Deforestation and current management practices reduce soil organic carbon in the semi-arid Chaco, Argentina

The soil is a fundamental component of the C cycle. Land use changes can alter the soil organic carbon (SOC) content, a key determinant of several regulation ecosystem services. Here, we analyzed the effects of land cover (e.g. from forest to cropland) and land use (i.e. management practices) change on SOC in the semi-arid Chaco of Argentina, a global deforestation hotspot. Using the CENTURY model, we analyzed SOC changes over 20 years for two sites with contrasting rainfall (600 and 800 mm). For each site, we evaluated the effect of different combination of management practices (e.g. fertilization, grazing intensity) and land uses (i.e. annual crops and sown pastures). 98.5% of the simulations performed for cropping systems showed a reduction in SOC, with an average reduction of 25% respect to the native forest. Wheat proportion in the crop rotation had the highest relative influence on SOC variation (54%), higher than the proportion of maize (26.4%) and nitrogen fertilization (9.8%). For sown pastures, <40% of the simulations showed SOC decreases up to 18%. Grazing intensity had the greatest relative influence on SOC variation in both sites (>60%), followed by burning (19%), temporary exclusion of cattle (12%) and nitrogen fertilization (4.3%). In most cases SOC changes were mainly explained by changes in C inputs (i.e. NPP) rather than by changes in outputs (i.e. respiration and erosion). Therefore, delta SOC showed a strong negative relationship with the Human Appropriation of NPP (R2 = 0.54 and 0.67 in cropping systems and pastures, respectively). Overall, our results suggest that land use change has negative effects on SOC regardless of the management practices implemented. The few combinations that balanced food production and carbon sequestration were maize monocultures with annual N fertilization, and cattle raising with temporary exclusion and burning suppression. Our results may be used to define management practices that allow maintaining soil carbon stocks in the upper soil layer.