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 δ13C 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 C4 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 δ13C value of −19.67‰. 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.
Highlights
Soil has long been recognized as the largest organic carbon reservoir of terrestrial systems of the Earth (Post et al, 1982)
The δ13C of the topsoil (0–10 cm) of these sites was equal to −15 ‰, −21.2 ‰, and −15.4 ‰, respectively. δ13C data from these sites were not included in the discussion that follows
The results found in our paired sites (Fig. 3) confirmed other studies that involved plot-scale comparisons, especially on pasture soils, that showed that depending on local conditions, the soil carbon stocks may increase compared to the local native vegetation (Guo and Gifford, 2002; Ogle et al, 2005; Zinn et al, 2005; Braz et al, 2013; Eclisa et al, 2012)
Summary
Soil has long been recognized as the largest organic carbon reservoir of terrestrial systems of the Earth (Post et al, 1982). It has been estimated that globally approximately 1500 Pg of carbon is stored in the first meter of topsoil. Tropical soils store approximately 40 % of this total, with tropical evergreen forests being the single largest reservoir of soil carbon (Jobbágy and Jackson, 2000). Soil carbon exchange with the atmosphere through soil respiration is an important component of the global carbon cycle and it was estimated to be approximately 80 Pg C yr−1 (Raich and Potter, 1995). It is well established that carbon pools in the soil have distinct residence times. There is a much faster cycling pool, that in lower amounts
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