Carbon and nitrogen stock of Acrisols and Nitisols in South Bahia, Brazil

Abstract

This study evaluated if the effect of secondary forest (SF) conversion into pasture (PAS) or rubber tree plantation (RT) and cacao + erythrina AFS (C + E) conversion into cacao + rubber tree AFS (C + RT) on the C and N storage in soils and aggregate-size fractions could be compensated by a high amount of organic residues deposited over the years by rubber tree plantation and agroforestry systems (above and belowground) and a high amount of belowground residues deposited by pasture. Soil samples were collected from six layers (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) and separated by wet-sieving into three fraction-size classes (2000-250 μm; 250-53 μm and < 53 μm) in soils under RT, PAS and SF sites (Argisols) and C + RT and C + E AFSs (Nitosols). The C and N were determined by dry combustion. The average soil organic carbon (SOC) stock in Acrisols was 207 Mg ha−1 up to 100 cm depth, and 72 Mg ha−1 was stored at the first 30 cm depth; while the average SOC stock in Nitosols was 224 Mg ha−1 up to 100 cm, and 116 Mg ha−1 at 30 cm depth. As expected, C and N stock in soils and in the different soil aggregate-size fractions revealed that the dissimilarity between sites was strongly influenced by soil order. There was high dissimilarity between the sites in Nitosols and those in Yellow Argisols, and the dissimilarity between the land use systems only occurred in the Yellow Argisol. The conversion effect of cacao + erythrina AFS into cacao + rubber tree AFS was restricted to the surface soil and did not promote dissimilarity among the AFSs regarding the C and N storage in the whole soil and aggregate size-fractions. Adopting no tillage and tree-based systems (such as the rubber tree plantation) would compensate the negative conversion effect in the amount of organic matter and promote C and N storage mostly in the soil aggregate size-fractions. The effect of land-use conversions was more evident in the aggregates than in the whole soil, regardless of the soil order. The conversions in both soil orders induced macroaggregate turnover and increased the yield of new free microaggregates.