Mineral Nitrogen Transformation and Microorganism Responses Affected by Organic and Biological Amendment in Intensive Cropping System

Intensive cropping systems often disbalance soil nitrogen (N) cycling due to high mineral fertilization and low organic matter return. We investigated the linkage between soil N transformation and microorganism responses to mineral N fertilization in long-term intensive cropping systems during 2019–2022. A three-factor experiment was established on loam soil, including varying mineral N rates (100, 150, 180, 230 kg N ha−1), organic fertilizer (0 or 300 kg ha−1), and a biological activator (0 or 0.1 L ha−1). Higher mineral N rates combined with organic fertilizer and biological activator significantly increased soil organic carbon and enhanced biologically transformed N (NH4+ + NO3−), with the strongest effects observed at N180. Despite increased mineral N inputs, total soil N did not rise, indicating limited long-term N retention. Increasing mineral N inputs reduced total microbial abundance and shifted the community toward bacterial dominance, lowering soil fungi and bacteria ratio and decreasing biological N immobilization. Organic fertilizer inputs counteracted these effects by increasing fungal abundance and improving biological N transformation efficiency. Overall, the study demonstrates that maintaining balanced mineral N inputs together with organic and biological amendments is essential for stable soil N transformation in intensive cropping systems. For sustainable N management, mineral N should not exceed 150 kg N ha−1 or 180 kg N ha−1, when supplemented with organic fertilizer to avoid soil microbial functioning disturbance and to improve N-use efficiency.