Microbial Role in Straw Organic Matter Depolymerization to Dissolved Organic Nitrogen Under Nitrogen Fertilizer Reduction in Coastal Saline Paddy Soil

This study examines the effects of reduced nitrogen (N) application on rice straw N depolymerization in coastal saline paddy soil to establish a scientific basis for optimizing N application strategies during straw incorporation in coastal paddy systems. A 360-day field straw bag burial experiment was conducted using four N application levels: N0 (control, without N fertilizer), N1 (225 kg N/ha), N2 (300 kg N/ha), and N3 (375 kg N/ha). The results indicated that applying 300 kg N/ha significantly (<i>p</i> < 0.05) increased dissolved organic N (DON) content, <i>apr</i> and <i>chiA</i> gene copies, and the activities of alkaline protease, chitinase, leucine aminopeptidase, and N-acetylglucosaminidase. In addition, the application of 300 kg N/ha enhanced the synergistic effects of alkaline protein- and chitin-degrading microbial communities. <i>Pseudomonas</i>, <i>Brevundimonas</i>, <i>Sorangium</i>, <i>Cohnella</i>, and <i>Thermosporothrix</i> were identified as keystone taxa predominant in straw N depolymerization. Straw N depolymerization occurred by two primary pathways: direct regulation of enzyme activity by straw properties of total carbon and electrical conductivity, and indirect influence on N hydrolase activity and DON production through modified microbial community structures. The findings suggest that an application rate of 300 kg N/ha is optimal for promoting straw N depolymerization in coastal saline paddy fields.