Distribution of organic carbon in the stable soil humic fractions as affected by tillage management

Soil humus comprises a large and stable pool of soil organic matter (SOM); hence a better understanding of the fate of C in soil humic fractions can provide valuable information for the development of alternative tillage practices that will lead to long-term soil C sequestration. We used δ 13 C techniques to investigate the effects of tillage on the dynamics of native (C 3 –C) and corn derived C (C 4 –C) in fulvic acid (FA), humic acid (HA) and humin fractions. Humic substances were extracted from soils cropped to corn for 11 yr and managed under either conventional (CT) or no-tillage (NT), and from a conventionally tilled soil under > 55 yr of tobacco/rye rotation. No-tillage resulted in higher proportions of C 4 –C in the upper 5 cm and generally lower C 4 –C proportions below 5 cm than CT. Up to 31, 27 and 34% of C 4 –C were assimilated into FA, HA and humin fractions, respectively, indicating that even the humin fraction, often described as passive, old or resistant, acted as a sink of recently added C, and that it is heterogeneous with some of its components being young. Recovery of large proportions of C 3 –C in the humic fractions demonstrated their importance in the long-term stabilization of SOM. Within each sampling depth, there were no unique differences in the distribution of C 3 –C among the three humic fractions, suggesting similar turnover of C 3 –C in all the fractions. Therefore, there was no unique active fraction corresponding with the concept of C pools with defined turnover characteristics used in models of SOM turnover. Key words: Soil humic fractions, corn derived C, native C, δ 13 C techniques, tillage practices