Long-term effects of tillage and corn stalk return on soil carbon dynamics

The ability to increase pools of soil organic C (SOC) in agricultural ecosystems is of interest both for sequestering atmospheric CO,, and for restoring organic matter pools important to soil health. It has been well established that tillage and harvest management regimes can influence SOC in cropland, but long-term, continuous experiments are rare. We investigated the dynamics of relic and new SOC pools using delta(13)C analysis in cornfields (Zea mays L.) established in 1972 at the University of Connecticut Research Farm. The plots have been tinder no-till (NT) or conventional till (CT) management, with residues returned (+) or residues removed (-) within each tillage treatment. After 28 yr of continuous management, NT increased SOC significantly by 48.3 +/- 9.9 g C m(-2) yr(-1) over CT in treatments with residue returned (i.e., NT+ compared with CT+), and by 60.1 +/- 13.8 g C m(-2) yr(-1) over CT in treatments with residue removed (i.e., NT- compared with CT-). Residue return in NT+ plots did not increase SOC relative to NT- plots (23.0 +/- 12.7 g C m(-2) yr(-1), N.S.), but residue return to the CT+ plots resulted in a significant increase in SOC of 34.8 +/- 11.1 g C m(-2) yr(-1) over SOC content in CT- plots. The greatest difference in SOC content was found between the CT- and NT+ treatments (83.1 +/- 10.7 g C m(-2) yr(-1)). Our results indicate that there may be a rapid cycling of the aboveground C4-C back to the atmosphere as CO, or lost as dissolved organic C from the soil profile. Such a rapid cycling of returned C4-C suggests that the annual return of aboveground biomass may not increase soil C storage over the long term once soils have reached a steady-state SOC level.