Changes in soil carbon stocks in Brazil due to land use: paired site comparisons and a regional pasture soil survey

In this paper we calculated soil carbon stocks in Brazil studying 17 paired sites where soil stocks were determined in native vegetation, pastures and crop-livestock systems (CPS), and in other regional samplings encompassing more than 100 pasture soils, from 6.58 to 31.53 degrees S, involving three major Brazilian biomes: Cerrado, Atlantic Forest, and the Pampa. The average native vegetation soil carbon stocks at 10, 30 and 60 cm soil depth were equal to approximately 29, 64, and 92 Mg ha(-1), respectively. In the paired sites, carbon losses of 7.5 Mg ha(-1) and 11.6 Mg ha(-1) in CPS systems were observed at 10 cm and 30 cm soil depths, respectively. In pasture soils, carbon losses were similar and equal to 7.5 Mg ha(-1) and 11.0 Mg ha(-1) at 10 cm and 30 cm soil depths, respectively. Differences at 60 cm soil depth were not significantly different between land uses. The average soil delta C-13 under native vegetation at 10 and 30 cm depth were equal to -25.4 parts per thousand and -24.0 parts per thousand, increasing to -19.6 parts per thousand and -17.7 parts per thousand in CPS, and to -18.9 parts per thousand, and -18.3 parts per thousand in pasture soils, respectively; indicating an increasing contribution of C-4 carbon in these agrosystems. In the regional survey of pasture soils, the soil carbon stock at 30 cm was equal to approximately 51 Mg ha(-1), with an average delta C-13 value of -19.6 parts per thousand. Key controllers of soil carbon stock in pasture sites were sand content and mean annual temperature. Collectively, both could explain approximately half of the variance of soil carbon stocks. When pasture soil carbon stocks were compared with the average soil carbon stocks of native vegetation estimated for Brazilian biomes and soil types by Bernoux et al. (2002) there was a carbon gain of 6.7 Mg ha(-1), which is equivalent to a carbon gain of 15% compared to the carbon soil stock of the native vegetation. The findings of this study are consistent with differences found between regional comparisons like our pasture sites and plot-level paired study sites in estimating soil carbon stocks changes due to land use changes.