High chemical weathering rates in first-order granitic catchments induced by agricultural stress

Chemical Geology 265 (2009) 369–380

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Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

High chemical weathering rates in first-order granitic catchments induced by agricultural stress A.-C. Pierson-Wickmann a,b,⁎, L. Aquilina a,b, C. Martin b,c, L. Ruiz b,c, J. Molénat b,c, A. Jaffrézic b,c, C. Gascuel-Odoux b,c a b c

CNRS – Université Rennes 1; CAREN Research Federation – Géosciences Rennes UMR 6118, Campus de Beaulieu, 35042 Rennes Cedex, France Université Européenne de Bretagne (UEB, European University of Brittany), France INRA – Agrocampus Ouest; CAREN research federation – Soil Agro and HydroSystem UMR 1069, 65 Rue de Saint-Brieuc, 35042 Rennes Cedex, France

a r t i c l e

i n f o

Article history: Received 25 July 2008 Received in revised form 11 March 2009 Accepted 27 April 2009 Editor: B. Bourdon Keywords: Chemical erosion Granitic catchment Agricultural inputs Cation release Soil acidification

a b s t r a c t Chemical erosion rates have been determined on two upland granitic catchments under agricultural pressure in Brittany, France. Intensive agriculture has been carried out for at least 30 years in this region. The influence of geochemical processes related to agriculture on the chemistry of streamwaters is determined through a geochemical mass balance. The elemental export fluxes from these two agricultural catchments are then compared with other catchments around the world. The volume and concentrations of the precipitation are taken into account, as well as the inputs of organic and chemical fertilizers, groundwaters and streamwaters, to estimate the relative influence on export fluxes, and then evaluate the elemental fluxes released by weathering. The relatively high Si flux of about 1.8 ± 0.9 kmol ha− 1 yr− 1 is directly attributed to the chemical weathering of soil and rock in the catchment system. However, the Si flux remains comparable to values found in both small and large-sized catchments under temperate and tropical conditions. On the other hand, extremely high fluxes of major cations (Ca, Na and Mg) are observed, ranging from 4.2 ± 2.6 to 8.0 ± 4.9 kmol ha− 1 yr− 1, which can be attributed to chemical weathering. These fluxes remain dramatically higher than those found in granitic catchments worldwide. Despite an integrated agriculture, the soil acidification induced by fertilizer application leads mainly to a release of major cations from the system, by processes of soil ion-exchange leaching as well as weathering of soil and rock. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Silicate mineral weathering is a natural mechanism in ecosystems that results in the neutralization of protons and the production of soluble base cations (Ca, Mg, Na and K), along with aluminium and silica (Drever, 1988; Likens et al., 1977), which, in turn, sustains vegetative growth (Taylor and Velbel, 1991; White and Brandley, 1995). The rates of chemical weathering of minerals and rocks in small catchments can be defined by a geochemical mass-balance method that describes input–output budgets (Bricker et al., 1994; Garrels and Mackenzie, 1967; Katz et al., 1985; Paces, 1983; Velbel, 1985). The input–output budgets reflect a balance between sources (atmosphere, biomass, exchangeable compounds in the soil, and mineral weath-

⁎ Corresponding author. CNRS – Université Rennes 1; CAREN Research Federation – Géosciences Rennes UMR 6118, Campus de Beaulieu, 35042 Rennes Cedex, France Tel.: +33 2 23 23 50 56; fax: +33 2 23 23 61 00. E-mail address: [email protected] (A.-C. Pierson-Wickmann). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.04.014

ering) and sinks (biomass and exchangeable compounds in the soil, and streamwaters). The mass-balance approach has been mostly applied to specific mineral weathering reactions (Blum et al., 1994; Drever, 1988; Drever and Clow, 1995; Garrels and Mackenzie, 1967; Katz et al., 1985; Mast et al., 1990; Paces, 1983), and especially on granitic catchments (i.e. White and Blum (1995), Oliva et al. (2003)). A number of studies show that chemical weathering is a major sink of protons (H+) (Driscoll and Likens, 1982; Van Breemen et al., 1984; Van Breemen et al.,1983). Human activities, through the combustion of fossil fuels and industrial production, have led to widescale acidification of rain waters (Schindler, 1988). Similarly, catchment budgets in agricultural systems are significantly influenced by major additional inputs of agrochemicals or livestock manures. In addition to nitrate pollution, which has been widely described, nitrogen fertilizers and timber harvesting result in the net production of acidity in ecosystems, thus promoting soil acidification. Relatively few catchment studies have investigated the effects of agricultural practices on element budgets. Correll et al. (1984) found that ionic outputs were lower for a forested site than in a catchment on which cereals were grown. Increased output

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Fig. 1. Location map of the Kerbernez and Kerrien catchments (Brittany, France), showing the different sampling sites (wells and streams).

fluxes of nutrients (Collins and Jenkins, 1996; Pereira, 1987), as well as accelerated rates of chemical weathering, have been attributed to the addition of fertilizers (Mayorga, 2008; Paces, 1983). However, the net effect of agriculture on cation fluxes remains poorly known, as well as the sources of chemical elements in agricultural catchments. The purpose of this study is to use a massbalance approach to assess the influence of agricultural practices and applications on weathering rates of silicate rocks in a temperate environment. This study focuses on two small adjacent catchments in Brittany, western France. Indeed, Brittany is well known for the development of intensive agriculture, especially over the last thirty years. The cationic chemical weathering rates for these catchments are estimated by monitoring precipitation, groundwater and streamwaters as well as fertilizers (volumes and chemistry) over the last ten years. The solute fluxes from these two small catchments are then compared with erosion rates given in the literature. 2. Study site description The experimental site (Fig. 1), described in previous studies (Martin et al., 2004; Ruiz et al., 2002a), consists of two adjacent first-order catchments, Kerbernez (0.12 km 2 ) and Kerrien (0.095 km2), located in south-western Brittany, France (47°57′N– 4°8′W, about 550 km west of Paris and 10 km from the Atlantic Ocean). These upland catchments share the same climatic and lithological characteristics. The regional climate is oceanic. Mean annual precipitation and potential evapotranspiration calculated for the last decade are 1179 and 643 mm, respectively (Legout et al., 2005; Ruiz et al., 2002a). Mean annual temperature is 11.4 °C, with a

monthly minimum of 6.1 °C in January and a maximum of 17.6 °C in July (Ruiz et al., 2002a). During the period 2000–2005, the average air/soil temperature was 12.0 °C. Table 1 Nature and application levels of organic and inorganic fertilizers on Kerbernez and Kerrien catchments over the 1992-2005 period. 2001-2005 average in kg/ha

1996-2005 average in kg/ha

1996-2000 average in kg/ha

1992-2005 average in kg/ha

Rate ⁎

Rate ⁎

Rate ⁎

σ

Rate ⁎

Kerbernez catchment Ammonitrate 98.5 NPK 0 Urea 2.6 CaOMg 5.7 KCl 0 4.7 Slurry (m3/ha) Manure (T/ha) 0.9 Kerrien catchment Ammonitrate NPK Urea CaOMg Trez KCl Slurry (m3/ha) Manure (T/ha)

144.0 39.0 2.7 12.3 236.8 28.4 3.0 4.8

σ

σ

σ

86.9 0 2.8 12.8 0 2.7 0.9

106.8 12.9 2.0 2.8 3.0 4.5 0.7

64.5 29.3 2.9 9.0 4.2 2.4 0.7

115.2 25.9 1.4 0 6.4 4.3 0.5

40.3 39.9 3.1 0 4.0 2.5 0.6

97.0 9.7 1.4 2.0 2.2 5.2 0.5

59.3 25.0 2.6 7.6 3.8 3.3 0.7

71.4 53.5 6.1 27.5 529.6 38.9 2.2 3.1

143.9 31.1 2.5 6.2 118.4 36.7 3.1 3.9

50.5 45.0 5.4 19.5 374.5 49.4 2.0 4.2

143.7 23.2 2.4 0 0 45.0 3.3 3.1

25.1 39.1 5.3 0 0 61.7 2.1 5.3

160.0 29.5 1.8 4.4 84.6 26.2 4.5 3.0

58.5 40.3 4.7 16.5 316.5 44.6 4.0 3.8

⁎: Except for slurry (m3/ha) and manure (T/ha). s represents the standard deviation over the average period. The application rates are reported for the total catchment area: 12 ha for Kerbernez and 9.5 ha for Kerrien.

A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380 Table 2 Hydrological context during the study period. Hydrological period

PET

P

R

QKerbernez

QKerrien

11/2000–10/2001 11/2001–10/2002 11/2002–10/2003 11/2003–10/2004 11/2004–10/2005 2000–2005 average

694 727 746 703 703 714 ± 21

1619 1017 1111 1074 690 1102 ± 333

1043 376 615 444 219 539 ± 315

626 197 278 221 129 290 ± 195

946 179 537 264 71 400 ± 351

Hydrological year is from October 1st to September 31st. A 5-year-average is expressed with the standard deviation. Precipitation (P), potential evapotranspiration (PET) and soil water drainage (R) are calculated for each hydrological year based on daily data and expressed in mm/yr. Stream specific discharge for both Kerrien (QKerrien) and Kerbernez (QKerbernez) outlets are also reported for the same period.

The basement of the basin is made up of Paleozoic rocks belonging to the Plomelin leucogranodiorite (Béchennec et al., 1999). This coarse-grained rock type consists of quartz (40%), plagioclase (albiteoligoclase) and K-feldspar (50%), muscovite and biotite (10%). Some clay minerals (kaolinite) are also present in the granitic sandy regolith (Béchennec et al., 1999). Accessory minerals can include apatite, garnet and zircon (Béchennec et al., 1999). The fresh granite is overlain by 1 to 20 m of regolith (Montoroi et al., 2001), which is slightly thicker at Kerbernez than at Kerrien (Legchenko et al., 2004). Soils are mainly brown sandy loam (distric cambisol, FAO classification) developed on a granitic sand. Soil profiles were dug down to the C or B/C horizon. The weathered granite appears between 0.7 and 1.2 m below the soil surface in Kerrien and Kerbernez (Legout et al., 2005). The C-horizon represents the weathered bedrock and contains sand (63%), silt (26%) and clay (11%). The B-horizon corresponds to a cambic horizon (BW), varying from 40 to 65 cm in thickness, with low organic carbon content (0.5 wt.% C) and enriched in silt particles (~64%) relative to sand (19%) and clay (17%). The sealed macropores are mostly generated by earthworms. The upper horizons correspond to mineral horizons altered by human-related activities (Ap) from 10 to 40 cm depth, formed of micaceous sands and silts with higher organic carbon content (2.8 wt.% C). The soil bulk density ranges from 1.3–1.5 g cm− 3 (A and B horizons) to 1.7 g cm− 3 (C-horizon). Soils are well drained except in the relatively narrow bottomlands where hydromorphic soils are found (Ruiz et al., 2002a). Land use is mainly agricultural (77%), with seven stockbreeding farms. Most arable fields (43% of cultivated surface-area), growing maize and cereals in rotation, are farmed intensively, including importation of pig slurry and cattle manure. Most of the grasslands (40% of the cultivated surface-area) are grazed intensively by dairy cows (Ruiz et al., 2002a). Farmers have exhaustively recorded all agricultural activities over the last decade. These data are presented as average values in Table 1. The typical total amount of inorganic

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mineral fertilizer applied varies between the Kerrien and Kerbernez catchments, and over the years. The inorganic mineral fertilizers consist of ammonitrates, NPK and KCl for both the Kerrien and the Kerbernez catchments. Some calcareous soil improvement (liming, local Trez) has also been carried out on the Kerrien catchment. Relative to the total catchment area, a higher amount of inorganic fertilizer is applied on the Kerrien than on the Kerbernez catchment (164–533 kg/ha as against 30–257 kg/ha). Organic fertilizers (pig slurry and cattle manure) are also applied on both catchments at different rates. On average, the Kerbernez catchment receives more pig slurry and less cattle manure than the Kerrien catchment (Table 1). These rates are comparable with fertilizer application on intensively farmed grassland in temperate regions (Oenema, 1990). Previous studies (Legout et al., 2005; Martin et al., 2004; Ruiz et al., 2002a) have investigated the relationships between stream water quality and annual nitrate fluxes, as a function of agricultural practices and groundwater transfer, as well as the elemental transfer from the unsaturated to the saturated zone. Both catchments are characterized by the presence of shallow groundwater in the weathered granitic material (Molénat et al., 2008). The groundwater feeds the stream throughout year. In the bottomland, the water table is near the soil surface and the uppermost layer of the groundwater flows through the soil. Along hill slopes, the water table is typically 2–8 m below the land surface (Molénat et al., 2008). 3. Material and methods 3.1. Water sampling An automatic weather station, located 500 m north of the catchment outlets in an open field, was used to record daily rainfall (P) and the different parameters required to calculate potential evapotranspiration (PET) by the Penman formula, assuming that the parameters are similar for both the Kerrien and the Kerbernez catchments. Stream discharge was measured continuously at the two catchment outlets equipped with V-notch gauging stations using pressure-sensor dataloggers. The monitoring period (in this study) extended over five hydrological years, from October 2000 to September 2005, including two very different years (2000–2001 and 2004–2005). The first year (from October 2000 to September 2001) was very wet, with a total precipitation of 1619 mm and a PET of 694 mm, while 2004–2005 was much drier, with a total precipitation of 690 mm and a PET of 703 mm (Table 2). Stream- and rainwater-samples were collected monthly over the two first years, and then every three months. To monitor groundwaters and stream waters, both catchments were equipped with several piezometers. In the Kerbernez catchment, two transects were equipped with four wells each (labelled A and B in Fig. 1). In the Kerrien catchment, eight wells were placed along one transect

Table 3 Average concentrations of elements in precipitation and Kerrien and Kerbernez streamwater during the period from October 1st 2000 to September 31st 2005. Chemical species Precipitation

Kerrien streamwater

Kerbernez streamwater

Mean (mmol L− 1) Relative standard deviation (%) Mean (mmol L− 1) Relative standard deviation (%) Mean (mmol L− 1) Relative standard deviation (%) pH Cl Ca Mg Na K Al Si Rb Sr Ba U

6.17 0.24 0.02 0.02 0.15 0.01 3.65 × 10− 4 1.01 × 10− 3 5.50 × 10− 6 5.23 × 10− 5 1.09 × 10− 5 1.00 × 10− 7

5.3 4.3 50.5 27.2 31.6 50.7 52.8 103.3 59.0 69.5 104.6 100.3

6.32 0.91 0.27 0.32 0.87 0.13 1.05 × 10− 3 0.274 5.84 × 10− 5 1.15 × 10− 3 1.86 × 10− 4 1.88 × 10− 6

5.6 8.3 20.2 13.3 7.5 23.7 83.7 9.9 21.5 13.8 20.1 45.2

6.05 1.00 0.37 0.43 1.07 0.11 5.83 × 10− 4 0.315 5.51 × 10− 5 1.81 × 10− 3 1.90 × 10− 4 2.93 × 10− 6

3.9 8.6 12.6 10.8 6.8 22.9 59.51 9.1 14.7 8.9 9.6 30.1

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(labelled F in Fig. 1). In this latter catchment, shallow wells were also placed close to the stream network along two lines (labelled C and D in Fig. 1). Well depths ranged from 1.5 to 20 m. Wells consisted of PVC tubes screened over an interval of 0.5 to 1 m at their base.

4.2. Chemical budget computation

3.2. Chemical analyses

NðiÞ = F ðiÞSO − F ðiÞSI = F ðiÞW + F ðiÞP + F ðiÞAGR F F ðiÞB + F ðiÞS

All water samples were filtered in the field through 0.20 µm Nylon Millipore filters and then stored acidified (HNO3) and nonacidified in the dark at less than 4 °C. Major and trace cation concentrations were determined using an Agilent Technologies™ HP4500 ICP-MS (Bouhnik-Le Coz et al., 2001; Yeghicheyan et al., 2001). The instrument was calibrated using both an external calibration based on multi-element synthetic standards and an internal calibration (indium) in all standards and samples. The international standard SLRS-3 was used to check the accuracy and reproducibility of the results (Dia et al., 2000). Typical uncertainties including all error sources are less than 5% for all trace elements, whereas, for major anions, the uncertainty lies between 2 and 5%, depending on the concentration level.

where, per unit of time, Ni is the net accumulation or depletion of solute i in the system, being the difference between the outputs (F(i)SO) and the inputs (F(i)SI) of solute i in the system, F(i)W is the mass of solute i generated by chemical weathering, F(i)P is the mass of solute i from atmospheric deposition in the system, F(i)AGR is the mass of solute i from anthropogenic inputs to the system, F(i)B is the mass of the solute i associated with biological processes in the system, F(i)S is the change in concentration of solute i on the soil exchange complex in the system. The chemical mass balance is calculated here over different average time-periods (2001–2005, 1996–2000, 1996–2005, and 1992–2005), taking into account the precipitation, streamwater data and fertilizer data. For this study, the water fluxes are available for the period 2001– 2005. Hence, the same average of water fluxes are used for the four periods of time considered. The main variable is the amount of fertilizer recorded over a 14-year period (Appendix A).

4. Chemical budget computation 4.1. Water mass balance The Kerrien catchment shows a high specific discharge in winter, but it almost dries out in summer and autumn. By contrast, the Kerbernez catchment maintains a relatively higher discharge during low-stage periods, but discharge remains moderate in winter (Martin et al. (2004). The water budget, calculated as the difference between precipitation, actual evapotranspiration and stream discharge (Table 2), indicates a water deficit of ranging from 8 to 30% of the input precipitation for the different years and outlets. Underflow not intercepted at the catchment outlets and significant deep losses may explain this imbalance in the water budget (Ruiz et al., 2002b). This implies that the measurement of stream discharge underestimates the total water outflowing from the catchment. To calculate chemical erosion rates, we need to know the total amount of water outflowing from the catchment, i.e., the sum of stream discharge, underflow and deep losses. The outflowing water flux is assumed to be equal to the soil water drainage, i.e. the water flux through the base of the soil layer. The soil water drainage is estimated from a simple reservoir model. The model treats the soil layer as a reservoir whose level is updated daily by adding rainfall and subtracting PET. When the reservoir is filled up, soil water drainage occurs at a rate corresponding to the amount of excess rainfall.

A conceptual model of mass balance, calculated from aqueous concentrations of solutes (Bricker et al., 1994; Drever, 1988; Mast et al., 1990), can be represented by the following equation: ð1Þ

4.2.1. Precipitation budgets Atmospheric inputs of chemical elements (F(i)P, mol ha− 1 yr− 1) are estimated using the volumetric measurements of precipitation together with chemical composition data, according to Eq. (2), where CP(i) (mmol L− 1) is the concentration of solute i (Table 3, Appendix A), QP (mm or L m− 2 yr− 1) is the mean annual precipitation. h i h i −3 4 F ðiÞP = CP ðiÞ × 10 × Q P = 10

ð2Þ

4.2.2. Agricultural inputs In an agricultural context, the anthropogenic input (F(i)AGR) is mostly composed of the flux of solute i from the application of chemical and organic fertilizers and liming. On both catchments, the main mineral and organic fertilizers (Table 1, Appendix A) are ammonium nitrate [NH4NO3], cattle manures and pig slurries. More rarely, KCl fertilizer is also applied to grasslands on the Kerrien catchment. Based on their chemical formula, ammonium nitrate and urea [CO(NH2)2] are not considered as base cation sources. The only base cation sources are represented by KCl fertilizers, cattle manures and pig slurries, and more occasionally liming. To estimate chemical fluxes due to agricultural inputs, we compiled a database of solute

Table 4 Element fluxes (F(i)AGR (mol ha− 1 yr− 1)) from fertilizer applications for different periods (2001-2005, 1996-2000, 1996-2005 and 1992-2005) for both Kerrien and Kerbenez catchments. FAGR (mol ha− 1 yr− 1)

Kerrien

Chemical species

2001–2005

1996–2000

1996–2005

1992–2005

2001–2005

1996–2000

1996–2005

1992–2005

Cl Ca Mg Na K Al Si Rb Sr Ba U

452 1724 191 58 653 9 32 1.4) × 10− 1 4.3 × 10− 1 7.5 × 10− 3 1.8 × 10− 3

658 61 74 55 823 9 33 1.1 × 10− 1 2.8 × 10− 1 8.1 × 10− 3 1.3 × 10− 3

555 892 132 56 737 9 32 1.2 × 10− 1 3.5 × 10− 1 7.7 × 10− 3 3.8 × 10− 3

373 608 485 3.2 473 1.7 3.6 6.2 × 10− 2 2.9 × 10− 1 1.2 × 10− 3 1.0 × 10− 3

38 57 91 63 174 11 41 8.7 × 10− 2 8.0 × 10− 2 1.1 × 10− 2 7.4 × 10− 4

53 68 90 60 270 12 42 1.3 × 10− 1 3.2 × 10− 1 1.0 × 10− 2 1.5 × 10− 3

45 63 90 61 222 11 42 1.1 × 10− 1 2.0 × 10− 1 1.0 × 10− 2 1.1 × 10− 3

47 68 102 69 233 13 472 1.2 × 10− 1 1.8 × 10− 1 1.2 × 10− 2 1.1 × 10− 3

Kerbernez

The elemental fluxes are expressed as average values for the different periods.

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Table 5 The chemical budget N(i) (mol ha− 1 yr− 1) of the chemical element supplied by atmospheric inputs (F(i)P) and solutes released from stream at the catchment outlet using the calculated specific discharge (FSO) for both Kerrien and Kerbernez catchments based on stream discharges for the four different periods of time. Chemical species

FP

Cl Ca Mg Na K Al Si Rb Sr Ba U

4.79 × 103 2.06 × 102 3.12 × 102 2.44 × 103 1.77 × 102 5.95 1.68 × 101 8.64 × 10− 2 9.51 × 10− 1 2.14 × 10− 1 2.03 × 10− 3

FSO (Kerrien)

FSO (Kerbernez)

2001–2005

N(i) in Kerrien 1996–2005

1996–2000

1992–2005

2001–2005

N(i) in Kerbernez 1996–2000

1996–2000

1992–2005

4.90 × 103 1.46 × 103 1.73 × 103 4.70 × 103 7.34 × 102 5.64 1.48 × 103 3.15 × 10− 1 6.23 1.00 1.02 × 10− 2

5.42 × 103 2.00 × 103 2.33 × 103 5.80 × 103 5.95 × 102 3.14 1.70 × 103 2.97 × 10− 1 9.76 1.02 1.58 × 10− 2

− 3.50 × 102 − 4.70 × 102 1.23 × 103 2.20 × 103 − 9.54 − 9.60 1.43 × 103 9.23 × 10− 2 4.85 7.82 × 10− 1 6.40 × 10− 3

4.53 × 102 3.62 × 102 1.29 × 103 2.21 × 103 − 1.80 × 102 −9.52 1.43 × 103 1.06 × 10− 1 4.92 7.82 × 10− 1 6.64 × 10− 3

− 5.56 × 102 1.19 × 103 1.34 × 103 2.21 × 103 − 2.66 × 102 − 9.53 1.43 × 103 1.18 × 10− 1 5.00 7.81 × 10− 1 6.86 × 10− 3

− 2.72 × 102 6.46 × 102 1.37 × 103 2.26 × 103 8.46 × 101 − 2.01 1.46 × 103 1.67 × 10− 1 4.99 7.88 × 10− 1 7.15 × 10− 3

5.87 × 102 1.73 × 103 1.93 × 103 3.30 × 103 2.44 × 102 −1.39 × 101 1.64 × 103 1.24 × 10− 1 8.73 7.99 × 10− 1 1.31 × 10− 2

5.80 × 102 1.73 × 103 1.93 × 103 3.30 × 103 1.96 × 102 − 1.43 × 101 1.64 × 103 9.98 × 10− 2 8.61 7.99 × 10− 1 1.27 × 10− 2

5.72 × 102 1.72 × 103 1.93 × 103 3.30 × 103 1.49 × 102 − 1.47 × 101 1.64 × 103 7.59 × 10− 2 8.49 7.99 × 10− 1 1.23 × 10− 2

5.78 × 102 1.72 × 103 1.92 × 103 3.29 × 103 1.85 × 102 − 1.57 × 101 1.64 × 103 9.43 × 10− 2 8.63 7.98 × 10− 1 1.27 × 10− 2

F(i)AGR used for these calculations are presented in Table 4. A positive value indicates the release of an element and a negative value indicates the storage or consumption of an element.

concentrations measured in organic and mineral fertilizers (Appendix B), including fertilizers directly applied to the studied catchments, as well as literature data (McBride and Spiers, 2001; Riou, 1995; Widory et al., 2001). The database shows relatively homogeneous values for the major cations. However, there are large differences in trace element concentrations in NPK fertilizers, which vary by a factor of 10 to 100 compared with the values from McBride and Spiers (2001) and Riou (1995). Such differences in organic fertilizers could be explained by the specific nature of regional products. As far as possible, we preferred to use regional data for budget calculations. For a given chemical element i, the agricultural input (F(i)AGR, mol ha− 1 yr− 1) is defined according to Eq. (3), where MAGR (kg ha− 1 yr− 1) is the annual amount of fertilizer and CAGR(i) is the solute concentration in fertilizers (g kg− 1 or g L− 1), and M(i) is the molar mass of element i (g mol− 1). The values of CAGR(i) used for this calculation are given in Appendix B. F ðiÞAGR =

X

MAGR × CAGR ðiÞ = MðiÞ

ð3Þ

Table 4 presents the results for the four different periods of time. 4.2.3. Biomass and water storage In addition to the data on fertilizers, the local farmers provided exportation data concerning grassland and crop cuts. The dairy cow uptakes are assumed to be compensated by their excreta. The storage of water is also considered to be unchanged. F(i)S is thus only related to concentration changes in the soil exchange complex.

from the catchment, and C(i)SO (mmol L− 1) is the mean annual stream solute i concentration. h i h i 4 −3 F ðiÞSO = RSO = 10 × C ðiÞSO × 10

ð4Þ

The data are presented in Table 5 for the different periods of time. The results for the individual years are presented in Appendix C. 4.2.5. Chemical budgets The chemical mass balance is computed using data presented previously and reported in Table 5. To determine the flux of elements released from soil and rock weathering, we assume that the catchment is at steady-state during a 5-year period. We then define the net budget (N(i)) as the difference between the solute exports at the catchment outlets and both the atmospheric and agricultural inputs, as expressed in Eq. (5):   −1 −1 = F ðiÞW + F ðiÞS = F ðiÞSO − F ðiÞAGR − F ðiÞP ð5Þ yr NðiÞ mol ha In Table 5, negative values for any element indicate storage in the system, while positive values indicate a release from weathering F(i)W and/or from soil exchange F(i)S. In Table 6, the Cationic Chemical Erosion Rate (CCER), commonly expressed in mm/1000 yr, is derived from the net export of solutes (Table 5) using a measured bedrock density of 2.65 g cm− 3 (Legout et al., 2007), and normalized to the catchment area (Drever and Clow, 1995). 4.3. Uncertainties

4.2.4. Solute outputs Solute output (F(i)SO, mol ha− 1 yr− 1) is estimated according to Eq. (4), using the mean annual solute concentrations measured in streamwater, where RSO (mm or L m− 2 yr− 1) is the calculated annual soil water drainage, assumed to be the sum of all outflowing water

4.3.1. Dry deposition As the catchments are mainly covered by grass with some trees lining their borders, throughfall is considered negligible. The precipitation sampling device, located in an open field, enables the

Table 6 Cationic element release rate (N, mol ha− 1 yr− 1, from Table 5) and cationic chemical erosion rate (mm/1000 yr) for the Kerrien and Kerbernez catchments. Period N (Ca + Na + Mg)(mol ha− 1 yr− 1) CCER (mm/1000 yr) N(Si) (mol ha− 1 yr− 1) CCER (mm/1000 yr)

In Kerrien

In Kerbernez

2001–2005

1996–2000

1996–2005

1992–2005

2001–2005

1996–2000

1996–2005

1992–2005

3430 ± 2514 3.9 ± 2.2 1428 ± 819 1.5 ± 0.9

3853 ± 2593 3.7 ± 2.3 1428 ± 822 1.5 ± 0.9

4744 ± 3459 5.0 ± 3.5 1428 ± 821 1.5 ± 0.9

4274 ± 2709 4.3 ± 2.4 1457 ± 841 1.5 ± 0.9

6961 ± 4811 7.4 ± 5.0 1645 ± 954 1.78 ± 1.0

6958 ± 4796 7.4 ± 5.0 1644 ± 952 1.8 ± 1.0

6954 ± 4785 7.4 ± 5.0 1644 ± 951 1.8 ± 1.0

6933 ± 4764 7.4 ± 5.0 1639 ± 945 1.8 ± 1.0

Cationic chemical erosion rate = CCER calculated assuming a rock density of 2.65 g/cm3. Standard deviations are also shown.

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collection of bulk precipitation, including both wet and dry deposition. 4.3.2. Biomass uptake and mineralization Biomass pools may modify the element budget via uptake of elements such as Ca, Mg, N, P or K from the soil solution and incorporation into the biomass. As local farmers provided information on crop exportation from fields (not shown here), we can estimate the quantity of Ca, Mg, K, Na and Si (F(i)B) removed from the soils by vegetation as less than 1% of the Net Cation Export (N(i)). This parameter is considered negligible. 4.3.3. Storm events In small catchments, the stream discharge can be rather sporadic with rapid increases of flow during prolonged heavy rains. The chemical composition of streamwater can vary substantially with the stream discharge. Previous investigations on the Kerrien and Kerbernez catchments have indicated that the contribution from surface waters is generally low (b5%) in small catchments on poorly permeable bedrock (Molénat et al., 1999). Thus, the measured chemical concentrations adequately represent the entire compositional range of the stream waters.

calculated as P/(P-PET) ranges from 1.7 to 4.0 over the 5 years, which implies theoretical concentrations for Cl and Na much lower than measured in the Kerbernez and Kerrien catchments. This suggests that, in addition to precipitation, soil exchange and rock weathering contribute to the chemistry of groundwaters and streamwaters. Although water budgets in the Kerrien and Kerbernez catchments have been measured over the last ten years (Martin et al., 2004; Ruiz et al., 2002a), no temporal trend can be observed. The chemical composition of streamwaters is strongly related to that of groundwaters, as the streams are mainly fed by groundwater discharge (Martin et al., 2004; Ruiz et al., 2002a). Groundwater chemical compositions (Fig. 2, Appendix C) are rather homogeneous and show extremely limited variation during the last 5 years of monitoring. Furthermore, the residence time based on CFC measurements (Ayraud et al., 2008) yields an apparent groundwater age of 0–10 years in the upper part of the catchment (5–10 m below surface) and up to 10– 25 years at 10–25 m depth. The streamwater compositions shows even smaller variations, and are different compared with precipitation, implying that the large variations of chemical fluxes are mainly related to specific discharge variations. 5.2. Agricultural inputs (F(i)ARG)

5. Results 5.1. Water chemistry 5.1.1. Precipitation (F(i)P) Table 3 lists the average concentrations and standard deviations of all cations in samples of precipitation and stream waters over the studied period. The concentrations in precipitation may show a significant temporal variation by a factor of 10 for some elements. In particular, the concentrations of Na, Mg are much higher in winter than in summer, while Ca has higher concentrations in summer. Due to the vicinity of the Atlantic Ocean, Na and Cl are the main components of the precipitation, representing 78 to 96% of the dissolved solids. Na alone represents 45 to 81% of the total dissolved cations. The average concentrations calculated over the 5-year study period, weighted by the volume of precipitation, are 0.24 ± 0.01 mmol L− 1 for Cl and 0.15 ± 0.04 mmol L− 1 for Na. However, the relatively low Na/Ca ratios are very different from seawater, indicating reactions between seasalt aerosols and chemical compounds in terrestrial dust (Reid et al., 1981). 5.1.2. Stream- and ground-waters The stream waters (Table 3) are slightly acidic, with a pH within the range of values found in rainwater. The Na and Cl concentrations are about 5 times higher in the stream- and ground-waters than in the precipitation. The concentrations of major cations vary by 7 to 20% over the last 5 years. This indicates that the streamwater chemical composition is only slightly sensitive to variations in precipitation or soil water drainage. The streamwaters of the Kerbernez catchment display, on average, 10 to 30% higher concentrations of Ca, Mg, Na and Si and lower K in comparison to Kerrien. In a previous study, Martin et al. (2004) suggest that the streamwater chemistry is controlled by mixing between shallow and deep groundwaters in the Kerrien catchment, while relatively concentrated deep groundwaters make up the main contribution in the Kerbernez catchment. Kerbernez streamwaters are indeed generally more concentrated than in the case of Kerrien. In the data presented in Fig. 2, the streamwater is bracketed by compositions representing shallow groundwater (3– 8 m) and deep groundwater (15–20 m), which confirms that streamwater is derived from a mixing of these end-members. As expected in such catchments, the stream concentration is very close to groundwaters, which indicates that the stream is directly related to the groundwater discharge. The evaporative water loss factor

Chemical elements supplied by agricultural inputs are highly variable from one year to another, due to the changing application of organic and mineral fertilizers (Table 4, Appendix A). The main inputs involve K and Cl (in the form of KCl for both catchments), while NPK fertilizers were not used on Kerbernez during the period 2001–2005, while Ca was twice applied as lime on Kerrien during the same period. In general, over the last 14 years, the Kerrien catchment received a larger volume of organic and mineral fertilizers, as shown in Appendix A. 5.3. Stream solute fluxes and chemical erosion (F(i)SO and N(i)) The dominant precipitation-derived inputs are Na and Cl, reflecting the marine influence on atmospheric deposition in this region. A large amount of base cations and chloride are exported from the catchments. During the 2001–2005 period, the following solutes, in order of decreasing abundance, were exported from the Kerrien and Kerbernez catchments: (Na, Cl) N Mg N (Ca, Si) N K. There was a larger export of cations by streamwaters in Kerbernez than in Kerrien, by about 15–30% depending on the element. 6. Discussion 6.1. Cation release and acidification processes Even considering the influence of agricultural and atmospheric inputs on stream solute fluxes, the net budgets (N(i), Table 5) of Si, Mg, Na and Ca remain particularly high. This suggests that the excess of cations are derived from a considerable leaching of weathering soil and rock products. Potassium either remains mainly stored in the catchment due to the formation of clay minerals in soils or is consumed by the local biomass. The storage/consumption of K is not balanced by the release of other cations and silica. This implies cationic exchange on the soil-clay fraction and/or weathering of rocks and soil. Along with Si, minor soluble cations such as Ba, Rb and U are also released by weathering. In view of contribution of atmospheric and agricultural inputs, up to 88% of Ca, 74–85% of Mg, 97% of Si, 47% of K and 54–62% of Na can be explained by soil/rock leaching and weathering. At the worldwide scale, a coupling between tectonics, erosion and chemical weathering has been shown from global compilations of riverine Si and base cation fluxes (Waldbauer and Chamberlain,

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375

Fig. 2. Major cation and chloride concentrations (µmol.L− 1) in rainwater (black diamonds), streamwater (open diamonds) and shallow groundwater sampled between 3 and 8 m depth (black squares) and deep groundwater sampled between 15 and 20 m depth (black triangles). The groundwater end-members correspond to the groundwater encountered in each catchment. All the sampled were collected over the same study period. The error bars represent the standard deviation of the different end-members.

2005), as well as by modelling (West et al., 2005) and field studies (Riebe et al., 2004). At a more local scale, cation fluxes may depend on several physical parameters of the catchment such as soil thickness or topography. Such parameters control water fluxes in the catchments

and may induce enhanced weathering. However, as observed in studies comparing chemical weathering in small catchments (b100 ha) (White and Blum, 1995; West et al., 2005; Oliva et al., 2003), extremely different rock types, topography and climate do not

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generate fluxes as high as those observed in Brittany. Furthermore, these studies have not emphasized physical parameters as a potential cause of variations in chemical weathering. Agricultural practices such as tillage may also enhance the mineral availability and weathering. Such an effect is relatively closely associated with fertilizer use, but the processes involved are difficult to distinguish. However, acid loading due to fertilizer application are extremely high and remain the most obvious control on chemical weathering. Acidic deposition related to agricultural fertilizers is defined (Van Breemen et al., 1984; Van Breemen et al., 1983) as an input/output − budget of the NH+ 4 /NO3 couple, which can be expressed as: H

+

  + + load = NH 4 deposition − NH 4 leaching −

ð6Þ

−

+ ðNO 3 leaching − NO 3 depositionÞ Nitrogen budgets for the Kerrien and Kerbernez catchments indicate mean nitrogen loads of the order of 240 Kg-N/ha/yr. Soil acidification induced by this level of ammonitrate and urea application is of the order of 10 to 20 kmol ha− 1 yr− 1. Such a value is even higher than those found in forested areas under atmospheric acidification, which give values ranging from 9 to 14 kmol ha− 1 yr− 1 (Van Breemen et al., 1984; Van Breemen et al., 1983). 6.2. Comparison of Kerrien and Kerbernez catchments The Kerrien and Kerbernez catchments display many interesting differences in terms of net cation release. According to our calculations (Table 5), the Kerbernez catchment releases far more cations during chemical weathering than the Kerrien catchment. While the export of Na + Mg + Ca cations in the Kerrien catchment varies from 3.4 ± 2.5 to 4.7 ± 3.4 kmol ha− 1 yr− 1 over the last 14 years, it remains stable in the Kerbernez catchment during the same period at a level of about 6.9 ± 4.8 kmol ha− 1 yr− 1. Similarly, a release rate of about 1.7 kmol ha− 1 yr− 1 for Ca and Mg in the Kerbernez catchment compares with a rate of less than 1.2 kmol ha− 1 yr− 1 in the Kerrien catchment. Higher amounts of potassium are stored and/or consumed in the Kerrien catchment, whereas we observe a minimum release rate of 1.5 × 102 mol ha− 1 yr− 1 in the Kerbernez catchment. Two mechanisms are proposed to explain these differences. Firstly, since the same meteorological parameters are assumed for calculating cation fluxes from both catchments, we need to consider the existence of internal differences. Temporal variations of stream discharge reflect the characteristics and behaviour of groundwaters in each catchment (Martin et al., 2006). The groundwater transmissivity determined by geophysical investigations varies over more than one order of magnitude, with values ranging from 1.8 ± 0.9 × 10− 4 m s− 1 to 2.6 ± 1.3 × 10− 5 m s− 1 in the Kerrien and Kerbernez catchments, respectively (Legchenko et al., 2004). The weathered material is thicker, and has a higher clay content in the Kerbernez catchment compared to the Kerrien catchment (Martin et al., 2004), which leads to lower hydraulic conductivity in the Kerbernez catchment. Such physical differences may induce a longer residence time of water in the soil and hence promote weathering. Secondly, another explanation may be the difference in agricultural inputs. However, neither the changes in cation export from the Kerrien catchment, nor the difference between the Kerrien and Kerbernez catchments are supported by any variation in fertilizer application rates over the last 14 years. Fertilizers have been used less on the Kerbernez than on the Kerrien catchment, which nevertheless shows a lower level of cation exports. However, the difference in agricultural activities may have occurred decades ago. According to the local farmers, the Kerbernez catchment received much more fertilizers in the 1980s. This would imply that high cation export from the Kerbernez would be a delayed effect of the fertilizers applied 15 to 30 years ago, rather than the result of applications over the last

15 years. The transfer time of elements from the inputs (aquifer recharge) to the outputs is longer than 10 years, which is in good agreement with the groundwater ages (Ayraud et al., 2008) and the strong connection between stream waters and groundwaters. The underestimation of fertilizer inputs in the past would enhance their effects on the chemical erosion of local soil and bedrock. Higher chemical weathering related to agricultural inputs would emphasize the importance of time-lag due to catchment memory effects (Kirchner et al., 2000; Molénat et al., 2002; Steinheimer et al., 1998). We suggest that Kerrien is at nearly steady-state considering the inputs and outputs of the catchments, while Kerbernez outputs do not represent the last 14 years of inputs. This interpretation is supported by the Cl budget for the Kerrien catchment, which is nearly balanced when corrected for the anthropogenic inputs (−5.56 × 102 to 4.52× 102 mol ha− 1 yr− 1 of Cl), whereas the Kerbernez catchment shows a net Cl export (5.56 × 102 mol ha− 1 yr− 1). Since the fertilizers represent a cation-poor source, the underestimation of fertilizer application does not invalidate the mass-balance conclusions, although the results should be considered with less confidence for the Kerbernez catchment. 6.3. Source of exported cations The neutralization of acidity from the fertilizers can be achieved by ion exchange reactions in the soil, whereby H displaces base cations from exchange sites. The leaching of soil during the draining period releases base cations. The weathering of fresh bedrock in deeper soil layers, which is less dependent on hydrological fluctuations, also releases some major elements. Both processes contribute to the major cation flux from the catchment. Corrections for atmospheric and agricultural inputs indicate that a large part of the fluxes of Ca, Mg and Si and half of the Na are due to rock weathering and/or soil ion-exchange desorption. The relative proportions of the three main cations (Na, Ca and Mg) in streamwaters are similar to solutes resulting from weathering processes (cf. soil leaching experiments, Pierson-Wickmann et al., submitted for publication). This contrasts with the major element composition of precipitation, indicating that weathering contributes significantly to streamwaters (Billett and Cresser, 1996; White et al., 1999). We estimate that chemical weathering contributes more than 76% to the output fluxes of Ca and Mg, and more than 40% in the case of Na.

Fig. 3. Comparison of the Kerbernez (stars) and Kerrien (hexagons) catchments with UK catchments from Stutter et al. (2002): mean %Na+ dominance (100x[Na+]/[Na+ +Ca2+ + Mg2+]) plotted as a log10 function for the some upland sites (open circles) and agricultural catchments (open squares) vs. the sum of annual weathering rate of Na+, Ca2+ and Mg2+. Units are molc.m− 2/yr− 1.

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377

Fig. 4. Si chemical erosion rate (mol.ha− 1.yr− 1) vs. calculated specific discharge for the Kerrien (grey squares) and Kerbernez (open circles). For comparison, small catchments (b 100 ha) from Oliva et al. (2003) are also shown (black diamonds).

Nevertheless, Na remains the dominant cation in Kerrien and Kerbernez streamwaters. The main sources of Ca, Mg and Na, as well as Si, are plagioclase and biotite, which are more easily weathered than K-feldspar or quartz. Such results are also supported by strontium isotope analyses presented elsewhere (Pierson-Wickmann et al., submitted for publication). Even considering the errors in estimating fertilizer application rates through the residence time of groundwaters, we cannot explain such high chemical fluxes without a high chemical weathering component. Fig. 3 compares the Kerrien and Kerbernez catchments to some agricultural and natural upland catchments in Scotland (Stutter et al., 2002). The %Na dominance

(calculated as % [Na]/[Na + Ca + Mg] in molc m− 2 yr− 1) increases and then stabilizes with increasing base cation weathering rate. Our results are in line with Stutter et al. (2002) indicating that Na dominance is higher in agricultural catchments as against nonagricultural catchments. 6.4. Chemical fluxes and chemical erosion rates In a previous study of granite catchments around the world, White and Blum (1995) showed a linear relationship between silica flux at catchment outlets and annual runoff on silicate catchments of highly

Fig. 5. Cationic chemical erosion rate (mol.ha− 1.yr− 1) calculated from the flux of Ca, Mg and Na vs. calculated specific discharge for the Kerrien (grey squares) and Kerbernez (open circles). For comparison, small catchments (b 100 ha) from Oliva et al. (2003) are also shown (black diamonds).

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variable size. Moreover, Oliva et al. (2003) demonstrated that temperature and runoff are the two main parameters controlling chemical weathering. However, these studies fail to detect any anthropogenic effect (i.e. acid rain) on chemical weathering. In our case, the relatively high silica flux of 1.8 ± 0.9 kmol ha− 1 yr− 1 remains in the range measured in natural catchments under tropical climates (India) or Alpine/temperate climates (Europe, North America and Japan) (Oliva et al., 2003) (Fig. 4). However, the fluxes of dissolved cations (Ca, Na and Mg) exported from the catchments are significantly outside the range of values presented in Oliva et al. (2003) for a comparable runoff (Fig. 5). The stoechiometry of chemical weathering reactions of silicate minerals (i.e. feldspars altered into clay minerals), as reflected by fluxes in other catchments, does not agree with the situation observed in Brittany, where there is a higher export of cations compared with silica. This implies that the chemical weathering of primary minerals is not the only contributor to the cation flux from the catchments, and we thus require an input from the soil ion-exchange complex. As suggested above, this contribution is enhanced by soil acidification caused by fertilizer application. Wright et al. (1988) studied the reversible effects of acid rain on small granitic catchments in Norway, showing that the exports of Al, SO4, Ca and Mg are the result of acid deposition, with a high proportion of Ca and Mg coming from the leaching of the exchangeable soil fraction and an increase in chemical weathering rates. Soil acidification can lead to irreversible clay loss with decreasing pH. The use of liming to prevent the soil from becoming too acidic is essential for preventing irreversible degradation of the soil. Essential nutrients (P, Ca, Mg and Mo) become unavailable at low soil pH, which leads to a reduction of plant production in farming systems. This would result in a reduced profitability and an increased reliance on fertilizers. The long-term effects of agricultural acidification could become a concern for plant production. 7. Summary and conclusions Chemical data were collected over a 5 year period on stream-, rain- and ground-waters, as well as concerning the quantity and type of agricultural fertilizers applied on two upland granitic catchments under a temperate climate in Brittany, France. This dataset allows us to investigate the influence of the application of organic and inorganic fertilizers on water quality. The region has been under

intensive agriculture for the last 30 years, and the impact of this system has significant consequences for the water quality of upland catchments. Although the Kerrien and Kerbernez catchments share similar physical, and climatic properties, they react differently to the agricultural pressures. The Kerrien catchment shows variable cation export due to chemical weathering of soil and rock depending on the level of fertilizer application. The Kerbernez catchment does not exhibit any variation in cation or silica export flux despite variable levels of fertilizer application. By comparing these two agricultural catchments in Brittany with other small and large-sized granitic catchments under diverse climate conditions, we show that high fluxes of Si are released, and even much higher in the case of base cations, compared with temperate and tropical conditions. The results of our study indicate that the role of agriculture is a major factor in the release of cations as well as the depletion of soil macronutrients. Agricultural practices affect cation exports owing to soil acidification. Organic fertilizers, such as urea, or agrochemicals (ammonitrates), provide protons to soil (ion-exchange complex), thus leading to the leaching of cations from clay minerals and soil ionexchange complexes. High cation export due to chemical weathering is also significant. The resulting depletion of agricultural soils may lead to an enhanced use of fertilizers and liming in the future. Such practices would lead to irreversible clay loss and cation depletion in soils.

Acknowledgments This work was supported by the French programme ACI “Eau et Environnement” of the French Ministry of Research, the French programme EC2CO of the INSU-CNRS and the SYSTERRA programme of ANR (ANR-08-STRA-01). The authors would like to thank M. Bouhnik-Le Coz and M. Faucheux for field and laboratory work, the “Lycée horticole de Kerbernez” and its staff for facilitating access to the site. We are also grateful to the farmers, who were always trustful and keen to provide information and allow access to their land. B. Bourdon is thanked for editorial handling and constructive comments. We are also much indebted to Bernhard Peucker-Ehrenbrink and an anonymous reviewer for a number of helpful comments on this manuscript. Michael Carpenter post-edited the English style.

Appendix A Application levels of chemical and organic fertilizers from 1992 to 2005 on the Kerrien and Kerbernez catchments. 1992

1993

Kerbernez catchment Ammonitrate 84.9 NPK 0 KCl 0 Urea 0 CaOMg 0 Slurry 2.5 Manure 0

24.7 5.0 0 0 0 9.9 0

Kerrien catchment Ammonitrate 183.2 NPK 0 KCl 0 Urea 0 CaOMg 0 Trez 0 Slurry 0 Manure 0

283.2 41.1 0 0 0 0 10.9 0

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

58.9 0 0 0 0 12.0 0

121.1 1.7 0 0 0 2.9 0

48.5 38.1 5.0 0 0 1.2 0

112.2 89.1 7.5 0 0 6.1 0.5

148.0 2.1 0 0 0 4.0 0.4

145.8 0 10.6 0 0 7.5 0

121.4 0 7.5 7.0 0 2.9 1.4

143.1 0 0 4.7 0 4.9 0

223.4 0 0 6.3 28.5 0 0.9

84.2 0 0 0 0 6.0 0.5

33.0 0 0 0 0 6.0 0.7

8.5 0 0 2.1 0 6.5 2.3

206.7 0 0 0 0 0 6.6 0

128.1 61.0 0 0 0 0 13.1 2.2

167.2 92.3 0 0 0 0 0 1.1

114.6 13.7 0 11.8 0 0 0.6 13.4

172.2 10 0 0 0 0 3.5 1.9

137.0 0 106.6 0 0 0 3.5 0

127.5 0 118.4 0 0 0 5.7 0

142.7 0 71.0 0 0 0 1.9 0

122.7 0 71.0 13.7 61.6 0 6.2 7.4

194.5 0 0 0 0 0 4.1 4.1

37.9 92.2 0 0 0 0 1.3 7.6

222.3 102.7 0 0 0 1184.2 1.3 4.8

Application levels are expressed in kg/ha, except for slurry (m3/ha) and manure (T/ha).

2004

2005

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379

Appendix B Solute concentrations measured in mineral and organic fertilizers. Elements

NPK

NPK

NH4NO3

Urea

Pig Slurry

Pig Slurry

Pig Slurry

Pig Slurry

Pig slurry average

Dairy manure

Dairy manure

References

(1)

(2)

(1)

(1)

(2)

(3)

(4)

(5)

⁎

(1)

(6)

Units

g.kg− 1

g.kg− 1

g.kg− 1

g.kg− 1

g.kg− 1

g.L− 1

g.L− 1

g.L− 1

g.L− 1

g.kg− 1

mg.L− 1

Cl Na Mg Al Si K Ca Rb Sr Ba U

nm nm nm nm nm nm nm 29.10− 3 37.10− 3 2.10− 3 65.10− 3

26.0 3.0 5.0 18.8 3.5 181.4 25.4 0.04 0.85 nm 8.10− 3

nm nm nm nm nm nm nm 7.10− 4 1.10− 3 1.10− 3 1.10− 4

nm nm nm nm nm nm nm 4.10− 4 1.10− 3 1.10− 3 3.10− 4

15.7 46.7 67.8 4.6 15.2 236.0 40.7 0.18 0.10 nm 1.10− 4

nm 3.5 2.9 nm nm 16.0 10.1 nm nm nm nm

nm 2.7 11.3 1.2 nm 12.2 42.6 1.10− 2 6.3.10− 2 4.0.10− 2 5.3.10− 3

nm 3.5 14.2 1.2 nm 15.2 35.9 1.3.10− 2 7.3.10− 2 3.6.10− 2 3.0.10− 3

15.7 17.9 28.3 3.9 15.2 89.1 28.9 1.10− 2 0.09 1.8 × 10− 2 2.2 × 10− 3

nm nm nm nm nm nm nm 0.02 0.07 0.05 1.10− 3

152.0 76.1 46.7 0.03 6.35 7.7 99.1 nm nm nm nm

(1): Mc Bride and Spiers (2001), (2): Riou (1995), (3): analyses performed on pig slurry used on Kerbernez and Kerrien catchments (unpublished data), (4): Kerbernez pig slurry S4LB-161-27, (5): Kerbernez pig slurry S1LB-161-12; (6): Widory et al. (2001), nm: not measured. Pig slurry average represents the average values of pig slurries ((2), (3), (4), (5)) applied to the Kerbernez and Kerrien catchments, used for the computation. Slurry and manure concentrations are based on dry weight. Underlined values are used in the calculation.

Appendix C Groundwater end-members used for Fig. 2. mmol.L− 1

Cl (±σ)

Na (±σ)

Mg (±σ)

Ca (±σ)

Kerrien catchment Shallow groundwater 0.91 ± 0.13 0.96 ± 0.17 0.34 ± 0.07 0.29 ± 0.09 Deep groundwater 0.95 ± 0.24 1.09 ± 0.28 0.41 ± 0.14 0.24 ± 0.06

K (±σ) 0.15 ± 0.13 0.11 ± 0.06

Kerbernez catchment Shallow groundwater 0.93 ± 0.20 0.98 ± 0.18 0.35 ± 0.09 0.44 ± 0.11 0.09 ± 0.03 Deep groundwater 1.16 ± 0.14 1.27 ± 0.30 0.51 ± 0.15 0.80 ± 0.50 0.16 ± 0.03 Precipitation Precipitation 0.20 ± 0.11 0.18 ± 0.12 0.02 ± 0.01 0.01 ± 0.01 0.02 ± 0.03 σ: standard deviation. The average concentration and standard deviation is determined over the study period (2000-2005).

References Ayraud, V., et al., 2008. Compartmentalization of physical and chemical properties in hard rock aquifers deduced from chemical and groundwater age analyses. Applied Geochemistry 23 (9), 2686–2707. Béchennec, F., Hallégouët, D., Thieblemont, D., 1999. Carte géologique de la France (1/ 50000), feuille de Quimper (346). BRGM, Orléans. Billett, M.F., Cresser, M.S., 1996. Evaluation of the use of soil ion exchange chemistry for identifying the origins of streamwaters in catchments. Journal of Hydrology 186 (375-394). Blum, J.D., Erel, Y., Brown, K., 1994. 87Sr/86Sr ratios of Sierra Nevada stream waters: implications for relative mineral weathering rates. Geochimica et Cosmochimica Acta 58, 5019–5025. Bouhnik-Le Coz, M., Petitjean, P., Serrat, E., Gruau, G., 2001. Validation d'un protocole permettant le dosage simultané des cations majeurs et traces dans les eaux douces naturelles par ICP-MS. Les cahiers techniques de Géosciences, vol. 1. Géosciences Rennes, Rennes. 84 pp. Bricker, O.P., Paces, T., Johnson, C.E., Sverdrup, H., 1994. Weathering and erosion aspects of small catchment research. In: Moldan, B., Cerny, J. (Eds.), Biogeochemistry of small catchments: a tool for environmental research. InWiley, Chichester, England, pp. 85–105. Collins, R., Jenkins, A., 1996. The impact of agricultural land use on stream chemistry in the Middle Hills of the Himalayas, Nepal. Journal of Hydrology 185 (1-4), 71–86. Correll, D.L., Goff, N.M., Peterjohn, W.T., 1984. Ion balances between precipitation inputs and Rhode River watershed discharges. In: Bricker, O.P. (Ed.), Geological Aspects of Acid Deposition. In: Acid Precipitation Series. Butterworth, Stoneham, Mass. Dia, A., et al., 2000. The distribution of rare earth elements in groundwaters: assessing the role of source-rock composition, redox changes and colloidal particles. Geochimica et Cosmochimica Acta 64 (24), 4131–4151. Drever, J.I., 1988. The Geochemistry of Natural Waters, 2nd ed. Prentice Hall, Englewood Cliff, NJ. 437 pp. Drever, J.I., Clow, D.W., 1995. Weathering rates in catchments. In: White, A.F., Brandley, S.L. (Eds.), Chemical weathering rates of silicate minerals. In: Reviews in Mineralogy. Mineralogical Society of America, pp. 463–483. Driscoll, C.T., Likens, G.E., 1982. Hydrogen ion budget of an aggrading forested ecosystem. Tellus 34, 283–292.

Garrels, R.M., Mackenzie, F.T., 1967. Origin of the chemical compositions of some springs and lakes. In: Stumm, W. (Ed.), Equilibrium Concepts in Natural Water Systems: American Chemical Society Advanced Chemistry Series, Washington, pp. 222–242. Katz, B.G., Bricker, O.P., Kennedy, M.M., 1985. Geochemical mass-balance relationships for selected ions in precipitation and stream water, Cotactin Mountains, Maryland. American Journal of Science 285, 931–962. Kirchner, J.W., Feng, X., Neal, C., 2000. Fractal stream chemistry and its implications for contaminant transport in catchments. Nature February 403 (6769), 524–527. Legchenko, A., et al., 2004. Magnetic resonance sounding applied to aquifer characterization. Ground Water 42 (3), 363–373. Legout, C., Molénat, J.S.L., Marmonier, P., Aquilina, L., 2005. Investigation of biogeochemical activities in the soil and unsaturated zone of weathered granite. Biogeochemistry 75 (2), 329–350. Legout, C., Molénat, J., Aquilina, L., Bariac, T., 2007. Solute transport with fluctuating water table. Journal of Hydrology 332, 427–441. Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S., Johnson, N.M., 1977. Biogeochemistry of a Forested Ecosystem, New-York. Martin, C., et al., 2004. Seasonal and inter-annual variations of nitrate and chloride in streamwaters related to spatial and temporal patterns of groundwater concentrations in agricultural catchments. Hydrological Processes 18, 1237–1254. Martin, C., et al., 2006. Modelling the effect of physical and chemical characteristics of shallow aquifers on water and nitrate transport in small agricultural catchments. Journal of Hydrology 326, 25–42. Mast, M.A., Drever, J.I., Baron, J., 1990. Chemical weathering in the Loch Vale watershed, Rocky Mountain National Park, Colorado. Water Resources Research 26 (12), 2971–2978. Mayorga, E., 2008. Harvest of the century. Nature 451, 405–406. McBride, M., Spiers, G., 2001. Trace element contents of selected fertilizers and dairy manures as determined by ICP-MS. Communications in Soil Science and Plant Analysis 32 (1-2), 139–156. Molénat, J., Davy, P., Gascuel-Odoux, C., Durand, P., 1999. Study of three subsurface hydrologic systems based on spectral and cross-spectral analysis of time series. Journal of Hydrology 222, 152–164. Molénat, J., Durand, P., Gascuel Odoux, C., Davy, P., Gruau, G., 2002. Mechanisms of nitrate transfer from soil to stream in an agricultural watershed of French Brittany. Water Air and Soil Pollution 133 (1-4), 161–183. Molénat, J., Gascuel-Odoux, C., Ruiz, L., Gruau, G., 2008. Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France). Journal of Hydrology 348 (3–4), 363–378. Montoroi, J.P., et al., 2001. Analysis of a piezometric network using an electrical imaging survey (Kerbernez watershed, Brittany, France). In: INRA (Ed.), Géophysique des sols et des formations, actes du 3ème colloque GEOFCAN, pp. 47–50. Oenema, O., 1990. Calculated rates of soil acidification of intensively used grassland in the Netherlands. Nutrient Cycling in Agroecosystems 26, 217–228. Oliva, P., Viers, J., Dupré, B., 2003. Chemical weathering in granitic environments. Chemical Geology 202, 225–256. Paces, T., 1983. Rate constants of dissolution derived from the measurements of mass balance in hydrological catchments. Geochimica et Cosmochimica Acta 47, 1855–1863. Pereira, H.C., 1987. Policy and Practice in the Management of Tropical Watersheds. Westview Press, Boulder. Pierson-Wickmann, A-C., Aquilina, L., Weyer, C., Molenat, J., Lischeid, G., in press. Acidification processes and soil leaching influenced by agricultural practices revealed by strontium isotopic ratios. Geochimica et Cosmochimica Acta. doi:10.1016/j.gca.2009.05.051. Reid, J.M., MacLeod, D.A., Cresser, M.S., 1981. The assessment of chemical weathering rates within an upland catchment in North-East Scotland. Earth Surface Processes and Landforms 6, 447–457.

380

A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380

Riebe, C.S., Kirchner, J.W., Finkel, R.C., 2004. Erosional and climatic effects on long term chemical weathering rates in granitic landscapes spanning diverse climate regimes. Earth and Planetary Science Letters 224, 547–562. Riou, C., 1995. Sources Des Excès de Phosphore Dans Les Eaux Superficielles de Bretagne. Master thesis Thesis, Université de Rennes 1, Rennes. Ruiz, J., et al., 2002a. Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: I. Annual nitrogen budgets. Hydrology and Earth System Processes 6 (3), 497–505. Ruiz, L., et al., 2002b. Effect on nitrate concentration in stream water of agricultural pratices in small catchments in Brittany: II. Temporal variations and mixing processes. Hydrology and Earth System Processes 6 (3), 507–513. Schindler, D.W., 1988. Effects of acid rain on freshwater ecosystems. Science 239, 149–157. Steinheimer, T.R., scoggin, K.D., Kramer, L.A., 1998. Agricultural chemical movement through a filed-size watershed in iowa: subsurface hydrology and distribution of nitrate in groundwater. Environmental Science & Technology 32 (8), 1039–1047. Stutter, M., Smart, R., Cresser, M., 2002. Calibration of the sodium base cation dominance index of weathering for the River Dee catchment in north-east Scotland. Applied Geochemistry 17 (1), 11–19. Taylor, A.B., Velbel, M.A., 1991. Geochemical mass balance and weathering rates in forested watersheds of the southern Blue Ridge. II. Effects of botanical uptake terms. Geoderma 51, 29–50. Van Breemen, N., Mulder, J., Driscoll, C.T., 1983. Acidification and alkalinization of soils. Plant and Soil 75, 283–308. Van Breemen, N., Driscoll, C.T., Mulder, J., 1984. Acidic deposition and internal proton sources in acidification of soils and waters. Nature 307, 599–604.

Velbel, M.A., 1985. Geochemical mass balances and weathering rates in forested watersheds in the southern Blue Ridge. American Journal of Science 285, 904–930. Waldbauer, J.R., Chamberlain, C.P., 2005. Influence of uplift, weathering and base cation supply on past and future CO2 levels. In: Ehleringer, J.R., Cerling, T.E., Dearing, M.D. (Eds.), A history of atmospheric CO2 and its effects on plants, animals and ecosystems. Ecological Studies. InSpringer Verlag, Berlin, pp. 166–184. West, A.J., Galy, A., Bickle, M., 2005. Tectonic and climatic controls on silicate weathering. Earth and Planetary Science Letters 235, 221–228. White, A.F., Blum, A.E., 1995. Effects of climate on chemical weathering in watersheds. Geochimica et Cosmochimica Acta 59 (9), 1729–1747. White, A.F., Brandley, S.L., 1995. Chemical weathering rates of silicate minerals: an overview. In: White, A.F., Brandley, S.L. (Eds.), Chemical weathering rates of silicate minerals. InMineralogical Society of America, Washington, D. C., pp. 1–22. White, C.C., et al., 1999. A novel index of susceptibility of rivers and their catchments to acidification in regions subject to a maritime influence. Applied Geochemistry 14, 1093–1099. Widory, D., et al., 2001. Traçage isotopique des sources de nitrates dans les eaux souterraines: cas du Bassin de l'Arguenon (Côtes-d'Armor). Rapport BRGM/RP51091 – FR. Wright, R.F., Lotse, E., Semb, A., 1988. Reversibility of acidification shown by wholecatchment experiments. Nature 334, 670–675. Yeghicheyan, D., et al., 2001. A compilation of some trace elements measured in the natural river water reference material SLRS4. Geostandard Newsletter 25 (3), 465–474.