FCRN member Dr. Adrian Muller co-authored a meta-analysis published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS). The authors looked at datasets from 74 studies from pairwise comparisons of organic vs. nonorganic farming systems to identify differences in soil organic carbon (SOC).
The objectives were to test whether adoption of organic farming resulted in 1. an increase in overall SOC concentration, 2. an increase in overall SOC stocks, and 3. higher SOC accumulation over time compared with nonorganic management.
The general results of the study indicated that soil organic carbon stocks were 3.5 metric tons per hectare higher in organic than in non-organic farming systems and that organic farming systems sequestered up to 450 kg more atmospheric carbon per hectare and year, through CO2 bound into soil organic matter. Metaregression did not deliver clear results on the drivers of this, but differences in external carbon inputs and crop rotations seemed important.
The abstract and citation are as follows:
It has been suggested that conversion to organic farming contributes to soil carbon sequestration, but until now a comprehensive quantitative assessment has been lacking. Therefore, datasets from 74 studies from pairwise comparisons of organic vs. nonorganic farming systems were subjected to meta-analysis to identify differences in soil organic carbon (SOC). We found significant differences and higher values for organically farmed soils of 0.18 ± 0.06% points (mean ± 95% confidence interval) for SOC concentrations, 3.50 ± 1.08 Mg C ha−1 for stocks, and 0.45 ± 0.21 Mg C ha−1 y−1 for sequestration rates compared with nonorganic management. Metaregression did not deliver clear results on drivers, but differences in external C inputs and crop rotations seemed important. Restricting the analysis to zero net input organic systems and retaining only the datasets with highest data quality (measured soil bulk densities and external C and N inputs), the mean difference in SOC stocks between the farming systems was still significant (1.98 ± 1.50 Mg C ha−1), whereas the difference in sequestration rates became insignificant (0.07 ± 0.08 Mg C ha−1y−1). Analyzing zero net input systems for all data without this quality requirement revealed significant, positive differences in SOC concentrations and stocks (0.13 ± 0.09% points and 2.16 ± 1.65 Mg C ha−1, respectively) and insignificant differences for sequestration rates (0.27 ± 0.37 Mg C ha−1 y−1). The data mainly cover top soil and temperate zones, whereas only few data from tropical regions and subsoil horizons exist. Summarizing, this study shows that organic farming has the potential to accumulate soil carbon.
Gattinger, A., Mueller, A., Haeni, M., Skinner, C., Fliessbach, A., Buchmann, N., Mäder, P., Stolze, M., Smith, P., El-Hage Scialabba, N., and Niggli U. (2012). Enhanced top soil carbon stocks under organic farming. Proceedings of the National Academy of Sciences vol. 109 no. 44 18226-18231. doi:10.1073/pnas.1209429109PNAS