Soil organic carbon fractionation in sandy soils in the semiarid grasslands and forested areas of Nebraska Sandhills

Abstract Soil organic carbon (OC) fractionation enhances our mechanistic understanding of the processes that govern OC storage. However, research on OC fractionation in sandy soils remains limited. Here, we sampled the sandy soils under three vegetation types: native grass, eastern redcedar ( Juniperus virginiana ), and ponderosa pine ( Pinus ponderosa ). We fractionated soils into free particulate organic matter (fPOM), occluded particulate organic matter (oPOM), mineral‐associated organic matter (MAOM), and water‐extractable organic matter (WEOM) at three soil depths (0–10, 10–30, and 30–100 cm). At the 0‐ to 10‐cm depth, the grassland had lower bulk soil OC compared to the cedar and pine forests (7.09 ± 0.88 vs. 12.84 ± 1.65 and 17.90 ± 2.53 g kg −1 , p < 0.05). At 10–30 cm, grassland had higher bulk soil OC than pine forest (4.24 ± 0.49 vs. 2.68 ± 0.32 g kg −1 , p < 0.05) but did not differ from cedar forest (4.51 ± 0.55 g kg −1 , p > 0.05). At 30–100 cm, vegetation cover did not have significant effects ( p > 0.05). The proportion of protected (MAOM or oPOM) to unprotected (fPOM) OC increased with soil depth, highlighting the increasing OC persistence with soil depth. The MAOM shows a finite capacity for OC. The POM rather than MAOM dominated soil OC because of limited mineral surface area and microbial transformation in these sandy soils. Our study enhances the mechanistic understanding of OC dynamics within fractions of sandy soils, an important component of terrestrial OC sequestration.