Long-term native forests-to-herbaceous conversion shifts soil organic carbon fractions but not total stocks in semiarid ecosystems of Argentina

Land-use change can substantially alter soil organic carbon (SOC) dynamics, yet responses may differ among carbon fractions, particularly where low rainfall and coarse-textured soils constrain carbon stabilization capacity. The impact of long-term (>37 years) native woodland-to-herbaceous conversion on SOC content and fractions was assessed along a precipitation gradient in two semiarid ecoregions of Argentina (Espinal and Monte). Using a paired-site design (three independent woodland–grassland pairs per ecoregion), total SOC (SOCt), particulate organic matter fractions (coarse POMc, 105–2000 μm; fine POMf, 53–105 μm), and mineral-associated organic matter (MAOM, <53 μm) were quantified at 0–20 and 80–100 cm depths. Across sites, SOCt, POMf, and MAOM concentrations and stocks did not differ significantly between vegetation covers. In general, effect sizes for ecoregion, vegetation cover, and their interaction were low to moderate, suggesting only limited variation in SOCt and these fractions between woodland and grassland soils. On average, SOC at 80-100 cm represented ≈31% of that measured at 0–20 cm. MAOM represented the majority (56–74%) of SOCt. However, in the more humid region, woodland soils exhibited higher surface POMc concentrations and stocks than grassland (2.6 vs. 1.0 g kg −1 ; 7 vs. 3 Mg ha −1 , respectively), indicating greater sensitivity of labile fractions to land-use change. The results suggest that under rainfall- and texture-limited semiarid conditions, long-term woodland-to-herbaceous conversion may alter labile fractions prior to measurable total SOC change. These findings highlight the relevance of particulate fractions as early indicators of soil carbon alteration in dryland ecosystems.