No detectable elevated soil carbon under organic farming in German croplands − results from two soil surveys

• New method using machine learning to explore soil management effects from soil inventory data. • Two large scale data sets representative for Central European agriculture covering more than 3000 croplands. • Surprisingly, croplands under organic farming did not store more carbon than conventionally farmed croplands. • Results were confirmed with the independent soil inventory data set. • Underlying mechanism for missing soil carbon accrual with organic farming are assessed. Organic farming contributes to multiple environmental targets in agriculture. Long-term field experiments found that organic farming cropland not only emit lower greenhouse gas emissions but can also increase soil organic carbon (SOC) stocks relative to conventionally farmed plots. It is yet unknown whether the same SOC effect can be found in agricultural reality. In this study, two independent large scale soil inventory datasets (2159 and 811 sites) including both organic and conventional farming sites in Germany were analysed. Differences in SOC due to pedo-climatic conditions between sites were corrected using a random forest model. Croplands under organic farming showed neither higher SOC content nor higher SOC stocks compared to those under conventional farming in both data sets. The amount of organic fertilisers applied was comparable between organic farming (0.47 Mg C ha −1 a -1 ) and conventional farming (0.45 Mg C ha −1 a -1 ). The crop rotations under organic farming contained more crops that are known to enhance SOC (e.g. grass-clover mixtures, 39 %) than in conventional farms (11 %). However, yields in organic farming were on average 31 % lower than in conventional farming, which most likely led to less crop residue returned to soil to build up SOC. Our results challenge the previous perception that organic farming builds up SOC. Agricultural practice reality may not be representatively captured in existing organic farming long-term field trials. Microbial N mining and lower yield resulting from lower N availability in organic farming may hamper the build-up of SOC in organic farming systems. Thus, the extension of organic farming in Central Europe will likely not contribute to additional SOC accrual. However, SOC in organic farms is built up with less environmental costs, e.g. via nitrogen losses, and can thus be considered as more sustainable than SOC in conventional systems.