Organic and inorganic carbon concentrations and fluxes from managed and unmanaged boreal first-order catchments

Science of the Total Environment 408 (2010) 1649–1658

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Science of the Total Environment j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v

Organic and inorganic carbon concentrations and fluxes from managed and unmanaged boreal first-order catchments Miitta Rantakari a,⁎, Tuija Mattsson a, Pirkko Kortelainen a, Sirpa Piirainen b, Leena Finér b, Marketta Ahtiainen c a b c

Finnish Environment Institute, P.O. Box 140, 00251 Helsinki, Finland Finnish Forest Research Institute, Joensuu Research Unit, P.O. Box 68, 80101 Joensuu, Finland North Karelia Regional Environment Centre, P.O. Box 69, 80101 Joensuu, Finland

a r t i c l e

i n f o

Article history: Received 23 June 2009 Received in revised form 30 November 2009 Accepted 14 December 2009 Available online 2 February 2010 Keywords: Streams Dissolved organic carbon Dissolved inorganic carbon Carbon dioxide Peatland drainage Climate change

a b s t r a c t Seasonal and between stream variation (catchment dependent variation) in losses of organic and inorganic carbon via downstream transport and outgassing of CO2 into the atmosphere were studied in 11 small boreal catchments situated in close proximity to each other. Of these catchments four were undrained peatland rich catchments, four drained peatland rich catchments and three managed mineral soil-dominated catchments. Downstream export of total inorganic carbon (TIC) varied between 870 and 1400 kg km− 2 a− 1 and was rather consistent between the catchments, except in the case of the mineral soil-dominated catchment Kangaslampi, where export was only 420 kg km− 2 a− 1. The export of total organic carbon (TOC) varied between 2300 and 14,800 kg km− 2 a− 1 and was highest in peatland rich catchments. Peatland drainage decreased TIC and TOC concentrations in the long term, but did not affect lateral carbon export due to increased runoff from the catchments. Partial pressure of CO2 in streams was the highest in undrained peatland rich catchments, but the outgassing of CO2 into the atmosphere was also high from drained peatlands due to the higher discharge rate and long ditch networks. In mineral soil-dominated catchments both downstream export of carbon and emission into the atmosphere were low. TOC exports were compared in two climatically different years (2003 and 2007). The results indicate that climate change might alter the timing of the TOC export from the catchments, the importance of the spring ice melt diminishing and both snow cover and snow free period export increasing. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Headwater catchments in the boreal zone are dominated by coniferous forests, which have high carbon (C) pools in vegetation and soil. Humid climate and low temperatures prevailing in the boreal zone retard complete decomposition and favour accumulation of organic matter on the soil surface. Carbon pools are high, especially in peatlands (Gorham, 1991; Kauppi et al., 1997; Minkkinen, 1999; Kortelainen et al., 2004). Typically, boreal catchments are complexes of peatlands and upland soils and consequently headwater streams have been shown to be highly variable with respect to many key chemical parameters (Mattsson et al. 2003; Finér et al., 2004; Kortelainen et al., 2006a; Buffam et al., 2007). Organic and inorganic carbon occur in boreal streams as particulate organic and inorganic carbon (POC, PIC), dissolved organic and inorganic carbon (DOC, DIC) and gaseous forms (free CO2 and CH4). In the northern latitudes, streams provide a pathway for C loss from peatlands via lateral transport downstream and degassing to the

⁎ Corresponding author. Tel.: +358 4 7468180; fax: +358 9 54902390. E-mail address: miitta.rantakari@ymparisto.fi (M. Rantakari). 0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2009.12.025

atmosphere. A number of recent studies have suggested that surface waters are an important conduit for C loss and can contribute to the C budget of both peatland and upland areas dominated by organic soils (e.g. Dawson et al., 2001; Billett et al., 2004; Hope et al., 2004). Boreal forests in Scandinavia and Russia are intensively managed. The catchment studies have mainly focused on total carbon losses from pristine catchments, and therefore there is little knowledge of how different forestry practices affect the stream water concentrations and export of carbon and especially the inorganic forms TIC and CO2. Compared to clear-cuttings and other forest operations, drainage has the most significant and long-lasting impact on carbon fluxes. Drawdown of the water level has been shown to increase the peatland storage of carbon due to subsidence of the original peat surface and increased litter and fine root production of the tree stand exceeding the enhanced oxidation of organic matter (Minkkinen and Laine, 1998). However, the carbon losses of a drained peatland forest via lateral water flow are still unknown. The more favourable conditions for decomposition after drainage may increase transport of the decomposition products via waterways. The extensive drainage network may also increase stream water contact time with air and potentially enhance the loss of gaseous carbon into the atmosphere.

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The annual water discharge cycle and the seasonal concentration changes of dissolved organic carbon (DOC) in small forested catchments (e.g. Lindström et al., 2002; Buffam et al., 2001; Buffam et al., 2007) as well as DOC export have been studied in many previous papers (e.g. Bishop and Pettersson, 1996; Kortelainen et al., 1997; Schiff et al., 1997; Mattsson et al., 2003; Laudon et al., 2004a; Kortelainen, 2006a; Ågren et al., 2007), whereas the export and seasonal concentration changes of TIC have attracted less interest until recent years (Hope et al., 2004; Worrall et al., 2005; Striegl et al., 2007; Waldron et al., 2007; Nilsson et al., 2008; Öquist et al., 2009). The behaviour of TIC is more complex than that of TOC due to the gaseous phase (CO2) (Dawson et al., 2001; Billett et al., 2004; Hope et al., 2004). In northern catchments runoff is usually highest during the snowmelt and autumn rain periods, and most solutes and particulates are transported during these high-flow periods. During storm events and other periods of high discharge the stream DOC concentrations are often highest (e.g. Hinton et al., 1997; Schiff et al., 1997; Ågren et al., 2007). In the boreal catchments most of the annual total organic carbon (TOC) export has been observed to occur during the snow melt in spring and during rain events in autumn, when a great part of the annual discharge also occurs (e.g. Kortelainen et al., 1997; Laudon et al., 2004; Köhler et al., 2008). However, climate change may alter this pattern by changing the distribution of rainfall during the year and especially by changing the annual temperatures, redefining how much of the rainfall occurs as snow and as water. According to the IPCC report (Lemke et al., 2007) the snow-covered period has shortened in the northern latitudes; in particular snow melt occurs earlier. As temperatures increase, the possibility of precipitation falling as rain rather than snow increases, especially in autumn and spring at the beginning and end of the snow season, and in areas where temperatures are close to freezing (Mellander et al., 2007). Moreover, precipitation in northern Europe has been increasing (Lemke et al., 2007). In this paper, we compare annual and seasonal TOC concentrations and export in two climatically different years in order to illustrate how changing climate might alter runoff and the carbon fluxes from small headwater catchments. Furthermore, we present stream TOC and TIC concentrations and export data for small forested headwater catchments in order to explore how the carbon fluxes are affected by the peatland percentage and the drainage status of peatlands. We present factors controlling the export and concentrations of TOC and TIC in different seasons from the small headwater catchments representing the whole range of boreal forests (managed and unmanaged peatland and upland soils) in Finland. We also estimate the annual CO2 flux from brooks into the atmosphere and compare it to the lateral inorganic carbon flux. The data was obtained from 11 small forested headwater catchments, situated in eastern Finland, that have been subjects of intensive forest research for several years; the

earliest studies were started already in the 1970's (Finér et al., 1988; Ahtiainen and Huttunen, 1999; Finér et al., 2004; Mannerkoski et al., 2005; Palviainen et al., 2004; Kortelainen et al., 2006a; Palviainen et al., 2007; Piirainen et al., 2007; Sarkkola et al., 2009). 2. Materials and methods 2.1. Description of the catchments The study sites represent small forested headwater catchments in the boreal zone. The data for the study was based on six Nurmes study catchments monitored since 1979 and five VALU catchments monitored since 1992 (Finér et al., 1997). All catchments are located in eastern Finland (63°45'–63°53'N, 28°30'–29°10'E). The forests in the catchments vary from pristine to managed. In four of the catchments the peatlands have been drained for forestry purposes (Table 1). The forestry practices have mainly taken place more than 10 years ago. The area of the peatlands was determined from forest inventories provided by the Finnish Forest Research Institute (Table 2). The proportion covered with peatlands ranged from 8 to 64%. The upland and peatland site types were classified into five fertility classes. An average site type, an average upland site type and average peatland site type were calculated for each catchment by weighting with the percentage area of each site type class. Class 1 consists of the most fertile site type, and class 5 the least fertile site type. Upland site types were classified according to Cajander (1909, 1949) (class 1 =OxalisMaianthemum-type and Oxalis-Myrtillus-type and related site types, class 2 =Myrtillus-type and related site types, class 3 =Vaccinium-type and related site types, class 4 =Calluna-type and related site types, and class 5 =Cladonia-type). Peatland site types were classified according to Huikari (1952, 1974) (class 1 = Eutrophic mires and herb-rich mires, class 2 =Vaccinium myrtillus and dwarf-shrub mires, class 3 =V. vitisidaea and small sedge mires, class 4 = Cotton grass dwarf-shrub mires, and class 5 =Sphagnum fuscum mires). The locations of the different mire types in the catchment were studied on maps. There was rather little of the most fertile mire type (Type 1) in the catchments (Table 2). Along the stream sides there was typically the second most fertile mire type belonging to class 2, whereas the more infertile mire types (classes 4 and 5) were situated in upper parts of the catchment, mainly above the area where the visible stream begins. The mires belonging to class 3 were scattered around the catchment, but typically not along the stream sides. Types 1–3 represent forested mires, whereas Types 4 and 5 are treeless mires (Finér et al., 1988). The catchments of the study streams can be divided into three categories: (i) catchments including a significant amount of undrained peatlands (peatland area >20% of the total area), (Välipuro, Liuhapuro, Kivipuro and Korsukorpi), (ii) catchments including a

Table 1 Catchment characteristics for the study catchments.

Kivipuro Välipuro Korsukorpi Liuhapuro Koivupuro Iso-Kauhea Suopuro Murtopuro Porkkavaara Kangasvaara Kangaslampi

Area km2

Slope %

Peatland %

0.54 0.86 0.69 1.65 1.18 1.76 1.13 4.94 0.72 0.56 0.29

5.3 4.4 4.1 4.7 3.6 4.4 3.1 3.8 9.2 12.8 11.2

28 52 56 48 53 50 64 54 16 8 9

Upland soil type, %

Average till soil type

1 (gravel)

2 (sandy)

3 (silty)

0 0 0 0 1 0 0 0 0 0 0

23 15 0 25 10 28 12 20 0 82 81

45 30 44 27 32 19 19 30 84 0 0

2.67 2.67 3.00 2.52 2.76 2.40 2.61 2.60 3.00 2.00 2.00

Peatland site type, % of the total catchment area

Average peatland site type

1

2

3

4

5

1 0 0 0 0 0 0 0 10 5 0

18 9 6 18 11 6 3 31 1 3 7

5 13 23 6 10 26 13 4 2 0 2

4 26 26 19 31 17 40 18 0 0 0

0 4 1 0 1 1 8 1 3 0 0

2.43 3.48 3.39 3.02 3.42 3.26 3.83 2.80 2.06 1.38 2.22

M. Rantakari et al. / Science of the Total Environment 408 (2010) 1649–1658 Table 2 Forestry practices carried out in the study catchments. Catchment

Kivipuro

Välipuro Korsukorpi

Liuhapuro Koivupuro

Iso-Kauhea

Suopuro Murtopuro

Total area (ha)

Year

54

1983 1986 1987

36

1998 2001 2000 2001 2002 No treatments 1983 1986 1987 1996 1997–1998

27 8 4

86 69

165 118

176

113 494

Porkkavaara

72

Kangasvaara

56

Kangaslampi

29

1999 2000 2005 1983 1996 1998 1999 1983 1983 1986 1987 2000–2002 1985–1987 1996 1998 1999 1983–1985

Clear-cut Drainage Other area area (ha) (ha) Soil scarification Replanting with pine

Soil scarification Pine planting 6

32 Soil scarification Pine planting

19

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Table 3 The climate (precipitation, temperature and the water equivalents of snow) in North Karelia/Eastern Finland in 2007 and 2003, and the long–2007.

Precipitation I–XII Precipitation I–III and XI–XII Precipitation IV–V Precipitation VI–X Mean air temperature I–XII Mean air temperature I–III and XI–XII Mean air temperature IV–V Mean air temperature VI–X Mean air temperature III In I–III and XI–XII the number of days when the mean air temperature >0 The average water equivalent of the snow cover (mm) The average water equivalent IV The average water equivalent V

2003

2007

1949–2007

690 240 110 350 2.2 − 7.4 4.0 11 −2.4 31

800 260 130 410 3.0 − 6.0 4.9 11 0.1 44

640 210 79 350 3.5 − 5.4 5.3 12 − 3.9 15

86

36

77

170 34

45 1.5

150 34

Soil scarification and pine planting 17.5 13 5 63 23 Soil scarification Pine planting 15 286 198 20 24

Soil scarification Pine planting Soil scarification and pine planting

19

17

Ploughing, mounting Replanting with pine Soil scarification and pine planting

significant amount of peatlands (peatland area >20% of the total area), but with part of the peatlands drained for forestry purposes (Murtopuro, Koivupuro, Suopuro and Iso-Kauhea), and (iii) catchments including mainly (> 80% of the area) upland soils, with undrained peatlands (Porkkasalo, Kangasvaara and Kangaslampi); (Tables 1 and 2). In the three upland soil-dominated catchments the soil was mainly sandy till in Kangasvaara and Kangaslampi, whereas in Porkkasalo the till was finer (silty) (Table 2). In the peatland dominated catchments the existing upland soils were mainly silty till in Välipuro, Kivipuro, Koivupuro and Korsukorpi, whereas the other catchments both silty and sandy tills were equally common (Finér et al., 1997). 2.2. Weather conditions in Finland in 2007 and 2003 For this study we selected runoff and water quality data for two hydrologically different years 2003 and 2007. The year 2007 was exceptionally rainy throughout Finland including the study area in eastern Finland (Table 3). The thickness of the snow cover was low in winter 2006–2007 due to the unusually warm December of 2006, when rainfall was mainly water. Furthermore, snow melted in 2007 earlier than in an average year, in some places earlier then ever recorded. The average mean daily temperatures in the weather station near the study catchments for December 2006 and March 2007 were 4–5° above the long-term average. In September the rainfall was 116 mm in the study area, which is almost twice the long-term average (1949–2007). October was also rainy in eastern Finland, and the precipitation was predominantly water. In 2007 the annual rainfall in Finland was higher than the long-term average; for example in eastern Finland the annual

rainfall was 800 mm, compared to the average long-term annual rainfall (1949–2007) of 640 mm in that district. Because 2007 was exceptional compared to the long term climatic records of Finland, a reference year 2003 was chosen to represent average weather conditions. TOC concentrations and the annual TOC export were compared between the 2 years and during the three seasonal periods: snow melt (April– May), snow free (June–October) and snow cover (November–March). Unfortunately, TIC results were available only for the year 2007. The hydrometeorological data of the study area for both years were obtained from the internet services of the Finnish Environment Institute (www. environment.fi) and the Finnish Meteorological Institute (www.fmi.fi) and from the Hydrological yearbook (Korhonen, 2007). 2.3. Runoff and water quality Daily runoff was recorded by a V-notch weir and a water level recorder. The water quality of the streams was sampled 11 times per year, but TIC was sampled only 7–9 times a year along with stream water temperature and pH measurements. The sampling was focused on high-flow periods. Stream water chemistry was analyzed in the accredited laboratory of the Regional Environmental Centre of Northern Karelia. TOC was analyzed from unfiltered samples by oxidation to CO2 followed by IR-measurement. Samples for TIC were taken into airtight bottles and placed in coolers while in transit to the laboratory. TIC was measured in the laboratory using infrared spectroscopy. Total nitrogen (TN) was analyzed colorimetrically after oxidation to NO3– N, the sum of NO3–N and NO2–N colorimetrically by auto analyzer after reduction to NO2–N, NH4–N colorimetrically with hypochlorite and phenol, and organic nitrogen (TON) was calculated as the difference between total and inorganic nitrogen. Total phosphorus (TP) was measured by a colorimetric method after oxidation, phosphate phosphorus (PO4–P) by spectrophotometric determination, calcium (Ca) and magnesium (Mg) were determined by flame atomic adsorption/ICP–MS, silicate (SiO2) by a colorimetric method or by FIA and sulphate (SO4) by ion chromatography, pH was analyzed from unfiltered samples and the measurement of pH was also carried out electrometrically at 25 °C with a pH meter in the laboratory. All the analyses were carried out in the laboratory of North Karelia Regional Environment Centre using standard methods of the environmental administration in Finland (National Board of Waters, 1981). CO2 concentrations were calculated from the measurements of TIC and pH with correction for actual water temperature (Stumm and Morgan, 1970; Butler, 1982; Kling et al., 1992). Partial pressure of CO2 (pCO2) was calculated by use of the appropriate Henry's law constant,

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corrected for temperature and atmospheric pressure (Plummer and Busenberg, 1982). The CO2 flux from the streams was estimated from CO2 concentrations on the basis of published equations by Hope et al. (2001) and Jones and Mulholland (1998) and using the gas transfer coefficients for CO2 calculated on the basis of the stream discharge as outlined by Jones and Mulholland (1998). In one of the study catchments, Murtopuro, there is a natural spring, which enabled the comparison of brook water with the assumed ground water quality. Water temperature, pH, SiO2, TOC, TIC and pCO2 concentrations were compared between stream and spring. 2.4. Statistical analyses The relationships between stream water TOC, TIC or pCO2 and catchment characteristics or stream water quality were studied by correlation analysis (PROC CORR; SAS Institute, 2001). Most of the concentration and catchment data were transformed into natural logarithms or square roots in order to improve the normality of the distributions. The differences in carbon (TOC, TIC or pCO2) concentrations between the catchment/treatment types (peatland rich, undrained; peatland rich, drained; mineral soil rich) were tested with randomized complete block design using different sampling dates as blocks and testing the differences by the F-test. The differences in carbon loads between the catchment/treatment types were tested by the t-test. 3. Results The discharge rates of the streams co-varied and the flow peaks occurred mainly at the same time in all streams (Fig. 1). The high precipitation in 2007 generated several high peaks in the discharge rate during the summer and the low-flow periods occurred in February–March and in August–September. The average daily temperature increased above 0 °C for several days at the turn of the years 2006–2007 and resulted in a flow peak in January, which is usually characterized by low flow. In 2003, which was chosen to represent the average climate in the long term, the spring flow peak

Fig. 1. Discharge rate of the study streams in 2007 and 2003.

Fig. 2. Streamwater TIC concentrations (mg l− 1) in the study streams and springwater TIC (mg l− 1) in the Murtopuro spring.

was slightly higher than in 2007 and the discharge in the summer was lower (Fig. 1). The annual discharge in the catchments was on average 23% (0–42%) higher in 2007 than in 2003. The stream water TIC concentrations in 2007 were highest in March and in August, i.e. in low-flow periods (Figs. 1 and 2). Annual concentrations varied between 0.8 and 6.9 mg l− 1. Generally, the TIC concentrations followed a similar annual pattern than that of CO2 (Fig. 3), with the exception that the CO2 concentrations were highest in August and those of TIC in March. This was due to the higher pH in streams in March compared to August (Fig. 4). The highest annual TIC concentrations were found in undrained peatland catchments. The difference compared with the drained peatlands was statistically significant (p < 0.001) in the F-test. In winter (March), TIC concentrations correlated positively with Fe concentrations (R2 = 0.54, p < 0.0001) and average till soil type in the catchment (R2 = 0.36, p < 0.0001), suggesting that the finer the grain size in soil, the greater

Fig. 3. Streamwater CO2 (µatm) in the study streams and springwater CO2 (µatm) in the Murtopuro spring.

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Fig. 4. Streamwater pH in the study streams and springwater pH in the Murtopuro spring.

were the TIC concentrations. In other seasons correlation coefficients between TIC and catchment or water quality variables were weak. The pCO2 in streams varied between 890 and 8320 µatm. In the streams in group (iii) (upland soil), the winter maximum of pCO2 was around 3000 µatm, which was significantly lower than in the catchments including higher percentages of peatlands (Fig. 3). In late summer (August) and in autumn, the maximum values for pCO2 were found in group (i) (undrained peatlands). In this group, the pCO2 values in late summer were close to 8000 µatm except in the pristine Liuhapuro catchment with the lowest summertime values of all streams. The differences in the annual pCO2 levels between the subgroups were statistically significant (p < 0.01), although the groups (iii) (upland catchments) and (ii) (drained peatlands) had similar pCO2 in late summer and autumn (Fig. 3). Stream water TOC concentrations were the highest in group (i) (undrained peatlands) and lowest in group (iii) (upland) in both 2003 and 2007 (Fig. 5). The differences between the groups in both study years were statistically significant in the F-test (p < 0.01). The seasonal dynamics in TOC concentrations were similar in all catchments in both study years. Generally, TOC concentrations increased with increasing flow, but the trend was not very clear. Summer and autumn 2007 were characterized by frequent heavy rains, thus leading to several flow peaks in streams, making it difficult to separate long low-flow periods (Fig. 1). The difference in TOC concentrations between undrained peatlands and drained peatlands was more pronounced in 2003 than in 2007, especially during the autumn months (September, October) (Fig. 5). In the winter low-flow situation (in 2007), mire proportion in the peatland site type class 3 (0–52% of the total peatland area, spread around the catchments) explained the concentrations of TOC well (R2 = 0.89), whereas the mire proportion in the site type class 2, (5– 78% of the total peatland area, mainly on the stream sides) had a much weaker correlation with stream water TOC (R2 = 0.29). In the rising spring flow in April, the correlation coefficient between the proportion of mires in class 2 and TOC concentration in streams improved (R2 = 0.66) (Fig. 6). A very similar pattern was observed in 2003 (data not shown). The situation was similar in summer, although the effect of the flowpeak on correlations was weaker. In mid-August low-flow period mires in class 3 explained the concen-

Fig. 5. Streamwater TOC concentrations (mg l− 1) in the study streams in 2007 and 2003 and springwater TOC concentrations (mg l− 1) in the Murtopuro spring in 2007.

trations of TOC well (R2 = 0.81) and the correlation was weaker with mires in class 2 (R2 = 0.48). With the flow peak in September due to heavy rain, TOC correlation with mire type 3 (R2=0.60) weakened and with type 2 became stronger (R2 = 0.60). pCO2 also had positive correlation with peatland type 3, especially at low-flow periods. There was a natural spring in the Murtopuro catchment, which enabled the comparison of stream water with assumed ground water quality. Water temperature in the spring showed very little variation during the year compared to the Murtopuro stream, suggesting that the groundwater was the main source of water in the spring and that the mixing of surface water was minimal. This was further supported by the higher pH in the spring, suggesting minimal mixing of acidic peat water (Fig. 4). The results also showed constantly higher pCO2, but very low TOC concentrations in spring compared to the stream (Figs. 3 and 5). The annual runoff from the catchments in 2007 ranged from 250 to 490 mm. The runoff was greater from the drained peatland catchments (470–490 mm) than from undrained peatland catchments (380–470 mm). The annual TIC loads from the catchments were rather consistent (870–1400 kg/km2), except from the mineral soil

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Fig. 6. The relationship between streamwater TOC concentrations (mg l− 1) and the proportion of peatland site type 2 or 3 (%) on the two sampling occasions 12 March and 16 April in 2007.

catchment Kangaslampi where the annual TIC load was only 420 kg/ km2 (Fig. 7). The correlation coefficients between annual TIC load and water quality or catchment variables were mainly weak (data not shown). Like the wintertime TIC concentrations, the annual TIC load also had a positive correlation with average soil type (R2 = 0.34). Although both TIC and CO2 stream water concentrations were the highest in undrained peatland catchments, the same pattern did not occur with TIC loads. This was due to greater runoff from the drained catchments, which compensated lower TIC concentrations, thus resulting in similar TIC loads in all peatland rich catchments. The results, however, gave some indication that the recently managed (clear cut, site preparation) forest catchments produce larger inorganic carbon loads compared to unmanaged or catchments in which management was ended many years ago, because the TIC load was significantly smaller from Kangaslampi (managed 24 years ago)

Fig. 7. The annual lateral stream TOC and TIC exports (kg km− 2) and annual CO2–C outgassing to the atmosphere (kg km− 2) from the study catchments.

(420 kg km–2 a–1) compared to the otherwise similar catchment Kangasvaara, with 34% of the area clear cut 11 years ago (1150 kg km–2 a–1). Correspondingly, TIC load was higher from the heavily managed Murtopuro catchment (950 kg km–2 a–1) compared to the pristine Liuhapuro catchment (870 kg km–2 a–1) with very similar soil situated next to it. The difference in the TIC export between the neighbouring catchments can also be derived from the differences in pH. In the catchments with higher TIC loads (Murtopuro, Kangasvaara) the streamwater pH is also higher (Fig. 4), suggesting that there are more carbonate minerals in those catchments. Another reason for the lower TIC loads in the catchments with lower pH could be the greater portion of TIC in gaseous form and thus exchangeable with the atmosphere. The annual TOC loads ranged from 2300 to 14,800 kg km− 2 (Fig. 7) and correlated positively with the peatland percentage of the catchments. The TC load from the catchments ranged from 3500 to 15700 kg km− 2 and consisted mainly of TOC (66–94%). The highest TOC load and the second lowest TIC load were recorded in the Liuhapuro catchment with a high proportion of unmanaged peatlands and pristine forests. As with TIC, the higher TOC concentrations in undrained peatlands were compensated by the higher runoff from the drained catchments resulting in similar TOC exports in both undrained and drained peatland rich catchments. TOC/TON ratio was higher (59 vs. 52) in the undrained sites compared to the drained sites indicating differences in the quality of the exported organic matter. The difference in the TOC/TON ratio was statistically significant when tested with the t-test. There was a great deal of variation in the annual CO2 release estimates between the streams, but the stream-specific CO2-C release into the atmosphere was always higher than the lateral TIC export (Fig. 7). Due to higher CO2 concentrations, the estimated CO2 fluxes were high from the peatland rich catchments, except for the Liuhapuro catchment with pristine forest. Although the partial pressure of CO2 in streams was the highest in undrained peatland rich catchments, the outgassing of CO2 was also high in drained peatland rich systems due to the higher discharge rate and long ditch

M. Rantakari et al. / Science of the Total Environment 408 (2010) 1649–1658

network. The highest CO2 outgassing estimates in our study were in the Koivupuro and Murtopuro catchments with extensive ditch networks. The annual TOC export from the catchments was higher in 2007 than in 2003 (Fig. 8), mainly due to the higher water discharge rate. The seasonal distribution of TOC export varied between the years. In 2003 the main part of TOC export occurred at the time of the snow melt in April–May when the runoff was also highest. In 2007 the TOC export was more evenly spread throughout the year. The export was high during the summer as a consequence of high precipitation and also during the winter due to frequent snow melt periods (Fig. 8). There was no significant difference in the average April–May runoff from the catchments (148 mm in 2007 vs. 168 mm in 2003), although there was substantially less now when the snow melt period started in spring 2007 than in 2003 (Table 3), and TOC export from the catchments during the snow melt period was rather similar (Fig. 8). The precipitation at the time of the snow melt was only 20 mm higher in 2003 than in 2007 (Table 3). Consequently, the difference in the annual TOC export (on average 17%) between the study years was mainly derived from the greater TOC export during the snow free and especially during the snow cover period in 2007. During the snow cover period TOC export was greater in 2007 in both drained and undrained peatland dominated catchments, whereas there were no major differences between 2003 and 2007 in the mineral soildominated catchments. Correspondingly, in the mineral soil-dominated catchments there were no major differences in the snow free period TOC export between the study years, but in the peatland dominated catchments the difference in the snow free period TOC export was on average greater in drained peatlands than in undrained peatlands (Fig. 8).

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4. Discussion 4.1. Organic and inorganic C concentrations and fluxes The temporal variation in TOC concentrations was similar in all catchments, the highest and lowest concentrations occurring at the same time in both mire dominated and in upland soils dominated catchments. Köhler et al. (2008) and Buffam et al. (2007) reported that temporal variation in stream TOC concentrations was primarily driven by changes in streamflow, but in the mire catchment stream water TOC was diluted with increased runoff during spring, whereas TOC from upland forests increased during runoff peaks irrespectively of season. In upland soils the water table is low and water is draining mineral layers of soil in normal flow situations, whereas in high-flow periods the rising water table reaches organic soil layers leading to increased TOC concentrations. However, the mixed peat/upland soil cover of our study catchments resulted in similar TOC response to the discharge changes in all catchments, despite the fact that peatland percentage varied greatly. Nyberg et al. (2001) observed that during the spring flood the soil profiles saturated from below and that groundwater levels rose to superficial soil layers especially in the riparian zones close to streams. In our study catchments the Type 3 forest mires were an important TOC source in stream water in the winter low-flow situation according to correlation analyses, whereas Type 2 along the stream sides had much less influence on TOC. This suggests that the stream banks were not connected to the stream in the low-flow times, whereas in high-flow situations the peat in the stream banks became connected to the stream water, delivering TOC to the stream. In a similar geographical area in Sweden, Ågren et al. (2008a) observed

Fig. 8. TOC export (kg km− 2) from the study streams in 2003 and 2007 during the whole year and during the three periods: snow melt (April–May), snow free (June–October) and snow cover (November–March).

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that different land cover areas of the catchment acted as organic carbon sources in low-flow and in high-flow situations. Furthermore, Bishop et al., 2004 observed in the Nyänget catchment in Sweden that the stream water concentration of TOC depended on the flowpath depth through riparian soils in the changing runoff. In Virginia, USA Buffam et al. (2001) concluded that the rapid response of stream water dissolved organic matter (DOM) concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. They also observed that DOC concentrations doubled during storms, with maximum concentrations occurring on the rising limb of storm hydrographs. Water pH was mainly below 5 in the peatland dominated streams, and consequently TIC was mostly in the form of CO2. Only in the March samples, and possibly throughout the winter, was the pH above 5, thus causing part of TIC to be in bicarbonate form. The higher winter pH might be due to the greater contribution of ground water to the stream flow compared to the other seasons. In winter and spring, stream water TIC concentrations were rather similar in peatland and upland soildominated catchments but pH was higher in upland streams, generating lower stream water CO2 contents in these catchments. Higher pCO2 in catchments with undrained peatlands compared to the drained peatlands may be a result of greater water storage and longer storage period in undrained peats and therefore a longer accumulation time for CO2. In a study by Kortelainen et al. (2006b), low oxygen level predicted high pCO2 in lakes, the oxygen deficiency typically occurring after long stagnation. The efficient loss of CO2 into air from the ditch network in the drained catchments might also be the reason for lower concentrations. The annual CO2 release into the atmosphere was higher than the lateral TIC load in all streams, as was also observed for a Scottish headwater catchment (Hope et al., 2001), for streams in the Mer Bleue peatland in Canada (Billett and Moore, 2007) and for Västrabäcken in Sweden (Öquist et al., 2009). The CO2 outgassing rates from the peatland catchments were of the same magnitude as observed for the Brocky Burn catchment in Scotland (Hope et al., 2001, 2004), and from the mineral soil sites of the same magnitude as observed in Västrabäcken northern Sweden (Öquist et al., 2009). Berggren et al. (2007) and Ågren et al. (2008b) concluded that although mires have high area-specific export of TOC, the forest soils contribute highly bioavailable carbon and are the main contributors of bacterial production supporting carbon in boreal streams. The bioavailability of carbon from forest sites is much higher than from mire sites. Furthermore, Ågren et al (2008a) showed that the absorbance ratio of DOC (A254/A365) and bioavailability increased in the soil profile towards the surface both in wetland soils and riparian forest soils. Therefore, it is also conceivable that quantity and quality of stream water organic matter and its availability for mineralization varies depending on whether it originates from drained or undrained peats. The main purpose of peatland drainage has been to lower the water table in order to enhance tree growth, and therefore most of the drained sites are covered by forests. Although TOC concentrations are higher in the drained peatland streams than in streams on mineral soils, the CO2 concentrations are similar. The TOC/TON ratio of the exported organic matter suggests more uniform quality between the drained peatlands and mineral soils (52 vs. 51) than between drained and undrained peatlands (52 vs. 59). However, decay of fresh organic matter was more efficient in an undrained site compared to drained sites in the study by Laiho et al. (2004), probably due to relative moisture deficiency at drained sites. The overall mineralisation rates have been shown to vary in different mires, the slowest mineralisation rates typically occurring in bogs, probably because the mineralisation has been shown to depend strongly on interstitial water nutrient richness and also on soil pH (Verhoeven and Toth, 1995; Verhoeven et al., 1996). Most of the CO2 in these small first-order streams probably originates from the surrounding catchment soils rather than from stream processes, because the water renewal time in streams can be expected to be short.

The natural spring in the Murtopuro catchment enabled the comparison of brook water with the assumed ground water quality. The results show constantly higher TIC and pCO2, but very low TOC concentrations in spring compared to the stream. This is in agreement with the results of Bishop et al. (1994), who found that even under summer low-flow condition, the stream TOC content was more than two-fold that seen in shallow groundwater in mineral soils and the difference was even greater during periods of high flow. Furthermore, in a headwater catchment in NE Scotland dissolved inorganic carbon (DIC) concentrations were highest during a dry summer when groundwater dominated runoff, suggesting that DIC concentration in groundwater is greater than in shallow surface runoff (Waldron et al., 2007). The spring pH was constantly higher than the stream pH except in March. The lower spring pH in March compared to the other seasons is most probably due to melting snow water mixing with the spring water. The spring water temperature is close to 4 °C all year round, thus melting the snow around the spring. In the stream the ground water contribution is at its maximum during the winter, leading to higher pH compared to the other seasons. The wide range of annual TOC loads from the catchments in 2003 and 2007 (2300–14,800 kg− 1 km2) was of the same magnitude as that observed before any forestry practices (2300 and 14,000 kg− 1 km2) (Kortelainen et al., 2006a). The amount of litter and dead organic matter increases in clear-cutting and Palviainen et al. (2004) showed that a large amount of carbon was rapidly released from logging residues. Clear-cutting and site preparation has been shown to increase the leaching of DOC for some years (Piirainen et al., 2007), but in the present study the forestry practices were mainly carried out more than 10 years ago, and therefore the effects of forest management on TOC export in this study are probably negligible. There was only little variation in TIC exports between the catchments. However, there was some indication that forest management (clear cut, site preparation) increases TIC export from the catchments for longer than TOC export. The higher TIC load from managed catchments can be related to soil ploughing, that exposes soil particles to weathering reactions. This is further supported by the positive correlation between average soil grain size and TIC load. The finer the soil fractions in the catchment, the higher the TIC load, suggesting that there is greater specific surface area for weathering in soils with finer particle size. The soil properties might have greater influence on C loads than management, because the highest TOC load and the second lowest TIC load were recorded in the pristine Liuhapuro catchment. High TOC load and low TIC load were also observed in the neighbouring heavily managed catchment Murtopuro. The total lateral C load (TOC+ TIC) from the catchments (3500– 15700 kg km− 2) in this study was significantly lower than that observed by Billett et al. (2004) in Scotland. In their study the mean annual downstream C flux was 30 400 kg km− 2, although in both studies the total load consisted mainly of TOC, and the TOC concentrations were of the same magnitude. However, in Scotland the annual runoff was significantly higher, leading to the higher C load compared to our study. 4.2. Differences between two climatically different years; proposed changes in seasonal patterns in the future climate In the small boreal catchments half of the long term annual runoff has occurred during the spring period, although it lasts only for 1–2 months (Kortelainen et al., 1997). In that sense the year 2007 was exceptional, because the spring peak in runoff was rather low due to the warm period in December 2006 when the snow cover partly melted, remaining thin for the rest of the winter. During the summer of 2007 the high rainfall led to high discharge, although in the long term records a low discharge rate is typical for summer. The weather in 2007 might give us a glimpse of the future climate. The climate change scenarios predict increasing rainfall and temperature in Northern Europe, which means shorter snow cover period,

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frequent snow melt events during winter, and increasing rain during summer (Lemke et al., 2007), similar conditions to those that were observed in Northern Karelia in 2007. The discharge and TOC export results were compared to the year 2003, with mean temperatures and rainfall close to the monthly long term averages observed in that district. The water discharge rate in the study streams was on average 23% higher in 2007 than in 2003 due to higher rainfall (13%) in 2007, and consequently, TOC export was on average also 17% higher in 2007. There was even greater variation between the years in the seasonal distribution of the TOC export. In 2007 the TOC export was rather evenly distributed between the snow cover period, spring snow melt and the snow free period, the greatest part of the TOC export occurring during the snow free period in most streams. In 2003 the snow melt period and the snow free period dominated the TOC export, the spring snow melt being the period of greatest TOC export. The importance of the snow cover period to the TOC export was minimal in 2003. Even though there was substantially less snow when the snow melt started in spring 2007 than in 2003, the April–May runoff was of the same magnitude in both years, and consequently the TOC exports from the catchments during the snow melt period were rather similar. This is in contrast to the results of Köhler et al. (2008), who found that snowmelt-related TOC export had a strong linear correlation with the water equivalent of the winter precipitation. Although the water equivalent of snow in April 2003 was 172 mm and in April 2007 42 mm, the average snow melt period runoff was rather similar in both study years (168 mm vs. 148 mm). The similar runoff may be due to a very dry year (annual precipitation 540 mm in 2002) preceding the year 2003 causing the snow melt water to raise the very low ground water table rather than generating heavy runoff. Laudon et al. (2007) showed that during the snowmelt period the main part of the runoff was pre-event water in the forested catchment and little new snowmelt directly entered the stream, whereas in the wetland dominated catchments snowmelt water contributed over 50% of the runoff. The large snowmelt water contribution in the wetland catchment was suggested to be due to continuous soil frost layer inhibiting melt water from infiltrating into the peat. However, the thick continuous soil frost is typical in wetlands with high groundwater levels in autumn before freezing. After a dry summer and autumn the groundwater level might lower also in peatlands, especially if they have been drained. Therefore, there can be differences also in the soil frost thickness and continuity. Mitchell et al. (2006) studied effects of storms on discharge in a northern hardwood forest watershed and observed that after a very dry summer autumn storms yielded very low discharge compared to the other years. TOC export in the snow cover period in 2007 was almost three-fold compared to the export in 2003. The greater export rate was mainly derived from greater runoff in 2007, because the TOC concentrations in the streams were of the same magnitude in both years. The greater snow cover period runoff in 2007 was a consequence of the temperature being frequently above zero during the five winter months. The redefined annual TOC export pattern from the catchments may have consequences in the downstream lakes and streams. Lindström et al. (2006) suggested that in many small boreal lakes most of the water volume is replaced by the spring flood. Allochthonous carbon is important for aquatic food webs, and the redefined timing of the carbon supply may have unexpected consequences for aquatic ecology, which will need further research. 5. Conclusions TIC concentrations were highest in low-flow situations, probably due to the greater contribution of ground water to the stream flow. Stream water pH strongly correlated with the proportion of peatland in the catchment, being significantly lower in the peatland dominated catchments. Consequently, CO2 content of the stream water was

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highest in peatland dominated catchments, although TIC concentrations were of the same magnitude in all catchments. Organic carbon dominated the total lateral C load. TIC load was rather consistent between the catchments, whereas TOC load was higher in peatland dominated catchments. In the low-flow situations fertile mire spots situated around the catchment were TOC sources, but in the rising flow stream banks also released TOC. The least fertile open mire types situated in upper reaches of catchments had only minor influence as stream water TOC sources. In the long term, peatland drainage decreases stream water TOC, TIC and CO2 concentrations but does not affect lateral TOC or TIC export from the catchments. However, drainage potentially enhances gaseous losses of CO2 as a consequence of turbulence caused by higher discharge rate and larger water surface area. Climate change might alter the timing and amount of TOC export from the catchments. Especially warmer winters affect the timing of the TOC export, the importance of the spring ice melt diminishing and that of the snow cover period and the snow free period TOC export increasing. Acknowledgements We thank the staff of the Finnish Forest Research Institute's Nurmes field station and the laboratory of North Karelia Regional Environment Centre for the field work and the laboratory analyses. This study was financially supported by the Academy of Finland. References Ågren A, Buffam I, Jansson M, Laudon H. Importance of seasonality and small streams for the landscape regulation of dissolved organic carbon export. J Geophys Res 2007;112:G03003. Ågren A, Buffam I, Berggren M, Bishop K, Jansson M, Laudon H. Dissolved organic carbon characteristics in a forest–wetland gradient during the transition between winter and summer. J Geophys Res 2008a;113:G03031. Ågren A, Berggren M, Laudon H, Jansson M. Terrestrial export of highly bioavailable carbon from small boreal catchments in spring flood. Freswater Biol 2008b;53:964–72. Ahtiainen M, Huttunen P. Long-term effects of forestry managements on water quality and loading in brooks. Boreal Environ Res 1999;4:101–14. Berggren M, Laudon H, Jansson M. Growth and respiration of freshwater bacteria on organic carbon from different terrestrial sources. Glob Biogeochem Cycles 2007;21 (4):GB4002. Billett MF, Palmer SM, Hope D, Deacon CM, Storeton-West R, Hargreaves KJ, et al. Linking land–atmosphere carbon fluxes in a lowland peatland system. Glob Biogeochem Cycles 2004;18:GB1024. Billett MF, Moore T. Supersaturation and evasion of CO2 and CH4 in surface waters at Mer Bleue Peatland, Canada. Hydrol Process 2007;22(12):2044–54. Bishop K, Pettersson C. Seasonal variations of total organic carbon, iron, and aluminium on the Svartberget catchment in Northern Sweden. Environ Int 1996;22 (5):535–40. Bishop K, Pettersson C, Allard B, Lee Y-H. Identification of the riparian sources of aquatic dissolved organic carbon. Environ Int 1994;20(1):11–9. Bishop K, Seibert J, Köhler S, Laudon H. Resolving the double paradox of rapidly mobilized old water with highly variable responses in runoff chemistry. Hydrol Process 2004;18:185–9. Buffam I, Galloway J, Blum L, McGlathery KJ. A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream. Biogeochemistry 2001;53:269–306. Buffam I, Laudon H, Temnerud J, Mörth C-M, Bishop K. Landscape scale variability of acidity and dissolved organic carbon during spring flood in boreal stream network. J Geophys Res 2007;112:GO1022. Butler JN. Carbon dioxide equilibria and their applications. Reading, Massachusetts: Addison-Wessley Publishing Company; 1982. 259 pp. Cajander AK. Über Waldtypen. Act For Fenn 1909;1:1-175. Cajander AK. Forest types and their significance. Act For Fenn 1949;56:1-71. Dawson JJC, Bakewell C, Billett MF. Is in-stream processing an important control on spatial changes in carbon fluxes in headwater catchments? Sci Total Environ 2001;265:153–67. Finér L, Heimala-Raimas R, Päivänen J. Tree stands and ground vegetation in two watersheds in the Nurmes-research area. Aqua Fenn 1988;18(1):47–60. Finér L, Ahtiainen M, Mannerkoski H, Möttönen V, Piirainen S, Seuna P, Starr M. Effects of harvesting and scarification on water and nutrient fluxes. A description of catchments and methods, and results from the pretreatment calibration period. Finnish Forest Research Institute, Research Papers 1997; 648, Joensuu Research Station. Finér L, Kortelainen P, Mattsson T, Ahtiainen M, Kubin E, Sallantaus T. Sulphate and base cation concentrations and export in streams from unmanaged catchments in Finland. For Ecol Manage 2004;195:115–28.

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M. Rantakari et al. / Science of the Total Environment 408 (2010) 1649–1658

Gorham E. Northern peatlands: role in the carbon cycle and probable responses to climate warming. Ecol Appl 1991;1:182–95. Hinton MJ, Schiff S, English MC. The significance of storms for the concentration and export of dissolved organic carbon from two Precambrian Shield Catchments. Biogeochemistry 1997;36:67–88. Hope D, Palmer SM, Billett MF, Dawson JJC. Carbon dioxide and methane evasion from a temperate peatland stream. Limnol Oceanogr 2001;46(4):847–57. Hope D, Palmer SM, Billett MF, Dawson JJC. Variations in dissolved CO2 and CH4 in a first-order stream and catchment: an investigation of soil–stream linkages. Hydrol process 2004;18(17):3255–75. Huikari O. Site quality estimation of forest land. Proc. Int. Symp. For. Drainage. Jyväskylä- Oulu, Finland, 1974; pp. 15–24. Huikari O. Suotyypin määritys maa-ja metsätaloudellista käyttöarvoa silmällä pitäen. On the determination of mire types, specially considering their drainage value for agriculture and forestry (in Finnish with English summary). Silva Fenn 1952;75:1-22. Jones Jr JB, Mulholland PJ. Carbon dioxide variation in a hardwood forest stream: an interactive measure of whole catchment soil respiration. Ecosystems 1998;1:183–96. Kauppi PE, Posch M, Hänninen P, Henttonen HM, Ihalainen A, Lappalainen E, et al. Carbon reservoirs in peatlands and forests in the boreal regions of Finland. Silva Fenn 1997;31:13–25. Kling GW, Kipphut GW, Miller MC. The flux of CO2 and CH4 from lakes and rivers in arctic Alaska. Hydrobiologia 1992;240:23–46. Hydrologinen vuosikirja 2001–2005, Hydrological yearbook 2001–2005 (in Finnish with English abstract). In: Korhonen J, editor. Suomen ympäristö 44/2007, Luonnonvarat. Suomen Ympäristökeskus/ Finnish Environment Institute; 2007. 216 pp. Kortelainen P, Saukkonen S, Mattsson T. Leaching of nitrogen from forested catchments in Finland. Glob Biogeochem Cycles 1997;11:627–38. Kortelainen P, Pajunen H, Rantakari M, Saarnisto M. A large carbon pool and small sink in boreal Holocene lake sediments. Glob Change Biol 2004;10:1648–53. Kortelainen P, Mattsson T, Finér L, Ahtiainen M, Saukkonen S, Sallantaus T. Controls on the export of C, N, P and Fe from undisturbed boreal catchments, Finland. Aquat Sci 2006a;68:453–68. Kortelainen P, Rantakari M, Huttunen JT, Mattsson T, Alm J, Juutinen S, et al. Sediment respiration and lake trophic state important predictors for the large CO2 evasion from small boreal lakes. Glob Change Biol 2006b;12:1554–67. Köhler SJ, Buffam I, Laudon H, Bishop KH. Climate's control of intra-annual and interannual variability of total organic carbon concentrations and flux in two contrasting boreal landscape elements. J Geophys Res 2008;113:G03012. Laiho R, Laine J, Trettin CC, Finér L. Scots pine litter decomposition along drainage succession and soil nutrient gradients in peatland forests, and the effects of interannual weather variation. Soil Biol Biochem 2004;36:1095–109. Laudon H, Köhler S, Buffam I. Seasonal TOC export from seven boreal catchments in northern Sweden. Aquatic Sciences 2004;66:223–30. Laudon H, Sjöblom V, Buffam I, Seibert J, Mörth M. The role of catchment scale and landscape characteristics for runoff generation of boreal streams. J Hydrol 2007;344:198–209. Lemke P, Ren J, Alley RB, Allison I, Carrasco J, Flato G, et al. Observations: changes in snow, ice and frozen ground. In: Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL, editors. Climate Change 2007: the physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press; 2007. Lindström G, Bishop K, Löfvenius MO. Soil frost and runoff at Svartberget, northern Sweden — measurements and model analysis. Hydrol Process 2002;16:3379–92. Lindström ES, Forslund M, Algesten G, Bergström A-K. External control of bacterial community in lakes. Limnol Oceanogr 2006;51(1):339–42. Mannerkoski H, Finér L, Piirainen S, Starr M. Effect of clear-cutting and site preparation on the level and quality of groundwater in some headwater catchments in Finland. For Ecol Manage 2005;220:107–17.

Mattsson T, Finér L, Kortelainen P, Sallantaus T. Brook water quality and background leaching from unmanaged forested catchments in Finland. Water Air Soil Pollut 2003;147:275–97. Mellander P-E, Ottosson Löfvenius M, Laudon H. Climate change impacts on snow and soil temperature in boreal Scots pine stands. Climatic Change 2007;85:179–93. Minkkinen K. Effect of forestry drainage on carbon balance and radiative forcing of peatlands in Finland. PhD thesis, Helsinki University, 1999, 42 pp. Minkkinen K, Laine J. Long-term effect of forest drainage on the peat carbon stores of pine mires in Finland. Can J For Res 1998;28:1267–75. Mitchell MJ, Piatek KB, Christopher S, Mayer B, Kendall C, McHale P. Solute sources in stream water during consecutive fall storms in a northern hardwood forest watershed: a combined hydrological, chemical and isotopic approach. Biogeochemistry 2006;78:217–46. National board of waters. Methods of water analysis employed by the Water Administration (in Finnish with English summary). Helsinki; 1981. 136 pp. Nilsson M, Sagerfors J, Buffam I, Laudon H, Eriksson T, Grelle A, et al. Contemporary carbon accumulation in a boreal oligotrophic minerogenic mire — a significant sink after accounting for all C-fluxes. Global Change Biol 2008;14:2317–32. Nyberg L, Stähli M, Mellander P-E, Bishop K. Soil frost effects on soil water and runoff dynamics along a boreal forest transect: 1. Field investigations. Hydrol Process 2001;15:909–26. Öquist MG, Wallin M, Seibert J, Bishop K, Laudon H. Dissolved inorganic carbon export across the soil/stream interface and its fate in a boreal headwater stream. Environ Sci Technol 2009;43(19):7364–9. Palviainen M, Finér L, Kurka A-M, Mannerkoski H, Piirainen S, Starr M. Decomposition and nutrient release from logging residues after clear-cutting of mixed boreal forest. Plant Soil 2004;263:53–67. Palviainen M, Finér L, Laurén A, Mannerkoski H, Piirainen S, Starr M. Development of ground vegetation biomass and nutrient pools in a clear-cut disc-plowed boreal forest. Plant Soil 2007;297:43–52. Piirainen S, Finér L, Mannerkoski H, Starr M. Carbon, nitrogen and phosphorus leaching after site preparation at a boreal forest clear cut area. For Ecol Manage 2007;243:10–8. Plummer LN, Busenberg E. The solubility of calcite, aragonite, and vaterite in CO2–H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for CaCO3– CO2–H2O equilibria. Geochim Cosmochim Acta 1982;46:1011–40. Sarkkola S, Koivusalo H, Laurén A, Kortelainen P, Mattsson T, Palviainen M, et al. Trends in hydrometeorological conditions and stream water organic carbon in boreal forested catchments. Sci Total Environ 2009;408:92-101. SAS Institute. SAS version 8.2 for Windows. Cary: SAS Institute Inc; 2001. Schiff SL, Aravena R, Trumbore SE, Hinton MJ, Elgood R, Dillon PJ. Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: clues from 13C and 14C. Biogeochemistry 1997;36:43–65. Striegl RG, Dornblaser MM, Aiken GR, Wickland KP, Raymond PA. Carbon export and cycling by the Yukon, Tanana, and Porcupine rivers, Alaska, 2001–2005. Water Resour Res 2007;43:WO2411. Stumm W, Morgan JJ. Aquatic Chem. An introduction emphasizing chemical equilibria in natural waters. New York: Wiley; 1970. 583 pp. Verhoeven JT, Toth E. Decomposition of Carex and Sphagnum litter in fens: effect of litter quality and inhibition by living tissue homogenates. Soil Biol Biogeochem 1995;27(3):271–5. Verhoeven JT, Keuter A, van Logtestijn R, van Kerkhoven MB, Wassen M. Control of local nutrient dynamics in mires by regional and climatic factors: a comparison of Dutch and Polish sites. J Ecol 1996;84:647–56. Waldron S, Scott ME, Soulsby C. Stable isotope analysis reveals low-order river dissolved inorganic carbon pools are highly dynamic. Environ Sci Technol 2007;41 (17):6156–62. Worrall F, Burt T, Adamson J. Fluxes of dissolved carbon dioxide and inorganic carbon from an upland peat catchment: implications for soil respiration. Biogeochemistry 2005;73:515–39.