Organic carbon pools and organic matter chemical composition in response to different land uses in southern Brazil

The adoption of conservation agriculture (e.g., no‐till system) has been recognized as pivotal to maintaining soil functions, but the potential of this system to enhance organic carbon (OC) quantity and quality and how this OC is stabilized in soils are not well established. In this study, we evaluated the effects of land‐use types (native vegetation (NV) vs. no‐till system (NT)) on OC stocks and on the chemical composition of organic matter (OM), and sought to understand the mechanisms that govern OC protection in the studied highly weathered soils. To achieve these objectives, we used an OC fractionation scheme in a combination of solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopic analyses in soils from six farms in southern Brazil. Our results showed smaller OC stocks (whole soil) under NT than under NV in four of the six sites. In addition, the OC stock differences between land‐use types were larger in coarser textured soils and in those where conventional tillage was used before the adoption of NT. Among fractions, particulate organic carbon (POC) represented only 8% of the whole OC stock but was the fraction most affected by land‐use type. In contrast, the humus organic carbon (HOC) fraction contributed 78% of the whole OC stock and was little altered by land‐use type. Resistant organic carbon (ROC) represented 14% of the whole OC stock and it was altered by land‐use type, demonstrating that this fraction is not as inert as previously thought. Overall, OM chemical composition was quite similar between land uses, with O‐alkyl‐C being the predominant C type. This labile component was further highly correlated with OC stock and silt + clay contents, indicating that the accumulation of OC in these highly weathered soils is mainly a response to the association between labile C compounds and minerals. HIGHLIGHTS: The OC build‐up in no‐till areas and the mechanisms of OC protection in soils are still uncertain. OC stocks under no‐till are still smaller than under native vegetation. Soils under no‐till showed untapped potential for OC sequestration. The association of labile compounds with soil minerals is the driving factor for OC protection.