Climate As a Confounding Factor in the Response of Surface Water to Nitrogen Deposition in an Area South of the Alps

Applied Geochemistry 22 (2007) 1122–1128 www.elsevier.com/locate/apgeochem

Climate as a confounding factor in the response of surface water to nitrogen deposition in an area south of the Alps

Michela Rogora ¤, Rosario Mosello

CNR Institute of Ecosystem Study, L.go Tonolli 50, I 28922 Verbania Pallanza, Italy

Available online 18 March 2007

Abstract

V Climate e ects on NO3 concentrations have been investigated for two small rivers south of the Alps draining forested catchments. The Pellino and Cannobino rivers are representative of diVerent stages of N saturation determined by high deposition of inorganic N (2.0–2.5 g N m¡2 a¡1). Long-term records of air temperature, precipitation, N deposition and V stream NO3 concentration were used to assess the relative e ect of N deposition and climate on NO3 export from the catchments. The climate signal was more evident in the river with the lower NO3 concentration. Prolonged dry and warm periods were the precondition for the occurrence of the highest peaks of NO3 concentration. © 2007 Elsevier Ltd. All rights reserved.

1. Introduction highest values in the South, close to the major emis- sion sources (2.5–3.0 g N m¡2 a¡1 as the sum of N– The subalpine areas of north-western Italy are NH4 and N–NO3) and decreasing deposition subject to high deposition of atmospheric pollu- towards the Alps (1.6–1.7 g N m¡2 a¡1) (Mosello tants. These mainly come from the South, where the et al., 2001). This huge Xux of N is causing satura- Po Plain, one of the most industrialised parts of tion of terrestrial catchments by N especially in the Italy, is located (Fig. 1). Atmospheric deposition of southern part of the area and NO3 enrichment of N compounds and the N content in surface water surface water (Rogora et al., 2001; Wright et al., have been monitored in the area of Lake Maggiore 2001). Nitrate concentrations are increasing in sev- catchment since the 1970s in the framework of eral rivers and lakes, and in Lake Maggiore itself national and international programs on acidiWcat- (Mosello et al., 2001). ion and N enrichment of surface water, such as the In addition to the atmospheric input of N, cli- ICP Waters (Mosello et al., 2000). Deposition of mate warming also plays a role in the long-term var- inorganic N (N–NH4 +N–NO3) in this area has not iation of NO3 in surface water. Several studies have changed signiWcantly in the last two decades emphasized the need to consider climatic variations (Mosello et al., 2001). The deposition of N com- when evaluating the response of forested ecosystems pounds shows a North–South gradient, with the to elevated atmospheric input of N (Mitchell et al., 1996; Murdoch et al., 1998). Both rates of N miner- W * Corresponding author. alisation and nitri cation are sensitive to changes in E-mail address: [email protected] (M. Rogora). temperature and moisture (Stark and Hart, 1997).

Lake Como Lake MILAN Garda

TURIN River Po River Po

River Cannobino

Pallanza LakeMaggiore

Lake Orta

River Pellino

Fig. 1. Location of the River Cannobino and River Pellino catchments and atmospheric deposition sampling sites (triangles). The upper right panel shows the location of the study area in Northern Italy.

Climate change, particularly increasing temperature, in relation to the temperature and precipitation could aVect biological processes, leading to regime of each year. increased release of N to surface waters in excess of that taken up by plants or immobilised in the soil 2. Study area and methods (Wright, 1998). Long-term records of air temperature, precipita- Two rivers were considered in this study: the Pel- tion, N deposition and stream NO3 concentration lino is a tributary of Lake Orta, while River Canno- were used in this paper to assess the relative eVect of bino, running further North, is a tributary of Lake N deposition and climate on NO3 export. The long- Maggiore (Fig. 1; Table 1). Both catchments are term trend and seasonality were assessed in detail sparsely populated and do not include any intensive for two rivers draining forested catchments. The industrial, stock-rearing or agricultural activity. The long-term trend of NO3 concentrations in these riv- morphological characteristics of the area do not per- ers has already been assessed in Wright et al. (2001). mit extensive agriculture, so that the use of N fertil- Here an additional 4 a of data are included (2000– izers is negligible (Boggero et al., 1996). 2003) and the relationships between climate drivers The River Cannobino is longer and has a larger (temperature and precipitation) and N levels in the catchment (110 km2) compared to the River Pellino two rivers focused on. Input/output N budgets were (17.5 km2) (Fig. 1; Table 1). Anthropogenic pressure performed on an annual basis and results evaluated in the River Cannobino catchment is very low (mean 1124 M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128

Table 1 cal quality for each analysis a comparison between Selected characteristics of the rivers considered and their catch- the sum of anions and cations and between mea- ments sured and calculated conductivity was performed. River River Further quality assurance measures involved the use Pellino Cannobino of control charts and the analysis of synthetic sam- Latitude N 45° 47Ј 46°04Ј ples on a regular basis. Participation in several inter- Ј Ј Longitude E 08° 04 08°42 laboratory comparisons on freshwater and rain Minimum altitude (m a.s.l.) 290 193 Maximum altitude (m a.s.l.) 942 2193 water analysis allowed a further quality check of the Mean slope (%) 5.6 7.4 results (Mosello et al., 1998). Length (km) 11.7 27.0 Nitrogen budgets were calculated on an annual Catchment area (km2) 17.5 110.4 basis for the period 1984–2003. The atmospheric ¡1 Average precipitation (m a ) 1.6–1.7 2.1–2.2 inputs of N to the river catchments were obtained by Annual catchment 1.65 1.52 discharge (m a¡1) interpolating chemical data of atmospheric deposi- Soil C/N ratio 16.6 15.3 tion collected at some sites in the study area by Soil base saturation (%) 17.9 14.7 means of wet-only samplers (Fig. 1) and precipitation volume, as described in Rogora et al. (2001). Dry deposition of N is important in the study area and may represent from 15% to 30% of total deposi- density is 9 inhabitants km¡2) and concentrated tion. To take dry deposition into account, the per- along the shoreline of Lake Maggiore. The catch- centage of each catchment covered by forest, ment has a steep altitudinal gradient with the highest agricultural land and lakes, rocks and settlements portion reaching 2000 m a.s.l. (Table 1). Vegetation was used to calculate a Wxed ratio between dry and occupies more of the catchment as the altitude rises, wet deposition of N. Output Xuxes of total N were with deciduous and coniferous forest covering about calculated from the concentration in each sample 77% and 3% of the total surface, respectively. Gneis- and the discharge measured on the sampling day ses, micaschists and paragneiss are very common, but (Rogora et al., 2001). in the upper part of the catchment basic rocks are Meteorological data (temperature and precipita- also present. The River Pellino catchment lies at a tion) were taken from the long-term daily records lower altitude (below 1000 m a.s.l.) and is more popu- available for Pallanza. Monthly maximum (Tmax), ¡2 lated (about 140 inhabitants km ). Forest covers minimum (Tmin) and mean temperature (Tmean) as 66% of the catchment area, and broadleaves domi- well as monthly precipitation amount (P) were con- nate over coniferous species. The catchment is made sidered. up mainly of granitic and granodioritic rocks, with River chemical data, as well as temperature and orthogneiss and micaschists (Boggero et al., 1996). precipitation were tested for trends by applying the Soils are acidic in both catchments, and have low Seasonal Kendall Test (SKT) to monthly blocks of base saturation and low C:N (Table 1). data. Trend slopes were calculated according to Sen, The two rivers have been sampled monthly since as described in Evans et al. (2001). Seasonality of N 1972 (Cannobino; since 1978 for total and organic concentrations in river water was tested by the N) and 1984 (Pellino). Water discharge was mea- Kruskall Wallis test (KW). Correlations between sured daily in the River Cannobino starting in 1978. variables were tested by the Spearman r correlation In the case of the River Pellino, the discharge has coeYcient. For the analysis of climate eVects, sea- not been directly measured but calculated as a Wxed sonal decomposition by Loess smoothing was percentage of the daily value measured for the outlet applied to N–NO3 and meteorological data (Cleve- of Lake Orta. land et al., 1990). All statistical analyses were per- River samples were analysed for N–NH4 by spec- formed with S-plus 2000 (Math Soft). trophotometry (indophenol blue) and N–NO3 by ion chromatography. Alkalinity was determined by 3. Results acidimetric titration using the Gran method. Water samples were also analysed for total N by spectro- 3.1. River chemistry photometry (persulphate digestion). Organic N was calculated as the diVerence between total N and The two rivers considered are characterised by inorganic N (N–NH4 +N–NO3). To check analyti- low solute concentrations (conductivity between 42 M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128 1125 and 47 Scm¡1 at 20 °C). River waters have a good Table 2 neutralizing capacity, as is shown by mean alkalinity Results of trend analysis of monthly mean values of temperature, values of 0.19–0.20 meq L¡1. The main diVerence precipitation and N compound concentrations in the Rivers Cannobino and Pellino between them is the level of NO3, which represent the main fraction of N compounds in these river Period p B T 1972–2003 ¤¤¤ 0.07 waters. Mean NO3 concentrations in River Canno- max bino in 2000–2003 (0.80 mg N L¡1) were about a half 1984–2003 ¤¤¤ 0.08 ¡1 Tmin 1972–2003 ¤¤¤ 0.05 of those measured in River Pellino (1.70 mg N L ). 1984–2003 ¤¤¤ 0.07 Nitrogen–NH4 concentrations were almost negligi- Precipitation 1972–2003 n.s. – ble (below 0.01 mg N L¡1). Organic N varies between 1984–2003 n.s. – ¡1 0.10 and 0.20 mg N L in both rivers, representing River Cannobino 11% and 15% of total N in River Cannobino and Total N 1978–2003 ¤¤¤ 0.0050 River Pellino, respectively. 1984–2003 ¤¤ 0.0050 ¤¤¤ According to Traaen and Stoddard (1995), the N–NO3 1972–2003 0.0081 1984–2003 ¤¤¤ 0.0059 Rivers Cannobino and Pellino are classiWed as levels N–NH4 1972–2003 ¤¤¤ ¡0.0006 2 and 3 of N saturation, respectively. A seasonal pat- 1984–2003 ¤¤ ¡0.0002 tern can be observed in River Cannobino (p < 0.001 Organic N 1978–2003 ¤¤ ¡0.0019 according to the KW test), with maximum concen- 1984–2003 ¤¡0.0004 trations in the winter period and minima during the River Pellino growing season. This means that N dynamics are Total N 1984–2003 ¤¤¤ 0.0272 still under the control of biological uptake in the N–NO3 1984–2003 ¤¤¤ 0.0291 ¤¤¤ ¡ Cannobino catchment. In contrast, seasonality of N–NH4 1984–2003 0.0005 Organic N 1984–2003 ¤¤ ¡0.0029 both NO and TN concentrations was almost absent 3 ¡1 ¡1 ¡1 in the River Pellino, conWrming N saturation of this B: trend slopes. Unit: °C a for temperature data; mg N L a for N compounds. SigniWcance level: ¤p <0.05; ¤¤p <0.01; catchment. Previous analysis performed on monthly ¤¤¤p < 0.001. data of N–NO3 in these rivers showed that seasonal variations have become less and less evident in slope of 0.029 mg N L¡1 a¡1 (Fig. 2; Table 2). The cor- recent years, conWrming the aggrading level of N responding increase in the River Cannobino was saturation (Rogora et al., 2001). from about 0.6 mg N L¡1 to 0.8 mg N L¡1 (Sen’s slope 0.006 mg N L¡1 a¡1) (Fig. 2; Table 2). The two rivers 3.2. Long-term trends showed quite distinct patterns of N–NO3 concentration in time (Fig. 2). After 1997, N–NO3 steeply In order to relate trends to each other, trend anal- increased in the River Pellino, reaching a maximum ysis for River Cannobino data and meteorological in the autumn of 2000. In the River Cannobino the data was performed both for the entire record increase of NO3 was gradual, but ended with a sharp (1972–2003) and for the period covered by River peak in September 2003 (1.76 mg N L¡1), which had Pellino data (1984–2003) (Table 2). Meteorological no equal over the whole record. This peak had no data collected at Pallanza showed no trend for pre- correspondence in the River Pellino, where the mea- cipitation, and signiWcant positive trends for temper- sured NO3 concentration was similar to those ature. The overall increases of minimum and recorded in the autumns of previous years (Fig. 2). maximum temperature in the period 1972–2003 The anomalous N–NO3 peak in the River Canno- were 1.75 and 2.20 °C, respectively. The temperature bino in 2003 was related to a prolonged dry and hot increase accelerated in the most recent period, as period, with a mean temperature (24.6 °C in May– shown by the steeper slope of the trend in the August) which was sharply above the long-term shorter period (Table 2). average for the same months (19.8 °C). The relative importance of N–NO3 in the N bud- V get of the two rivers has increased in time. Nitrate 3.3. Climate e ects on N–NO3 concentrations and concentrations have increased signiWcantly, while exports W both N–NH4 and organic N have decreased signi - cantly (Table 2). Nitrogen–NO3 in the River Pellino The relationships between meteorological data ¡1 increased from 1.1–1.2 mg N L in the mid of the and N–NO3 concentrations in river water were 1980s to 1.6–1.7 mg N L¡1 in 2000–2003, with a trend investigated using both monthly and yearly values 1126 M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128

2.4

2.0 River Pellino

1.6 ) -1

1.2 (mg L 3

River Cannobino N-NO 0.8

0.4

0.0 1970 1975 1980 1985 1990 1995 2000 2005

Fig. 2. Trends of monthly N–NO3 concentrations in the Rivers Pellino (above) and Cannobino (below). Thick lines are 12-point running averages. of T , T , T and P. Mean monthly tempera- min max mean a g N m-2 a-1 tures and precipitation of one and two months 6.0 Input (inorganic N; wet + dry) before the river sampling were also considered. River Pellino Output (total N) No signiWcant correlations were found between 5.0 meteorological variables and N–NO3 in the River 4.0 Pellino. The N–NO3 pattern in this river is much more controlled by the atmospheric input of N than 3.0 W by climatic factors. This was con rmed by the 2.0 results of the N budget performed on an annual basis (Fig. 3). The N exports from the catchment are 1.0 related to the total N input (wet + dry) from atmo- 0.0 spheric deposition, even though a signiWcant rela- 1983 1986 1989 1992 1995 1998 2001 2004 tionship was found only when considering the b g N m-2 a-1 period between 1984 and 1999 (p <0.05; r D 0.59). 6.0 An aggrading condition of N saturation can be River Cannobino observed in recent years, with N exports equal to or 5.0 above the inputs in 1999, 2001 and 2003 (Fig. 3). 4.0 In the River Cannobino N–NO3 export is not related to atmospheric N inputs (Fig. 3). The outputs 3.0 of N were always below the inputs. A very low atmo- 2.0 spheric input was observed in 2003, due to reduced precipitation (1130mm compared to a long-term 1.0 mean value of 2150 mm). The climate signal appeared 0.0 in both the seasonal and annual patterns of N–NO3 1983 1986 1989 1992 1995 1998 2001 2004 W concentration in the River Cannobino. A signi cant Fig. 3. Yearly N inputs and outputs for the River Pellino (a) and correlation was found between de-seasonalised data River Cannobino (b) catchments. of N–NO3 and monthly temperature (rD0.12; W p< 0.05). Highly signi cant negative correlations were Seasonally-adjusted time series of Tmax, P and N– also found between N–NO3 and precipitation NO3 are compared in Fig. 4 after normalisation of recorded one (rD¡0.24; p<0.001) or two (rD¡0.25; the data. Both Tmax and N–NO3 series showed the p< 0.001) months before the river sampling. steepest increase in the periods 1980–1990 and M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128 1127

-1

-2

-3 N-NO3 Tmax P

-4 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006

Fig. 4. Seasonally-adjusted and normalised time series of maximum temperature (Tmax), precipitation (P) and N–NO3 concentrations in the River Cannobino.

1994–2003. The eVect of meteorological factors on ecosystem demand. In the North, catchments are N–NO3 concentrations was mainly evident in the mainly at a medium stage of N saturation, and N most recent period (1994–2003). Spring and summer retention in terrestrial and aquatic ecosystems is still temperatures above the long-term mean values for able to cope with N inputs (Mosello et al., 2001). this area have often been recorded in the last 10 a. The River Pellino catchment shows clear signs of Some of these periods were followed by relative N saturation. Nitrogen export is no longer controlled V maxima in N–NO3 concentrations (e.g. 1994, 1997, by biological processes, and seems una ected by cli- 2000, 2002, 2003) (Fig. 4). However, temperature mate, both when considering episodic events or long- alone was not suYcient to explain the variability in term trends. On the other hand climate played an N–NO3 data. The precipitation regime partly deter- important role in the response of the River Canno- mined the N–NO3 pattern in river water. For exam- bino catchment to atmospheric N inputs. Both tem- ple, the N–NO3 peaks recorded in 2000, 2002 and perature and precipitation were important in 2003 were preceded by warm but also relatively dry determining seasonal and long-term variation in N– periods. On the other hand, high precipitation NO3 concentrations. In particular, prolonged dry and explained some of the relative minima in the N–NO3 warm periods have been the precondition for the series, e.g. 1977, 1992–1993. In these cases the dilu- occurrence of N–NO3 peaks in the last few years of tion eVect due to heavy rainfall was the main cause the record (2000, 2002, 2003). It has been observed for the temporary decrease in N–NO3 concentra- that extended periods of drought are frequently fol- tions (Fig. 4). lowed by many years of high N–NO3 export (Mur- doch et al., 2000). These elevated losses may reduce 4. Discussion and conclusions the N availability in the soil, and lead to low N levels in the following years (Aber and Driscoll, 1997). The two rivers considered in this study showed According to this hypothesis, the high N export from V distinctly di erent patterns of N–NO3 concentra- River Cannobino catchment in 1989–1990 could tions, both at a seasonal level and in the long-term. explain the temporary decrease of N–NO3 levels in They are representative of surface water status in the early 1990s. Low N–NO3 levels can also be the subalpine areas of north-western Italy with explained by elevated precipitation: it was observed regards to N inputs: N saturation prevailed in lake that wet periods determined relative minima in N– V and river catchments in the southernmost part of NO3 values by a dilution e ect. The strongest evi- this area, where N inputs are clearly above the dence of this was in 1977 when precipitation amount 1128 M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128 far above the mean values for this area caused a sharp 2001. Recovery from acidiWcation in European surface waters. ¡1 Hydrol. Earth Syst. Sci. 5, 283–297. decline of N–NO3 concentrations (0.2–0.3 mgN L ). These Wndings suggest that rates of N mineralisa- IPCC, 2001. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, W X M., van der Linden, P.J., Xiaosu, D. (Eds.), Climate Change tion and nitri cation rather than deposition uxes 2001: The ScientiWc Basis. Contribution of Working Group I are the main factors controlling N–NO3 leaching in to the Third Assessment Report of the Intergovernmental the River Cannobino catchment. Changes over time Panel on Climate Change (IPCC). 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Climatic control of nitrate losses from for- last few years (2000–2003) (Rogora et al., 2004). ested watersheds in the Northeast United States. Environ. Sci. Most regional climate models predict a further Technol. 30, 2609–2612. increase of temperature and a higher frequency of Mosello, R., Bianchi, M., Geiss, H., Marchetto, A., Serrini, G., extreme events such as drought and heavy rainfall in Serrini Lanza, G., Tartari, G.A., Muntau, H., 1998. AQUA- CON-MedBas Subproject No 5. Freshwater analysis. Inter- the next decades (IPCC, 2001). These variations may comparison 1/97. Joint Res. Centre European Commission, exacerbate the eVect of elevated N deposition Rep. EUR 18075 EN. Mosello, R., Marchetto, A., Brizzio, M.C., Rogora, M., Tartari, increasing N–NO3 leaching to surface waters. Studies on the climate–chemistry relationship at the catch- G.A., 2000. 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