Interrelationships among soil total C and N, microbial biomass, trace gas fluxes, and internal N-cycling in soils under pasture of the Amazon region

The aim of this study was to investigate the interrelationships and seasonal weather variations on soil total C and N concentrations, microbial biomass C pool, trace gas fluxes, and rates of internal N-cycling from soils under degraded pasture at Nova Vida ranch, Rondonia, Brazilian Amazon. We collected samples from soils and gas fluxes at the 5 soil types in April 2000 (end of the wet season) and in October 2001 (end of the dry season). Samples were analyzed in both seasons for soil microbial biomass C, soil total C and N, inorganic N, net N mineralization and net nitrification rates and some other soil properties. Overall, the effects of the season events upon soil properties were highly variable. The studied soils with high rates of CO, evolution but low rates of net N mineralization had high rates of N immobilization. The high immobilization rates suggest that the high CO2 evolution rates were associated with a growing microbial biomass. Moreover, in the wet season, the lower NO3--N pools and rates of net mineralization and net nitrification we measured are indicative of lower pasture N2O fluxes. In general, net nitrification rates exceeded net N mineralization rates as the relatively NH4+-N pools. The interrelationships were investigated in order to understand the major biogeochemical cycles that influence soil fertility under pasture, which is important for devising management technologies that enhance the sustainability of these areas and thus slow further deforestation.