Effects of clearfell harvesting on soil CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O fluxes in an upland Sitka spruce stand in England

Abstract. The effect of clearfell harvesting on soil greenhouse gas (GHG) fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) was assessed in a Sitka spruce forest growing on a peaty gley organo-mineral soil in northern England. Fluxes from the soil and litter layer were measured monthly by the closed chamber method and gas chromatography over four years in two mature stands, with one area harvested after the first year. Concurrent measurements of soil temperature and moisture helped to elucidate reasons for the changes in fluxes. In the three years after felling, there was a significant increase in the soil temperature, particularly between June and November (3 to 5 °C higher), and in soil moisture which was 62 % higher in the felled area, and these had pronounced effects on the GHG balance, in addition to the removal of the trees and their carbon input to the soil. Annual soil CO2 effluxes reduced to almost a third in the first year after felling (a drop from 24.0 to 8.9 t CO2 ha−1 yr−1) and half in the second and third year (mean 11.8 t CO2 ha−1 yr−1) compared to before felling, while those from the unfelled area were little changed. Annual effluxes of N2O more than doubled in the first two years (from 1.0 to 2.3 and 2.5 t CO2e ha−1 yr−1, respectively), although by the third year they were only 20 % higher (1.2 t CO2e ha−1 yr−1). CH4 fluxes changed from a small net uptake of −0.03 t CO2e ha−1 yr−1 before felling to a small efflux increasing over the 3 years to 0.34 t CO2e ha−1 yr−1, presumably because of the wetter soil after felling. Soil CO2 effluxes dominated the total net GHG emission calculated using the global warming potential (GWP) of the three gases, but N2O contributed up to 20 % of the total annual emissions. This study showed fluxes of CO2, CH4 and N2O responded differently to clearfelling due to the significant changes in soil biotic and abiotic factors and showed large variations between years. This demonstrates the need for multi-year measurements of all GHGs to enable a robust estimate of the effect of the clearfell phase on the GHG balance of managed forests. This is one of a very few multi-year monitoring studies to assess the effect of clearfell harvesting on soil GHG fluxes.