Effects of extensive-to-intensive pasture conversion on soil nitrogen availability and CO2 and N2O fluxes in a Brazilian oxisol

Abstract

Brazilian beef production has been recognized as an important path to achieve environmental, social and economic sustainability, and to ensure food security. Although, traditionally, beef production in Brazil has relied on unmanaged pastures (i.e. extensive pasture), a growing demand for beef combined with increased competition for land to grow crops without increasing deforestation has led to the incremental conversion of pastures from unmanaged to intensive systems. To better understand the implications of changing pasture management on nitrogen availability and CO2 and N2O fluxes, a field experiment simulating the extensive-to-intensive pasture conversion was set up in São Paulo State, Brazil. Soil and gas samples were collected during two 6 months periods (October, 2013 to March, 2014 and December, 2014 to May, 2015) in an area of extensive pasture (EP), which served as a control, and also in two treatment areas where pasture intensification was simulated using practices typically adopted in the region. The treatment areas included conventionally tilled pasture without inorganic fertilizer (TP) and tilled pasture with inorganic fertilizer (IP). Environmental conditions and soil characteristics were also determined during the study period. The results clearly illustrated the impacts of commonly adopted pasture management systems in Brazil. Fluxes of CO2 and N2O fluxes increased with the application of fertilizer, with CO2 averaging about 4.1, 4.0 and 5.2 g m-2 day-1, and N2O fluxes averaging 0.2, 0.3 and 0.98 mg m-2 day-1 in EP, TP and IP, respectively. Despite equal rates of fertilizer application, differences in weather conditions and soil management resulted in different emission rates and dynamics for each pasture system type. Depending on the sampling period, emissions factors were 1.55% or 0.59%. The adoption of conservation management practices is vital to prevent “hot” moments of N2O emission arising from pasture intensification in Brazil.