CARBON DIOXIDE EXCHANGE ON CULTIVATED AND AFFORESTED BOREAL PEATLANDS

Drainage and agricultural management practices fundamentally alter the moisture and nutrient conditions in peat soils, leading to high carbon dioxide (CO2) losses. Uncertainties in the emission estimates of CO2 from cultivated peatlands are high, and the effects of different management practices on the carbon fluxes are not well understood. One possible land-use option for peat fields removed from agricultural use is afforestation, the effect of which on ecosystem CO2 balance is practically unknown. The aim of this work was to improve the understanding of the biosphere-atmosphere exchange and annual balances of CO2 on agriculturally-managed boreal peatlands. Micrometeorological eddy covariance measurements of CO2 exchange were conducted above two drained peat soil sites with an agricultural history: a cultivated peat soil growing spring barley or forage grass and a 30-year old Scots pine plantation established on organic-soil cropland. Respiration fluxes at both sites were measured with the chamber technique. Partition of total ecosystem respiration (Rtot) into autotrophic and heterotrophic components revealed that the soil respiration in the cultivated peat field was two to three times as high as that in cultivated clay and sandy soils. In addition to temperature, photosynthesis was a central factor controlling the dynamics of Rtot, and should be included in respiration models. While the wintertime emissions of CO2 from the cultivated peatland were mainly controlled by the soil temperature, temporary deviations from this pattern were observed, especially during snow and soil thawing. At or immediately after the snow melt, decreased CO2 efflux rates were observed, probably because of a reduction in gas diffusion rates in wet soil. In winter, significantly different temperature responses in CO2 efflux were not found in bare and grass-covered soil. The number of snow-free days, as well as soil and air temperatures, seemed to control the year-to-year variation in winter CO2 emission. The minimum and maximum values of daily net ecosystem exchange (NEE) during the growing season were rather similar for both of the studied ecosystems and for both crops, although slightly higher daily net CO2 emission and lower net CO2 uptake were observed in the peat soil growing grass. However, large differences in annual balances were observed. These were more attributable to the length of the season with net carbon uptake than to the magnitude of soil respiration. This length was controlled by the management of the ecosystems, being shorter with the annual crop than with the perennial crop, and being longest with the afforested peat field. All of the studied ecosystems acted as sources of CO2 on an annual basis, NEE equalling 770 and 290 g CO2 m yr in the barley and grass fields, respectively, and 50 g CO2 m yr in the afforested organic-soil cropland. When taking into account the carbon in harvested biomass, the total annual carbon loss (excluding leaching and airborne erosion) in the barley and grass fields was about 340 and 450 g C m yr, respectively. Despite the much smaller annual NEE at the afforested site, the results indicate that even this ecosystem, and particularly the peat soil there, is losing carbon. This study suggests that the CO2 loss from agricultural peat soils may be decreased with perennial crops or afforestation. These management strategies do not, however, turn these ecosystems into sinks of CO2 as the carbon loss from decomposition of the old peat continues. Publishing unit Finnish Meteorological Institute, Climate Change Research Classification (UDK)