Short-term changes in nitrogen availability, gas fluxes (CO2, NO, N2O) and microbial biomass after tillage during pasture re-establishment in Rondonia, Brazil

Anthropogenic conversion of primary forest to pasture for cattle production is still frequent in the Amazon Basin. Practices adopted by ranchers to restore productivity to degraded pasture have the potential to alter soil N availability and N gas losses from soils. We examined short-term (similar to 35 days) effects of tillage prior to pasture re-establishment on soil N availability, CO2, NO and N2O fluxes and microbial biomass C and N under degraded pasture at Nova Vida ranch, Rondonia, Brazilian Amazon. We collected soil samples and measured gas fluxes in tilled and control (non tilled pasture) 12 times at equally spaced intervals during October 2001 to quantify the effect of tillage. Maximum soil NH4+ and NO3- pools were 13.2 and 6.3 kg N ha(-1) respectively after tillage compared to 0.24 and 6.3 kg N ha(-1) in the control. Carbon dioxide flux ranged from I IS to 181 mg C-Co-2 m(2)h(-1) in the control (non-tilled) and from 110 to 235 mg C-CO, m 2 h(-1) when tilled. Microbial biomass C varied from 365 to 461 mu g g(-1) in the control and from 248 to 535 mu g g(-1) when tilled. The values for N,O fluxes ranged from 1.22 to 96.9 mu g N m(-2) h(-1) in the tilled plots with a maximum 3 days after the second tilling. Variability in NO flux in the control and when tilled was consistent with previous measures of NO emissions from pasture at Nova Vida. When tilled, the NO/N2O ratio remained < 1 after the first tilling suggesting that denitrification dominated N cycling. The effects of filling on microbial parameters were less clear, except for a decrease in qCO(2) and an increase in microbial biomass C/N immediately after tilling. Our results suggest that restoration of degraded pastures with tillage will lead to less C matter, at least initially. Further long-term research is needed. (c) 2007 Elsevier B.V. All rights reserved.