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 catch- ments. 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). 0883-2927/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.03.003 M. Rogora, R. Mosello / Applied Geochemistry 22 (2007) 1122–1128 1123 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 precipita- tion 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).