Conversion from forests to pastures in the Colombian Amazon leads to contrasting soil carbon dynamics depending on land management practices

Abstract

Strategies to mitigate climate change by reducing deforestation and forest degradation (e.g. REDD+) require country- or region-specific information on temporal changes in forest carbon (C) pools to develop accurate emission factors. The soil C pool is one of the most important C reservoirs, but is rarely included in national forest reference emission levels due to a lack of data. Here, we present the soil organic C (SOC) dynamics along 20 years of forest-to-pasture conversion in two subregions with different management practices during pasture establishment in the Colombian Amazon: high-grazing intensity (HG) and low-grazing intensity (LG) subregions. We determined the pattern of SOC change resulting from the conversion from forest (C3 plants) to pasture (C4 plants) by analysing total SOC stocks and the natural abundance of the stable isotopes (13) C along two 20-year chronosequences identified in each subregion. We also analysed soil N stocks and the natural abundance of (15) N during pasture establishment. In general, total SOC stocks at 30 cm depth in the forest were similar for both subregions, with an average of 47.1 ± 1.8 Mg C ha(-1) in HG and 48.7 ± 3.1 Mg C ha(-1) in LG. However, 20 years after forest-to-pasture conversion SOC in HG decreased by 20%, whereas in LG SOC increased by 41%. This net SOC decrease in HG was due to a larger reduction in C3-derived input and to a comparatively smaller increase in C4-derived C input. In LG both C3- and C4-derived C input increased along the chronosequence. N stocks were generally similar in both subregions and soil N stock changes during pasture establishment were correlated with SOC changes. These results emphasize the importance of management practices involving low-grazing intensity in cattle activities to preserve SOC stocks and to reduce C emissions after land-cover change from forest to pasture in the Colombian Amazon.

Large uncertainty in carbon uptake potential of land‐based climate‐change mitigation efforts

“…ORCHIDEE and JULES represent fewer management processes and, therefore, may underestimate soil C uptake potential in ADAFF (but also losses in BECCS); the incorporation of harvest (not included in ORCHIDEE pastures) and the representation of crops by specific crop PFTs (including tillage), instead of grasses, substantially increases soil C depletions on agricultural land in LPJ‐GUESS (Pugh et al., ). However, there are also observations suggesting that moderately intensive grazing might actually increase soil C stocks in C4‐dominated grasslands (Mcsherry & Ritchie, ; Navarrete, Sitch, Aragao, & Pedroni, ), a process the DGVMs likely do not capture well.…”

Soil Carbon Dynamics in Soybean Cropland and Forests in Mato Grosso, Brazil

“…Models that best fit the data include at least an active (or fast) pool that cycles on an annual to decadal timeframe and a passive (or slow) pool that turns over in centuries or millennia (Baisden et al, ). Stable isotope analysis of C (δ 13 C) can also be used to assess turnover of SOM following a change from C 3 to C 4 vegetation or vice versa (Ehleringer et al, ; Navarrete et al, ; Neill et al, ; Torn et al, ). Because C 3 plants have lower δ 13 C values than C 4 plants because of greater fractionation during photosynthesis, replacement of C 3 trees with C 4 pasture grasses increases soil δ 13 C and allows for calculation of turnover times (Ehleringer et al, ; Neill et al, ; Pendall et al, ; Torn et al, ).…”

Increasing beef production won't reduce emissions

“…present no evidence to support their assumption that higher beef production would result in more carbon captured in pastures. While higher profits might allow investment in pasture restoration, higher stocking rates can instead result in reduced soil organic carbon stocks (Navarrete et al ., ). Improved pasture management, if implemented, could increase grassland productivity, but this increased productivity will only translate into increased carbon storage if it outpaces the higher amount of carbon removed in the form of beef.…”