From forest to pastures and silvopastoral systems: Soil carbon and nitrogen stocks changes in northeast Amazônia

The Cerrado-Amazon ecotone has been under intense pressure over the years from agricultural and urban expansion, both of which are land uses that directly affect soil quality. The objective of this work was to evaluate the responses of soil carbon and nitrogen stocks, soil organic matter (SOM) quality, carbon isotopic composition (¹³C) from C₃ and C₄ plants and carbon sequestration and emission to silvopastoral systems, natural vegetation (NV), fallow pasture (FP), and intensive management pasture (IMP). Silvopastoral systems had different shading levels: 25 % (SP25), 50 % (SP50) and 75 % (SP75). The grass cultivated in all treatments was Megathyrsus maximus cv. Mombaça. The experimental design consisted of four replicates, and collection sites were distributed in strips throughout the study areas at different soil layers (0.00-0.05; 0.05-0.15; 0.15-0.30; 0.30-0.60 and 0.60-1.0 m deep). The conversion of natural vegetation in FP areas and silvopastoral systems (SP25 and SP75) led to increases in total C and N stocks (up to 1.0 m) when compared to other land use systems (SP50, IMP, and NV), which did not occur with total labile-C and C-POM. FP, SP25 and SP75 significantly increased labile C stocks in MAOM found in the 0.30-0.60 m layer. A greater enrichment of ¹³C in MAOM was observed with increasing depth for silvopastoral systems (SP25, SP50 and SP75) and NV. C-MAOM stocks derived from C₃ plants were higher in soils under SP25 and SP75 and from C₄ plants under FP. C-POM stocks were higher in all silvopastoral systems under study. IMP and FP affected δ¹³C values in MAOM and POM, especially in the 0.00-0.05 m layer. C sequestration increased under FP and SP25, with greater contributions from C₄ and C₃ plants, respectively. Of the silvopastoral systems, SP25 had the highest C stock in soil and contributed to the sequestration of 1.67 Mg C ha^-1 yr^-1.