Fertilizer nitrogen (N) turnover is highly controlled by soil aggregation. However, the functions of the various aggregates that regulate long-term fertilizer N retention under conservation management remain unexplored. In this study, N-labeled fertilizer was initially applied to investigate the effects of maize straw mulching on fertilizer N allocation in soil aggregates at a decadal scale. The topsoil was fractionated into macroaggregate, microaggregate, and silt-clay (SC) fractions. Macroaggregate was further divided into particulate organic matter (POM) and mineral-associated organic matter (MAOM). A higher enrichment factor of fertilizer N than of soil total N in macroaggregate indicated that the fertilizer N was more apt to incorporation into macroaggregate. The fertilizer N in the bulk soil declined gradually to 84.0% by the 13 year. Temporally, the reduction proportion of fertilizer N in the SC fraction was the largest before 5 years, whereas macroaggregate was the main reactive spot for fertilizer N transformation from 9 to 13 years. Therefore, the function of aggregates was time-dependent in controlling fertilizer N retention and turnover via the release of previously entrapped fertilizer N, but encapsulated the subsequently applied N (i.e., unlabeled fertilizer), whereas mineral adsorption contributed to the long-term stabilization of fertilizer N. Compared with fertilization alone, straw mulching improved aggregates stability, favored the initial fertilizer N retention in macroaggregate by enriching fertilizer N in POM, and reduced the proportion of N loss in MAOM after 9 years. These finding indicate that the improvement in fertilizer N stability related to straw decomposition was sequentially attributed to the enhancement of aggregate encapsulation and persistent interaction with soil minerals. Therefore, this study provides new insights into the functional heterogeneity of soil aggregates at different time stages and the intricate interplay between carbon availability-controlled fertilizer N retention and the improvement in soil aggregation.

中文翻译：

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土壤团聚体对肥料氮保留的时间依赖性调节及秸秆覆盖的影响


肥料氮 (N) 周转很大程度上受土壤团聚控制。然而，在保护管理下调节长期肥料氮保留的各种聚集体的功能仍有待探索。本研究首先应用氮标记肥料来研究十年尺度上玉米秸秆覆盖对土壤团聚体中肥料氮分配的影响。表土分为大团聚体、微团聚体和粉土-粘土 (SC) 部分。大团聚体进一步分为颗粒有机物（POM）和矿物伴生有机物（MAOM）。肥料氮在大团聚体中的富集系数高于土壤全氮，表明肥料氮更容易掺入大团聚体中。到第13年，大块土壤中的肥料氮逐渐下降至84.0%。从时间上看，SC组分中肥料氮的减少比例在5年前最大，而大团聚体是肥料氮转化的主要反应点，从9年到13年。因此，团聚体的功能在控制肥料氮保留和周转方面具有时间依赖性，通过释放先前截留的肥料氮，但封装随后施用的氮（即未标记的肥料），而矿物质吸附有助于肥料氮的长期稳定。与单独施肥相比，秸秆覆盖提高了团聚体稳定性，通过富集POM中的肥料氮，有利于大团聚体中初始肥料氮的保留，并减少了9年后MAOM中氮的流失比例。 这些发现表明，与秸秆分解相关的肥料氮稳定性的改善依次归因于团聚体封装的增强以及与土壤矿物质的持续相互作用。因此，这项研究为不同时间阶段土壤团聚体的功能异质性以及碳有效性控制的肥料氮保留与土壤团聚改善之间复杂的相互作用提供了新的见解。