Forest and grassland cover types reduce net greenhouse gas emissions from agricultural soils

摘要

Western Canada's prairie region is extensively cultivated for agricultural production, which is a large source of greenhouse gas emissions. Agroforestry systems are common land uses across Canada, which integrate trees into the agricultural landscape and could play a substantial role in sequestering carbon and mitigating increases in atmospheric GHG concentrations. We measured soil CO_2, CH_4 and N_2O fluxes and the global wanning potential of microbe-mediated net greenhouse gas emissions (GWP_m) in forest and herbland (areas without trees) soils of three agroforestry systems (hedgerow, shelterbelt and silvopasture) over two growing seasons (May through September in 2013 and 2014). We measured greenhouse gas fluxes and environmental conditions at 36 agroforestry sites (12 sites for each system) located along a south-north oriented soil/climate gradient of increasing moisture availability in central Alberta, Canada. The temperature sensitivity of soil CO_2 emissions was greater in herbland (4.4) than in forest (3.1), but was not different among agroforestry systems. Over the two seasons, forest soils had 3.4% greater CO_2 emission, 36% higher CH_4 uptake, and 66% lower N_2O emission than adjacent herbland soils. Combining the CO_2 equivalents of soil CH_4 and N_2O fluxes with the CO_2 emitted via heterotrophic (microbial) respiration, forest soils had a smaller GWP_m than herbland soils (68 and 89 kg CO_2 ha~(-1) respectively). While emissions of total CO_2 were silvopasture ＞ hedgerow ＞ shelterbelt, soils under silvopasture had 5% lower heterotrophic respiration, 15% greater CH_4 uptake, and 44% lower N_2O emission as compared with the other two agroforestry systems. Overall, the GWP_m of greenhouse gas emissions was greater in hedgerow (88) and shelterbelt (85) than in the silvopasture system (76 kg CO_2 ha~(-1)). High GWP_m in the hedgerow and shelterbelt systems reflects the greater contribution from the monoculture annual crops within these systems. Opportunities exist for reducing soil greenhouse gas emissions and mitigating climate change by promoting the establishment of perennial vegetation in the agricultural landscape.