Minimum tillage practices are known for increasing soil organic carbon (SOC). However, not all environmental situations may manifest this potential change. The SOC and N stocks were assessed on a Mollisol in central Ohio in an 8-year-old tillage experiment as well as under two relatively undisturbed land uses; a secondary forest and a pasture, on the same soil type. Cropped systems had 51 +/- 4 (equiv. mass) Mg ha(-1) lower SOC and lower 3.5 +/- 0.3 (equiv. mass)Mg ha(-1) N in the top 30cm soil layer than Linder forest. Being a secondary forest, the loss in SOC and N stocks by cultivation may have been even more than these reported herein. No differences among systems were detected below this depth. The SOC stock in the pasture treatment was 29 +/- 3 Mg ha(-1), greater in the top 10 cm layer than in cultivated soils. but was similar to those tinder forest and no-till (NT). Among tillage practices (plow, chisel and NT) only the 0-5 cm soil layer under NT exhibited higher SOC and N concentrations. An analysis of the literature of NT effect on SOC stocks. using meta-analysis, suggested that NT would have an overall positive effect on SOC sequestration rate but with a greater variability of what was previously reported. The average sequestration rate of NT was 330 kg SOC ha(-1) year(-1) with a 95%, confidence interval ranging from 47 to 620 kg SOC ha(-1) year(-1). There was no effect of soil texture or crop rotation on the SOC sequestration rate that could year explain this variability. The conversion factor for SOC stock changes from plow to NT was equal to 1.04. This suggests that the complex mechanisms and pathways of SOC accrual warrant a cautious approach when generalizing the beneficial changes of NT on SOC stocks. (C) 2004 Elsevier B.V. All rights reserved.
