Carbon remineralization in a north Florida swamp forest: Effects of water level on the pathways and rates of soil organic matter decomposition

Water level controlled gas emissions from North Florida swamp forests. Under flooded conditions, CO 2 and CH 4 were the principle carbon gases transported to the atmosphere by bubble ebullition and molecular diffusion. The respective emission rates were for CO 2 , 29.3 ± 16.4 (13% by means of ebullition, 87% by means of diffusion, error is ± 1 standard deviation throughout) and for CH 4 , 2.16 ± 2.24 (45% by means of ebullition, 55% by means of diffusion) mol m −2 yr −1 . Methane emissions were significantly attenuated by CH 4 oxidation which occurred primarily at the sediment‐water interface. Forty‐six ± 22 % (n=19) of the belowground CH 4 diffusing to this interface was oxidized before it could escape to the atmosphere. Under dry conditions, CO 2 was the principle carbon gas released and atmospheric CH 4 was consumed by microbes in the soil. The respective rates were 101.2 ± 26.80 and −0.015 ± 0.005 mol mr −2 yr −1 . A carbon budget for the degradation of soil organic matter was developed for a swamp forest site under flooded and dry conditions. Assuming that live root respiration accounted for 67% (value determined in a swamp forest and is at the upper range of literature values) of the total CO 2 emissions (given above), we calculate that under flooded conditions carbon remineralization proceeded at a total rate of 11.9 mol C m −2 yr −1 . Forty‐nine percent of the remineralization was by means of nonmethanogenic processes which produce CO 2 ; the balance was by means of methanogenic processes, which produce both CH 4 and CO 2 . Under dry conditions, remineralization was dominated by aerobic processes at a rate of 33.7 mol C m −2 yr −1 . Carbon inputs to the soil occurred by aboveground and belowground production. Aboveground litter production contributed 25.6 mol C m −2 yr −1 . If belowground production contributed an equal amount, then over the course of this study organic carbon accumulated in the soils at rates of 39.3 and 17.5 mol C m −2 yr −1 under flooded and dry conditions, respectively. If root respiration accounted for only 6% (lowest value in literature) of the total CO 2 emissions, organic carbon would accumulate in the soil at a rate of 21.6 mol C m −2 d −1 under flooded conditions and be lost from the soil at a rate of 43.8 mol C m −2 d −1 under dry conditions.