Objective This study aimed to elucidate the characteristics of soil microbial biomass nitrogen (SMBN) turnover, dynamic changes in key enzyme activities, and their interaction mechanisms under long-term organic fertilizer application at different substitution ratios in paddy field derived from yellow soil.

Method Based on a long-term fertilization experiment conducted on yellow soil fertility and fertilizer efficiency in Guiyang, five treatments were employed: CK (no fertilizer), NPK (nitrogen, phosphorus, potassium balanced chemical fertilizer), M (organic fertilizer alone), 0.25M + chemical fertilizer (25% organic fertilizer N replacing chemical N) and 0.5M + chemical fertilizer (50% organic fertilizer N replacing chemical N). The soil microbial biomass nitrogen (SMBN) and key enzyme activities were measured throughout the entire rice growth period, and their dynamic changes and interrelationships were analyzed.

Result Organic fertilizer application increased the average SMBN content throughout the rice growth period by 3.7% - 9.4% compared to NPK treatment. Partial substitution of chemical fertilizer with organic fertilizer significantly enhanced SMBN turnover, storage capacity, annual turnover intensity, and nitrogen content supplied by microbial nitrogen. Compared to NPK, these indicators increased by 12.6% - 18.3%, 6.5% - 11.1%, 5.3%, and 10.7% - 12.9%, respectively. Organic fertilizer application improved soil catalase and urease (UE) activities, and NIR activity in the later growth stage was notably higher under organic fertilizer treatments compared to the early stage.

Conclusion Chemical N fertilizer substituted by organic N in yellow-soil derived paddy fields increased SMBN content, turnover amount, storage capacity, annual turnover intensity, and N content supplied by microbial N, while enhanced catalase and UE activities.