Crop residue and tillage effects on carbon sequestration in a Luvisol in central Ohio

Soils play a key role in the global carbon cycle. They can be a source or a sink of carbon and influence CO₂ concentrations in the atmosphere. In order to calculate the carbon budget of a region, the effect of soil management practices on carbon sequestration in soils needs to be quantified. Objectives of this experiment were to determine: (i) the effects of ridge till, plow till and no-till on the soil organic carbon (SOC) pool; (ii) the SOC loss or sequestration for mulch rates of 016 Mg ha^-1 year^-1 wheat (Triticum aestivum L.) straw applied in combination with each tillage method, and (iii) impacts of tillage and crop residue treatments on soil physical quality, including aggregation and porosity. The experiment was initiated in 1989 on a Crosby silt loam (Stagnic Luvisol) in Central Ohio. Seven years after initiation of the experiment, there was a positive, linear effect of residue application rate on SOC contents in all tillage treatments. In the eighth year of the experiment these trends were confirmed for plow and no-till, but not for ridge till. Linear-regression equations, relating SOC content for the 0-10 cm soil depth to mulch rate, were: for no-till: SOC (Mg ha^-1) = 15.21 + 0.32 [Residue (Mg ha^-1 year^-1)] (r = 0.68) and for plow till: SOC = 11.95 + 0.27 [Residue] (p = 0.72). The carbon conversion efficiencies were 8% per year for plow till and 10% per year for no-till. Detailed sampling at different depths revealed that increases in SOC content were only significant for the 0-5 cm depths of plow and no-till treatments. Effects of crop residue application on water stable aggregation in the 0-10 cm layer were most pronounced with plow till and ridge till but not with no-till. Water retention characteristics, a measure of pore size distribution, was not influenced by tillage system, but crop residue application had a significant effect on water retention in the 0-10 cm layer at matric suctions of 30-300 kPa. This means that residue application increased macropores of diameters 1-10 μm. It is concluded that, depending on the amount of crop residue returned to the soil, the large numbers of farmers converting from plow to no-till cultivation in the Corn Belt may create an important sink for atmospheric CO₂.