Organic carbon stocks across various land use systems in Sandy Ultisols and Oxisols in Brazil

Abstract

Soil organic carbon (SOC) is a major carbon pool that can be easily affected by land use change, especially in tropical humid sandy soils, where organic decomposition is fast and SOC stabilization is weak. Here, we assessed SOC stocks in coarse-textured Oxisols and Ultisols under six contrasting land uses in western São Paulo, Brazil, an area marked by warm temperatures, within the Atlantic Rainforest biome. Soils were sampled for bulk density and SOC content to a 1-m depth in areas under a) primary native forest, b) native forest restoration, c) pastures, d) sugarcane, e) cassava, and f) annual crops. Even under primary forests, SOC stocks were low relatively to other Brazilian soils under cooler climates and with finer textures, and the effect of land use changes varied with soil type. In both soil types, the lowest SOC stocks occurred under cassava cultivation, a traditional indigenous crop with low soil coverage and which requires intensive soil disturbance for harvesting. In Ultisols, forest restoration and sugarcane presented the highest SOC stocks, whereas in Oxisols, the highest stocks occurred under primary forests and pastures, suggesting that gramineous perennial crops with little soil disturbance are effective in preserving or even sequestering SOC in these areas. When only primary forests are compared, SOC stocks in Oxisols were higher than in Ultisols, probably due the effect of higher soil bulk densities on the Ultisols, rendering root development and thus SOC accrual in subsoil more difficult.