Soil carbon varies between different organic and conventional management schemes in arable agriculture

The effects of organic versus conventional farming systems on changes in soil organic carbon (SOC) has long been debated. The effects of such comparisons may depend considerably on the design of the respective systems and climate and soil conditions under which they are performed. Here, we compare a range of arable organic and conventional crop systems at three sites (Jyndevad, Foulum and Flakkebjerg) in Denmark through long-term experiments initiated in 1997. The experimental treatments in the organic farming systems included use of whole-year green manure crops, catch crops and animal manure (as cattle, pig or digested slurry). Data on plant residues and animal manure were used to estimate C inputs to the soil. This was compared with measured changes in topsoil (0-25 cm) SOC content over 4-8 years. During 1997-2004, green manure, catch crops and animal manure enhanced estimated C input by 0.9, 1.0 and 0.7 Mg C ha(-1) yr(-1) respectively, across all locations. Based on measured SOC changes, green manure enhanced SOC by 0.4 Mg C ha(-1) yr(-1) and catch crops by 0.2 Mg C ha(-1) yr(-1), while animal manure by insignificantly 0.1 Mg C ha(-1) yr(-1). After 2005, advantages of using green manure (grass-clover) on SOC change disappeared, because cuttings of the grass-clover was removed whereas before 2005 they were mulched in the field, albeit there was still a small extra estimated C input of 0.2 Mg C ha(-1) yr(-1). An estimated higher C input of 0.7 Mg C ha(-1) yr(-1) with catch crops did not result in significant increase in measured topsoil SOC. From 2005-2008, the first 4 years of comparison between organic and conventional farming at all three sites, organic farming with animal manure had 0.3 Mg C ha(-1) yr(-1) higher estimated C input, but SOC measurements showed that conventional farming accumulated 0.4 Mg C ha(-1) yr(-1) more SOC than organic farming. At Foulum from 2005 to 2012, organic farming with animal manure had 0.7 Mg C ha(-1) yr(-1) more input, and topsoil SOC measurements showed a higher accumulation of 0.4 Mg C ha(-1) yr(-1) in organic compared with conventional fanning. Regressions of changes in topsoil SOC against estimated C inputs showed that 10-20% of C inputs were retained in topsoil SOC over the experimental period. There was no clear indication that belowground C input contributed more to SOC than aboveground C inputs. Despite consistently higher estimated C inputs in organic versus conventional systems, we were not able to detect consistent differences in measured SOC between the systems.