Higher NO ₃ ⁻ ‐N Levels Under Mulberry ( Zi Yang, Weiguo Zhao, Jinjin Yang, Zhenjiang Wang, Weijun Zhu, Changyu Qiu, Qiang Lin, Meina Zhu, Mengdi Zhao

ABSTRACT

Afforestation is widely advocated for boosting carbon (C) sequestration. However, the stability of soil C pools in economically and culturally significant mulberry plantations remains poorly understood. The soil organic C (SOC) contents in different aggregates and the oxidizable OC fractions were measured. We analyzed the mechanism of C pool stability change through a framework that incorporated soil nitrogen properties (ammonium nitrogen: NH ₄ ⁺ ‐N, nitrate nitrogen: NO ₃ ⁻ ‐N, alkali‐hydrolyzed nitrogen: AN, total nitrogen: TN), carbohydrate‐active enzyme (CAZyme) characteristics (reflecting the decomposition of plant‐ and microbial‐derived C), and climatic factors (mean annual temperature: MAT and mean annual precipitation: MAP). (1) Analysis of pooled data showed a 94.59% increase in the recalcitrant SOC (fraction‐4) content of mulberry soils relative to wasteland soils, accounting for 100.92% of the increase in total SOC (27%, p  < 0.05; TSOC). For different aggregates, the increase in TSOC was primarily driven by the SOC content in macroaggregates (contributing 65.92%; Mac‐SOC), (2) Compared to wasteland soils, mulberry soils NO ₃ ⁻ ‐N content increased by 107.29%, while NH ₄ ⁺ ‐N content increased by 47.16% ( p  < 0.05); among the soil microbial CAZyme families responsible for degrading plant‐ and microbial‐derived C, 32 out of 50 showed significantly greater abundance, (3) In mulberry soils, MAP and MAT exerted an indirect positive influence on CAZymes abundance via soil TN, AN, and NO ₃ ⁻ ‐N. Subsequently, CAZymes abundance exerted direct positive effects on fraction‐4, Mac‐SOC, and TSOC content, with a coefficient of 0.559. In wasteland soils, MAP and MAT indirectly increased CAZymes abundance via their effects on soil pH and EC. CAZymes strongly and directly increased labile SOC, SOC in silt + clay, and TSOC content (coefficient = 0.859). The findings of this research establish a practical framework for improving ecosystem C sequestration through the optimized management of mulberry planting systems.