Leaching of nitrogen from forested catchments in Finland

Transcription

1 GLOBAL BOGEOCHEMCAL CYCLES, VOL. 11, NO. 4, PAGES , DECEMBER 1997 Leaching of nitrogen from forested catchments in Finland Pirkko Kortelainen Finnish Environment nstitute, Helsinki Sari Saukkonen Pfiijfinne Nature Center, Vfifiksy, Finland Tuija Mattsson Finnish Environment nstitute, Helsinki Abstract. This study provides an assessment on the spatial variability of the long-term leaching (8-23 years) of nitrogen and organic carbon from 22 forested catchments ( km2). The catchments are located throughout Finland excluding the northernmost regions. The Kruunuoja catchment is located in a national park; the other catchments representypical Finnish forestry land. The leaching from the 21 forestry land catchments can be considered to represent average leaching from Finnish forestry land since the most important forestry practices (ditching, clear-cutting, scarification, and fertilization) since the 1960s have affected about 2.4% of the catchment area per year (compare 2.5% in the entire country in 1980 and 2% in 1991). Moreover, the mean annual runoff from the catchments, mm yr -, agree with the mean annual runoffrom Finland (301 mm yr - from 1931 to 1990). The major part of the nitrogen transported from the catchments consisted of organic nitrogen (on average 79%). The average inorganic nitrogen proportion ((NO3-N + NH4-N)/CNtot) was lowest (7.3%) in the Kruunuoja catchment and was highest (54%) in the southernmost Teeressuonoja catchment located in the highest anthropogenic nitrogen deposition area. The median C/N ratio in the study streams was high, ranging from 34 to 66. Nitrate leaching from the catchments varied between 2.8 (Kruunuoja) and 100 kg km - - yr - (Teeressuonoja) and was negatively related to C/N ratio in stream water and latitude. The stepwise multiple regression model selected C/N ratio and nitrogen deposition which together explained 72% of the variation in NO3-N leaching. Retention of NO3-N deposition (calculated as ((input-output)/input) was high in all catchments, ranging from 0.99 in Kruunuoja to 0.67 in Teeressuonoja. 1. ntroduction Catchments in Finland are predominantly covered by coniferous forests, and forestry can be considered the largest-scale human impact to have taken place in Finnish catchments. Over half of the peatlands which originally covered one third of the land area has been ditched mostly for forestry. About 86% of the total land area of Finland (78% of the total area) is presently classified as forestry land [Aarne, 1994]. Presently forestry practices (ditching, clearcutting, scarification, and fertilization) annually affect about 2-3% of the catchment area. Forest soils in Finland usually contain large amounts of nitrogen ( kg N ha 4) of which only about 1% is present in mineralized form [Viro, 1969]. Forest ecosystems accumulate considerable amounts of N in biomass and soil organic matter. However, there is increasing concern that in Copyright 1997 by the American Geophysical Union. Paper number 97GB /97/97 GB $ high-deposition areas forests can be saturated with N resulting in increased nitrate leaching [e.g., Dise and Wright, 1995]. n some forests, chronically high N deposition has exceeded the assimilation capacity of the ecosystem, resulting in increasing leaching of nitrogen (usually as nitrate). n Finland the bulk deposition of total nitrogen increases from less than 100 kg km -2 in the northernmost regions to over 1000 kg km -2 in the southernmost high-deposition regions [Jiirvinen and Viinni, 1994]. This study provides an assessment on the long-term leaching of total nitrogen (Ntot), total organic carbon (TOC), nitrate (NO3-N), and ammonium (NH4-N) from 22 forested catchments located throughout Finland excluding the northernmost regions. The catchments represent typical Finnish forestry land with variable peatland proportion. The longterm monitoring (8-23 years) and the regional representa- riveness of the catchments provide an assessment on the average leaching of nitrogen from forested catchments in Finland under variable hydrological conditions. The impact of catchment characteristics, climatic, and anthropogenic factors (deposition and forestry) on the spatial and temporal variability will be considered.

2 628 KORTELANEN ET AL.' NTROGEN LEACHNG FROM FORESTED CATCHMENTS 2. Material and Methods The proportion of the catchments covered by peatlands 2.1. Study Catchments ranges from 10 to 87%. Ditching has been conducted in 20 catchments, and the proportion of the ditched peatlands in The 22 study catchments ( km 2) are located the catchments ranges from 0 to 100% (2-53% of the throughout Finland excluding the northernmost regions catchment area). The ditching activity was most intensive in (Figure 1). n the study catchments, forestry and atmospheric the 1960s and in the 1970s. n Syd inmaanoja (catchment 4) deposition can be considered the only significant human and Kesselinpuro (catchmerit 7) ditching had been started as impacts; less than 6% of the catchments is covered by early as the 1930s. n Joutenpuro (catchment 10) and agricultural fields (Table 1). The Kmunuoja catchment Hein ist6nluoma (catchmerit 3), small-scale ditching still (number 5) is located in a national park and has been in a occurred in the early 1990s. natural state for the last 30 years. The other catchments n Joutenpuro (number 10) catchmerit, clear-cutting represent "normal" Finnish forestry land where a number of occurred in 87% of the catchment area in the late 1960s. n mostly small-scale forestry practices have occurred. No data Kirsioja (number 11), 48% of the catchment had been clearbank with continuous information of forest management is cut from the late 1960s to the early 1980s. n other catchavailable. The forestry practices since the 1960s were ments, clear-cutting has occurred on less than 20% of the collected from several authorities. n some cases the catch- catchment area. Scarification has been carried out in less ments were divided among tens of landowners. The most than 15% of the catchment area. important forestry practices have been thoroughly described Fourteen of the catchments have been fertilized with by Saukkonen and Kortelainen [1995]. The proportion of the nitrogen (Table 1). n T611inoja (catchment 6) all the N catchment area ditched (mostly drained for forestry), clear- fertilization (1600 kg km '2) occurred in n other cut, and scarificated (soil tilling, a side preparation method catchments with the highests N fertilization doses, a smallerconnected with forest regeneration) as well as total nitrogen scale fertilization has been going on during several years. fertilization during the last 30 years are summarized in Table 1. SWEDEN 20 ø NORWAY o e lo Gulf of Finland 7 1 N'ctic Cirde RUSSA Northern Southern,31 ø 2.2. Sampling and Analytical Methods Daily runoff has been recorded in 14 catchments by a V notch overfall weir and a water recorder; in the remaining eight catchments, discharge calculations were based on runoff data from nearby catchments (Table 2). The manual stream water quality monitoring has been going on in the catchments since However, all catchments have not been monitored continuously. There were many missing values in the NHa-N and NO3-N results in the 1960s. Moreover, the accuracy of the earlier NHa-N and NO3-N methods cannot be considered satisfactory. Therefore the study period in the present paper was restricted to the years The total study period in the 22 catchments ranges from 8 to 23 years (Table 2). The sampling frequency has been approximately 12 samples per year. n early years, samples were collected once a month. n 1981, sampling strategy was changed, and sampling was concentrated in spring and autumn high-flow periods. Ammonium was measured by a spectrophotometric method with hypochlorite and phenol, and the sum of NO 2- N and NO3-N was measured by the cadmium amalgam method [Erkomaa et al., 1977]. Total nitrogen was analyzed as NO3-N after oxidation with K2S208 [National Board of Waters, 1981]. Organic N fractions were calculated as the difference between total nitrogen and inorganic nitrogen Leaching Calculations Annual leaching and spring leaching were calculated separately for each catchment. Spring was defined to begin when runoff started to increase because of snowmelt. Spring was considered to be completed when runoff decreased to less than the mean annual runoff or the recession runoff was Figure 1. Study catchments. Open circles indicate peatland over. These definitions were made graphically for each study percentage < 35. Solid circles indicate peatland percentage > year in all catchments. 35. Demarcation between southern and northern Finland The annualeaching of total nitrogen, nitrate, ammonium, (compare Tables 1-3 and 6 is shown). and total organic carbon were calculated as follows:

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4 630 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS D 0 ' (' O0 0 D 0,-- Oh Oh D (' 0 O0 ',. ' O O o o o O0 D 0 0 (' Oh D O D ' O0 O0 0,. ' O0 ' t ' 0

5 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS 631 Table 3. Median p H and C/N Ratio (TOC(laeq L'l)/(Ntot - NO3-N - NH4-N)(lae q L-l)), Average NO3-N Retention ((nput-output)/nput), norganic N (NO3-N + NH4-N) Net Retention (nput- Output), and the Length of the Spring Period (Days) p H C/N Ratio NO3-N norganic N Spring Period, Retention Net Retention days 1 Huhtisuonoja Katajaluoma Heinast6nluoma Sydanmaanoja Kruunuoja ! T611inoja Kesselinpuro a ! Vertailualue Pahkaoja ! 0 Joutenpuro Kirsioja Kotioja Ylijoki Mean Teeressuonoja Paunulanpuro Hein/ijokP Kellojoki Myllypuro V/i ir ijoki V/iha-Askanjoki Kuusivaaranpuro Myllyoja Mean Southern Finland Northern Finland Southern Finland. N t,. Z c(t,) q[ti] (1) i=l where N is number of samples, c(q is the concentration value at regularly spaced sampling times tv and q[t ] is the discharge for the period T = [r 4, r ] with r, = «(ti + t + ); that is the runoff period around sampling was used Multiple Regression Analyses Stepwise multiple regression models were developed in order to explain spatial variation in nitrogen leaching. The average hydrological and meteorological data, deposition, and forestry practices from a several-year study period were used to describe the average conditions (see Table 1). Precipitation was obtained from local climatological stations, run by the Finnish Meteorological nstitute. Atmospheric deposition of nitrogen (N Deposition), obtained from monthly monitoring of bulk deposition of nitrate and ammonium, was used as an index of total deposition. Averages of measurements during the years from the nearby stations were used as an estimate for nitrogen deposition. The areas are located far from emission sources, and it is unlikely that there would be major differences in the dry deposition fraction of the total deposition. 3. Results 3.1. Runoff The average runoff from the catchments ranged from 230 to 420 mm yr 4 (Table 1), and it increased to the north (runoff versus latitude: r2=0.61). Precipitation is higher in southern Finland. However, lower precipitation in northern Finland is compensated by lower evapotranspiration in colder climatic conditions resulting in higher runoff. The runoff was slightly higher (310 versus 340 mm yr ' ) in catchments with a high proportion of peatlands (> 35%, see Table 1) compared to catchments with a low peatland proportion (< 35%).

6 632 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS 8OO 1 7oo Peatland percentage > 35 Peatland percentage < 35,,. 600, max E,,, 500 " o loo o '., E ' ' ' o _ ' o ' ' ' o o o. ' ; > i ure. The ean annualeachin of N o (rain is minimum annual N o leachin and max is maximum annual N o leaching). Within the two oups the catchments a e classified f om south to north. The interannual and seasonal variation in runoff was high in all catchments. The average spring period lasted from 38 southern Finland compared to northern Finland (82 versus 17 txg N L4). (Ylijoki) to 56 days (Hein[ijoki) (Table 3). n southern Finland the average spring period lasted 51 days; in northern 3.3. Nitrogen Leaching Finland the spring period was shorter, an average 44 days. The mean annual leaching of Nto t from the 22 catchments Spring runoff represented about 50% of the annual runoff, ranged from 82 to 300 kg km -2(Figure 2). The leaching was although the spring period consisted only of about 10-15% higher (p<0.05) in the catchments with a high peatland of the whole year. proportion compared to those with a low peatland proportion 3.2. Nitrogen Concentrations The median concentration of total nitrogen in the 22 streams ranged from 180 to 900 txg L -1(Table 2). The concentrations were higher (p<0.05) in the catchments with a high proportion of peatlands compared to those with a low (210 versus 150 kg km-2). Moreover, the leaching was higher (p<0.01) in southern Finland compared to northern Finland (210 versus 150 kg km -2 yr4). About half of the annual leaching occurred in spring, although the spring period represented only 10-15% of the whole year. The major part of the nitrogen transported from the peatland proportion (620 versus 440 [tg N L'i). Moreover, catchments consisted of organic nitrogen; the average the concentrations were higher (p<0.001) in southern Finland compared to northern Finland (670 versus 400 txg N L' ). inorganic nitrogen proportion ((NO3-N + NH4-N)/Ntot) was 21%. The inorganic nitrogen proportion was lowest (7.3%) The variation in nitrate and ammonium concentrations in the natural Kruunuoja catchment and was highest (54%) was much larger than evident for total nitrogen. The median nitrate in the 22 streams ranged from 5 to 420 txg N L -1. The combination of higher deposition, higher temperature, and more fertile soils in southern Finland results in slightly higher nitrate concentrations (p>0.05) compared to northern Finland (85 versus 35 txg N L-l). Nitrate concentrations were highest in the southernmost Teeressuonoja catchment located in the highest deposition area. Besides atmospheric deposition the only anthropogenic impact in this catchment since the 1960s has been some excavation of sand and gravel. The catchment-scale effects of these human generated impacts are probably small compared to deposition. The median ammonium in the 22 streams ranged from 5 to 240 txg N L -1. The concentrations were higher (p<0.05) in the catchments with a high peatland proportion compared to those with a low peatland proportion (78 versus 15 [tg N L-l). Moreover, the concentrations were higher (p<0.05) in in the southernmost Teeressuonoja catchment (Table 2 and Figure 3). Nitrate leaching varied between 2.8 and 100 kg km -2 yr '. The leaching was highest in the Teeressuonoja catchment and was lowest in the Kruunuoja catchment (Figure 4). The NH4-N leaching varied between 5 and 82 kg km '2 yr 4 and was higher (p<0.01) in southern Finland compared to northern Finland (31 versus 14 kg km '2 yr4.) The C/N ratio in the study streams, calculated by dividing TOC ([teq L 4) with organic nitrogen (txeq L 4) (TOC/(Nto t - NO3-N - NH4-N)), was high in all catchments. The median C/N ratio in the catchments ranged from 34 to 66 (Table 3). The ratio was slightly higher in catchments with a high peatland proportion compared to catchments with a low peatland proportion (51 versus 46). The C/N ratio was lowest in the Kruunuoja catchment, where the NO3-N leaching was very low (2.8 kg km -2 yr4). Retention of NO3-N deposition was high in all catch-

7 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS El= Org-N leaching El= NH4-N leaching TM NO3-N leaching 250 E 200,, c O Figure 3. The mean annual leaching of different nitrogen fractions. ments. Nitrate retention (calculated as ((input-output)/input) kg N km -2 yr -1. This net retention (ignoringaseous losses) ranged from 0.67 to Retention was highest in the was lowest in the northernmost catchmerits (Table 3). natural Kruunuoja catchmerit and was lowest in the south- The impact of catchmerit characteristics, climatic factors, ernmost Teeressuonoja catchmerit. The difference between deposition, and forestry practices on the leaching was studied inorganic nitrogen input and output ranged from 150 to 600 by correlation analysis. As in the survey of Dise and Wright 3OO!._1 z 25O O Peatland percentage > 35 max Peatland percentage < 35 C 0 0 C C C =o _= _=._ '-. -r' '6 03 Figure 4. The mean annual leaching of NO3-N (min is minimum annual NO3-N leaching and max is maximum annual NOs-N leaching). Within the two groups the catchmerits are classified from south to north.

8 634 KORTELANEN ET AL.' NTROGEN LEACHNG FROM FORESTED CATCHMENTS Table 4. Stepwise Multiple Regression Model for the Study Catchments (n=22) Predicting the Leaching of NO3-N, NH4-N, Organic N, Ntot, and TOC (kg km -2 yr 4) From Selected Variables Equation r 2 p NO3-N, kg km '2 yr -1 = C/N ratio N Deposition, kg km -2 yr p< NH4-N, kg km -2 yr -1 = TOC, kg km -2 yr p<0.05 Organic N, kg km -2 yr -1 N,ot, kg km -2 yr TOC, kg km -2 yr C/N ratio TOC, kg km -2 yr C/N ratio N Deposition, kg km -2 yr p< p<0.001 TOC, kg km -2 yr -1 = 16, ph 0.53 p< [1995], a large number of variables were not significantly 3.4. Organic Carbon Concentrations and Leaching correlated to nitrogen output. Latitude and inorganic nitrogen The median TOC concentration in the 22 streams ranged deposition were more important contributors to the variation from 5 to 31 mg C L -1(Table 2). The concentrations were in NO3-N and Nto t than forestry practices (the proportion of higher (p<0.01) in the catchments with a high peatland the catchment area ditched, clear-cut, or scarificated or the proportion compared to those with a low peatland proportion amount of N fertilization per surface unit). (20 versus 12 mg L-l). Moreover, the concentrations were Forestry practices were not significantly correlated to the higher (p<0.01) in southern Finland compared to northern leaching. Moreover, exact N fertilization amounts were Finland (20 versus 13 mg L-l). missing from four catchments, clear-cutting data were The mean annual leaching of TOC from the catchments missing from seven, and scarification data were missing ranged from 2600 to 8800 kg km -2(Table 2 and Figure 5). from nine catchments. Therefore stepwise multiple regression The leaching was higher (p<0.01) in the catchments with a models were developed for all 22 catchments including the high peatland proportion compared to those with a low proportion of the catchment area ditched but excluding N peatland proportion (6400 versus 4600 kg km -2 yr-1). Morefertilization, clear-cutting, and scarification data. over, the leaching was slightly higher (p>0.05) in southern NO3-N leaching was negatively related to C/N ratio Finland compared to northern Finland (6200 versus 5100 kg (p<0.01) and latitude (p<0.05). NH4-N leaching was posikm -2 yr-1). Average TOC leaching during spring period was tively related to TOC leaching (p<0.05) and was negatively 44% of that for the total year in southern Finland and was related to latitude (p<0.05). Organic nitrogen leaching was 53% in northern Finland. positively related to TOC leaching (p<0.0001) and precipita- TOC leaching was negatively related to ph in the stream tion (p<0.05). Nto t leaching was positively related to TOC water (p<0.0001) and positively related to peatland proporleaching (p<0.01), precipitation (p<0.05), and N deposition tion in the catchment area (p<0.01), C/N ratio in the stream (p<0.05) and was negatively related to latitude (p<0.01). water (p<0.01), precipitation (p<0.05), and the proportion of The most significant two-variable regression model comditched peatlands (p<0.05). The stepwise multiple regression bined C/N ratio with N deposition and explained 72% of the model selected only the most important predictor, ph, which variation in NO3-N leaching (n=22 and p<0.0001). For explained 53% of the variation in TOC leaching (n=22 and NH4-N the most important predictor was TOC leaching, p<0.0001). which explained 25% of the variation in NH4-N leaching (n=22 and p<0.05). There was a strong positive relationship between organic nitrogen leaching and TOC leaching 4. Discussion (r2=0.60). The stepwise multiple regression model combined 4.1. Regional and Temporal Variation in the Leaching TOC leaching with C/N ratio which together explained as much as 84% of the variation in organic nitrogen leaching (n=22 and p<0.0001). The most significant three-variable regression model combined TOC leaching, C/N ratio, and N deposition and explained 84% of the variation in Nto t leaching (n=22 and p<0.001) (Table 4). The interannual variation in the annualeaching of Ntot, NO3-N, and TOC was large in most catchments (Figures 2, 4, and 5) probably partly because of variation in natural climatic and hydrological conditions. t is probable that in many catchments the natural interannual variation in runoff

9 ._ KORTELANEN ET AL.' NTROGEN LEACHNG FROM FORESTED CATCHMENTS Peatland percentage > 35 i Peatland percentage < 35 max o Figure 5. The mean annualeaching of total organic carbon (TOC) (min is minimum annual TOC leaching and max is maximum annual TOC leaching). Within the two groups the catchments are classified from south to north. and leaching is obscurred by the changes due to forestry practices. To avoid too much effect of temporal variability of climatic and hydrological factors, average values from a long period were used for leaching calculations (8-23 years). regional variation in the long-term leaching was not higher than the variation in interannualeaching in the catchments. Runoff from the study catchments increased to the north (r2=0.61). This trend compensated for the lower concentra- The seasonal variation in leaching was high in all catch- tions recorded for the northernmost catchments such that the ments. Half of the annual runoff and leaching occurred in spring although the spring period represented only 10-15% of the whole year. Nutrient leaching during spring is important to lake ecosystem since increasing temperature and daylight favor production. During cold winters, precipitation normally enters the catchments as snow and does not leave the catchments until spring melt occurs. f climate change results in higher winter temperatures, winter runoff and leaching will increase because of more frequent melt periods and more rain in winter. The corresponding decrease in spring leaching of nutrients may cause changes in biological processes in lakes since nitrogen often limits biological productivity in Finnish coastal waters [Project Pelag, 1996]. Consequently, a change in the timing of the major stream water transport of nitrate may have at least local importance for the productivity of coastal waters. Higher overall temperatures due to climate change can result in earlier biological productivity. Continuous monitoring data sets from 10 catchments during the years were divided into cold, mild, and warm years. n the Huhtisuonoja catchmenthe NO3-N concentrations and leaching were significantly higher during warm years compared to cold years (p<0.05). n other catchments the difference between cold and warm years was not statistically significant. Considering the differences in catchment size, location, forest type, and peatland type as well as different forest practices, the regional differences in the average long-term leaching of Nto t and TOC were not large (Table 2). The regional variation in the mean annual leaching was rather small. Moreover, the concentrations were higher in the catchments with a high peatland proportion, while runoff from these catchments was slightly lower compared to the catchments with a low peatland proportion. n 1993, nitrogen deposition in Finland ranged from 90 to 1060 kg km -2 yr - [J irvinen and l/ inni, 1994]. n the study catchments the total nitrogen input from fertilization since the 1960s has ranged from 0 to 2600 kg km -2 (Table 1). Consequently, deposition can be considered the main longterm nitrogen source in the catchments. n the two southernmost catchments, Teeressuonoja and Huhtisuonoja, nitrate leaching was significantly higher in the 1980s and in early 1990s compared to the 1970s (Table 5). n other catchments located in lower deposition area, increasing nitrate leaching has not been found (Table 5). Turnover of nitrogen can be expected to be more rapid in the southern catchments because of higher temperatures, more productive stands, and a higher nitrogen deposition. However, the strong relationship between temperature and atmospheric N deposition in Finland make it difficult to distinquish the effect of one variable from the other. Presently, the leaching of nitrate to watercourses can be expected to increase only from the most fertile ecosystems in southern parts of the country, where N deposition is also highest. Climate change might contribute to large leaching of nitrate owing to mineralization of organic nitrogen. The amount of nitrogen leached from forest ecosystems is small compared to total nitrogen pools in forested ecosys-

10 636 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS Table 5. Decennial Average NO3-N Leaching for the Eight Deposition of nitrogen compounds currently plays a minor Catchments With Representative Water Quality Data Since role in the long-term acidification of the study catchments. the 1970s The median ph in the streams ranged from 4.4 to 7.1 (Table 3). The lowest median ph was found in the natural Kruu- 1970s 1980s Early 1990s nuoja catchment. The average ph was lower in southern Finland compared to northern Finland (5.7 versus 6.4). NO3-N, NO3-N, NO3-N, Moreover, the ph was lower in the catchments with a high kg km '2 yr q kg km '2 yr ' kg km '2 yr ' peatland proportion compared to those with a low peatland proportion (5.8 versus 6.4). Stream water acidity was 1 Huhtisuonoja dominated by organic acids in the majority of the catch- 7 Kesselinpuro ments. Organic anions exceeded sulphate not only in autumn 9 Pahkaoja but also during spring snowmelt in 18 catchments. n the highest deposition area in southern Finland, however, 21 Teeressuonoja nonmarine sulphate was more important. The lowest ph values were dominated by organic acids in 20 catchments, 22 Paunulanpuro and in most catchments TOC better explained ph than non- 23 Heinajoki marine sulphate. Nitrate made a minor contribution to the 25 Myllypuro acidity; only in the Teeressuonoja catchment was nitrate contribution significant [Kortelainen and Saukkonen, 1995]. 27 Vaha-Askanjoki This agrees with the Finnish Lake Survey (978 statistically selected lakes which covered the entire country and were sampled in autumn 1987). Even in southernmost Finland, tems. Therefore it is often difficult to identify the most important factors contributing to the leaching. Much of the 73% of the lakes had nitrate concentrations < 5 [teq L - [Forsius et al., 1990]. nitrogen transported from the study catchments was bound to organic carbon. The median stream water C/N ratio was high, ranging from 34 to 66. This resulted in low NO3-N leaching. n catchments with lower C/N ratios the leaching can be supposed to increase. n general, nitrification is favored by low soil C/N ratios, high temperature, and moisture as well as high ph. The soil C/N ratio from the catchments was not available. The stream water C/N ratio, which can be supposed to be closely related to soil C/N ratio, was, however, the most important single predictor of NO3-N leaching (negative relationship), followed by latitude (negative relationship) and ph (positive relationship). Retention of inorganic N deposition was high in all catchments. Nitrate retention (calculated as ((input-output)/input) in the catchments ranged from 0.67 to Retention was highest in the natural Kruunuoja catchment and was lowest in the southernmost Teeressuonoja catch mpact of Land Use The leaching values presented in this study representhe combined net effect of deposition, natural leaching, and leaching due to forestry practices. The relationships between nitrogen and organic carbon leaching and forestry practices (ditching, clear-cutting, and scarification percentages and nitrogen fertilization doses per catchment area; Table 1) were not statistically significant, suggesting that large-scale forestry practices are needed before clear effects on spatial variability can be detected. Largest catchments were divided between tens of landowners, making exact forestry practice estimates difficult. Exact scarification data were missing from nine catchments, clear-cutting data were missing from seven catchments, and N fertilization data were missing from five catchments. ment. Mass balances of Finnish catchments with high This database does not enable the exact evaluation of a percentages of organic soils suggest high retention of NH4-N and NO3-N [e.g., Kallio and Kauppi, 1990]. Moreover, most northern forests efficient ly retain added nitrogen, and only in the most fertile forest types has significantly increased nitrate leaching been shown to occur, even after major disturbances [Vitousek et al., 1979]. A recent analysis of an input-output database covering 65 forest ecosystem studies (plots and catchments) across Europe showed that nitrate leaching was close to zero below a deposition threshold of single forestry practice. The study catchments represent typical Finnish forestry land where a large number of smallscale forestry practices have been carried out during the past 30 years. Many forestry practices have been carried out simultaneously in different parts of the catchments. Moreover, in modern forestry the different forestry practices typically follow each other: cutting is followed by scarification, and ditching is followed by fertilization. Many of the practices have impacted only small parts of the study about 10 kg N ha - yr -, highly variable at intermediate catchments, and the impact of a single practice is dependent levels of kg N ha - yr 4, whereas considerable part of on the distance of the treated area from the stream. Furtherthe input (10-35 kg N ha 4 yr 4) was leached in all systems more, the natural variation in hydrological conditions makes with inputs above 25 kg N ha 4 yr 4 [Dise and Wright, 1995]. Among 41 different variables tested in the survey, output nitrogen was most highly correlated with input it difficult to separate the changes due to forestry from natural interannual variation. LepistO et al. [1995] studied 10 Finnish and 12 Swedish forested catchments during the years nitrogen (r2=0.69), followed by input SO4, soil ph (negative n some of these catchments, forestry practices correlation), percentage of slope, bedrock type, and latitude (negative correlation). A combination of input N and soil ph explained 87% of the variability in output N (n=20). affected most of the catchment area, and large-scale forestry practices were needed before any clear effect on the spatial variability was detected.

11 KORTELANEN ET AL.' NTROGEN LEACHNG FROM FORESTED CATCHMENTS 637 Table 6. Leaching of Nto t, NO3-N, NH4-N, and TOC From Finnish Forestry Land Southern Finland Leaching, kg km -2 yr 'l Leaching, t yr 't Northern Finland Leaching, kg km '2 yr 'l Leaching, t yr 't Leaching from Finland, t yr 'l Nto t , ,000 47,000 NO3-N 30 3, ,900 5,700 NH4-N 31 3, ,900 5,800 T OC 6, ,000 5, ,000 1,500,00 Forestry land in southern Finland is 125,440 km -2, in northern Finland is 138,100 km '2, and is 86 % of the total land area of Finland, 263,540 km '2. Ditching was the largest-scale forestry practice in the study catchments. The ditching intensity was already high in the early 1960s, whereas most of the catchments with a high peatland proportion were not monitored until the 1970s. This makes it difficult to evaluate the overall impact of ditching. The comparison of different catchments with each other does not enable reliable assessment of the impacts of ditching. The peatland types in ditched and unditched peatlands are not comparable. n many regions the ditched peatlands were on average more minerogenic with rather thin peat deposits while some of the unditched peatlands were nutrient poor and wet. Reliable assessment of the impacts of ditching would require representative reference catchments which are located close to the study catchments as well as long-term frequent monitoring before and after ditching. n earlier studies, ditching has been found to result in a short-term increase of organic carbon concentrations [Heikurainen et al., 1978; Moore, 1987; Ahtiainen, 1988]. n the long run, ditching lowers the groundwater level and can result in decreased TOC leaching. Sallantaus [1994] did not find differences in the leaching of TOC between natural fen, natural bog, drained fen, and drained bog several years after treatment Leaching From Finnish Forestry Land The long-term monitoring and the regional representativeness of the catchments enable an assessment of the leaching of nitrogen and total organic carbon from Finnish forestry land. The mean annual runoff from the catchments, mm yr 4 (Table 1), agrees with the mean annual runoff from Finland, 301 mm yr 4, from 1931 to 1990 [Kuusisto, 1992]. Moreover, the forestry practices in the study catchments (ditching, clear-cutting, scarification, and fertilization) have consisted annually of about 2.4% of the catchment area (compare 2.5% in the entire country in 1980 and 2% in 1991) (Aarne 1994). When the average Ntot, NO3-N, NH4-N, and TOC leaching from southern and northern Finland are multiplied with the area of forestry land in southern and northern Finland, respectively, the following estimates for total annual leaching from Finnish forestry land are obtained: 47,000 t of Ntot, 5700 t of NO3-N, 5800 t of NH4-N, and 1.5 million t of TOC (Table 6). The estimated total annual Nto t leaching from Finnish forestry land (47,000 t) is close to the background leaching (including contributions of forestry, deposition, and natural leaching; agriculture, industry, and municipalities are excluded) to Finnish coastal waters calculated by Pitkiinen [1994]. n his study the Finnish coastal waters (which receive about 70% of the annual runoff from Finnish territo- ry) were estimated to receive annually on average 79,000 t of total nitrogen; the contribution of background leaching was estimated to be 41,000 t. Rekolainen et al. [1995] have recently suggested that nutrient losses from small agricultural areas mostly enter coastal waters with negligible retention in river channels. This was connected to the fact that most of the nutrient losses occur in spring, autumn, or early winter in connection with high water flows, short residence times of water, and low intensities of biogeochemical processes. The results from agricultural areas are supported by the present study from forested catchments with average spring leaching about 50% and the total estimated annual Nto t leaching (47,000 t) from Finnish forestry land comparable to the riverine background nitrogen input (41,000 t) to Finnish coastal waters. Rekolainen [1989] has estimated that the total annual leaching from agriculture in Finland would be 20,000-40,000 t for nitrogen. This suggests that although the leaching from forested areas is much lower per unit surface area, the total leaching from Finnish forestry land (86 % of the total land area) is comparable or exceeds the leaching from agriculturaland. 5. Conclusions The leaching from the study catchments representing the combined net effect of deposition, natural leaching, and leaching due to forestry practices can be considered to describe the average long-term leaching from forested catchments in Finland. The major part of the nitrogen transported from the catchments consisted of organic nitrogen. The average inorganic nitrogen proportion was lowest in the natural Kruunuoja catchment and was highest in the southernmost Teeressuonoja catchment located in the highest nitrogen deposition area. n the Teeressuonoja catchment, NO3-N leaching was higher in the 1980s and in the early 1990s compared to the 1970s. The C/N ratio in the study streams was high, resulting in low nitrate leaching. Retention of inorganic N deposition was generally high, and most catchments can still accumulate the present nitrogen deposition. Spatial variation in nitrogen leaching was not closely related to forestry practices, although lowest nitrate leaching

12 638 KORTELANEN ET AL.: NTROGEN LEACHNG FROM FORESTED CATCHMENTS was found from the natural Kruunuoja catchment. This suggests that intensive forestry is needed before a clear, longterm impact on spatial variability in nitrogen leaching can be detected. References Aarne, M. (Ed.), Yearbook of Forest Statistics , SVT Agriculture and forestry, 7, 348 pp., Finn. For. Res. nst., Helsinki, Ahtiainen, M., Effects of clear-cutting and forestry drainage on water quality in the Nurmes study, in Symposium on the Hydrology of Wetlands in Temperate and Cold Regions, Publ. Acad. Finland, 4, , Dise, N.B., and R.F. Wright, Nitrogen leaching from European forests in relation to nitrogen deposition, For. Ecol. Manage., 71, , Erkomaa, K.,. M ikinen, and O. Sandman, Analytical methods Moore, T., A preliminary study of the effects of drainage and harvesting on water quality in ombrotrophic bogs near Sept-les, Quebec, Water Resour. Bull., 23, , National Board of Waters, Methods of water analyses employed by the water administration (in Finnish), Rep. 213, 136 pp., Helsinki, Pitk inen, H., Eutrophication of the Finnish coastal waters: Origin, fate and effects of riverine nutrient fluxes, Publ. Water Environ. Res. nst., 18, Project Pelag, Nitrogen discharge, pelagic nutrient cycles, and eutrophication of northern Baltic coastal environments, final report , (in press) (in Finnish), Finn. Environ. nst., Helsinki, Rekolainen, S., Phosphorus and nitrogen load from forest and agricultural areas in Finland, Aqua Fenn., 19(2), , Rekolainen, S., H. Pitk inen, A. Bleeker, and S. Felix, Nitrogen and phosphorus fluxes from Finnish agricultural areas to the Baltic Sea, Nord. Hydrol., 26, 55-72, Sallantaus, T., Response of leaching from mire ecosystems to used by authorized and water authority laboratories (in Finnish), changing climate, in The Finnish Research Programme on Rep. 121, 54 pp. Nat. Board of Waters, Helsinki, Climate Change, Second Prog. Rep., edited by M. Kanninen and Forsius, M., J. K im iri, P. Kortelainen, J. Mannio, M. Verta, and K. P. Heikinheimo, Publ. Acad. Finland 1, , Helsinki, Kinnunen, Statistical lake survey in Finland: Regional estimates of lake acidification, in Acidification in Finland, edited by P. Saukkonen, S., and P. Kortelainen, mpact of forestry practices on Kauppi, P. Anttila and K. Kentt imies, pp , Springer- the leaching of nutrients and organic matter (in Finnish), in The Verlag, New York, Effects of Forestry on Waters and their Abatement. Final Report Heikurainen, L., K. Kentt imies, and J. Laine, The environmental of METVE-project, edited by S. Saukkonen and K. Kentt imies, effects of forest drainage, Suo, 29(3-4), 49-58, Finnish Environment 2, Finn. Environ. nst., Helsinki, J irvinen, O., and T. V inni, The quality of deposition in Finland in (in Finnish), Mimeogr. Ser., 579, Nat. Board of Waters and Viro, P.J., Prescribed burning in forestry, Commun. nst. For. Fenn., the Environ., Helsinki, (7), 49 pp., Kallio, K., and L. Kauppi, on budgets of small forested basins, in Vitousek, P.M., J.R. Gosz, C.G. Grier, J.M Melillo, W.A. Reiners, Acidification in Finland, edited by P. Kauppi, P. Anttila and K. and R.L. Todd, Nitrate losses from disturbed ecosystems, Kentt imies, pp , Springer-Verlag, New York, Science, 204, , Kortelainen, P., and S. Saukkonen, Organic versus minerogenic acidity in headwater streams in Finland, Water Air Soil Pollut., P. Kortelainen and T. Mattsson, Finnish Environment nstitute, 85, , P.O. Box 140, Helsinki, Finland. ( pirkko.kortelainen Kuusisto. E., Runoff from Finland in the period of tuija.mattsson@vyh.fi) Aqua Fenn. 22(1), 9-22, S. Saukkonen, P iij inne Nature Center, Tallukantie 1, Lepist6, A., L. Andersson, B. Arheimer, and K. Sundblad, nfluence V i iksy, Finland. of catchment characteristics, forestry activities and deposition on nitrogen export from small forestes catchments, Water Air Soil (Received November 1, 1996; revised June 11, 1997; Pollut., 84, , accepted June 30, 1997.) 1997.