Soil organic matter degradation in an agricultural chronosequence under different tillage regimes evaluated by organic matter pools, enzymatic activities and CPMAS ¹³C NMR

• Soil organic matter quality under different tillages was evaluated in a chronosequence.
• Particulate organic matter, humic acids and bulk soil samples were analyzed by 13C NMR.
• POM, enzymatic activities and TOC were the most reliable soil quality indices.
• SOM is protected against degradation under conservation tillages.
• Conservation tillages improve soil quality and carbon storage especially at long-term.

Conservation agriculture (CA) is an important strategy to improve the quality of surface soils under Mediterranean conditions and its primary intent is to minimize the mineralization of soil organic matter (SOM). The focus of this manuscript is to evaluate how soil quality indices (e.g., enzymatic activity, total organic carbon (TOC), and labile carbon pools) are inter-connected and how tillage could affect cycles of storage and degradation of SOM. To achieve this objective, five different enzymatic activities, TOC, water soluble carbon and microbial biomass carbon were measured on soil samples collected at an experimental farm situated in the southwest region of Spain. Furthermore, the relative abundance of organic compound classes was measured, using CPMAS 13C NMR on bulk soil samples, humic acids (HA), and particulate organic matter (POM) fractions separated by density. Our results show that conservation tillage enhances soil quality at the surface for mid- and long-term experiments. The NMR analyses demonstrate that conservation tillage led to a preservation of less degraded SOM and revealed a high amount of proteinaceous material preserved from degradation. It is clear that TOC, β-glucosidase and alkaline phosphatase activities are reliable soil quality indices and we further propose that the CPMAS 13C NMR analyses of the particulate organic matter (POM) fraction of soil is an excellent evaluator for changes that occur in soil status.