Soil attributes and c and n variation in histosols under different agricultural usages in the state of Rio de Janeiro, Brazil

Resumo

Histosols are a natural reservoir of C in the soil, and their drainage followed by other farming practices leads to subsidence and soil organic matter transformations. The objective of this study was to evaluate the influence of use and management of Histosols, by means of: characterizing chemical and physical properties, and the content of SOM and humic fractions; and quantifying C and N stocks. Also, to obtain preliminary data on greenhouse gas emissions (CO₂, N₂O) in Histosol areas with different agricultural practices. Three areas were selected with similar soil and environment, two in Macaé municipality, under pasture, and with bean annual crop rotation, and the third in Santa Cruz, Rio de Janeiro city, cultivated with cassava (Manihot esculenta). The attributes evaluated were: physical - bulk density (BD), particle density (Dp), organic matter density (OMD), mineral matter (MM), mineral residue (MR), aggregate stability; and chemical - pH, exchangeable cations, soil organic matter (SOM), carbon in the humin (HUM-C), humic acid (HAF-C) and fulvic acid (FAF-C) fractions; stocks of C and N; and flux of CO₂ and N₂O. In general, the area cultivated with cassava had the highest values for exchangeable cations, as a result of fertilizer and soil management practices. The cassava site showed the highest values of BD and Dp; total volume of pores; MM, MR and OMD and higher degree of transformation of SOM; indicating higher alteration of Histosols properties under this usage. In all sites, the C levels indicated dominance of humin fraction. The SOM and C and N stocks were highest in the pasture, indicating preservation of organic matter, with values from 115.92 to 99.35Mg ha^-1 of C e 8.35 to 4.45 Mg ha^-1 for N. The values of CO₂-C flux were within the range proposed by the IPCC, where the highest emission was 0.09 Mg CO₂ ha^-1 day^-1 in the pasture site. The values of N₂O-N flux were lower than proposed by the IPCC, with the highest value (270 g N₂O-N m^-2 day^-1) in the area under beans (crop rotation). In general, the multivariate analyses discriminated the sites and the pasture was the usage that least affected the Histosols properties.