Knowledge for better food systems

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

Photo: Matthias Ripp, Agriculture, Flickr, CC by 2.0

This paper, by researchers from the US and the Netherlands, presents the findings of a model analysis that estimates how much soil organic carbon (SOC) has been lost, and from where, as a result of land use and land cover change (LU-LCC) associated with human agricultural activities.

In the introduction, the authors note that approximately 50 million km2 of soils are currently managed by humans to some degree for agricultural purposes, and that these activities have far-ranging consequences for soils including accelerated erosion, desertification, salinisation, acidification, compaction, biodiversity loss, nutrient depletion and loss of soil organic matter. The dominant component of the last-named is SOC loss. Knowing how much SOC loss has taken place through agriculture is stated as essential for estimating global carbon sequestration potential, but prior to this paper, only point measurements of SOC loss have been made (i.e. very localised).

In order to produce a reliable, global-scale estimate of SOC losses, the researchers developed a statistical model, based on machine learning, which integrates data from over 150,000 soil profile observations and samples collected over the last decade to predict SOC stocks as a function of climatic, topographic, geologic and land-use characteristics. This then allowed them to input historical values for these predictors, based on the History Database of the Global Environment (HYDE) (a database of historical human land use patterns), into their model to predict historical SOC levels. By comparing historical and current SOC stocks, this then allowed for a calculation of the change in SOC levels.

The key findings of the paper were:

  • The model projects that global SOC stocks for the year 2010 were 863 billion metric tons C in the upper 0.3 m soil; 1,824 billion tons C in the upper 1 m; and 3,012 billion tons in the top 2 m. Cropland contained on average 63, 127 and 198 metric tons per hectare in the top 0.3, 1 and 2 m respectively.
  • Reprojecting the SOC model to a no land-use condition with all other variables held constant produces global SOC stocks of 899 billion, 1,899 billion and 3144 billion metric tonnes in the upper 0.3, 1 and 2 m of soil respectively.
  • Subtracting current (2010) SOC stocks from historic (no land use) SOC stocks revealed a net SOC loss of 37 billion, 75 billion and 133 billion metric tons of SOC loss due to LU-LCC in the upper 0.3, 1 and 2 m of soil respectively.
  • Across all parts of the land where there is at least some degree of land-use, there was a mean absolute loss of SOC of 17.7 metric tons C per hectare from the top 2 m of soil.
  • Cropland was associated with higher per hectare SOC losses than grazing land, but grazing land covers a much greater area and so accounts for more of the total SOC loss globally.
  • In areas of naturally very poor soils (e.g. the Cerrado in Brazil), land use can actually result in net gains in SOC.
  • The model was validated at every step of the way by comparison of outputs with existing localised measurements, and the authors acknowledge the limitations of their method.

The authors state that the amount of SOC that has been lost historically (133 billion tons of carbon from the top 2 m of soil by their estimates) can be thought of as the carbon sink potential of the soil, with it often repeated that two-thirds of this could theoretically be achieved (88 billion tons of carbon, equating to 322 billion tons of CO2) if best management practices are adopted. They note that this is highly unlikely and that other studies have suggested biophysical limits on carbon sequestration on farmland of between 0.4 and 1.4 billion tons of C per year. All things considered the authors tentatively suggest that the amount of the potential carbon sink that could be filled is, optimistically, 10-30%.

The paper concludes that this type of modelling provides a basis for policies to target SOC restoration efforts, but also that more historical soil profile data is needed to refine the projections of SOC dynamics.
 

Abstract

Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133Pg C for the top 2m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semi arid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts.

Reference

Sanderman, J., Hengl, T., and Fiske, G.J. (2017). Soil carbon debt of 12,000 years of human land use. Proceedings of the National Academy of Sciences, 114(36), 9575-9580.

Read the full article here. For a discussion of soil carbon and the role of grazing systems, see the 2017 FCRN report Grazed and Confused.

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