally decreased in the order MNPK > SNPK > NPK > CK, while soluble organic N was the highest in the MNPK followed by the SNPK treatment. There was no significant difference in soluble organic N in the NPK and CK treatments throughout most of the maize growing season. Changes in soluble organic N occurred along the growing season and were more significant than those for soluble organic C. Soluble organic N was the highest at grain filling and the lowest at harvest. Overall, our results indicated that microbial biomass and soluble organic N in the surface soil were generally the highest at grain filling when maize growth was most vigorous. Significant positive relationships were found between soluble organic C and SMBC and between soluble organic N and SMBN.