Environmental controls and long-term changes on carbon stocks under agricultural lands

Improved understanding of changes in soil organic carbon (SOC) stocks is critical for developing strategies that ensure effective climate change mitigation and the long-term productivity. Changes in SOC are likely to vary across soil and climate conditions, yet long-term data to elucidate these trends across different ecosystems remains limited. In this study we evaluated long-term changes in SOC across a gradient of climate conditions (from arid to hyper humid), soil orders, and land uses (non-cultivated, woody perennial, and cultivated) in central Chile. Thus, we sought to find evidence for SOC changes in the agricultural lands over past three or four decades. Surface soils (8-29 cm depth) were sampled between 2014 and 2016 and analyzed for total C and N content, aggregate stability, texture, bulk density, pH as well as spectral properties using Mid-infrared (MidIR) and Near-infrared spectroscopy. SOC stocks were compared to those previously measured at the same sites between 1968 and 1994, covering a wide range of SOC values (from 12 Mg C ha(-1) to 128 Mg C ha(-1)). Our findings show that the largest SOC losses occurred in semiarid and subhumid areas for the time frame considered, decreasing from their initial C stocks by 24.7% and 26.1%, respectively. Moreover, cultivated soils in semiarid regions were more vulnerable than those in arid regions to SOC losses. The results also indicated that in cooler and humid regions, SOC stocks were stable or increased over time. Among soil orders, Mollisols showed the largest losses (29.9% reduction between sampling dates). The MidIR results indicate that the mineral bands for clays and silicates were associated with these sites demonstrating SOC conservation, suggesting that mineral protection played an important role in the long-term SOC storage in semiarid areas. This study provides a better understanding of temporal changes of SOC to address the restoration of degraded land and adaptation for future trends in global change.