N deposition and elevated CO_2 on methane emissions: Differential responses of indirect effects compared to direct effects through litter chemistry feedbacks

摘要

Increases in atmospheric CO_2 concentration and N deposition are expected to affect methane (CH_4) production in soils and emission to the atmosphere, directly through increased plant litter production and indirectly through changes in substrate quality. We examined how CH_4 emission responded to changes in litter quality under increased N and CO_2, beyond differences in CH_4 resulting from changes in litter production. We used senesced leaves from 13C-labeled plants of Molinia caerulea grown at elevated and ambient CO_2 and affected by N fertilization to carry out two experiments: a laboratory litter incubation and a pot experiment. N fertilization increased N and decreased C concentrations in litter whereas elevated CO_2 decreased litter quality as reflected in litter C and N concentrations and in the composition of lignin and saturated fatty acids within the litter. In contrast to our expectations, CH_4 production in the laboratory incubation decreased when using litter from N-fertilized plants as substrate, whereas litter from elevated CO_2 had no effect, compared to controls without N and at ambient CO_2. Owing to high within-treatment variability in CH_4 emissions, none of the treatment effects were reflected in the pot experiment. C mineralization rates were not affected by any of the treatments. The decrease in CH_4 emissions due to indirect effects of N availability through litter quality changes (described here for the first time) contrast direct effects of N fertilization on CH_4 production. The complex interaction of direct effects with indirect effects of increased N on litter quality may potentially result in a net decrease in CH_4 emissions from wetlands in the long term.