Effect of no-tillage and amendments on carbon lability in tropical soils

• Soil organic matter represents the primary reservoir of carbon and plant nutrients.
• The effects of organic matter in soils is dependent on its content and quality.
• The highest carbon content was found in the free light organic fraction.
• NMR detected predominance of the C-alkyl and CO-alkyl organic radicals.
• CMI should be considered only as an indicator of soil quality.

The effects of organic matter on soil properties depend on its content and quality. Understanding the carbon dynamics and soil organic matter (SOM) quality is crucial for evaluating the sustainability of agricultural systems, the global carbon cycle, elemental weathering, and soil capacity to withstand physical damage. The objective of this study was to assess SOM quality in four Brazilian locations, two of them under no-till (NT) conditions in which soils were amended with lime and gypsum, and the other two soils amended with sewage sludge or compost under conventional system. Soil samples were collected at a depth of 0–0.1 m in four long-term field experiments: (i) a NT system with limestone amendment and re-amendment; (ii) a NT system with gypsum amendment and re-amendment; (iii) a soil amended with sewage sludge for 13 consecutive years; and (iv) a soil amended with just one sewage sludge and composted sludge. Physical and chemical fractionation of SOM and analyzed samples were performed by laser-induced fluorescence (LIF; soil) and nuclear magnetic resonance (NMR; HS). In addition, the Carbon Management Index (CMI) was calculated to evaluate the impacts of soil management practices on organic matter quality. The highest carbon content was found in the free light organic fraction in all experiments, followed by the silt + clay fraction. NMR detected predominance of the C-alkyl and CO-alkyl organic radicals. Both fluorescence and LIF techniques generated Humification Indexes with similar trend. There were differences between the experimental sites when SOM granulometric fractions were analyzed, but no differences in the predominant organic compounds were observed. Soil quality, assessed by CMI, was generally improved with limestone, gypsum, compost and sludge.