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 delta C-13 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.