Soil Strength and Structural Stability Are Mediated by Soil Organic Matter Composition in Agricultural Expansion Areas of the Brazilian Cerrado Biome

A growing demand for resources has led to the expansion of agricultural areas worldwide. However, land conversion associated with poor soil management might lead to soil physical degradation. We investigated the effects of land conversion on soil physical properties in the Brazilian Cerrado region, under native Cerrado vegetation (NV)—pasture (PA) and NV—cropland (CL) conversion scenarios. Soil physical properties related to compaction, pore size distribution, and structure stability were assessed up to a 30 cm depth. Additionally, carbon levels of soil organic matter fractions (particulate and mineral-associated organic matter) were determined. Our results indicate that the compaction process equivalently reduced the soil porosity in PA and CL. However, soil penetration resistance was higher in PA (~2.5 MPa) than in CL (~1.5 MPa), as well as the stable mean weight diameter of soil aggregates. The highest total and labile organic carbon levels were observed in CL, while the lowest levels of total and labile organic carbon occurred in PA (smaller than in CL). These results suggest that the higher structural stability found in PA was mediated by the predominance of stabilized carbon (a decrease in the proportion of soil labile carbon), causing the gaining of soil strength under negligible soil volume variation (in comparison with CL). Our results suggest that the reduction in the soil porosity by compaction due to PA and CL uses can equivalently reduce macropore space and soil hydraulic functioning, and that soil carbon quality alterations (i.e., labile vs. stabilized fractions) are responsible for the gain in soil strength in long-term degraded PA areas. Future research should focus on understanding the magnitude in which soil organic matter controls soil physical attributes, such as soil strength in these expansion areas, and whether this gain in soil strength limits plant development and compromises productivity in the long term.