Repercussion of pastoral systems in C and N fractions stock in northeast Amazonia

• Higher tree shading pasture systems increases soil C and N stocks. • Full- sun pasture systems promote C and N soil stocks reduction. • Physical SOM fractionation shows pasture systems differences and shading effects. • C 3 /C 4 SOM contribution reflect past uses, tree shading and full-sun pastures. Changes with land use and management are the second largest cause of greenhouse gas emissions (E-GHG) into the atmosphere. So, it is important to better understand agricultural systems regarding C and N stocks, which are directly associated with soil quality. The objective of this work was to evaluate the impacts of pastoral use systems on C and N stocks and the natural abundance of 13 C (δ 13 C) in the fractions of soil organic matter in the Brazilian Amazon biome. The pastoral systems evaluated involved two silvopastoral systems with 30% (SP30) and 60% (SP60) of shading, two full-sun pasture systems, one in use and under intensive management (IMP) and other in fallow and is considered degraded (DP), as well as a control area under forest (NV). Dominant the grass of the pasture areas was Mombasa grass ( Megathyrsus maximus ). The experiment design consisted of four replicates where the collection sites (trenches) were systematically distributed within the areas (SP30, SP60, NV, DP and IMP) and the layers (0–5, 5–15, 15–30, 30–60 and 60–100 cm). Silvopastoral system with 60% shading had the greatest carbon stock, even greater than the control forest at some depths. In comparison to the control condition (NV) in soil profile, SP30, DP and IMP, reduced C-total stocks by 24%, 17% and 20%, and N-total by 14%, 10% and 18%, respectively. The δ 13 C values were higher in the IMP and DP systems and lower in SP60 and NV, both in organic matter associated with minerals (MaOM) and particulate organic matter (POM). An isotopic enrichment occurred in all systems in the soil profile.