Different land uses in the Brazilian semi-arid: impacts and drivers on organic matter fractions and soil carbon

The Caatinga biome, Brazil, is an semi-arid zone with climatic and socioeconomic vulnerability, which makes the soils more susceptible to degradation under cultivation and management conditions that hinder the maintenance of soil organic matter (SOM). In this sense, the objective of the study was to evaluate the impact of converting native vegetation to pasture and agricultural cultivation on the levels and stocks of total organic carbon and granulometric fractions of SOM and the existing correlations between these parameters in the Caatinga biome, Brazil. Composite soil samples were collected from 11 agricultural crops (Cr) under conventional management with implementation times ranging from 2 to 90 years and six pasture areas (Pa) with implementation times from 4 to 30 years, in addition to nine areas of native vegetation (VN) used as the reference. The soil samples were physically fractionated to isolate the soil organic matter into two fractions, particulate organic matter – labile (POM > 53 μm) and mineral associated organic matter - stable (MAOM <53 μm). In the POM and MAOM the total carbon content was quantified and additionally, the sulfur content for the MAOM fraction was determined. The total carbon contents and stocks of the soil and the mineral-associated organic matter fraction (MAOM-C) and particulate organic matter (POM C), also, the total of the soil and the nitrogen of the mineral-associated organic matter (MAOM-N) followed the order VN = Pa > Cr. The conversion of native vegetation to conventional agriculture significantly reduces the stocks of POM-C and MAOM-C in the first 15 years. Pastures, on the other hand, show an initial increase in SOC, followed by a decline, while shows recovery in areas with longer cultivation periods. In addition to land use, factors such as soil texture and precipitation influence carbon dynamics in this region of Brazil, with clayey soils and areas with higher rainfall favoring the accumulation of organic matter. There is an emphasis on the duration of cultivation and management in the conservation of soil carbon, indicating the need for the application of practices with strategies aimed at the recovery of organic matter in agricultural systems. • Pasture management can contribute to the maintenance and recovery of carbon stocks. • Pasture renovation every 10 or 15 years is important for the recovery of SOM. • Conversion of NV to crop reduce the stocks of POM-C and MAOM-C in the first 15 years. • Soil texture and precipitation influence carbon dynamic in the Caatinga biome, Brazil.