Soil N index-C

Nitrogen (N) is the major limiting nutrient determining yield and quality of crops. There are many factors related to the N cycle that jointly determine its overall functioning and plant uptake, which farmers aim to manipulate with different fertilization types (e.g., organic versus mineral) and fertilizer application rates. However, the soil N cycle, encompassing various elements, remains elusive within different soil aggregate fractions for each fertilization type, where the most significant portions of N are stored. This study established a soil N index combining N substrates (physiochemical properties and microbial residual N), N drivers (functional gene abundance and diversity) and N processes (N transformation rates and extracellular enzyme activities) to evaluate the effects of fertilizer types (organic vs. mineral) and application rates on N cycle in three fractions of soil aggregates. Compared to mineral fertilizer application, organic fertilizer application increased the soil N index in microaggregates rather than in large or small macroaggregates. Higher rates of fertilizer application also increased the soil N index in microaggregates instead of in large and small macroaggregates. We conclude that organic fertilizer application boosts the soil N index, which is dependent on aggregate size. Our study indicates that examining soil within microaggregates may be crucial for tracking functional changes in the soil N cycle.

氮（N）是决定作物产量与品质的核心限制养分。与氮循环相关的诸多因子共同调控其整体循环进程与植物吸收效率，农户可通过不同肥料类型（如有机肥与矿质肥）及施肥量对其进行人为调控。然而，土壤团聚体组分（soil aggregate fractions）是氮的主要储存场所，但不同施肥类型下各土壤团聚体组分中的土壤氮循环过程仍有待阐明。本研究构建了一套土壤氮指数，该指数整合了氮底物（理化性质与微生物残体氮）、氮驱动因子（功能基因丰度与多样性）及氮过程（氮转化速率与胞外酶活性），用于评估不同肥料类型（有机肥与矿质肥）及施肥量对三种土壤团聚体组分中氮循环的影响。与矿质肥施用相比，有机肥施用可提升微团聚体（microaggregates）中的土壤氮指数，而非大团聚体（macroaggregates）或小团聚体。更高施肥量同样可提升微团聚体中的土壤氮指数，而非大、小团聚体。本研究结论表明，有机肥施用可提升土壤氮指数，且该效应依赖于团聚体粒径。本研究提示，针对微团聚体中的土壤开展分析，或可为追踪土壤氮循环的功能变化提供关键切入点。