Patterns of organic carbon and nitrogen stocks in soil particle-size fractions along an aridity gradient in Northern China’s deserts

• Organic C in < 100 μm soil particles accounted for 57.8 %-99.2 % of SOC in northern China’s drylands. • Organic C in < 100 μm soil particles and its contribution to SOC decreased with increasing aridity. • SOC in bulk soil was more associated with organic C in < 100 μm soil particles rather than with < 53 μm. • Organic C in < 100 μm soil particles and its contribution to SOC were more controlled by vegetation cover than MBC. Clay and silt fractions (<53 μm in particle size) play a critical role in soil organic carbon (SOC) storage and are used to characterize SOC storage in terrestrial ecosystems. However, there is unclarity about SOC content and stock in soil particle-size fractions in deserts where the soil is immature. In this study, we investigated the composition of soil particles and the stocks of SOC and N in two particle-size fractions of 0–53 μm and 0–100 μm at the top (0–30 cm) and deeper (30–50 cm) layers along an aridity gradient. The very fine sand (53–100 μm in particle size) was the major component of bulk soil. The C and N stocks in the top 50 cm of soil decreased linearly with increasing aridity, from 63.5 MgC ha −1 to 4.1 MgC ha −1 and from 4.6 MgN ha −1 to 0.8 MgN ha −1 , respectively. Organic C content in two soil particle-size fractions and their contribution to bulk SOC decreased linearly with increasing aridity. Organic C content in < 100 μm soil particle-size fraction accounted for 57.8 %-99.2 % of bulk SOC, and it was only 0.6 %-24.2 % in the < 53 μm fraction. SOC stock was estimated by < 100 μm soil particle-size fractions compared with clay and silt (<53 μm). Organic C content in two soil particle-size fractions was higher in the alpine and steppe deserts than in the Gobi and sandy deserts; it was significantly higher at 0–30 cm than at 30–50 cm for all deserts. Vegetation cover was the first predictor for the organic C content in two soil particle-size fractions and their contribution to bulk SOC, addressing the direct contribution of plant-derived C inputs to soil particle-size fraction. Our study provides strong empirical evidence that organic C and N in deserts are better estimated by soil particles < 100 μm rather than clay and silt-sized fraction (<53 μm in particle size).