Land-use dominates climate controls on nitrogen and phosphorus export from managed and natural Nordic headwater catchments

Agricultural, forestry-impacted and natural catchments are all vectors of nutrient loading in the Nordic countries. Here, we present concentrations and fluxes of total nitrogen (totN) and phosphorus (totP) from 69 Nordic headwater catchments (Denmark: 12, Fin-land:18, Norway:17, Sweden:22) between 2000 and 2018. Catchments span the range of Nordic climatic and environmental conditions and include natural sites and km − 2 year − 1 ) from agricultural catchments was found, and countries showed contrasting patterns. Trends in annual concentrations and fluxes of totP and totN could not be explained in a straightforward way by changes in runoff or climate. Explanations for the totN decline include national mitigation measures in agriculture international policy to reduced air pollution and, possibly, large-scale increases in forest growth. Mitigation to reduce phosphorus appears to be more challenging than for nitrogen. If the green shift entails intensification of agricultural and forest production, new challenges for protection of water quality will emerge possible exacerbated by climate change. Further analysis of headwater totN and totP export should include seasonal trends, aquatic nutrient species and a focus on catchment nutrient inputs.

km −2 year −1 ) from agricultural catchments was found, and countries showed contrasting patterns. Trends in annual concentrations and fluxes of totP and totN could not be explained in a straightforward way by changes in runoff or climate. Explanations for the totN decline include national mitigation measures in agriculture international policy to reduced air pollution and, possibly, large-scale increases in forest growth. Mitigation to reduce phosphorus appears to be more challenging than for nitrogen. If the green shift entails intensification of agricultural and forest production, new challenges for protection of water quality will emerge possible exacerbated by climate change. Further analysis of headwater totN and totP export should include seasonal trends, aquatic nutrient species and a focus on catchment nutrient inputs.

K E Y W O R D S
agriculture, bioeconomy, forest, forestry, long-term trend, mitigation, monitoring, stream

| INTRODUCTION
Reconciliation of increasing reliance on agricultural and forestry products with water quality protection is becoming more urgent under the green growth policies that are being developed in Europe. The so-called "green shift" towards a low-carbon and resource-efficient society to mitigate climate change is expected to require wood and crop-based biomass for replacement of fossil resources by renewable energy-sources (Scarlat, Dallemand, Monforti-Ferrario, & Nita, 2015).
During the green shift, production of meat and cereals in Europe are expected to intensify-partly as consequence of income growthwith adverse effects on water quality (Rosegrant, Ringler, Zhu, Tokgoz, & Bhandary, 2013). Agricultural production in Europe is one of the main pressures identified under the Water Framework Directive (WFD) that reduces ecological status of surface and coastal waters, by increasing runoff of nitrogen and phosphorus (EEA, 2019). The WFD is implemented nationally with approaches targeted at climate, soil, hydrological conditions and agricultural production (Ulen, Bechmann, Folster, Jarvie, & Tunney, 2007). The WFD aims for good ecological, chemical and hydromorphological status by the end of 2027. However, river basin management and restoration actions could be insufficient to reach the WFD aims within 2050. In particular, diffuse nutrient loadings should be reduced significantly (Hering et al., 2010;Räike, Taskinen, & Knuuttila, 2020). National agro-environmental legislation in the Nordic countries since 1990 has led to country-specific reductions of the N and P field balances (inputs from fertilizer substracted with removal by harvest) on agricultural land. As an example, Danish regulations decreased the N and P field balance with 45% and 76% during the period 1990(Blicher-Mathiesen et al., 2020 and in Finland with 35% and 60%, respectively, during the period 1995(Aakkula & Leppänen, 2014. These country-specific reductions in field nutrient balances are expected to affect nutrient runoff from agricultural catchments although the presence of legacy nutrient stores will confound such relationships (Tattari et al., 2017).
Forest management to increase biomass for bioenergy is another pressure with potential effects on nutrient loadings to surface waters (Kreutzweiser, Hazlett, & Gunn, 2008). Increased extraction of forest biomass and intensified forestry have trade-offs with ecosystem services like biodiversity (Eyvindson, Repo, & Monkkonen, 2018) and are likely to also impact water quality (Laudon et al., 2011). Local increases of nitrogen and phosphorus concentrations in forestryimpacted catchments is well-documented Lofgren, Ring, von Bromssen, Sorensen, & Hogbom, 2009) but its impact on a wider temporal and spatial scale is less clear . In areas with intensive peatland forestry like Finland, harvesting operations in forest on organic soils are of specific concern Nieminen et al., 2018). Furthermore, intensified forestry by conducting whole tree harvesting may require application of a fertilizer to avoid reduced forest growth (Akselsson, Westling, Sverdrup, & Gundersen, 2007;Merila et al., 2014). In freshwaters, phosphorus is usually considered to be the limiting nutrient (Schindler, 1977) although more recent studies suggest a role for nitrogen as well (Bergstrom & Jansson, 2006). In marine waters, production typically is limited by nitrogen (Howarth & Marino, 2006). Agriculture and forestry are important pillars of Nordic societies.
Forestry is of particular interest to Sweden and Finland, both of which have a high percentage of productive forest and a large forestry sector (Verkerk et al., 2019). Agriculture is a dominant land use in Denmark and across the Nordic region, nutrient loadings from agriculture is a large concern for eutrophication of freshwater and marine ecosystems (Frigstad et al., 2020;Karlson, Rosenberg, & Bonsdorff, 2002). These waters are valuable natural resources and essential for Nordic societies, economies and human wellbeing as they provide multiple ecosystem services (Kronvang et al., 2008;Marttila et al., 2020;Ulen et al., 2007).
A solid understanding of processes driving catchment export of nitrogen and phosphorus is thus at the basis of predictions of effects of the green shift on water quality. In addition to agricultural and forest management, catchment export of nitrogen and phosphorus can be accelerated by extreme hydrological events (Borgesen & Olesen, 2011;Mellander et al., 2018) despite the measures and efforts made particularly in agriculture to reduce loading. Hydrological conditions are changing in the Nordic region (Oygarden et al., 2014) and are expected to continue to do so in the future (Arheimer & Lindstrom, 2015;Huttunen et al., 2015). Nutrient-runoff mitigation measures at the catchment scale (e.g., buffer zones, artificial wetlands) in a changed climate Haygarth et al., 2012;Laudon et al., 2016) may not be sufficient for adequate protection of water quality.
Small headwater catchments without point sources of nutrients provide an ideal framework to assess land use and climate impacts on nutrient export to surface waters, because nutrient retention in small streams is of lesser importance compared with larger river basins (Weigelhofer, Ramiao, Pitzl, Bondar-Kunze, & O'Keeffe, 2018). Nitrogen deposition from air pollution is a driver of changes in nitrogen runoff in such small catchments (Vuorenmaa et al., 2018), mitigated by international policy to reduce emissions of nitrogen to the atmosphere. Long-term changes in diffuse nutrient fluxes from managed catchments are strongly influenced by a complex combination of temporal and spatial factors, such as fluctuating climatic and hydrological conditions, land cover, soil characteristics, crop cycles and land-use practices in forestry and agriculture Kyllmar, Carlsson, Gustafson, Ulen, & Johnsson, 2006;Tattari et al., 2017;Vuorenmaa, Rekolainen, Lepisto, Kenttamies, & Kauppila, 2002). Similarly, natural catchments are governed by climatic and hydrological conditions (Vuorenmaa et al., 2018) but lack the confounding influence of management. Such catchments provide a reference that enables distinguishing between interacting pressures, that is, intensified agricultural and forestry-related land use, and climate change (Skarbovik et al., 2020). Combined records from natural and managed catchments, from a wide range of environmental gradients and under common management regimes, are therefore potentially useful for assessing water quality responses to environmental change and evaluation of mitigation measures to protect water quality.
Here, we present concentrations and export of total nitrogen (totN) and total phosphorus (totP) and other species of nitrogen and phosphorus, in 69 Nordic headwater catchments for the period 2000-2018. The catchments are representative of Nordic natural (unmanaged) and agricultural landscapes and include also forestryimpacted sites. They cover a wide range of climate, soil type, runoff and management patterns. The primary focus is on totN and totP because all study sites have full records suitable for trend analysis (10 years or more) for these parameters. We test effects of land use, climate, runoff and land cover on spatial variation and temporal trends on concentrations and fluxes of totN and totP.
Because nutrient runoff is potentially sensitive to differences in national mitigation measures (Hellsten et al., 2019;Kronvang et al., 2008;Ulen et al., 2007), we analyse for patterns by country in addition to examining patterns within land-use categories across the Nordic region.

| MATERIALS AND METHODS
Study sites and monitoring programs Data records on water chemistry, discharge and climate were compiled for 69 small catchments in Denmark (n = 12), Finland (n = 18), Norway (n = 17) and Sweden  Table 1). Detailed catchment-specific information is available (Table SI-1). All catchments are included in national monitoring programs, designed to assess longterm effects of air quality, agriculture and forestry on water quality.
The monitoring programs follow standardized national or international procedures for sampling and chemical analysis, including QA-QC procedures. In all countries the analytical programs have changed over time, depending on funding and monitoring priorities. All sites have records of totN and totP for at least 10 full calendar years. Shorter time series were available for some variables (see Table SI-2). All catchments were attributed to a land-use category, that is, agriculture, forestry-impacted and natural.
Discharge was measured daily, using an open channel stagedischarge relationship in Danish streams. V-notch profiles or crump weir with a stage-height relationship were used elsewhere. Water was predominantly sampled by grab sampling, except for agricultural catchments in Sweden and Norway where flow-proportional composite sampling was used Kyllmar, Forsberg, Andersson, & Martensson, 2014b). Sampling frequency varied from weekly to every second week to monthly and was stable throughout the entire monitoring period, with some exceptions as described below. In agricultural catchments, field management is monitored on an annual basis, except in Finland where annual management data are not available from all sites (Tattari et al., 2017). In all catchments, point sources are of minimal importance for annual loading of nitrogen and phosphorus. ). Catchment soils are mainly sand and sandy loams but one agricultural catchment is on clay soil (Svendsen et al., 2005). Subsurface drainage depends on soil texture (rare in sandy soils, common in fine-textured, especially clay soils).
For Finland, six agricultural catchments, eight forestry-impacted and four natural catchments are included (Seuna, 1983). Water discharge monitoring was initiated in 1957 and monitoring of water quality in 1962 (Vuorenmaa et al., 2002). Catchment boundaries are welldefined, and typically groundwater and surface water boundaries coincide. Most arable land in the agricultural catchments is located on graded soils, with high proportions of silt and clay except for one site (Haapajyrä) where acid sulphate soils dominate, related to post-ice age de WIT ET AL. land uplift of marine sediments, which leach considerable amounts of sulphuric acids. Crop cultivation includes mostly cereals and root crops.
The nine Norwegian agricultural catchments are monitored under the Norwegian Agricultural Environmental Monitoring program (JOVA) (Bechmann et al., 2008). Monitoring at all but one catchment started between 1990 and 1995. The catchments represent the main farming systems in Norway; cereal production in the eastern and middle parts of the country, vegetable production in the south, intensive dairy farming in the west, and more extensive grass production in the southern mountains and in northern Norway (Wenng, Bechmann, Krogstad, & Skarbøvik, 2020). Soil texture varies from clay loam (dominated by surface runoff), loam and sand F I G U R E 1 Map of study catchments in the Nordic countries. The land cover information bases on CORINE land cover information (https:// land.copernicus.eu/pan-european/corine-land-cover). Colour coding refers to land use category T A B L E 1 Key site characteristics, grouped by country and land use category, presented as median (minimum-maximum) values Abbreviations: MAP, mean annual precipitation; MAQ, mean annual discharge; MAT, mean annual temperature.
de WIT ET AL.
The seven natural catchments in Norway are part of the national program for monitoring effects of long-range transported air pollution (Garmo & Skancke, 2018 Eight of the 10 Swedish agricultural catchments are from the Swedish national agricultural monitoring program, and two are from a less intensively monitored regional program (Kyllmar et al., 2014a).
The monitored catchments were established between 1988 and 1996. All catchments have a large proportion of agricultural land, most of which is tile drained. Catchments cover the main variations in climate and geological characteristics in Sweden and hence in agricultural production (Kyllmar et al., 2014b). In south-west Sweden, catchments have sandy loam soils and are characterized by intensive crop production including cereals, rape seed, potatoes and vegetables.
In the southern inland highlands, where precipitation is higher and soils are coarse, grass and dairy production are typical. Swedish catchments with clay soils, mainly located in the central agricultural areas, are characterized by production of cereals and rape and a low number Forested Ecosystems (IM), established in the late 1980s and mid-1990s (Vuorenmaa et al., 2018). Six catchments are included the PMK5 longterm monitoring program (Folster & Wilander, 2002), one is part of the Krycklan Catchment Study (Laudon et al., 2013) and one is included in the Integrated Studies of the Effects of Liming Acidified Waters, ISELAW program (Appelberg, Lingdell, & Andren, 1995) where liming took place around 1990. Catchments are hydrologically well-defined and several of the catchments include a small lake or pond (0-13% water). All Swedish forested catchments are dominated by coniferous stands and the granitoid bedrock is covered with till-soils interspersed with mires and small lakes.

| Water chemical data and analytical methods
Common variables in all monitoring programs were totN and totP. In the natural catchments, monitoring programs also included nitrate (NO 3 ), ammonium (NH 4 ) (allowing computation of total organic N, TON) and total organic carbon (TOC). In Finnish catchments and some Danish natural catchments, dissolved reactive phosphorus (DRP) and suspended solids (SS) were included. In agricultural catchments, NO 3 was most often included in addition to DRP and SS. All monitoring programs used accredited laboratories and standardized analytical programs (Bechmann et al., 2008;Folster et al., 2014;Garmo & Skancke, 2018;Kortelainen et al., 2006;Kyllmar et al., 2014a;Pengerud et al., 2015).  Median concentrations of totN and totP declined in the following order: agriculture (totN 4.2 mg L −1 ; totP 0.14 mg L −1 ) > forestry (totN 0.6 mg L −1 ; totP 0.02 mg L −1 ) > natural (totN 0.28 mg L −1 ; totP 0.007 mg L −1 ) ( Table 1). Ranges in concentrations of totN and totP overlapped between countries, with a tendency towards highest totP in Sweden and Norway in the agricultural catchments and the lowest totP in natural Norwegian catchments (Figure 2). For totN, countries were more similar than for totP. In Denmark, ranges in totP and totN of agricultural and natural catchments were more similar than in other Nordic countries.

| Calculation of fluxes
In all land-use categories, NH 4 made up a very small part of totN concentrations (<5%) ( data not shown) and negatively with TOC (p = .053), suggesting that totP here was particle-bound rather than of organic origin. Where DRP was measured in natural catchments (14 sites), it made up 18-42% of totP, which indicates that a considerable part of totP was bio-available. The DRP fraction is traditionally thought to identify inorganic reactive fractions of P but may also include labile organic fractions (e.g., Haygarth & Sharpley, 2000). SS were correlated with totP in agricultural catchments (data not shown, r 2 = 0.38, p < .001), indicating that a considerable part of totP was particle-bound. Natural catchments had less variation in soil type than agricultural catchments (SI Table 1), that is, they were dominated by moraine soils while the agricultural sites had clay, loam, peat and sandy soils. Grouping of sites by management and soil type did not result in a better explanation of factors driving N and P export (data not shown).   regional Mann-Kendall test (Table 3). The sen-slopes in Table 3  With regard to fluxes, the regional M-K results (Table 4)  Thus, the regional M-K results indicated downward trends in the Nordic countries in N-species across land use categories, downward trends in concentrations of P-species in natural and forestry-impacted sites, and increases in totP export from agricultural catchments.

| Trends in concentrations and fluxes
In a correlation matrix, we tested if the trends in concentrations and fluxes of N and P species could be related to trends in climate (temperature, precipitation) and hydrology (discharge) within each land use category but except for forestry-impacted sites, few significant correlations were found ( Figure SI-3). In the forestry-impacted sites (n = 8), trends in runoff correlated negatively with trends in nutrient species, but its significance largely depended on one site and we do not regard this result as particularly robust. Thus, regional patterns of change in totN and totP concentrations for 2000 to 2018 could not be explained with simple relationships with climate and runoff.  (Blann, Anderson, Sands, & Vondracek, 2009;Pengerud et al., 2015), forestry-impacted (Kreutzweiser et al., 2008) or natural catchments (Vuorenmaa et al., 2018). Significant proportions of nutrient loadings from Nordic countries to marine recipients originate from agriculture, natural and semi-natural ecosystems (HELCOM, 2018;Lepisto, Granlund, Kortelainen, & Raike, 2006) suggesting that changes in nutrient runoff from both managed and unmanaged ecosystems is pertinent to the ecological status of receiving waters.
We found that long-term averaged nutrient concentrations and export were an order of magnitude higher from agricultural catchments compared with forestry-impacted and natural catchments, and that forestry-impacted catchment delivered significantly more nutrients than natural catchments. The high nutrient export from agricultural catchments is primarily driven by long-term surpluses of N and P, as indicated by statistics on gross nutrient balances (calculated from inputs of manure and fertilizer and removal from harvest) for agricultural land which vary roughly between 30 and 120 kg ha year −1 for N, and 0 to 12 kg ha year −1 for P in the Nordic countries after 2000 (Eurostat, 2020). In natural catchments, the main source of nutrient loading is atmospheric deposition, typically between 1 and 10 kg N ha −1 year −1 (Vuorenmaa et al., 2017) and usually retained for 90% within the catchment (Vuorenmaa et al., 2017;Watmough et al., 2005). In some agricultural catchments, losses of nitrogen may be close to a steady-state between inputs and outputs Thompson, Basu, Lascurain, Aubeneau, & Rao, 2011). However, catchment characteristics such as tile drainage, topography, texture and mitigation measures to reduce nutrient runoff will control the fate of the nutrient excess, that is, runoff, groundwater or soil storage (Hellsten et al., 2019;Kronvang et al., 2005a;Kronvang, Vagstad, Behrendt, Bogestrand, & Larsen, 2007). Forest management typically consists of a mosaic of numerous treatments (harvesting, drainage, fertilization, soil tillage) with considerable temporal and spatial variations (Ahtiainen & Huttunen, 1999;Kreutzweiser et al., 2008;Tattari et al., 2017). In forestry-impacted catchments, elevated nutrient runoff compared with natural catchments can be related both to application of fertilizer and to mobilization of soil nutrient stores.
When considering all land use categories simultaneously, we found that spatial variations in site-specific totN and totP concentrations were largely driven by a land-use gradient which partly overlapped with a climate gradient, demonstrating that climate and land use are confounded. The highest nutrient concentrations were associated with warmer, agricultural regions located in the south. Natural land cover types (i.e., forests, peatlands, lakes and shrublands) with lower nutrient concentrations were associated with cooler and wetter conditions. Agricultural land cover was the single-most powerful explanation for describing spatial variation in nutrient concentrations and can be considered as a proxy for gross nutrient balances as discussed earlier. Consistent with our study, totN and totP concentrations in Norwegian lakes, including natural and agriculturally impacted systems, were found to be positively related to terrestrial productivity and negatively to runoff (Hessen, Andersen, Larsen, Skjelkvale, & de Wit, 2009). Hessen and co-authors also highlighted nitrogen deposition as a strong driver of aquatic concentrations of N-species. This factor is likely to be most easily detectable in catchments with low nitrogen retention capacity, that is, with little soil and vegetation cover (Kaste, Austnes, & de Wit, 2020).
The positive correlations between spatial variation in totN concentrations and summer temperature in the natural and forestryimpacted catchments were mirrored by positive correlations between TON and summer temperature, and TOC and summer temperature.
This suggests these correlations are a demonstration of the strong T A B L E 3 Results of regional Mann-Kendal test for concentrations of totN, NO 3 (in μg N L −1 year −1 ), totP and DRP (in μg P L −1 year −1 ), grouped by agricultural (AGR), Forestry-impacted (FOR), Natural NAT) sites and country Note: Significance levels: *<.05, **<.01, ***<.001, n.s., p > . 05. n.d., no data. links between the element cycles of nitrogen and carbon in forested ecosystems (Mattsson, Kortelainen, & Raike, 2005). Surface water concentrations of dissolved organic matter are highest in carbon-rich catchments (Sobek, Tranvik, Prairie, Kortelainen, & Cole, 2007), which are found in Nordic regions with higher average temperatures (Callesen et al., 2003). Temperature, particularly during summer season, can be interpreted as a proxy for terrestrial productivity at least in climate where moisture is mostly not a limiting factor (Piao et al., 2011). By contrast, significant relationships between nutrient concentrations and summer temperature were not found in agricultural catchments, suggesting that combined effects of crop, management practices and soil type are stronger controls on element cycling than temperature in these systems (Bechmann et al., 2008).
Danish natural catchments had three to five times higher nutrient concentrations than other Nordic natural catchments, which could be indicative of profound differences in natural reference conditions (Skarbovik et al., 2020). The EU WFD (EC, 2000) defines reference conditions as "water bodies with no, or only very minor, anthropogenic alterations compared with conditions normally associated with undisturbed conditions". However, legacies of former land use (Hamilton, 2012) combined with lateral groundwater flow (Brunke & Gonser, 1997) in flat landscapes complicate interpretations of catchment effects on streamwaters. Geology, soils, climate and land use history in Denmark contrast with other Nordic countries (Emanuelsson, 2009). Deeper, sandy soils which predominate in the flat Danish landscape are probably associated with a relatively larger proportion of precipitation feeding groundwater, while the Fennoscandian shield is characterized by thinner soils, greater relief and more superficial hydrological pathways. Most loamy and clayey agricultural soils in Denmark are tile-drained which directs part of the precipitation surplus directly to surface waters (Møller, Børgesen, Bach, Iversen, & Moeslund, 2018). Skarbovik et al. (2020) also suggest that bank erosion may be more important in Danish than in other Nordic streams. In Norway and Sweden, agricultural soils are often tiledrained, especially clayey soils with low hydraulic conductivity that are more exposed to the risk of surface runoff. Steeper slopes in Norway also contributes to higher erosion and P export (Bechmann et al., 2008). In addition, higher temperatures and longer growing seasons make evaporative losses relatively more important in the hydrological cycle in southern parts of the Nordic countries (Kortelainen, Saukkonen, & Mattsson, 1997). The different composition of totN and totP species in agricultural and natural sites suggests a wide variation in susceptibility to hydrological and management impacts on their transport and leaching, for example, diffuse and particulate transport, and transport of nutrients in inorganic versus organic forms. Variation in soil type and texture was higher in agricultural catchments, with clay, loam, peat and sandy soils than in (semi-) natural catchments which were dominated by moraine soils.
We found a general decline in concentrations and fluxes of total nitrogen and NO 3 from agricultural and natural catchments in the Nordic region as a whole, for the period 2000-2018. These downward trends for agricultural catchments agree with the downward trends found in the gross N budget for agricultural land during the period 2000-2016, which is highest for Sweden (−27%) and lowest for Norway (−3%), with Denmark (−15%) and Finland (−14%) being in between (Eurostat, 2020). However, the reductions in both gross N budget and stream concentrations and fluxes of N were substantially higher during the 1990s in Denmark, Finland and Sweden (Hellsten et al., 2019;Windolf, Blicher-Mathiesen, Carstensen, & Kronvang, 2012). The reduction in gross N budget was driven by intensive mitigation campaigns to minimize N losses, especially in Denmark (Hansen, Thorling, Schullehner, Termansen, & Dalgaard, 2017;Kronvang et al., 2005b) and Sweden, where catch crop and spring ploughing were implemented .
The totN load to 10 estuaries (catchments covering 35% of the Danish land area) decreased by 39% during the period 1990-2009 (Windolf et al., 2012) following mandatory national regulations on agricultural production (Kronvang et al., 2005b). Agricultural extensification in two Norwegian sites reduced totN export here, although there was little national focus on mitigation measures to reduce nitrogen losses (Hellsten et al., 2019). Earlier studies demonstrated a predominance of declines in nitrogen export from agricultural headwater catchments in Denmark and Sweden, but lack of change in Norway (Kyllmar et al., 2014a;Stålnacke et al., 2014).

Changes in fertilizer application explained downward totN trends in
Finnish agricultural catchments while upward trends were related to crop distribution (Tattari et al., 2017).
Declines in NO 3 concentrations in natural catchments dominated over change in totN. In natural catchments, most totN consists of organic N and the dynamics of organic N are closely linked to those of DOM, which is usually elevated during the summer and autumn (Lepisto, Kortelainen, & Mattsson, 2008) whereas NO 3 is highest during the dormant season  and likely to be more strongly linked to atmospheric deposition (Vuorenmaa et al., 2018).
The widespread decline in annual totN concentrations in agricultural and natural sites could not be explained by simple relationships with climate or runoff. Spatial variation in totN export was strongly related to annual runoff in agricultural catchments, however, and implies that increases in runoff could lead to increased element export (e.g., Oygarden et al., 2014). It is likely that investigations of climatic and hydrological impacts on water quality and element export would benefit from a focus on seasonal trends and/or from including a longer time period Jeppesen et al., 2009;Jeppesen et al., 2011;Wenng et al., 2020).
Given the profound contrasts in N concentrations between land use categories, and the lack of correlations between trends in N species and trends in climatic and hydrological variables, it is likely that the decline in N species is the concerted effect of various interplaying factors, including environmental policy. Other studies document longterm declines in reactive nitrogen from unmanaged and managed Nordic landscapes (Garmo et al., 2014;Rekolainen, Mitikka, Vuorenmaa, & Johansson, 2005) Kreutzweiser et al., 2008;Nieminen et al., 2018), but the duration of increased nutrient export following disturbance is usually short relative to the forest rotation cycle . Furthermore, appropriate, context-sensitive (Ring et al., 2017) forest management can safeguard water quality (Sundnes et al., 2020;Sponseller et al., 2016).
Patterns of long-term change in totP concentrations and export were more varied temporally and spatially than for nitrogen; Denmark and Finland had significant declines while Sweden and Norway had significant increases in DRP, resulting in an overall neutral trend for all agricultural catchments (Kronvang, Tornbjerg, Hoffmann, Poulsen, & Windolf, 2016). To what extent climate can explain the patterns of long-term change in P export is unclear. Management of agricultural P-losses is complicated by increased precipitation intensity with subsequent increased erosion (Farkas, Beldring, Bechmann, & Deelstra, 2013) combined with a strong legacy of soil-P in the fields (Sharpley et al., 2013). Effects of mitigation of phosphorus sources might thus take several decades to document in monitoring programs (Bol et al., 2018;Mellander et al., 2018). In Norway, agricultural mitigation measures focused primarily on phosphorus, however, measures implemented to reduce totP export were changed in 2013 from general measures to measures only for high risk areas and increasing focus on production (Bechmann, Greipsland, & Falk Øgaard, 2019).
Removal of subsidies to abstain from autumn tillage, followed by an increase in autumn ploughing and erosion appears to be an explanation for increases in totP export in two Norwegian catchments . Hardly any sign of significant change was found in the forestry-impacted catchments except for reduced P concentrations likely related to reductions in P fertilizing (Tattari et al., 2017). There was a small overall decline in totP for natural catchments, primarily related Finland and Sweden. Huser, Futter, Wang, and Folster (2018) suggest that the totP-decline in Swedish lakes could be a combined effect of climate change and increased uptake of phosphorus by forests, similar to the explanation provided for declining NO 3 runoff in northern Swedish rivers (Lucas et al., 2016). Increased forest growth may explain trends in Finnish natural catchments while the lack of change in totP in Norwegian natural catchments could be related to a lower proportion of forest in these catchments. The significant decline in P export from Finnish forestry-impacted catchments was, however, attributed to reduced fertilizer use.
The lack of widespread reductions in totP export is substantiated by long-term records from rivers in Finland (Räike et al., 2020) and Norway (Skarbovik, Stalnacke, Kaste, & Austnes, 2014), and from marine recipients (Frigstad et al., 2020;Kuss et al., 2020). Our study implies that mitigation of P is more challenging than for N, because of more variation in sources (Kronvang et al., 2007), complexity of hydrological pathways  and delays in responses to mitigation as a consequence of legacy pools of phosphorus in soils (Bol et al., 2018;Jeppesen et al., 2009) and lake sediments (Couture et al., 2018).
Nutrient export from agricultural, forestry-impacted and natural ecosystems, in declining order of importance, is a strong control of freshwater and marine ecological status in the Nordic countries. Mitigation measure have been effective especially for reduction of nitrogen runoff, but the effect of mitigation can be counteracted by climate change (Crossman et al., 2013). If the green shift will be associated with intensification of agricultural and forest production and increased use of fertilizer, this will pose new challenges for protection of water quality . Long-term monitoring records of small headwaters under varied combinations of land use, climate and land cover are valuable and necessary for assessing combined effects of stressors on water quality and nutrient cycling and retention at the landscape level. We recommend sustained funding of longterm monitoring of managed and unmanaged, natural catchments.
Further analysis should consider (a) further inclusion of nutrient species (e.g., nitrate, particulate P, organic forms, and so forth) for investigation of possible contrasting responses to climate, runoff and mitigation and their impacts on aquatic ecology, (b) examine long-term patterns in seasonal variation, (c) incorporate information about nutrient inputs (including atmospheric deposition), and agricultural and forestry management.