Jun-Aug/annual mean T precip.sum (degC) (mm) / / / / / / / /810 Jun-Aug/annual mean T precip.sum (degC) (mm) / / / / / /730 Arctic circle FINLAND RUSSIA SWEDEN Contrasting impact of forestry-drainage on CO 2 balance at two adjacent peatlands in Finland Lohila A 1*., Minkkinen K 2., Penttilä T 3., Launiainen S 3., Koskinen M 2., Ojanen P 2., Laurila T 1. 1) Finnish Meteorological Institute, Helsinki, Finland; 2) University of Helsinki, Department of Forest Sciences, Helsinki, Finland; 3) Finnish Forest Research Institute, Helsinki/Joensuu, Finland. Figure 1. The two sites are located 20 km from each other Lettosuo fenKalevansuo bog Tree C store (kg C m -2 ) Tree C fixation (kg C m -2 yr -1 ) Soil C% (0-10 cm55.2 ± ± 0.4 Soil N% (0-10 cm)2.2 ± ± 0.1 Soil CN (0-10 cm)25 ± 334 ± 2 Bulk density 0-10 cm (g cm -3 ) 0.13 ± ± 0.02 Bulk density cm 0.19 ± ± 0.02 Average water level in June-August (cm below the surface ) (± stdev) 50 ± 9 43 ± 5 (56 ± 12 in 2006) Kalevansuo bog Lettosuo fen Introduction Drainage of peatlands is likely to alter the carbon (C) biogeochemistry and may threaten their huge peat C store. Forestry is the most important land-use type of peatlands in Finland. The impact of the water level drawdown on the C balance in forestry-drained peatlands is, however, not well understood. Here we present a comparison of CO 2 fluxes at two adjacent pine forests growing on peatlands which were drained about 40 years earlier. A fen type peatland, Lettosuo (Fig.1, left), is more nutrient-rich as compared to the bog-type site, Kalevansuo (Fig.1, bottom). We measured the net ecosystem exchange (NEE) using the eddy covariance method for 4 and 3 years at the bog and fen sites, respectively. In addition, we have built transparent chambers (Fig. 2) at both sites to understand the role of the forest floor NEE in the whole ecosystem NEE. Peat depth at both sites varies from 1 to 3 m. The differences in the fertility are reflected in: *element concentrations in peat :higher at the fen site *tree growth rate: higher at fen *abundance and species composition of the forest floor vegetation: biomass of mosses and shrubs at the bog 2-3 times higher as at the fen *higher tree C store and CO 2 fixation rate at the fen *CN ratio was lower, and the bulk density of the surface peat higher at the fen site *June-August water level slightly lower at the fen site, also year-to year variation smaller; however in dry year 2006 the mean water level was very low at the bog site, 56 cm below the surface (see Table 1) Figure 1. Transparent automatic chamber at Kalevansuo bog forest with the lid (60 x 60 x 30 cm) open. Table 1. Tree carbon store and fixation rate and peat properties at the two sites. Figure 2. Cumulative daily NEE at a) Kalevansuo in and b) Lettosuo in Negative (positive) value indicate net CO 2 uptake (emission) by the ecosystem. Number panels within the figure show the mean weather conditions during the measurement years. Results Annual and growing season NEE *Annual NEE varied from -580 to g CO 2 m -2 yr -1 at Kalevansuo bog and from -10 to +50 g CO 2 m -2 yr -1 at Lettosuo fen (Fig 2) *Considering the average tree C fixation rates (620 and 1000 g CO 2 m -2 yr -1, at the bog and fen, respectively), we can conclude that C was accumulated in the peat soil at Kalevansuo bog every year. The accumulation rate was, however, much smaller in 2006 when -[NEE] was close to the average tree C fixation rate. At Lettosuo fen, C was lost from the peat soil each year at a rate of about 1000 g CO 2 m -2. *At Kalevansuo bog, the lowest annual CO 2 uptake rate was observed in 2006 with an exceptionally dry summer *In a dry year, both gross primary production (GPP) and ecosystem respiration (R eco ) were suppressed, GPP more. *At Lettosuo fen, the annual NEE in 2010 (warm and dry summer) and 2012 (normal T and precip.) were rather similar, whereas in 2011 (warm and wet summer) the CO 2 uptake was considerably higher. Forest floor CO 2 exchange We tested the difference in forest floor CO 2 exchange rates between the sites by using the gap-filled NEE and GPP data from chambers. In June-August 2013, the NEE (including the tree root respiration, which was assumed to be similar based on the approximately same fine root mass) was 500 and 1120 g CO 2 m -2 at Kalevansuo bog and Lettosuo fen, respectively, indicating considerably higher peat respiration and lower GPP at the Lettosuo fen. For the same period, the cumulative GPP equalled -390 and -50 g CO 2 m -2 at Kalevansuo and Lettosuo, respectively. Conclusions -The drained bog is accumulating C in trees and peat, while the fen has a neutral C balance, i.e., the same amount of C is annually sequestered into tree stand that is lost from the peat (270 g C m -2 ). -This is due to higher GPP by the forest floor vegetation and lower peat respiration at the bog site -Dry summer conditions reduced CO 2 sink in bog, but increased the sink of fen -Tree C fixation at the bog is 60% of that at the fen but the forest floor GPP is 800% of that at the fen, which is related to the sparser canopy at the bog site -CH 4 and N 2 O fluxes at both sites were negligible if compared to CO 2 -The nutrient status of forestry-drained peatlands is likely to determine the fate of peat carbon, with high peat loss rates in fen type peatlands, while bogs may instead remain C sinks ACKNOWLEDGEMENTS Funding was provided by: *Maj and Tor Nessling foundation *EU projects NitroEurope and GHG Europe *Ministry of Agriculture and Forestry *Ministry of Transport and Communications *Academy of Finland (FCoE) Site conditions