Impacts of Wet–Dry Cycles and a Range of Constant Water Contents on Carbon Mineralization in Soils under Three Cropping Treatments

Soil water content and cropping history play important roles in soil organic carbon inputs, decomposition, and nutrient cycling. However, variations in soil water content including wetting and drying events can influence organic carbon transformations in soils. The impacts of constant soil water contents (30, 45, 60, 75, and 90% water-filled pore space, WFPS), wet–dry (W–D) cycles (five 10-d cycles varying between 90 and 30% WFPS) and cropping treatments on carbon mineralization (CO₂ emissions) and dissolved organic carbon (DOC) were investigated using repacked cores of a clay loam soil incubated for 50 d. The cropping treatments included monoculture corn (Zea mays L.), a 2-yr corn–soybean (Glycine max L. Merr.) rotation (C–S), and a 3-yr corn–soybean–winter wheat (Triticum aestivum L.) rotation (C–S–WW) as these were believed to impact the carbon status of the soils. The carbon mineralization rates increased with increasing soil water content, and generally achieved the highest rates after 10 d. Cumulative carbon mineralization was greatest with the wettest constant soil water content treatment (i.e., 90% WFPS) and decreased with decreasing constant WFPS values. The average water content over the 10 d drying process for the W–D treatment was 63% WFPS. Cumulative carbon mineralization during the W–D cycles was similar to that for the constant 60% WFPS treatment. The corn phase of the C–S–WW rotation produced lower carbon mineralization rates than the corn phase of the C–S rotation and monoculture corn for each constant soil water content treatment. The DOC levels dramatically decreased over the first 10 d and then remained between 36.5 and 69.8 mg C kg⁻¹ for the subsequent 40 d. Cropping treatments did not significantly affect the DOC levels. In general, the effect of the W–D cycles on carbon mineralization appeared to be related more to the average soil water content during the drying process than to soil carbon release as a result of soil drying and rewetting.