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

Abstract

Abstract. 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 • 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 δ 13 C under native vegetation at 10 and 30 cm depth were equal to −25.4 ‰ and −24.0 ‰, increasing to −19.6 ‰ and −17.7 ‰ in CPS, and to −18.9 ‰, and −18.3 ‰ 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 δ 13 C value of −19.6 ‰. 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.

Changes in soil carbon, nitrogen, and phosphorus due to land-use changes in Brazil

“…A full description of the study area can be found in Assad et al (2013). Briefly, we conducted two types of surveys: one at the regional level, exclusively in pasture soils, and another in which 17 plot-level paired sites were sampled encompassing soils of pastures, CPSs and native vegetation.…”

“…A bias in this scheme is that sampling sites were not randomly selected. A second bias is that, although all pastures were in use at the time they were sampled, it was difficult to visually assess their grazing conditions or stocking rates, which may affect the soil nutrient stocks (Maia et al, 2009;Braz et al, 2012;Assad et al, 2013). Paired sites were selected by the EMBRAPA (Empresa Brasileira de Pesquisa Agropecuária) regional offices and sampled between November and December 2011.…”

“…Additionally, the Brazilian law (Law no. 12187 of 29 December 2009) encourages the adoption of good agricultural practices to promote low carbon emission (Low Carbon Emission Program -ABC Program) and stipulates that mitigation should be conducted by adopting (i) recovery of degraded pastures, (ii) a no-tillage system, (iii) integrated livestock-crop-forest systems, and (iv) re-forestation in order to reduce approximately 35 to 40 % of Brazil's projected greenhouse gas emissions by 2020 (Assad et al, 2013).…”

“…Most studies in Brazil on the effects of agricultural practices on soil properties deal with soil carbon stocks due to its importance for a low-carbon agriculture (Sá et al, 2001;Bayer et al, 2006;Marchão et al, 2009;Maia et al, 2009;Braz et al, 2012;Assad et al, 2013;Mello et al, 2014). On the other hand, there are fewer studies on agricultural practices affecting soil nitrogen concentration, and especially stocks, and even fewer studies on changes in soil phosphorus stocks.…”

“…On the other hand, there are fewer studies on agricultural practices affecting soil nitrogen concentration, and especially stocks, and even fewer studies on changes in soil phosphorus stocks. Based on this, this paper aims to investigate effects of agricultural practices on carbon, nitrogen and phosphorus soil concentration and stocks, plus the soil stoichiometry (C : N : P ratio), in several Brazilian regions using the same study sites and methodology used by Assad et al (2013), who evaluated changes in soil carbon stocks due to different land uses. Two sampling approaches were used in Assad et al (2013): the first, at the plot level, addressed 17 paired sites, comparing soil stocks among native vegetation, pasture and CPSs, and the second was a regional survey of pasture soils in more than 100 sites.…”