Hu, Teng; Sørensen, Peter and Olesen, Jørgen E
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 farming.
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.
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