Patterns of woody plant-derived soil carbon losses and persistence after brush management in a semi-arid grassland

Aims: Woody encroachment and subsequent brush management aimed at reducing woody plant cover can alter soil organic carbon (SOC) pools. However, brush management influences on the sources and stability of SOC is unknown. Using a space-for-time approach in a site with closely co-located patches representing unencroached grassland, woody encroachment, and brush management, we coupled stable isotopes and plant-derived biomarkers to quantify how woody encroachment alters input sources and stability of SOC and how these patterns change with brush management. Methods: Stable isotopes, lignin-derived phenols, substituted fatty acids, and carbon content of density fractions were measured in soils collected in shrub canopy interspaces, under live shrubs, and under shrubs killed 8 or 52 y previously. Results: Bulk SOC and C₃-derived SOC were higher in shallow soil (0–5 cm) under live shrubs than in interspace soil. Long-term brush management showed a decline in total SOC stocks, substituted fatty acids, and C₃-derived SOC that were associated with the soil light fraction. Despite declines in bulk SOC following brush management, accumulated C₃-derived SOC pools in the dense fraction remained largely stable following brush management. Conclusions: Woody encroachment increases the proportion of poorly-protected light fraction and loosely-mineral associated SOC derived from leaf litter, which is lost within several decades after brush management. However, woody encroachment produced a stabilized mineral-bound pool of C₃- and C₄-derived SOC that remained 52 years after brush management, suggesting that woody encroachment has the potential for long-term SOC stabilization.