Soil organic carbon saturation in cropland-grassland systems: Storage potential and soil quality

Highlights

• Mineral-associated organic C deficits in croplands and grasslands were estimated.

• The relationship between soil C-saturation and soil physical quality was assessed.

• Challenges to the C-saturation concept and the SOC:clay ratio were highlighted.

• C-saturation in grassland was positively linked to SOC accrual history.

• SOC accrual is more beneficial for soil physical quality in croplands.

Abstract

Reliable estimations of soil organic carbon (SOC) deficits in agroecosystems are crucial in evaluating the atmospheric C sequestration potential of agricultural soils and supporting management decisions. Nonetheless, the co-benefit on soil quality resulting from SOC accrual is rarely considered. Here, we assessed SOC saturation and soil physical quality in permanent grasslands (PG) and croplands (CR) by applying the C-saturation concept and the SOC:clay ratio as an indicator of soil physical quality to a set of long-term monitoring sites in western Switzerland. For this goal, we produced a new relationship between the silt + clay (SC) particles and the C stored in the mineral-associated fraction (MAOM_C) and we assessed the assumption that grasslands can be used as C-saturated reference sites. The saturation in PG was not coincidental as it depended on the C accrual history. Hence, PG with the lowest MAOM_C have not reached their C-saturation level and present a potential SOC storage under optimal management. The MAOM_C saturation in CR was low (62 ± 4%) and corresponded to a deficit of −8.8 ± 1.2 mg C g⁻¹ soil as compared to the current level in PG. The saturation was mainly affected by the proportion of temporary grassland in the crop rotation. The relative distribution of C between MAOM (∼80%) and the fine and coarse particulate organic matter (POM) was not affected by land-use types. The MAOM_C saturation in this study (MAOM_C = 0.372 × SC + 4.23) was similar to that reported in the litterature, but discrepancies appeared when the silt and clay contents were considered separately. SC was by far the main factor explaining MAOM_C amount in PG (semi-partial R²: 0.66). In contrast to other studies, the C content of MAOM in PG (43 mg C g⁻¹ SC) was not related to the SC content, suggesting a fixed maximal value in C-saturated soils. Nonetheless, MAOM_C saturation may be underestimated as the least saturated PG might still accumulate MAOM_C. Finally, the SOC:clay ratio was correlated with MAOM_C saturation level in CR, but not in PG suggesting that targeting SOC accrual in CR optimizes the benefits between soil C storage and soil quality.