Legume-based silvopastoral systems drive C and N soil stocks in a subhumid tropical environment

Abstract

Silvopastoral systems include trees or shrubs and grasses and are considered a sustainable method of restoring degraded pastures and increasing carbon and nitrogen stocks in tropical soils. We hypothesize that establishing shrub-tree legumes in a signalgrass pasture has the potential to increase soil C and N stocks. Thus, we investigated a silvopastoral system over four years in a subhumid tropical zone in northeastern Brazil to determine the soil C and N stocks and the organic matter chemical fractions. The experimental design consisted of i-pure signalgrass (Brachiaria decumbens), ii-signalgrass intercropped with gliricidia (Gliricidia sepium) and iii-signalgrass intercropped with sabiá (Mimosa caesalpiniaefolia). Measurements were taken at 0, 4 and 8 m away from the legume rows and at random in the pure signalgrass pasture. Soil samples were obtained in the second and fourth years after silvopastoral system implementation at 0–10, 10–20, 20–40, 40–60 and 60–100 cm soil depths at each distance in triplicate to produce compound samples. These were used to determine the C and N contents and stocks, as well as the soil organic matter chemical fractions. Within two years, the gliricidia and sabiá soil C stocks directly below the legume row at 100 cm depth increased by 45% and 35%, respectively, while the soil N stock tripled at all distances. The soil organic matter increased over the two years, and the humin proportion in the soil organic matter (SOM) increased in the silvopastoral system soils. Overall, after four years of silvopastoral system establishment, this system showed the potential to store N and achieve soil C stock levels that are within the global average range for these systems.