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。

Irrigated desert soil samples in the Hexi Corridor of China were collected over a period of 23 years from a site where different fertilization methods had been used. Changes of soil organic carbon (SOC) and its water stable aggregate (WSA) size fractions were studied. The effects of various fertilization methods on the distribution of added organic carbon (OC) in different WSA size fractions were also analyzed. The results showed that the applied fertilizations for 23 years improved SOC concentrations and OC concentrations in all WSA size fractions compared to the non-fertilized treatment (CK). In addition, fertilization obviously increased the OC stocks of <2 mm WSA size fractions compared to the CK. The average OC stock of <0.053 mm WSA fraction was 1.7, 1.6 and 3.2 times higher than those of >2 mm, 0.25-2 mm and 0.053-0.25 mm WSA fractions, respectively. A significant positive correlation was found between soil C gains and OC inputs (r=0.92, P<0.05), indicating that SOC may have not reached the saturation point yet at the site. The C sequestration rate was estimated by 14.02% at the site. The OC stocks in all of the <2 mm WSA fractions increased with the increase of OC input amounts; and the conversion rate of the input fresh OC to the OC stock of <0.053 mm WSA fraction was 1.2 and 2.6 times higher than those of the 0.25-2 mm and 0.053-0.25 mm WSA fractions, respectively. Therefore, the <0.053 mm WSA fraction was the most important component for soil C sequestration in the irrigated desert soil.