Tuija Mattsson

Leaching of nitrogen from forested catchments in Finland

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 (0.69-56 km 2 ). The catchments are located throughout Finland excluding the northernmost regions. The Kruunuoja catchment is located in a national park; the other catchments represent typical 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, 230-420 mm yr -1 , agree with the mean annual runoff from Finland (301 mm yr -1 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 ((NO 3 -N + NH 4 -N)/N tot ) 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 -2 yr -1 (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 NO 3 -N leaching. Retention of NO 3 -N deposition (calculated as ((input-output)/input) was high in all catchments, ranging from 0.99 in Kruunuoja to 0.67 in Tceressuonoja.

Trends in hydrometeorological conditions and stream water organic carbon in boreal forested catchments

Temporal trends in stream water total organic carbon (TOC) concentration and export were studied in 8 forested headwater catchments situated in eastern Finland. The Seasonal Kendall test was conducted to identify the trends and a mixed model regression analysis was used to describe how catchment characteristics and hydrometeorological variables (e.g. precipitation, air and stream water temperatures, and atmospheric deposition) related to the variation in the concentration and export of stream water TOC. The 8 catchments varied in size from 29 to 494 ha and in the proportion of peatland they contained, from 8 to 70%. Runoff and TOC concentration were monitored for 15-29 years (1979-2006). Trends and variation in TOC levels were analysed from annual and seasonal time series. Mean annual TOC concentration increased significantly in seven of the eight catchments. The trends were the strongest in spring and most apparent during the last decade of the study period. The slopes of the trends were generally smaller than the variation in TOC concentration between years and seasons and between catchments. The annual TOC export showed no clear trends and values were largely determined by the temporal variability in runoff. Annual runoff showed a decreasing trend in two of the eight catchments. Mean annual air and stream water temperatures showed increasing trends, most clearly seen in the summer and autumn series. According to our modeling results, stream water temperature, precipitation and peatland percentage were the most important variables explaining annual and most seasonal TOC concentrations. The atmospheric deposition of SO4, NH4, and NO3 decreased significantly over the study period, but no significant link with TOC concentration was found. Precipitation was the main hydrometeorological driver of the TOC export. We concluded that stream water TOC concentrations and exports are mainly driven by catchment characteristics and hydrometeorological factors rather than trends in atmospheric acid deposition.

Brook Water Quality and Background Leaching from Unmanaged Forested Catchments in Finland

The water chemistry of 21 outlet brooks in undisturbed, forested catchments (0.074–38 km2) in Finland was monitored during three years (1997–1999) in order to estimate the background levels and leaching of total organic carbon (TOC), iron, nitrogen and phosphorus, and examine the relationships between catchment characteristics and brook water quality in pristine forested areas. The studied catchments are located throughout Finland except the northernmost parts of the country. The concentrations of TOC were relatively high, on average 20 mg L-1. Annual leaching of TOC ranged from 3000 to 10 000 kg km-2. The average total nitrogen concentration and annual leaching were 430 μg L-1 and 140 kg km-2, respectively. The average total phosphorus concentration and annual leaching were 15 μg L-1 and 5.4 kg km-2, respectively. On a national level temperature and discharge conditions were the most important variables for predicting total phosphorus, total nitrogen and TOC concentrations. Both total nitrogen and total phosphorus concentrationsdisplayed positive correlation with temperature, and thus the concentrations were higher in the south than in the north. Nitrogen was mainly organic and showed strong correlation with TOC. On the catchment level, both TOC and total nitrogen concentrations and export had a strongpositive correlation with the abundance of Norway spruce (Piceaabies Karsten) and a strong negative correlation with the abundance of Scots pine (Pinus sylvestris L.). Nitrate concentrations and leaching were related to average site type. The more fertile the average site type was in the catchment, the higher the nitrate concentrations and export were.

Export of dissolved organic matter in relation to land use along a European climatic gradient

The terrestrial export of dissolved organic matter (DOM) is associated with climate, vegetation and land use, and thus is under the influence of climatic variability and human interference with terrestrial ecosystems, their soils and hydrological cycles. We present a data-set including catchments from four areas covering the major climate and land use gradients within Europe: a forested boreal zone (Finland), a temperate agricultural area (Denmark), a wet and temperate mountain region in Wales, and a warm Mediterranean catchment draining into the Gulf of Lyon. In all study areas, DOC (dissolved organic carbon) was a major fraction of DOM, with much lower proportions of DON (dissolved organic nitrogen) and DOP (dissolved organic phosphorus). A south-north gradient with highest DOC concentrations and export in the northernmost catchments was recorded: DOC concentrations and loads were highest in Finland and lowest in France. These relationships indicate that DOC concentrations/export are controlled by several factors including wetland and forest cover, precipitation and hydrological processes. DON concentrations and loads were highest in the Danish catchments and lowest in the French catchments. In Wales and Finland, DON concentrations increased with the increasing proportion of agricultural land in the catchment, whereas in Denmark and France no such relationship was found. DOP concentrations and loads were low compared to DOC and DON. The highest DOP concentrations and loads were recorded in catchments with a high extent of agricultural land, large urban areas or a high population density, reflecting the influence of human impact on DOP loads.

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

Seasonal and between stream variation (catchment dependent variation) in losses of organic and inorganic carbon via downstream transport and outgassing of CO(2) 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 1400kgkm(-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 420kgkm(-2)a(-1). The export of total organic carbon (TOC) varied between 2300 and 14,800kgkm(-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 CO(2) in streams was the highest in undrained peatland rich catchments, but the outgassing of CO(2) 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.

36 year trends in dissolved organic carbon export from Finnish rivers to the Baltic Sea

Increasing dissolved organic carbon (DOC) concentrations in lakes, rivers and streams in northern mid latitudes have been widely reported during the last two decades, but relatively few studies have dealt with trends in DOC export. We studied the export of DOC from Finnish rivers to the Baltic Sea between 1975 and 2010, and estimated trends in DOC fluxes (both flow normalised and non-normalised). The study encompassed the whole Finnish Baltic Sea catchment area (301,000 km(2)) covering major land use patterns in the boreal zone. Finnish rivers exported annually over 900,000 t DOC to the Baltic Sea, and the mean area specific export was 3.5 t km(-2). The highest export (7.3t km(-2)) was measured in peat dominated catchments, whereas catchments rich in lakes had the lowest export (2.2 t km(-2)). Inter-annual variation in DOC export was high and controlled mainly by hydrology. There was no overall trend in the annual water flow, although winter flow increased in northern Finland over 36 years. Despite the numerous studies showing increases in DOC concentrations in streams and rivers in the northern hemisphere, we could not find any evidence of increases in DOC export to the northern Baltic Sea from Finnish catchments since 1975.

Iron concentrations are increasing in surface waters from forested headwater catchments in Eastern Finland

Observations of increased water colour have been made in lakes and rivers all across the northern mid-latitudes of Europe and North America, particularly during the last 10-20 yr. This water browning or brownification has been attributed to the increased organic carbon concentrations due to climate change and decreased acid atmospheric deposition. Given that iron (Fe) may also increase water colour, the contribution of Fe to water brownification has received small attention. Our aim was to study the temporal trends of Fe in forested headwater catchments in eastern Finland, where an increasing air temperature and total organic carbon (TOC) trend had been observed in an earlier study. We found a statistically significant increasing trend also in stream water Fe concentrations and a strong correlation between the trends of TOC and Fe. The average increase in TOC and Fe concentrations between 1995 and 2006 was 0.5 mg l(-1) yr(-1) (2.5%), and 34.6 μ gl(-1) yr(-1) (3.5%), respectively. These results indicate that the increased water colour or brownification in Northern Europe may not only be due to increased concentrations of organic matter but also increased concentrations of Fe. The change in precipitation and temperature conditions, particularly during late autumn and early winter periods, appeared to be the main environmental factor behind increasing Fe trends. The strong correlation between the trends of Fe and TOC indicated that the increased Fe-organic matter complexation is the mechanism behind increasing Fe trends, but further research is needed to assess the chemical forms of increased Fe that coupled with increased TOC concentrations would enhance water brownification.

Long-term base cation balances of forest mineral soils in Finland

Long-term base cation balances (Ca, Mg and K) for forest mineral soils in Finland were calculated with mass balance methods. The aim of the study was to identify the areas in which weathering and base cation deposition do not support leaching and uptake of base cations by vegetation. The effect of stem harvesting and whole-tree harvesting on the base cation balances was studied and preliminary calculations were made about the amounts of ash needed for compensation of the potential base cation depletion. The effect of sulphur emission reduction till 2010 was also considered. The study demonstrated that there would be depletion of base cations in forest mineral soils in southern, central and northeastern Finland (40–50% of the grids) in the long term if whole-tree harvesting would be practised. Theoretical calculations showed that ash application would be most useful in those areas to compensate the base cation depletion. If stem harvesting is practised, only small areas (20%) in southwestern and southeastern Finland and northeastern Lapland would show depletion of base cations in the long term.

Spatial and temporal variability of organic C and N concentrations and export from 30 boreal rivers induced by land use and climate

Climate change scenarios for northern boreal regions indicate that there will be increasing temperature and precipitation, and the changes are expected to be larger in winter than in summer. These precipitation and discharge patterns, coupled with shorter ice cover/soil frost periods in the future would be expected to contribute significantly to changing flow paths of organic matter over a range of land use patterns. In order to study the impact of climate change on the seasonality of organic matter export we compared total organic carbon (TOC) and total organic nitrogen (TON) concentrations and export, during different seasons and climatically different years, over 12 years for 30 Finnish rivers separated into forest, agriculture and peat dominated catchments. The mean monthly TOC concentrations were highest during autumn and there was also a peak in May during the highest flow period. The mean monthly concentrations of TON were lowest during winter, increased in spring and remaining high throughout summer and autumn. The TOC/TON ratios were lowest during summer and highest during winter, and in all seasons the ratios were lowest in catchments with a high proportion of agricultural land and highest in peat-dominated catchments. The seasonality of TOC and TON exports reflected geographical location, hydrology and land use patterns. Most of the TOC and TON were transported during the high flow following the spring snowmelt and during rainfall in autumn. In all catchments the relative importance of the spring snowmelt decreased in wet and warm years. However, in peat-dominated catchments the proportion of spring period was over 30% of the annual export even in these wet and warm years, while in other catchments the proportion was about 20%. This might be linked to the northern location of the peat-dominated catchments and the permanent snow cover and spring snowmelt, even in warm years.

Dissolved organic carbon fractions in Finnish and Maine (USA) lakes

Abstract Dissolved organic carbon (DOC) was examined in 17 Finnish lakes, 6 Finnish streams, and 30 Maine (USA) lakes by fractionating DOC into hydrophobic acids and neutrals, and hydrophilic acids, bases, and neutrals. A procedure using XAD-8, anion-exchange, and cation-exchange resins was used. In every sample, organic acids dominated, averaging 82% of DOC. No major differences in DOC fractions between Maine lakes and Finnish lakes were found. However, differences in DOC fractions were found between lakes with low and high DOC. The proportion of hydrophobic acids increased with increasing DOC, whereas the proportion of hydrophilic acids decreased with increasing DOC. Low DOC lakes had a greater proportion of their DOC as hydrophilic neutrals compared with high DOC lakes. On the basis of the fractionation results, linear equations were developed for the relationships between the total DOC and the proportions of hydrophobic and hydrophilic acids of the total DOC.