Tillage legacy, grasses and soil texture shape soil organic carbon recovery in semi-arid woodland.

Soil degradation from intensive tillage poses a critical barrier to carbon sequestration, particularly in semi-arid regions where low rainfall limits ecosystem recovery. As climate change expands arid zones globally, understanding the long-term potential for soil organic carbon (SOC) recovery in degraded lands becomes increasingly urgent. Here, we analyze reforested chronosequences (4-36 years post-abandonment) in formerly cultivated subtropical thornscrub woodlands to assess SOC dynamics across gradients of soil texture and degradation. We introduce a novel degradation index (DI), integrating SOC deficit and grass dominance, to evaluate how invasive grasses, commonly overlooked in restoration metrics, impede nitrogen (N2)-fixing tree establishment and, consequently, SOC recovery in N-depleted soils. SOC accumulation was greatest (0.46 Mg C ha⁻1 yr⁻1) in moderately degraded, loamy floodplain soils where native N2-fixing trees thrived, reducing SOC deficits by 17% and coinciding with a δ15N decline from 7.18 to 5.27‰. In contrast, sandy uplands exhibited slower recovery and persistent grass cover, while clay-rich sites maintained high baseline SOC with minimal gains. δ13C trends confirmed a shift from C4 to C3 inputs across sites. Our findings demonstrate that site-specific texture and degradation state, especially grass competition, shape SOC trajectories, underscoring the need for targeted reforestation strategies in an increasingly arid world.