From forest to cropland and pasture systems: a critical review of soil organic carbon stocks changes in Amazonia

Abstract

The impact of deforestation on soil organic carbon (SOC) stocks is important in the context of climate change and agricultural soil use. Trends of SOC stock changes after agroecosystem establishment vary according to the spatial scale considered, and factors explaining these trends may differ sometimes according to meta-analyses. We have reviewed the knowledge about changes in SOC stocks in Amazonia after the establishment of pasture or cropland, sought relationships between observed changes and soil, climatic variables and management practices, and synthesized the δ C measured in pastures. Our dataset consisted of 21 studies mostly synchronic, across 52 sites (Brazil, Colombia, French Guiana, Suriname), totalling 70 forest-agroecosystem comparisons. We found that pastures (n = 52, mean age = 17.6 years) had slightly higher SOC stocks than forest (+6.8 ± 3.1 %), whereas croplands (n = 18, mean age = 8.7 years) had lower SOC stocks than forest (-8.5 ± 2.9 %). Annual precipitation and SOC stocks under forest had no effect on the SOC changes in the agroecosystems. For croplands, we found a lower SOC loss than other meta-analyses, but the short time period after deforestation here could have reduced this loss. There was no clear effect of tillage on the SOC response. Management of pastures, whether they were degraded/nominal/improved, had no significant effect on SOC response. δ C measurements on 16 pasture chronosequences showed that decay of forest-derived SOC was variable, whereas pasture-derived SOC was less so and was characterized by an accumulation plateau of 20 Mg SOC ha after 20 years. The large uncertainties in SOC response observed could be derived from the chronosequence approach, sensitive to natural soil variability and to human management practices. This study emphasizes the need for diachronic and long-term studies, associated with better knowledge of agroecosystem management.

Soil carbon debt of 12,000 years of human land use

“…Because of the naturally infertile soil state in the Cerrado, agricultural expansion has resulted in little loss in SOC (Fig. 1B), and there are large areas that have actually accumulated modest amounts of SOC after the advent of agriculture, which is consistent with results from field studies (17,32,33). Historic SOC Loss Trend.…”

Impacts of land-use history on the recovery of ecosystems after agricultural abandonment

“…Generally, agriculture significantly reduces C and, in the absence of supplementary sources, nitrogen (N) pools due to initial deforestation, reduced soil litter input, and accelerated soil decomposition and erosion (Davidson and Ackerman, 1993;Fujisaki et al, 2015;Guo and Gifford, 2002;McLauchlan, 2006;Murty et al, 2002). Pasture soils can be an exception as they have been found to accumulate C, depending on location and management (McSherry and Ritchie, 2013;Milchunas and Lauenroth, 1993).…”

Brazil’s Amazonian forest carbon: the key to Southern Amazonia’s significance for global climate

“…In Amazonian soils, the fast turnover organic matter is concentrated near the soil surface (de Marques et al 2015), which is where increases in soil temperature are greatest when forests are cleared, and this would also be the layer undergoing the greatest effects of climate change. After deforestation, changes in soil organic matter depend heavily on management, with a variety of techniques resulting in enhanced organic matter retention (e.g., Fujisaki et al 2015;Maia et al 2009Maia et al , 2010Perrin et al 2014). Nevertheless, the dominant land use in Brazilian Amazonia continues to be cattle pasture with minimal management (Fearnside 2005).…”