Organic farming is often considered a strategy that increases croplands’ soil organic carbon (SOC) stock. However, organic farms currently occupy only a small fraction of cropland, and it is unclear how the full-scale expansion of organic farming will impact soil carbon inputs and SOC stocks. Here we use a spatially explicit biogeochemical model to show that the complete conversion of global cropland to organic farming without the use of cover crops and plant residue (normative scenario) will result in a 40% reduction of global soil carbon input and 9% decline in SOC stock. An optimal organic scenario that supports widespread cover cropping and enhanced residue recycling will reduce global soil carbon input by 31%, and SOC can be preserved after 20 yr following conversion to organic farming. These results suggest that expanding organic farming might reduce the potential for soil carbon sequestration unless appropriate farming practices are implemented.
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All data on crop areas, soil carbon inputs and soil organic carbon stocks for any of the scenarios and organic shares considered in this paper are available on a public repository57.
The model code for GOANIM is available in its most recent version at https://github.com/Pie90/GOANIM_public/, together with a full model documentation. All analyses were done using R x64 3.5.3. For RothC we used the ‘cin_month’ and ‘runExplicitSol’ functions from the RothC package to respectively estimate SCI0 and SOC stock evolution across time.
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We thank R. Girault and Y. Behara for help regarding carbon losses in manure management process; D. Angers, E. Ceschia and C. Poeplau for inputs on how to consider cover crops. This work was funded by ADEME, Bordeaux Sciences Agro (Univ. Bordeaux), INRAE’s committee on organic farming (MP Métabio) and Aberdeen University. M.K. and P.S. acknowledge support from the CIRCASA project, which received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no 774378.
The authors declare no competing interests.
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Annual organic to baseline ratios of plant-based residues (a) and manure (b) carbon inputs (in both normative and optimal organic scenarios) and the additional carbon inputs from cover-crops (c) and from enhanced root/shoot ratio and residues recycling (d) in the optimal organic scenario.
Extended Data Fig. 2 Organic to baseline annual SOC stock change in global croplands (% per ha and per year) and in the normative organic scenario.
Annual SOC stock change is reported as a map and a density curve for each period after global transition to organic farming (0–20 years, 20–50 years, 50–100 years). In the density curves, the red dashed lines indicate the estimated global mean of organic to baseline ratio of annual SOC stock change per ha, and the blue dashed lines indicate the value 1.
Conventional manure surpluses available for organic croplands (Mg C.ha-1).
Extended Data Fig. 4 Changes in global SOC stocks (PgC) over time using directly SCIbaseline and SCIorg as inputs to the RothC model.
Changes in global cropland SOC stocks are reported for the baseline (black line) and the normative organic scenario (red line). Values at the right end of each curve represent the SOC stocks after 100 years. The black dashed lines represent the current global SOC stock for croplands.
Extended Data Fig. 5 Global changes in soil organic carbon (SOC) stocks (PgC) in grasslands over time, and maps of the SOC stock ratios between the 100% organic scenario and the baseline at 20 years.
Changes in global SOC stocks in grasslands and spatial distribution are reported for the 100% normative organic scenario. The black dashed line represents the global SOC stocks for grasslands in the baseline.
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Gaudaré, U., Kuhnert, M., Smith, P. et al. Soil organic carbon stocks potentially at risk of decline with organic farming expansion. Nat. Clim. Chang. 13, 719–725 (2023). https://doi.org/10.1038/s41558-023-01721-5