本研究选取黄土高原旱地农田系统三个长期定位试验：杨凌（25年）、天水（35年）、平凉（37年），研究长期不同施肥对SOC及其组分，例如：矿质结合态有机碳（MOC）和颗粒有机碳（POC，包含轻组颗粒有机碳和重组颗粒有机碳）的影响。每个试验点包括三个处理：对照（不施任何肥料，CK），施化肥（CF）和化肥与有机肥配施（MCF）。结果表明：施肥显著增加了耕层（0-20 cm）土壤固碳速率，杨凌、天水和平凉三个点化肥处理较CK分别提高134、89和129 kg ha^-1 yr^-1，有机无机配施 分别提高418、153和384 kg ha^-1 yr^-1。杨凌、天水和平凉三个点MOC库分别占总SOC储量的72、67和64%；三个地点CF处理的固碳速率分别为76、57和83 kg ha^-1 yr^-1，MCF分别为238、118和156 kg ha^-1 yr^-1且呈现出MOC饱和的趋势。相应的POC库储量在三个试验点分别占总SOC储量的27、33和36%，其中Light-POC占SOC总量的11、17和22%，Heavy-POC占17、16和15%。三个点CF处理POC库固碳速率分别为58、32和46 kg ha^-1 yr^-1，MCF分别为181、90和228 kg ha^-1 yr^-1。其中，Light-POC在CF处理中的贡献占POC的59、81和72%，在MCF处理中的贡献占POC的60、40和69%，其余则为Heavy-POC的贡献。相比CK处理，单施化肥对MOC和POC占总SOC的比例没有影响，而MCF处理显著降低了MOC占SOC的比例，增加了POC所占比例，主要表现为Light-POC组分的增加。SOC在不同库中的分布与土壤团聚体的分布和稳定性密切相关。本研究表明，施用有机肥不仅能够提高土壤有机碳储量，还可以显著改变土壤有机碳的组成，从而提高土壤有机碳的质量，可能与土壤有机碳的饱和水平有关。

本研究探讨了长期施用化肥以及有机肥下大团聚体对有机碳的固存机制。本研究共设置4个处理，分别是不施肥的对照（CK）、施化肥（NPK）、单施有机肥（M）和有机无机肥配施（MNPK）。土壤样品采自连续施肥11年后0-20 cm耕层原状土壤，采用物理分组方法将大团聚体筛分为4个组分：未受保护的粗颗粒有机质（cPOM）和细颗粒有机质（fPOM）、受物理保护的大团聚体中微团聚体内颗粒有机质（iPOM）以及受生物化学保护的矿质结合态有机质组分（MOM）。研究结果表明：与CK相比，长期施用NPK对土壤有机碳（SOC）、全氮（TN）含量以及大团聚体中各有机质组分的OC和TN储量均无显著影响。但是，配施有机肥后（MNPK）可显著提高SOC（45.7%）和TN含量（24.3%），而且还显著提高了cPOM、fPOM和iPOM组分中OC和TN储量，其中cPOM-C、fPOM-C和iPOM-C储量分别提高了292.2%、136.0%和124.0%，cPOM-N、fPOM-N和iPOM-N储量分别提高了607.1%、242.5%和127.6%，而对MOM-C和MOM-N储量均无显著影响。线性回归分析显示土壤有机碳含量与未受保护的组分中碳储量呈显著正相关，其中与cPOM-C储量关系密切，表明石灰性土壤上，有机碳固存首先累积在大团聚体中的cPOM-C组分中。可见，有机无机肥配施（MNPK）通过增加的cPOM-C和cPOM-N储量来提高SOC的储量和稳定性，意味着有机无机肥配施（MNPK）是农业可持续发展的有效措施。

This study investigated the effects of three contrasting soil management regimes and different nutrient treatments on the distribution of water-stable aggregates (>2, 1–2, 0.5–1, 0.25–0.5, and <0.25 mm) and associated soil organic carbon (SOC) and total nitrogen (TN) content in loess soil. A 21-yr long-term experiment was performed, in which soil management regimes include cropland abandonment (Abandonment), bare fallow (Fallow) and wheat-fallow cropping (Cropping). Under Cropping, the following nutrient treatments were employed: control (CK, no nutrient input), nitrogen only (N), nitrogen and potassium (NK), phosphorus and potassium (PK), NP, NPK, and manure (M) plus NPK (MNPK). Results demonstrated that Abandonment significantly increased the content of soil macro-aggregates (>0.25 mm) and mean weight diameter (MWD) at 0–10 and 10–20 cm soil horizons compared with Cropping, whereas Fallow yielded lower values of above two parameters. Abandonment increased SOC and TN contents in all aggregate sizes by 17–62% and 6–60%, respectively, at 0–10 cm soil layer compared with Cropping. Conversely, Fallow decreased SOC and TN contents in all aggregates by 7–27% and 7–25%, respectively. Nevertheless, the three soil management regimes presented similar SOC contents in all aggregates at 10–20 cm soil horizon. Only Cropping showed higher TN content in >0.5 mm aggregates than the two other regimes. Consequently, Abandonment enhanced the partitioning proportions of SOC and TN in >1 mm macro-aggregates, and Fallow promoted these proportions in micro-aggregates compared with Cropping. Under Cropping, long-term fertilization did not affect the distribution of aggregates and MWD values compared with those under CK, except for NPK treatment. Fertilizer treatments enhanced SOC and TN contents in aggregates at all tested soil depths. However, fertilization did not affect the partitioning proportions of SOC and TN contents in all aggregates compared with CK. Comprehensive results showed that different soil management regimes generated varied patterns of SOC and TN sequestration in loess soil. Abandonment enhanced soil aggregation and sequestered high amounts of SOC and TN in macro-aggregates. Long-term amendment of organic manure integrated with NPK maintained soil aggregate stability and improved SOC and TN sequestration in all aggregates in loess soil subjected to dryland farming.