Impacts of Constant Versus Fluctuating Water-Filled Pore Space On Carbon Mineralization In Soils.

Soil water content and cropping history play important roles in soil organic carbon inputs, decomposition and nutrient cycling.  However, variations in soil water content within and between seasons add to the complexity of organic carbon transformations in soils.  We investigated the impacts of constant soil water contents (30%, 45%, 60%, 75% and 90% water-filled pore space, WFPS), wet-dry (WD) cycles (varying between 90 to 30% WFPS) and crop rotation (monoculture corn [Zea mays L.], a 2-yr corn-soybean [Glycine max (L.) Merr.] rotation, or a 3-yr corn-soybean-winter wheat [Triticum aestivum L.] rotation) on carbon mineralization and dissolved organic carbon (DOC) using repacked soil cores incubated for 50 d.  The carbon mineralization rates increased with increasing soil water content, and the CO₂ emission generally peaked after about 10 d. Cumulative carbon mineralization was greatest with the wettest constant soil water content treatment (i.e. 90% WFPS).  Compared with the constant water content treatments, cumulative carbon mineralization in the repeated WD cycles was similar to that of the 60% WFPS.  The average water content over the 10 d drying process for the WD treatment was 63%. The soil from C-S-WW rotation had the lower cumulative carbon mineralization than the soils from both the C-S and monoculture corn treatments for each constant soil water content treatment.  DOC levels dramatically decreased over the first 10 d and then remained fairly steady for the subsequent 40 d. Crop rotation did not significantly affect the DOC levels. In general, the effect of the WD cycles on carbon mineralization appeared to be primarily related to the average water content during the drying process more so than any indirect effect of the wetting and drying on C release from the soils.