PARENT SESSION
Contributed Oral Session 75: Biogeochemistry: Soil Chemical Flux
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Impacts of afforestation on soil methane fluxes.

McNamara, Niall ¹, Helaina, Helaina¹, Piearce, Trevor², Stott, Andrew¹, Chamberlain, Paul¹, Ineson, Phil³, ¹ Centre for Ecology and Hydrology, Lancaster, Lancashire, United Kingdom² Department of Biological Sciences, Lancaster, Lancashire, United Kingdom³ Department of Biology, York, Yorkshire, United Kingdom

ABSTRACT- There is accumulating evidence that land use influences the direction of soil methane (CH₄) fluxes and afforestation has been proposed as a potential tool for mitigating CH₄ emissions from organic soils through stimulating microbial CH₄ oxidation and/or inhibiting microbial CH₄ production. To investigate, we measured soil CH₄ flux rates under four tree species, Norway spruce, Scots pine, oak, alder and grassland controls at two weekly intervals over 12 months. Our results indicated an overall trend of CH₄ production from grassland plots (mean rate: 52 ± 48.9 g CH₄ m^-2 hr^-1). There was, however, consistent CH₄ consumption in one grassland plot that may reflect plot differences in water table depth. Afforested soils were weak sinks for CH₄; -5.7 ± -2.9 (oak), -10.4 ± -7.7 (alder), -7.9 ± 6.6 (S. pine) and -10.8 ± -5.9 (N. spruce) g CH₄ m^-2 hr^-1. Overall, CH₄ fluxes across the treatments were correlated with water table depth (r² = 0.84) with lower water tables (forest plots) limiting CH₄ emissions. In the laboratory we incubated forest soil cores under high and low water tables for 15 months. Measurements of CH₄ fluxes and the natural abundance ¹³C stable isotope values of CH₄ showed that under high water table conditions CH₄ production predominated (1925 ± 1702 g CH₄ m^-2 hr^-1) while under low water tables some CH₄ uptake occurred (-3.5 ± 6.8 g CH₄ m^-2 hr^-1). The ¹³C of CH₄ from high water table cores was depleted (-54.6 ± 3.6 per mille) compared to ambient air (47.5 per mille) indicative of methanogenic processes. The negligible deviation from ambient air of ¹³C of CH₄ from low water table cores (¹³C -47.6 ± 0.4 per mille) suggested little soil atmosphere exchange of CH₄ in either direction. These results show that reduced CH₄ emissions from afforested soils were as a result of decreased CH₄ production rates, rather than enhanced CH₄ oxidation rates.

Key words: methane, soil, land use, microbial