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...n of partially decomposed organic matter (OM) as peat.sThis accumulated soil C accounts for a considerable share of the global terrestrial C pool withsan estimated C reservoir of 250–455 Pg (1015 g) (=-=Gorham 1991-=-, Turunen et al. 2002). Rising concentrations of greenhouse gases (GHG) in the atmosphere are causing thesEarth’s climate to change (IPCC 2007). This climate change is expected to have the mostspronou...

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...mical reactions, strongly related to temperature.sThis positive relationship between soil temperature and CO2 efflux originating from soil isswell established in mineral soils (Singht and Gupta 1977, =-=Raich and Schlesinger 1992-=-, Lloydsand Taylor 1994, Knorr et al. 2005, Davidson and Janssens 2006). Also in peatlands the importance of temperature in explaining temporal variation in heterotrophic peat soil respirations(Minkki...

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...reduces the thickness of soil water films, thus inhibiting diffusion of extracellular enzymes and soluble organic C substrates which causes reduction in substrate availability at reaction microsites (=-=Davidson and Janssens 2006-=-). Drought also limits the amountsof moisture available to decomposer organisms for use as a medium for tissue growth andsimpedes the activity of aquatic organisms. Thus soil moisture conditions along...

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...mperature,sWL, vegetation structure and photosynthetic activity as well as from the input rates of littersand root-derived photosynthetic products (root exudates) to the soil (Howard and Howards1993, =-=Lloyd and Taylor 1994-=-, Silvola et al. 1996a, Davidson et al. 1998, Buchmann 2000,sKuzyakov et al. 2000, Högberg et al. 2001) to which they may respond differently (Boone etsal. 1998). Thus, it is difficult to estimate the...

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...ng the autotrophic respiration of plant roots andsrespiration from the processes occurring in the rhizosphere (RROOT). The relative contributionsof these components to RTOT varies between ecosystems (=-=Hanson et al. 2000-=-). These components are also controlled by a range of different biotic and abiotic factors such as temperature,sWL, vegetation structure and photosynthetic activity as well as from the input rates of ...

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...ecosystem or soil in question. However, studies in field and laboratorysconditions have demonstrated great variability in the temperature sensitivity of soil respirationsbetween different ecosystems (=-=Boone et al. 1998-=-, Fierer et al. 2006) and soil characteristicss(Moore and Dalva 1993, Yavitt et al. 1997, Davidson and Janssens 2006, Hardie et al. 2011).sThe fixed temperature sensitivity of OM decomposition in the ...

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...e input rates of littersand root-derived photosynthetic products (root exudates) to the soil (Howard and Howards1993, Lloyd and Taylor 1994, Silvola et al. 1996a, Davidson et al. 1998, Buchmann 2000,s=-=Kuzyakov et al. 2000-=-, Högberg et al. 2001) to which they may respond differently (Boone etsal. 1998). Thus, it is difficult to estimate the decomposition rate of peat and its dependenceson environmental factors by using ...

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...emoval of a tree stand alters the site’s microclimate, vegetation structure, litter input rates, the amount of photosynthesising and respiring biomass andsconsequently the carbon balance of the site (=-=Johnson and Curtis 2001-=-, Kowalski et al. 2004,sJandl et al. 2007). In drained peatland forests removal of trees most probably causes thesessites to turn into a source of C to the atmosphere as tree growth no longer compensa...

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...itudes (IPCC 2007) where the majority of northern peatlands areslocated (Gorham, 1991). Indeed, longer and drier growing seasons have in fact already beensrecorded in these areas (Keyser et al. 2000, =-=Goetz et al. 2005-=-). Responses of peatland C poolssto climate change have significant uncertainties partly because of the difficulties in predictingsthe decomposition rate of peat in these forthcoming conditions (Laiho...

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...ated increase in the volume of the oxic peatslayer has been considered to increase the decomposition rate of the accumulated peat layers(Armentano and Menges 1986, Silvola 1986, Silvola et al. 1996a, =-=Alm et al. 1999-=-, Moores2002). This increase in decomposition rate of peat related to WL drawdown has been clearlysdemonstrated in laboratory studies which show that CO2 emission from peat samples, an indicator of mi...

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...iration rate leading to an overestimation of heterotrophicsrespiration (Subke et al. 2006). Nevertheless, the initial effects of trenching have been shownsto last only a few months (Ewel et al. 1987, =-=Bowden et al. 1993-=-, Komulainen et al. 1999).sFigure 6. Annual heterotrophic peat soil respiration (RPEAT) in afforested organic soil croplandssversus peat ash content (r2=0.133). 32 In this study, we reported RPEAT fro...

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...organisms. Thus soil moisture conditions along with oxygensavailability can regulate microbial growth rates (Barros et al. 1995, Alexandre et al. 1999)sand rates of organic matter (OM) decomposition (=-=Howard and Howard 1993-=-, Reichstein etsal. 2005). Only a few studies have demonstrated that drought would affect OM decomposition rates in peatlands. Within these studies the effect of drought has been limited to the freshs...

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...atesbut also human-induced forestry activities and changes in land use may threaten their large Csstocks. The primary effects of drainage for forestry (Glenn et al. 1993, Silvola et al. 1996a,s1996b, =-=Minkkinen and Laine 1998-=-, Minkkinen et al. 2002, Hargreaves et al. 2003, Laiho et al.s2004, von Arnold et al. 2005a, 2005b, Byrne and Farrell 2005, Minkkinen et al. 2007, Lohilaset al. 2010, 2011) and for agriculture (Nykäne...

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...ands are reaching maturity and forest activities arescarried out in these areas with increasing rate (Hökkä et al. 2002). Removal of trees changessthe environmental conditions and litter input rates (=-=Jandl et al. 2007-=-), which could alter peatsdecomposition rates and potentially cause a substantial C release to the atmosphere. Bettersunderstanding of the effects of management practices on peat decomposition rates c...

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... conditions (Davidson and Janssens 2006). 1.3. Impacts of human-induced land use changes on peat decomposition rate Drained peatlands have become a large land use category in many northern countriess(=-=Paavilainen and Päivänen 1995-=-, Maljanen et al. 2009), where drainage of peat soils hassbeen commonly used to stimulate the productivity of the peatland forests but also to increasesthe area suitable for agriculture (Maljanen et a...

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...ying LR is enhanced by the input of fresh organicsmatter in the form of LR. Laboratory studies have recently demonstrated the existence andsimportance of priming in soil organic matter decomposition (=-=Fontaine et al. 2004-=-, 2007). Thisspriming effect seems to be especially relevant if old soil organic matter, which itself consistssof recalcitrant compounds with low energy content, is exposed to excessive amounts of fre...

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...e 1998, Minkkinen et al. 2002, Hargreaves et al. 2003, Laiho et al.s2004, von Arnold et al. 2005a, 2005b, Byrne and Farrell 2005, Minkkinen et al. 2007, Lohilaset al. 2010, 2011) and for agriculture (=-=Nykänen et al. 1995-=-, Langeveld et al. 1997, Maljanenset al. 2001a, 2004, Lohila et al. 2004 ) on ecosystem and the soil C balance in peatlands havesdrawn great interest during the last decades. However, only scattered i...

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...s measured using a closedchamber system with air circulating in a loop between the chamber and an external infraredsgas analyser (EGM-4 CO2 analyser + modified SRC-1 soil respiration chamber (NSF-11,s=-=Pumpanen et al. 2004-=-), PP-Systems Ltd.) equipped with a water vapour equilibrator. A metallic chamber (diameter 31.5 cm, height 16.9 cm) equipped with a fan was manually placed onsthe sample plot for a measurement period...

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...he most sustainable forestry activities would besselected in the future. Reclaiming peatlands for agricultural use have caused these areas to lose considerablesamounts of C from their peat C storage (=-=Armentano and Menges 1986-=-, Nykänen et al. 1995,sLangeveld et al. 1997, Maljanen et al. 2001a, 2004, 2009, Lohila et al. 2003, 2004). Furthersland use change from agricultural lands into forests through afforestation has been ...

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...clearfelling. After clearfelling soil temperatures may rise and face higher diurnal fluctuations, as thesremoval of the tree canopy increases the amount of direct solar radiation to the soil surfaces(=-=Edwards and Ross-Todd 1983-=-, Londo et al. 1999). Increased soil temperature as such shouldsaccelerate decomposition of peat if other factors remain unchanged. In drained peatland forests,showever, the felling of trees and thus ...

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.... Reclaiming peatlands for agricultural use have caused these areas to lose considerablesamounts of C from their peat C storage (Armentano and Menges 1986, Nykänen et al. 1995,sLangeveld et al. 1997, =-=Maljanen et al. 2001a-=-, 2004, 2009, Lohila et al. 2003, 2004). Furthersland use change from agricultural lands into forests through afforestation has been recognisedsas a useful means to reduce soil degradation and increas...

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...ters the site’s microclimate, vegetation structure, litter input rates, the amount of photosynthesising and respiring biomass andsconsequently the carbon balance of the site (Johnson and Curtis 2001, =-=Kowalski et al. 2004-=-,sJandl et al. 2007). In drained peatland forests removal of trees most probably causes thesessites to turn into a source of C to the atmosphere as tree growth no longer compensates forsthe decomposit...

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...d Dalva 1993, 1997,sBlodau et al. 2004). Observations in field conditions, however, show considerable variabilitysin the relationship between WL and measured soil CO2 emissions (Silvola et al. 1996a, =-=Lafleur et al. 2005-=-). Studies have further shown that lowering the WL within sites increases CO2semissions only to a certain depth (Silvola et al. 1996a, Chimner and Cooper 2003, Flanagansand Syed 2011), and that WL may...

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...n question. However, studies in field and laboratorysconditions have demonstrated great variability in the temperature sensitivity of soil respirationsbetween different ecosystems (Boone et al. 1998, =-=Fierer et al. 2006-=-) and soil characteristicss(Moore and Dalva 1993, Yavitt et al. 1997, Davidson and Janssens 2006, Hardie et al. 2011).sThe fixed temperature sensitivity of OM decomposition in the models could thus ca...

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...ship of the entire RPEAT dataset (r2 = 0.47) (Equation 1, see Section 2.3.).s25 decrease in soil moisture content has been observed both in laboratory and in field conditionss(Howard and Howard 1993, =-=Reichstein et al. 2005-=-, Rey et al. 2005). The observed variation in temperature sensitivity of decomposition could be related tosWLAVE which was the strongest factor of the measured variables to explain the changes insmicr...

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... of these surface layers for peat soil CO2 releaseshas been demonstrated earlier in laboratory studies by Hogg et al. (1992). The WL fluctuationssin deep peat layers that had recalcitrant substrates (=-=Bridgham and Richardson 1992-=-) and lowstemperatures are likely to have had a minor effect on the observed CO2 fluxes, which wouldsexplain the lack of a relationship between WL and RPEAT. This observation is in agreementswith Chim...

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...cultural history had clearly transformedsthe peat properties on the studied sites (I, V, Table 1). Peat bulk density and ash content wassconsiderably higher than in peatlands drained for forestry (V, =-=Kaunisto and Paavilainen 1988-=-,sMinkkinen et al. 1999). These effects of cultivation practices on peat properties, and also onspeat decomposition rates, were still evident even decades after afforestation with the annualsRPEAT bei...

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... to an overestimation of heterotrophicsrespiration (Subke et al. 2006). Nevertheless, the initial effects of trenching have been shownsto last only a few months (Ewel et al. 1987, Bowden et al. 1993, =-=Komulainen et al. 1999-=-).sFigure 6. Annual heterotrophic peat soil respiration (RPEAT) in afforested organic soil croplandssversus peat ash content (r2=0.133). 32 In this study, we reported RPEAT from the trenched plot one ...

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... of net soil carbon storage in changing climate,sparticularly since these simulations have been shown to be rather sensitive to even smallschanges in temperature sensitivity of OM (Jones et al. 2003, =-=Lenton and Huntingford 2003-=-). Even though the temperature sensitivity of decomposition has been fixed in current simulation models, there are several known factors that can cause variation in temperature sensitivitysof decompos...

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...d 1983).sThis drying of the surface soil is proposed to be the reason for the lower litter decompositionsrates on clearfelled mineral soil sites compared to those under uncut stands (Yin et al. 1989,s=-=Prescott et al. 2000-=-).sIt appeared that changes in environmental factors following clearfelling caused rather smallschanges in RPEAT and that there was no increase in old peat decomposition rates (III). However,swhen the...

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...d changes in land use may threaten their large Csstocks. The primary effects of drainage for forestry (Glenn et al. 1993, Silvola et al. 1996a,s1996b, Minkkinen and Laine 1998, Minkkinen et al. 2002, =-=Hargreaves et al. 2003-=-, Laiho et al.s2004, von Arnold et al. 2005a, 2005b, Byrne and Farrell 2005, Minkkinen et al. 2007, Lohilaset al. 2010, 2011) and for agriculture (Nykänen et al. 1995, Langeveld et al. 1997, Maljanens...

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...s. Trenching was done 1 year before the actual measurements started. The 30 cm trenching depth was considered to be adequate as most of the rootssin peatlands are situated in the topmost peat layers (=-=Saarinen 1996-=-, Laiho and Finér 1996).sThe aboveground parts of the ground vegetation were removed from the sample plots andsplots were kept free of vegetation by regular clippings in order to eliminate their respi...

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...depth (Silvola et al. 1996a, Chimner and Cooper 2003, Flanagansand Syed 2011), and that WL may occasionally sink to a depth at which drying of the peatssurface may start to limit decomposition rates (=-=Lieffers 1988-=-, Laiho et al. 2004). Soil drought reduces the thickness of soil water films, thus inhibiting diffusion of extracellular enzymes and soluble organic C substrates which causes reduction in substrate av...

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...ther methods include component integrations(Sapronow and Kuzyakov 2007), girdling (Högberg et al. 2001), root exclusion (Lalondesand Prescott 2007), clipping (Fu and Cheng 2004) and isotopic methods (=-=Ostle et al. 2000-=-).sThe measurement of soil processes and their responses to environmental drivers are highlyschallenging (Subke and Bahn, 2010) and all these techniques have their own caveats whichsmost studies ackno...

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...one third of the land area consists of peatlands (8.9 milj. ha)sand half of this original area has been altered by land use; Peatlands have been drained, forsagriculture, but especially for forestry (=-=Keltikangas et al. 1986-=-, Virtanen et al. 2003, Hökkäset al. 2002). The forestry-drained peatlands are reaching maturity and forest activities arescarried out in these areas with increasing rate (Hökkä et al. 2002). Removal ...

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...ural use have caused these areas to lose considerablesamounts of C from their peat C storage (Armentano and Menges 1986, Nykänen et al. 1995,sLangeveld et al. 1997, Maljanen et al. 2001a, 2004, 2009, =-=Lohila et al. 2003-=-, 2004). Furthersland use change from agricultural lands into forests through afforestation has been recognisedsas a useful means to reduce soil degradation and increase sequestration of C to the ecos...

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...ling soil temperatures may rise and face higher diurnal fluctuations, as thesremoval of the tree canopy increases the amount of direct solar radiation to the soil surfaces(Edwards and Ross-Todd 1983, =-=Londo et al. 1999-=-). Increased soil temperature as such shouldsaccelerate decomposition of peat if other factors remain unchanged. In drained peatland forests,showever, the felling of trees and thus the elimination of ...

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...s of drainage for forestry (Glenn et al. 1993, Silvola et al. 1996a,s1996b, Minkkinen and Laine 1998, Minkkinen et al. 2002, Hargreaves et al. 2003, Laiho et al.s2004, von Arnold et al. 2005a, 2005b, =-=Byrne and Farrell 2005-=-, Minkkinen et al. 2007, Lohilaset al. 2010, 2011) and for agriculture (Nykänen et al. 1995, Langeveld et al. 1997, Maljanenset al. 2001a, 2004, Lohila et al. 2004 ) on ecosystem and the soil C balanc...

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...exposed to direct solar radiation and thereby to more extreme temperatures. This, in turn, may have causedsenhanced evaporation rates and drying of the soil surface where decomposition mainly occurss(=-=Hogg et al. 1992-=-). This may have been the case even though the water level was closer tosthe peat surface on the clearfell site compared to that on the control site: findings on mineralssoils show that reduction in s...

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...ithoutsthe effect of root respiration. Furthermore, following clearfelling the addition of fresh OM to the soil in the form of logging residue (LR) may alter soil temperature and moisture conditions (=-=Roberts et al. 2005-=-) andsthus microbial respiration rates of the under laying soil layers (Edwards and Ross-Todd 1983).sLogging residue may also provide additional nutrients and energy sources to soil microbes,swhich co...

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...cts by altering the observedstemperature sensitivity of peat decomposition. This indirect effect of WL on decomposition rates could be caused by WL driven variation in microbial population structure (=-=Jaatinen et al. 2007-=-, 2008). Fluctuations in microbialspopulation structure can affect enzyme concentration in reaction micro sites, which can thensaffect the temperature sensitivity of decomposition (Davidson and Jansse...

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... von Arnold et al. 2005a, 2005b, Byrne and Farrell 2005, Minkkinen et al. 2007, Lohilaset al. 2010, 2011) and for agriculture (Nykänen et al. 1995, Langeveld et al. 1997, Maljanenset al. 2001a, 2004, =-=Lohila et al. 2004-=- ) on ecosystem and the soil C balance in peatlands havesdrawn great interest during the last decades. However, only scattered information on theseffects of silvicultural practices (Zerva and Mencucci...

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...rthremore in these measurements usually only the effects of momentary interactions between temperature and WL are created and observed. Thus, transfer of results to thesecosystem level seems dubious (=-=Reichstein et al., 2000-=-). Measurements in field conditions aresrare mainly due to the difficulties in separating CO2 evolved from peat decomposition fromstotal soil CO2 efflux. These measurements would, however, cause small...

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...er table level (WL). Within these ecosystemssWL is considered the main factor that controls the peat decomposition. Water table levelsdirectly regulates the volume of the oxic peat layer (Lähde 1969, =-=Silins and Rothwell 1999-=-),swhere decomposition can occur in aerobic conditions. In water saturated soil layers the O2sdiffusion rate is much slower (104 times slower) compared to that in air (Wild 1981). Withinsthese conditi...

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... by high water table level (WL). Within these ecosystemssWL is considered the main factor that controls the peat decomposition. Water table levelsdirectly regulates the volume of the oxic peat layer (=-=Lähde 1969-=-, Silins and Rothwell 1999),swhere decomposition can occur in aerobic conditions. In water saturated soil layers the O2sdiffusion rate is much slower (104 times slower) compared to that in air (Wild 1...

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...that cause variation in the soil respiration rate insfield conditions have included often only the effects of momentary water level on soil respiration (Chimner and Cooper 2003, Tuittila et al. 2004, =-=Riutta et al. 2007-=-). This procedure hassbeen successful on wet pristine sites, but the effects of water level on the respiration rate onswell-drained sites have remained either small or insignificant (Lafleur et al. 20...

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...sition of peat if other factors remain unchanged. In drained peatland forests,showever, the felling of trees and thus the elimination of canopy evapotranspiration results insrising water table level (=-=Heikurainen and Päivänen 1970-=-, Marcotte et al. 2008). The associated decrease of the volume of the aerated peat layer (Lähde 1969, Silins and Rothwell 1999)sshould reduce the decomposition of peat. A decrease in soil respiration ...

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... use as a medium for tissue growth andsimpedes the activity of aquatic organisms. Thus soil moisture conditions along with oxygensavailability can regulate microbial growth rates (Barros et al. 1995, =-=Alexandre et al. 1999-=-)sand rates of organic matter (OM) decomposition (Howard and Howard 1993, Reichstein etsal. 2005). Only a few studies have demonstrated that drought would affect OM decomposition rates in peatlands. W...

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...tlands, not only the forthcoming changes in climatesbut also human-induced forestry activities and changes in land use may threaten their large Csstocks. The primary effects of drainage for forestry (=-=Glenn et al. 1993-=-, Silvola et al. 1996a,s1996b, Minkkinen and Laine 1998, Minkkinen et al. 2002, Hargreaves et al. 2003, Laiho et al.s2004, von Arnold et al. 2005a, 2005b, Byrne and Farrell 2005, Minkkinen et al. 2007...

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... related to WL drawdown has been clearlysdemonstrated in laboratory studies which show that CO2 emission from peat samples, an indicator of microbial decomposition, rises with increased WL (Moore and =-=Dalva 1993-=-, 1997,sBlodau et al. 2004). Observations in field conditions, however, show considerable variabilitysin the relationship between WL and measured soil CO2 emissions (Silvola et al. 1996a, Lafleur et a...

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... dataset (r2 = 0.47) (Equation 1, see Section 2.3.).s25 decrease in soil moisture content has been observed both in laboratory and in field conditionss(Howard and Howard 1993, Reichstein et al. 2005, =-=Rey et al. 2005-=-). The observed variation in temperature sensitivity of decomposition could be related tosWLAVE which was the strongest factor of the measured variables to explain the changes insmicrobial community s...

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...on practices such as ploughing and harrowing, fertilization, liming and addition of mineralssoil, which change the physical, chemical and biological properties of the old peat (Wall andsHytönen 1996, =-=Hytönen and Wall 1997-=-) and further increase the decomposition rates. As asresult, converting natural peatlands to agricultural use turns the peatland from a CO2 sinksinto a large source (Nykänen et al. 1995, Maljanen et a...

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... in the predictions of net soil carbon storage in changing climate,sparticularly since these simulations have been shown to be rather sensitive to even smallschanges in temperature sensitivity of OM (=-=Jones et al. 2003-=-, Lenton and Huntingford 2003). Even though the temperature sensitivity of decomposition has been fixed in current simulation models, there are several known factors that can cause variation in temper...

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... remain unchanged. In drained peatland forests,showever, the felling of trees and thus the elimination of canopy evapotranspiration results insrising water table level (Heikurainen and Päivänen 1970, =-=Marcotte et al. 2008-=-). The associated decrease of the volume of the aerated peat layer (Lähde 1969, Silins and Rothwell 1999)sshould reduce the decomposition of peat. A decrease in soil respiration following clearfelling...

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...his positive relationship between soil temperature and CO2 efflux originating from soil isswell established in mineral soils (Singht and Gupta 1977, Raich and Schlesinger 1992, Lloydsand Taylor 1994, =-=Knorr et al. 2005-=-, Davidson and Janssens 2006). Also in peatlands the importance of temperature in explaining temporal variation in heterotrophic peat soil respirations(Minkkinen et al. 2007, Ojanen et al. 2010) and s...

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...re sensitivity of soil respirationsbetween different ecosystems (Boone et al. 1998, Fierer et al. 2006) and soil characteristicss(Moore and Dalva 1993, Yavitt et al. 1997, Davidson and Janssens 2006, =-=Hardie et al. 2011-=-).sThe fixed temperature sensitivity of OM decomposition in the models could thus cause asmajor source of uncertainty in the predictions of net soil carbon storage in changing climate,sparticularly si...

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...rsand root-derived photosynthetic products (root exudates) to the soil (Howard and Howards1993, Lloyd and Taylor 1994, Silvola et al. 1996a, Davidson et al. 1998, Buchmann 2000,sKuzyakov et al. 2000, =-=Högberg et al. 2001-=-) to which they may respond differently (Boone etsal. 1998). Thus, it is difficult to estimate the decomposition rate of peat and its dependenceson environmental factors by using measurements of RTOT....

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...ate change and associated temperature increase could severely affect soilsC storages. This would be especially harmful if a warmer climate would not cause an equalsincrease in net primary production (=-=Kohlmaier et al. 1990-=-), which would result a positivesfeedback to climate change. These predictions of the effects of a changing climate on soil C stocks mainly rely on current terrestrial carbon models, such as Roth-C, C...

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...ilityslowers their respiration rate. In the water saturated layers with anoxic conditions decomposition is slow as anaerobic decomposition proceeds much more slowly than aerobic (Bergmanset al. 1999, =-=Šantrůčková et al. 2004-=-). After drainage WL drawdown and the associated increase in the volume of the oxic peatslayer has been considered to increase the decomposition rate of the accumulated peat layers(Armentano and Menge...

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...ty are closely related tosthe amount of mineral soil added to the site during the agricultural phase (Wall and Hytönens1996). The addition of mineral soil has been shown to increase soil temperature (=-=Pessi 1956-=-)sas well as nutrient availability (Pessi 1962) both of which can accelerate microbial activity and thus affect RPEAT. To study the effect of soil properties on RPEAT in afforested organics19 soil cro...