Carbon emissions from cropland expansion in the United States

Abstract

After decades of decline, croplands are once again expanding across the United States. A recent spatially explicit analysis mapped nearly three million hectares of US cropland expansion that occurred between 2008 and 2012. Land use change (LUC) of this sort can be a major source of anthropogenic carbon (C) emissions, though the effects of this change have yet to be analyzed. We developed a data-driven model that combines these high-resolution maps of cropland expansion with published maps of biomass and soil organic carbon stocks (SOC) to map and quantify the resulting C emissions. Our model increases emphasis on non-forest-i.e. grassland, shrubland and wetlandabove and belowground biomass C stocks and the response of SOC to LUC-emission sources that are frequently neglected in traditional C accounting. These sources represent major emission conduits in the US, where new croplands primarily replace grasslands. We find that expansion between 2008-12 caused, on average, a release of 55.0 MgC ha −1 (SD spatial =39.9 MgC ha −1 ), which resulted in total emissions of 38.8 TgC yr −1 (95% CI=21.6-55.8 TgC yr −1 ). We also find wide geographic variation in both the size and sensitivity of affected C stocks. Grassland conversion was the primary source of emissions, with more than 90% of these emissions originating from SOC stocks. Due to the long accumulation time of SOC, its dominance as a source suggests that emissions may be difficult to mitigate over human-relevant time scales. While methodological limitations regarding the effects of land use legacies and future management remain, our findings emphasize the importance of avoiding LUC emissions and suggest potential means by which natural C stocks can be conserved.

Mapping co-benefits for carbon storage and biodiversity to inform conservation policy and action

“…While the relation between stock and emissions potential can approach one-to-one for biomass carbon stocks where land-use changes result in the loss of nearly all carbon stocks, the relationship is more nuanced for soil organic carbon stocks. The response of soil carbon stocks to land-use change varies with soil type, climate, depth and subsequent land use among other factors [79][80][81][82] and cannot accurately be inferred simply from the size of the existing stock. Given the uncertain future of a given piece of land, our map is agnostic to the vulnerability of soil organic carbon and thus represents a grid cell's absolute maximum potential emission.…”

Estimating the Potential for Conservation and Farming in the Amazon and Cerrado under Four Policy Scenarios

“…Mean above-ground biomass carbon stocks (AGBC) were calculated from an AGBC map [43]. Below-ground biomass carbon stocks (BGBC) were derived from a matching map of below-ground BGBC that we created using a regression model [44] that predicts BGBC based on covariance with mean annual temperature (MAT), tree phylogeny (angiosperms or gymnosperms), and the history of forest management, as used and described in Spawn et al [45]. Spatial estimates of MAT (1970MAT ( -2000 were taken from the WorldClimV2 data set [46], and management history was inferred from our synthetic land cover map.…”

“…Committed emissions from SOC stocks to a depth of 30 cm were mapped by spatially applying expected SOC stock change estimates to SOC maps from the SoilGrids250v2 dataset [47] following the general approach of Spawn et al [45]. Mean stock change estimates represent the fraction of the initial SOC stock that is lost upon conversion to cropland and were taken from Don et al [48].…”

Impacts of tillage practices on soil carbon stocks in the US corn-soybean cropping system during 1998 to 2016

“…Clearing, tilling and draining these lands for food production directly intensified global climate change through releasing large amount of CO 2 into the atmosphere (Lal et al 1999). Large C loss under agricultural activities has been well-documented in both observational evidence from long-term monitoring experiments (Huggins et al 1998, Matson et al 1997 and model simulations , Spawn et al 2019. In the US cropland, a total of approximate 5.0 Pg C (1 Pg=1000 Tg=10 15 g) was lost as a result of cultivation (Lal et al 1999, accounting for 12.3% of the C loss from global agricultural land since 1850 (Pugh et al 2015).…”