Legacy of severe soil degradation hinders the buildup of mineral-associated soil organic carbon

Global efforts target a soil organic carbon (SOC) enhancement rate of 2.4 ‰ y^-1 in the upper 30 cm of agricultural soils to address declining soil productivity associated with declining SOC stocks. We explored a unique chronosequence of homogeneous soils formed after mining in Germany, which serve conventional agriculture and exhibit a large margin for SOC storage, but limited SOC accrual, to study SOC protection mechanisms and accrual constraints. We hypothesized that limited SOC accrual is associated to insufficient nitrogen rather than to minerals saturation. Soil samples (0-30 and 30-60 cm) were collected across the chronosequence (0-56 years) and compared to an original non-mined soil (OS) managed similarly. The mean residence time (MRT) of SOC and its protection mechanisms were studied using soil incubation, organic matter density fractionation, and δ¹³C measurements. After 56 years, total and mineral-associated SOC (MAOC) stocks remained 18 % and 28 % lower than in the OS at 0-30 cm, with estimated replenishment times of 93 and 129 years, respectively. Mineral-associated nitrogen (MAN) stocks stagnated along recultivation time below OS level. Together with significant linear correlation of MAOC with total SOC and MAN stocks, these results indicate that nitrogen rather than saturation of minerals limits SOC accrual. In fact, the MAOC stock deficit to saturation was estimated at 316.4 Mg ha^-1. At 30-60 cm, SOC and nitrogen stocks were restored within 30 years, due to comparatively lower initial losses. The initial MRT of SOC at 0-30 and 30-60 cm (15.3 and 27.9 years) declined before finally becoming comparable to OS (11.7 and 7.7 years). This reflected new carbon entering the soil that initially contained predominantly MAOC (78-82 %), followed by its stabilization as MAOC. Due to their susceptibility to nitrogen losses, degraded soils require tailored nitrogen management to restore SOC stocks and comply with European laws requiring agricultural SOC accrual.