NET NITROGEN MINERALIZATION AND NET NITRIFICATION RATES IN SOILS FOLLOWING DEFORESTATION FOR PASTURE ACROSS THE SOUTHWESTERN BRAZILIAN AMAZON BASIN LANDSCAPE

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the origina l forest. In this study, we sought to determine the generality of thes e patterns by examining soil inorganic N concentrations, net mineraliz ation and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of s oil textures and spanned a 700-km geographical range in the southweste rn Brazilian Amazon Basin state of Rondonia, We sampled each site duri ng October-November and April-May. Forest soils had higher extractable NO3--N and total inorganic N concentrations than pasture soils, but s ubstantial NO3--N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils . Greater concentrations of soil organic matter in finer textured soil s were associated with greater rates of net N mineralization and net n itrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil t ype or texture. Net N mineralization and net nitrification rates per u nit of total soil organic matter showed no pattern across the differen t forest sites, suggesting that controls of net N mineralization may b e broadly similar across a wide range of soil types. Similar reduction s in rates of net N transformations in pastures 3 years old or older a cross a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in ext ent. Lower net N mineralization and net nitrification rates in establi shed pastures suggest that annual N losses from largely deforested lan dscapes may be lower than losses from the original forest. Total ecosy stem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately followin g forest clearing.