Water-Soluble Carbon and the Carbon Dioxide Pulse are Regulated by the Extent of Soil Drying and Rewetting

It is well established that rewetting dry agricultural soils causes a pulse of CO2 to be emitted due to rapid mineralization of organic C ("Birch effect"). The factors controlling the size of the CO2 pulse are not well known or understood. The objective of this laboratory study was consequently to determine the main factors affecting CO2 pulse size resulting from drying repacked cores of a clay loam soil (fine, loamy, mixed, mesic Typic Argiaquoll) to 45, 30, 20, or 10% water-filled pore space (WFPS) and subsequent rewetting to 75 or 90% WFPS. A distinct CO2 pulse was observed after rewetting soil dried to 10 and 20% WFPS, but no pulse was emitted after rewetting soil dried to 30 or 45% WFPS. Significant positive linear correlations occurred between CO2 pulse size and water-soluble soil C content in the form of dissolved organic C (DOC) and hot water extractable C (HWEC); the greatest increase in both DOC and HWEC occurred with the most extensive soil drying (i.e., drying to 10% WFPS). There were no significant correlations between CO2 pulse size and peak intensity of functional groups in HWEC, as determined using Fourier transform infrared (FT-IR) spectroscopy. However, the magnitude of the C-O stretching peak associated with soil polysaccharides was significantly related to the amount of CO2 emitted in the first few hours after rewetting dry soil. The total amount of C mineralized over the entire drying-rewetting process was found to be lower than that when soils were maintained at 75 or 90% constantly. Hence, the CO2 pulse following rewetting did not compensate for the reduced CO2 emission during the drying phase. It was also concluded that the amount of CO2 emitted on rewetting this clay loam soil depended primarily on the amount of water-soluble C (WSC; DOC, HWEC) released from the soil drying-rewetting process.