Effects of increased deposition of atmospheric nitrogen on an upland moor: leaching of N species and soil solution chemistry

Environmental Pollution 135 (2005) 29–40 www.elsevier.com/locate/envpol

Effects of increased deposition of atmospheric nitrogen on an upland moor: leaching of N species and soil solution chemistry M.G. Pilkingtona,), S.J.M. Capornb, J.A. Carrollb, N. Cresswellc, J.A. Leed, T.W. Ashendene, S.A. Brittaine, B. Reynoldse, B.A. Emmette a

Department of Environmental and Leisure Studies, Manchester Metropolitan University, MMU Cheshire, Crewe Green Road, Crewe, Cheshire, CW1 5DU, UK b Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK c Department of Biological Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK d Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2UQ, UK e Centre for Ecology and Hydrology (Bangor), Deiniol Road, Bangor, Gwynedd, LL57 2UP, UK Received 10 June 2004; accepted 15 October 2004

Nitrate leaching from an upland moor podsol was significantly increased only in response to rates of N deposition in excess of 96 kg N haÿ1 yrÿ1. Abstract This study was designed to investigate the leaching response of an upland moorland to long-term (10 yr) ammonium nitrate additions of 40, 80 and 120 kg N haÿ1 yrÿ1 and to relate this response to other indications of potential system damage, such as acidification and cation displacement. Results showed increases in nitrate leaching only in response to high rates of N input, in excess of 96 and 136 kg total N input haÿ1 yrÿ1 for the organic Oh horizon and mineral Eag horizon, respectively. Individual N additions did not alter ammonium leaching from either horizon and ammonium was completely retained by the mineral horizon. Leaching of dissolved organic nitrogen (DON) from the Oh horizon was increased by the addition of 40 kg N haÿ1 yrÿ1, but in spite of increases, retention of total dissolved nitrogen reached a maximum of 92% and 95% of 80 kg added N haÿ1 yrÿ1 in the Oh and Eag horizons, respectively. Calcium concentrations and calcium/aluminium ratios were decreased in the Eag horizon solution with significant acidification mainly in the Oh horizon leachate. Nitrate leaching is currently regarded as an early indication of N saturation in forest systems. Litter C:N ratios were significantly lowered but values remained above a threshold predicted to increase leaching of N in forests. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nitrate leaching; Upland moor; C:N ratio; Nitrogen saturation; Mor humus

1. Introduction Roughly half of the area of upland Britain is covered by moorland, with more than 80% found in Scotland (UK ) Corresponding author. Tel.: C44 161 247 5250; fax: C44 161 247 6372. E-mail address: [email protected] (M.G. Pilkington). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.10.016

BAP, 2001). Smaller pockets of moorland in England and Wales contain heather in poor condition, having less than 50% cover (Bardgett et al., 1995) and the encroachment of grasses into areas once dominated by heather has been linked to increases in nitrogen deposition over recent decades (NEGTAP, 2001), although inappropriate management may also play a role. Moreover, underlying soils may be acidified as a result of the displacement of base

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cations by ammonium accumulation (White and Cresser, 1998) and their loss through leaching with nitrate (INDITE, 1994), leading to increases in the concentration of toxic metals such as aluminium (NEGTAP, 2001). However, most research into the effects of nitrogen deposition on upland semi-natural systems has focussed on managed forests where an increase in nitrate leaching is now considered to be a universal and sensitive response to increased N inputs, heralding a potential for longer term reductions in productivity as well as for the eutrophication and acidification of other sensitive systems (Wilson and Emmett, 1999). Sustained chronic inputs of N lead to ‘‘nitrogen saturation’’, defined as the point when N availability is greater than combined plant and microbial demand (Aber et al., 1989). Studies involving European and North American forests across a wide range of N deposition rates found a good correlation between N deposition and inorganic N leaching (Dise and Wright, 1995) above a threshold input value of w10 kg N haÿ1 yrÿ1. However, over a narrower range of N inputs typical for Europe (between 10 and 25 kg N haÿ1 yrÿ1), N outputs were more related to soil N content or C/N ratio (Dise and Wright, 1995; Wilson and Emmett, 1999). Several studies suggest that the ability of heather moorland systems to retain nitrogen inputs exceeds that of forests. Nitrate concentrations in surface water draining upland catchments were negatively related to the catchment area occupied by peat (Harriman et al., 1998), a substrate commonly associated with Calluna vulgaris in the British uplands (Cresser et al., 2002). In an incubation study of peat cores taken from the mor layer of a Danish Calluna heathland, rates of immobilisation of inorganic N far exceeded that of an adjacent Quercus forest soil (Kristensen and Henriksen, 1998; Kristensen and McCarty, 1999). Notwithstanding a variety of factors causing wide variations in soil and vegetation stores of nitrogen as well as nitrate outputs from catchments (Chapman and Edwards, 1999), some peat and moorland systems in particular appear to have the ability to retain relatively large amounts of atmospherically deposited N. However, little work of this nature has been carried out in British heaths, and, in the context of upland seminatural systems, where N inputs are dominated by wet deposited nitrate and ammonium (NEGTAP, 2001), there is not enough evidence to show whether the leaching response would provide a useful indicator of future damage to the system or at what level of deposition these systems might show signs of N saturation. In this study therefore, the effects of increasing N deposition on N leaching and soil solution chemistry are investigated using long-term plots on an upland moor which had been treated with N additions for 10 yr.

2. Materials and methods 2.1. Site description and treatments The study site was situated on an upland moor (470 m asl) near Ruabon in Denbighshire, North Wales. The soil was an iron pan stagnopodsol (Hiraethog series) overlying lower Palaeozoic sediments with a humified, black peaty surface horizon (Oh) of massive structure and approximately 8 cm in thickness. This horizon was covered by the same depth of Calluna litter containing a fibrous root mat (L/F). An elluviated, grey coloured, stony Eag horizon, approximately 5 cm thick, extended below the Oh layer. Beneath this horizon, at a depth of 21 cm, was a discontinuous iron pan (Bf), and then a bright yellow/orange Bs horizon, with a silt texture. There were few roots in this wet horizon, which extended below 50 cm. The pH in the leachate from the Oh horizon and the soil solution from the Eag horizon was 4.0 and 4.4, respectively (details below). Vegetation cover consisted almost exclusively of C. vulgaris (L.) with a British National Vegetation Classification of H12; C. vulgaris–Vaccinium myrtillus heath (Rodwell, 1991). At the time of sampling there was a sparse understorey of moss beneath heather that could be described as ‘mature’ to ‘degenerate’ (Gimingham, 1972) and due for burning as part of the local gamekeeper’s management strategy for shooting of Red Grouse (Lagopus lagopus). The study made use of long-term N addition plots (1 m ! 1 m), established in 1989 (Caporn et al., 1994) and arranged in a randomised design with 4 blocks representing 4 replicates of each treatment on a south-facing 4  slope. Finely sprinkled ammonium nitrate solution at rates of 40, 80 and 120 kg N haÿ1 yrÿ1 had been applied using a watering can at monthly intervals since establishment for 10 yr prior to the start of the soil solution sampling reported here (Table 1). Over the sampling period a knapsack sprayer was used to apply the N solutions, which were dissolved in 1 l of collected rainwater per square metre, representing a 0.1% increase in annual rainfall amount. 2.2. Meteorology and total N deposition For the period 1998/1999, mean annual rainfall, mean monthly soil temperature (ca. 10 cm depth) and mean monthly air temperature at a height of 50 cm amounted to 1010 mm, 6.3  C and 6.1  C, respectively. Soil moisture (‘Theta’ soil moisture probe type HH1 (DeltaT Devices, Cambridge)) (ca. 10 cm depth) remained saturated and close to maximum instrument output for most of the year, with substantial drying during July/ August. Each plot was furnished with a single thermocouple and air and soil temperature recorded every 4 h using a Campbell Scientific datalogger.

M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40 Table 1 Mean annual background deposition of N species (kg N haÿ1 yrÿ1) and additional N in the form of ammonium nitrate in the ‘low’ (C40), ‘middle’ (C80) and ‘high’ (C120) treatments Species

Background

N additions Low (C40)

Middle (C80)

High (C120)

Dry deposition NH3-N NO2-N

1.1 1.5

1.1 1.5

1.1 1.5

1.1 1.5

Wet deposition DON NH4-N NO3-N

5.8 6.1 1.9

5.8 26.1 21.9

5.8 46.1 41.9

5.8 66.1 61.9

10.6 16.4

50.6 56.4

90.6 96.4

130.6 136.4

Total N input (ÿDON) Total N input (CDON)

All values in kg N haÿ1 yrÿ1. Total inorganic N deposition is given for comparison with critical load mapping values. The N content of the tank rainwater used for diluting each treatment prior to application amounted to an additional 0.1 kg N haÿ1 yrÿ1 or less than 0.5 % of the total N input. The concentration of NH4NO3 in the spray solutions at each application of the low, middle and high treatments was 14.3, 28.6 and 42.9 mM, respectively.

Monthly concentrations of ammonia and nitrogen dioxide were measured using passive diffusion samplers; in the former case as part of the National Ammonia Monitoring Network (Sutton et al., 2001) and for the latter, according to the triethanolamine method (Atkins et al., 1986). Gaseous flux input was then calculated using deposition velocities of 22 mm sÿ1 and 2 mm sÿ1, respectively (upland moorland values, supplied by CEH Edinburgh). Wet deposition was measured using a rain gauge with added thymol to prevent microbial activity. Occult deposition was not measured. Estimated total background deposition of N based on 20 months data from August 1998 to March 2000 was 16.4 (including DON) or 10.6 (excluding DON) kg N haÿ1 yrÿ1 (Table 1). This compared with 130 meq mÿ2 yrÿ2 or 17 kg N haÿ1 yrÿ1 (excluding DON) as modelled for the relevant 5 km2 by CEH Bush. The contribution of dry deposited ammonia to the composition of precipitation measured in continuously open gauges may have amounted to an overestimation of approximately 0.5 kg N haÿ1 yrÿ1 (Cape and Leith, 2002), while that from dry deposited nitric acid may have amounted to an additional 0.2 kg N haÿ1 yrÿ1; however, underestimation from omitting contributions from dry deposited nitric acid to vegetation may have amounted to 1 kg N haÿ1 yrÿ1 (N. Cape, Pers Comms). 2.3. Soil solution sample collection and monitoring equipment Leachate from the Oh horizon was sampled using a single zero-tension lysimeter made from rectilinear

31

PVC guttering cut into a 40-cm length with a stopend and installed beneath the Oh horizon at the downslope end of each plot. The soil solution from the mineral Eag horizon was sampled using a single suction lysimeter consisting of P80 porous ceramic cups (CeramTec UK Ltd., Colyton, Devon) installed beneath the Eag horizon at the downslope end of each plot and evacuated to approximately 70 kPa. The use of the downslope end of the plots minimised potential losses by lateral flow. Three months soil stabilisation was allowed, during which time samples were drawn and discarded, before sampling began in July 1998. Samples were taken from collecting vessels buried beneath the organic horizon of each plot and were analysed for pH within 24 h, before being filtered through 0.45 mm pore size cellulose nitrate filters, and stored at 2  C. Rainwater and soil solution samples were collected every two weeks and bulked to give monthly samples from each plot, either in proportion to the volume collected (organic leachate samples from the gutters) or in approximately equal proportions (mineral soil solution samples from the porous cups). 2.4. Chemical analyses Prior to analysis, soil solution samples were filtered through ‘OnGuard-P’ cartridges (Dionex (UK) Ltd., Camberley, Surrey) to remove phenolics and analysed for ammonium, nitrate, sulphate, chloride and phosphate using ion exchange chromatography (Dionex (UK) Ltd., Camberley, Surrey). Aluminium concentrations were determined using the pyrocatechol violet method of Dougan and Wilson (1974). Sodium and potassium concentrations were measured using flame emission spectrophotometry (Corning M410, Corning, Halstead, Essex) and magnesium and calcium using atomic absorption spectrometry (Perkin Elmer 3110, Perkin Elmer, Seer Green, Buckinghamshire). The method for estimating dissolved organic N (DON) and organic phosphorus (DOP) was from Williams et al. (1995): Briefly, soil solution samples were digested with alkaline persulphate to oxidise organic and inorganic forms of N and P prior to measurement of concentration by colorimetric analysis in a segmented flow auto analyser (Skalar (UK) Ltd., Wheldrake, York). The concentration of inorganic N and P was then subtracted. 2.5. Calculations and assumptions for soil solutions Annual flux output of N (kg haÿ1 yrÿ1) from the two horizons was calculated from a total sampling period of 21 months (July 1998–March 2000). A detailed comparison of two calculations for annual flux, firstly using total annual rainfall volume (adjusted for 30% evapotranspiration) and secondly using annual volumes in the

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M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40

collecting vessels, suggested that use of the first method avoided problems such as higher annual volumes recorded from a number of overflowing collectors situated beneath pooling rain and greater variability generally, probably associated with differences in lysimeter function. 2.6. Litter C and N status Six cores were taken from each of the plots in June 1998, divided into litter (LFH) layer, peat at 0–1 cm and 1–2 cm depths and dried at 80  C. The core sections were roughly ground in a mortar and pestle and weighed after large stones were removed and then finely ground in a Spex freezer mill. All samples were then analysed for C and N by oxidative digestion using an Elementar Vario – EL analyser. (Elementar Analysensysteme GmbH, Hanau, Germany). 2.7. Statistical analysis Differences between treatment responses were determined using analysis of variance with the general linear models (GLM) procedure and Tukey’s test for comparisons between treatments. Loge transformations were used to normalise data as appropriate. Linear or curvilinear responses to increasing treatment were determined using analysis of variance, breaking the treatment effect down into its orthogonal polynomial components (linear, quadratic, cubic), with only linear and quadratic responses quoted and represented graphically using linear or polynomial regression plots. All statistical tests were carried out with n Z 4 using MINITAB Release 13.30 (Minitab Inc. State College, U.S.A).

3. Results 3.1. Fluxes of N and P 3.1.1. Oh horizon In response to background deposition of 16.4 kg N haÿ1 yrÿ1 (Table 1), flux output of nitrate-N and ammonium-N from the upper organic layer amounted to 0.3 and 2.9 kg N haÿ1 yrÿ1, respectively, which, with a further 2.8 kg N haÿ1 yrÿ1 of DON, amounted to a total measurable output of 6.0 kg N haÿ1 yrÿ1, equivalent to 63% retention of inputs. While nitrate-N flux from the Oh layer significantly increased in response to the middle and high treatment (total inputs of 96.4 and 136.4 kg N haÿ1 yrÿ1, respectively), with a significant response to the linear component of increasing treatment (Table 2 and Fig. 1a), that of ammonium-N was not changed, although a significant response to the quadratic component of increasing treatment suggested a reduction in leaching with N additions of the low and middle treatment (Table 2 and Fig. 1b). Leaching of DON increased in the low treatment plots only (Table 2 and Fig. 1c). Leaching of total inorganic nitrogen (nitrate and ammonium) was not changed by individual treatment, although there was an increase in response to the linear component of increasing treatment (Table 2 and Fig. 1d). Expressed as a percentage of N inputs, the flux of total N (CDON) from this horizon amounted to 14%, 8% and 10% of inputs in response to the low, middle and high treatments, respectively. For P and DOP, relatively low volume-weighted annual concentrations (approximately 0.03 and 0.05 mg P lÿ1, respectively) and fluxes (approximately 0.2 and 0.3 kg P haÿ1 yrÿ1, respectively) did not respond either to individual or increasing N treatments (Table 2).

Table 2 Analyses of variance on the effects of individual N treatments (with Tukey post hoc comparisons) and of increasing N treatment (broken down into orthogonal polynomial components) on annual flux of N and P species (kg haÿ1 yrÿ1) and N-derived HC (kequiv. haÿ1 yrÿ1) in Oh horizon leachate Eag horizon solution Parameter

ANOVA (treatments)

Tukey comparison (with control)

df

F

P

Treatments

3.4 11.3

0.07 0.002

3 3 3 3 3

3.0 5.2 1.8 0.4 5.1

0.09 0.02 0.22 0.77 0.03

ns control–middle control–high ns control–low ns ns control–middle

Eag horizon solution 3 NHC 4 -N NOÿ 3 3 -N Tot Nin. 3 DON 3 N-derived HC 3

1.2 6.2 2.9 1.0 5.3

0.38 0.01 0.09 0.46 0.03

ns control–high ns ns control–middle

Oh horizon leachate NHC 3 4 -N 3 NOÿ 3 -N Tot Nin. DON PO3ÿ 4 -P DOP N-derived HC

T

ANOVA (increasing treatment) P ! 0.05

F

P ! 0.05

Orthogonals

9.2 32.5

0.01 !0.001

quadratic linear

1.41 5.44

0.01 0.00

3.77

0.02

5.6 4.8

0.04 0.06

linear linear

3.79

0.020

10.80

0.010

quadratic

4.15

0.01

18.3 8.3

0.002 0.02

linear linear

ÿ3.63

0.03

12.56

0.01

quadratic

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M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40 10

(a) 8

Oh: R2 = 0.55; P = 0.01 Eag: R2 = 0.23; P = 0.19

NH4+-N (kg ha-1yr-1)

NO3--N (kg ha-1yr-1)

10

6 4 2

(b)

8

Oh: R2 = 0.41; P = 0.03 Eag: R2 = 0.02; P = 0.88

6 4 2 0

0 0

50

100

0

150

50

100

150

Total N input (kg ha-1yr-1) 20

(c)

8

Oh: R2 = 0.35; P = 0.06 Eag: R2 = 0.20; P = 0.22

Total N in. (kg ha-1 yr-1)

DON (kg ha-1 yr-1)

10

6 4 2

(d) 15

Oh: R2 = 0.46; P = 0.02 Eag: R2 = 0.21; P = 0.22

10 5 0

0 0

50

100

150

0

50

100

150

Fig. 1. Effect of N input on the volume-weighted annual flux of (a) nitrate-N, (b) ammonium-N, (c) dissolved organic N (DON) and (d) total inorganic N (nitrate-N C ammonium-N) in the Oh horizon leachate (filled circles, solid line) and the Eag horizon solution (hollow circles). Trend lines were fitted by polynomial regression (P ! 0.05) on non-transformed full dataset with R2 Z regression coefficient. Other statistical tests are given in Table 2.

There were no relationships between the flux of N and the flux of P species. 3.1.2. Eag horizon In response to background N deposition in the control plots, output flux of total N from the underlying Eag horizon was 4.6 kg N haÿ1 yrÿ1, equivalent to 28% of atmospheric N deposition. Inorganic species made up 50% of total N in the Oh horizon leachate, reduced to 4% in the Eag horizon solution. The flux of ammonium-N was particularly reduced in the Eag horizon; an increase in response to the quadratic component of increasing N treatment in the Oh horizon leachate was no longer present (Table 2 and Fig. 1b, dotted line). The flux of Nitrate-N, although decreased in the Eag horizon, (Fig. 1a, dotted line) retained a similar significant response to that observed in the Oh leachate, both as an increase in response to the highest N treatment, as well as an increase in response to the linear component of increasing N treatment (Table 2). As found in the Oh horizon leachate, there was an increase in total inorganic N flux from the Eag horizon by the linear component of increasing treatment (Table 2). P species were not analysed in the Eag horizon solution because porous ceramic cups have a high sorption capacity for inorganic P. 3.2. Base cations, hydrogen ions, total inorganic Al, pH and calcium/aluminium ratios Base cation and aluminium volume-weighted annual concentrations in the Oh horizon leachate did not

respond to N treatment (Tables 3 and 4). Acidity, expressed as volume-weighted annual concentration of hydrogen ions, was increased by the N inputs of the middle treatment and, for this as well as the original direct measurements of pH, there were increases in response to the quadratic component of increasing N treatment (Table 4). The solution of the underlying Eag horizon was more alkaline, with pH decreased by the linear component of increasing treatment. Potential acidification was also calculated from N species entering or leaving soil horizons according to the proton budget approach of Van Breemen et al. (1984). Input of one mole of ammonium to a horizon represented a source of one proton as did output of one mole of nitrate. Similarly, output of one mole of ammonium from a horizon represented a sink for one proton as did the input of one mole of nitrate. The resulting N-derived proton concentration showed a similar increase in response to the middle N treatment and to the quadratic component of increasing treatment as HC concentration in the Oh horizon leachate (Table 2). In both horizons there were similar curvilinear relations to those of HC concentration in response to increasing N treatment (Fig. 2a, b). Calcium volume-weighted annual concentrations in the Eag horizon solution were reduced by N treatment (Table 3 and Fig. 3a). Although the linear increase in total aluminium volume-weighted annual concentration was not significant (P Z 0.07, Table 4), regression plots of aluminium concentrations against treatment provided a significant increasing slope (P Z 0.047, Fig. 3b).

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M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40

Table 3 Volume-weighted annual concentrations of base cations (mequiv. lÿ1 in Oh horizon leachate and Eag horizon solution) Treatments

Mg2C

Ca2C

Oh horizon leachate Control 35.6 42.9 3.7 28.7 Low

Middle

High

P1 P2

35.1

27.2

35.3

Low

Middle

High

P1 P2

61.1

48.5

50.2

0.13 0.31

19.7

18.3

32.5 2.8 21.3

13.3

46.8 5.7 19.7

18.6

25.7 2.2 16.3

16.8

27.2 3.0 14.2

20.3

21.3 2.9 9.1

16.5

22.4 2.0 13.1

24.8

0.07Q 0.08

Eag horizon solution Control 59.7 68.7 5.3 46.6

NaC

Treatments pH

39.9 1.9 31.1

0.54 0.23

KC

Table 4 pH and volume-weighted annual concentrations of hydrogen ions (mequiv. lÿ1), aluminium (mmol. lÿ1) and the molar ratio of Ca and Al in Oh horizon leachate and Eag horizon solution

8.8

69.5 3.9 52.4

5.7

68.3 7.0 36.9

4.0*

53.5 1.6 46.1

5.6*

0.01&Q 0.01

30.8 5.0 7.1

170.6

40.2 7.6 8.0

172.8

24.6 3.4 8.8

195.0

55.2 10.3 9.1

162.6

0.18 0.43 11.1 0.9 6.6

12.9

7.6 1.0 3.2

12.1

5.5 0.7 2.4

8.1

7.1 1.1 2.3

7.7

189.9 10.6 144.1 189.8 10.1 143.4

182.5

24.7 4.3 6.6

193.3

10.7 1.4 4.6

203.5

11.2 1.3 5.1

182.8

0.11 0.39

Oh horizon leachate Control 3.9 4.2 138.2 0.1 3.8

Al

Ca/Al

159.9 5.9 15.5 92.7

7.8 1.85 0.9 4.2

3.09 0.47 1.08

Low

3.9

222.2 12.0 166.1

4.0 197.4 0.0 3.8

222.9 5.8 14.1 158.0

6.6 1.77 0.6 3.9

3.47 0.57 1.15

Middle

3.8

181.6 18.0 108.6

3.9 216.4* 0.0 3.7

260.9 6.7 20.1 170.5

9.6 1.09 1.2 3.8

1.71 0.25 0.49

High

3.9

4.1 156.7 0.1 3.8

230.6 5.6 25.4 114.6

8.3 1.73 0.9 3.9

2.31 0.22 1.24

P1 P2

0.02Q 0.05

0.97 0.38 18.4 2.0 9.2

HC

209.9 11.2 162.0 198.4 3.5 183.4

0.004Q 0.02

Eag horizon solution Control 4.3 4.5 65.6 0.1 4.1

0.99 0.85

0.64 0.27

95.1 23.3 26.6 0.20 11.3 1.4 47.7 19.6

0.28 0.03 0.12

Low

4.4

214.9 5.2 190.1

4.7 59.0 0.1 4.0

106.2 29.9 35.7 0.10* 16.3 2.8 31.2 22.5

0.15 0.02 0.05

Middle

4.2

193.6 10.5 151.4

4.4 71.4 0.1 4.0

99.7 32.2 39.1 0.06* 11.7 3.4 43.3 24.4

0.10 0.01 0.05

High

4.1

4.2 87.2 0.0 4.0

104.6 29.9 32.6 0.10* 7.2 1.8 69.6 25.1

0.13 0.02 0.04

P1 P2

0.03 0.07

0.04Q 0.13

P1 denotes the P-value obtained from analysis of variance on the effect of increasing N treatment (broken down into linear and quadratic (Q) components) and marked by bold type where P1 ! 0.05). P2 denotes the P-value obtained from analysis of variance on the effect of individual N treatments (with Tukey post hoc comparisons) and marked by an asterisk where the parameter value is different from that of the control at P2 ! 0.05). The right-hand cell in each case contains: maximum (top); minimum (bottom); standard error (middle) of n Z 4 replicates.

Nevertheless, calcium/aluminium molar ratios were decreased by all individual N treatments and by the linear component of increasing treatment (Table 4 and Fig. 3c). There was a negative relation between pH and the concentration of aluminium in the Eag horizon solution (Fig. 3d). 3.3. Litter C and N status and leaching rates of N species Although N treatment had no effect on carbon concentration in the litter layer, nitrogen concentration was raised by the middle and high treatments and by the linear component of increasing treatment, with similar

0.08 0.20

0.07 0.13

0.004&Q 0.01

Symbols and layout are explained in Table 3.

reductions in C:N ratio (Table 5 and Fig. 4). A linear correlation was found between %N and %C in the litter layer (Fig. 4d). N treatment effects on C:N ratio at 0–1 cm depth of the Oh horizon were less strong, with no effects at 1–2 cm depth (not shown). Linear correlations were found between litter C:N ratio and nitrate leaching from both horizons (Table 6 and Fig. 5a), with no relation between litter C:N ratio and ammonium leaching from either horizon. There was a marginal relation between litter C:N ratio and DON leaching from the Oh horizon. For total inorganic N, a marginal linear relation with C:N ratio in the Eag horizon (Fig. 5b) reflected the weaker response of ammonium here. Increases in nitrate leaching from the Oh and the Eag layer occurred when litter N reached 1.7% and 1.8%, respectively, or below a C:N ratio of 31–32. These measures of litter N status corresponded to N inputs of approximately 100 kg haÿ1 yrÿ1 (Pilkington, 2003).

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M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40 2.5

H+ (keq ha-1 yr-1)

2.5

(a) Oh horizon

2.0

(b) Eag horizon

2.0 1.5

1.5 1.0

Flux: R2 = 0.20; P = 0.25

1.0

Flux: R2 = 0.46; P = 0.02

0.5

0.5 0.0

0.0 N derived flux: R2 = 0.52; P = 0.01

-0.5 0

50

100

N derived flux: R2= 0.42; P = 0.03

-0.5 0

150

50

Total N input (kg ha-1yr-1) Fig. 2. Effect of N input on volume-weighted annual hydrogen ion flux (filled circles, solid line) with annual volume-weighted N-derived hydrogen ion flux as calculated from Van Breemen’s equation (hollow circles, dotted line) in (a) the Oh horizon leachate and (b) Eag horizon solution. Trend lines were fitted by polynomial regression (P ! 0.05) on non-transformed full dataset with R2 Z regression coefficient. Other statistical tests are given in Table 4.

4. Discussion 4.1. Leaching and immobilisation of N Reduction of nitrogen inputs after passage through the Oh horizon increased from 63% at ambient N deposition (16.4 kg N haÿ1 yrÿ1) to a maximum of 92% for an additional 80 kg N haÿ1 yrÿ1 in the middle treatment. Although this increase was not significant, stimulation of retention in response to periodic treatments was also noted in forest studies by Duckworth and Cresser (1991) and Johnson (1992). With losses by denitrification from moorland systems estimated

between 0.1 and 0.3 kg N haÿ1 yrÿ1 (Macdonald et al., 1997; Sozanska et al., 2002), it was likely that these rates of reduction were mainly the result of retention within the system. High rates of retention of atmospherically deposited nitrogen equivalent to those found in this study have been previously noted by Nielsen et al. (1999) after passage through ericaceous soil in a Danish heathland, and by Adamson et al. (1998), at a depth of 10 cm in deep peat of the Moorhouse National Nature Reserve. Using 15N tracers in incubated Danish heathland soil, Kristensen and McCarty (1999) found immobilisation rates of 15 kg N haÿ1 over a 48-h period, sufficient to account for the rates of retention in the

50

(a)

12

R2 = 0.53; P = 0.01

Al (micromol. l-1)

Ca (microequiv. l-1)

14

10 8 6 4 2 0

(b)

40 30 20 10

R2 =0.37; P = 0.05

0 0

50

100

150

0

50

Total N input (kg ha-1yr-1) 0.30

150

50 R2 = 0.65; P = 0.001

(c )

0.25

Al (micromol. l-1)

Ca/Al (molar ratio)

100

Total N input (kg ha-1yr-1)

0.20 0.15 0.10 0.05 0.00 0

50

100

150

Total N input (kg ha-1yr-1)

40

(d)

30 20 R = -0.59 10 0 3.8

P = 0.02

4.0

4.2

4.4

4.6

4.8

pH

Fig. 3. Effect of N input on the volume-weighted annual concentrations of (a) calcium and (b) aluminium in the Eag horizon solution and (c) their molar ratio. Trend lines were fitted by polynomial regression (P ! 0.05) on non-transformed full dataset with R2 Z regression coefficient. Also included is (d), the relation between pH and aluminium in the Eag horizon solution, with R Z Pearson correlation coefficient. Other statistical tests are given in Table 4.

36

M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40

Table 5 Analyses of variance on the effects of individual N treatments (with Tukey post hoc comparisons) and of increasing N treatment (broken down into orthogonal polynomial components) on %C, %N and C:N ratio in the litter layer Parameter

ANOVA (treatments)

Tukey comparisons (with control)

ANOVA (increasing treatment)

df Total litter (LFH)

F

P

Treatment

T

P ! 0.05

F

P ! 0.05

Orthogonals

%C %N

3 3

1.3 11.7

0.33 0.002

linear

13.9

0.001

0.03 0.002 0.01 0.001

!0.001

3

3.52 5.33 ÿ4.10 ÿ6.11

34.2

C:N

ns control–middle control–high control–middle control–high

41.6

!0.001

linear

present study, even if maintained for only short periods in the field. Observations made whilst applying the treatments suggested that most of the ammonium nitrate solution in the spray adhered initially to the dense canopy and stems of the heather plants. Losses of N from surfaces in semi-natural systems to the atmosphere by volatilisation are relatively low (Phoenix et al., 2003) and canopy residence times may have been sufficient for considerable canopy uptake of nitrate (Edwards et al., 1985), as well as ammonium (Bobbink et al., 1990). In the Ruabon plots, treatment-related increases in cover (Pilkington, 2003) implied similar increases in plant productivity (Johnson et al., 1998) and root exudates rich in amino acids (Williams and Silcock, 1997). This may have accounted for the increase in the flux of amino acid-rich DON leaching from the organic layer of the low treatments over that of the control (Table 2 and Fig. 1). Root exudates are also rich in soluble carbon

(Jaeger et al., 1999), effective in promoting microbial activity and compatible with a treatment-related increase in microbial biomass and a change in the microbial community toward amino acid utilization observed in a study conducted on the same plots a few years earlier than the present investigation (Johnson et al., 1998). Indeed, soil and litter microbial communities have high affinity for low molecular weight amino acids (Jones and Hodge, 1999; Jones et al., 2004), accounting for low leaching losses of DON relative to N inputs as well as the observed pattern of peaks of DON followed by peaks of ammonium fluxing from the Oh horizon through time, especially in the control and low treatment plots (Pilkington, 2003). Although N inputs of the higher treatments may have escaped canopy uptake and been directly taken up by the same heterotrophic microbes, the key to the high retention of N in ericaceous soils may depend on the subsequent fate of this heterotrophic microbial biomass: There 40

2.0

(a)

1.8

(b) 35

C/N

1.6 1.4 R2 = 0.62

1.2

30 R2 = 0.66

25

P = 0.001

P = 0.002 20

1.0 0

50

100

150

0

50

Total N input (kg ha-1 yr-1)

100

150

Total N input (kg ha-1 yr-1) 35

54

(c)

(d)

30

53

25 20

52

15 R2 = 0.20

51

P = 0.24

10

R = 0.71 P< 0.001

5 0

50 0

50

100

150

Total N input (kg ha-1 yr-1)

0

20

40

60

N

Fig. 4. Effect of N input on (a) %N, (b) %C and (c) the C:N ratio of the litter layer. Trend lines were fitted by polynomial regression (P ! 0.05) on non-transformed full dataset, with R2 Z regression coefficient. Also included is (d), the relationship between %N and %C in the litter layer (with R Z Spearman correlation coefficient on rank-order transformed data). Other statistical tests are given in Table 5.

37

M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40 Table 6 Relations between litter nutrient status (%C, %N and C:N ratio) and the flux of N species from the Oh and Eag horizons NOÿ 3 -N

Litter parameters

C NOÿ 3 -N C NH4 -N

NHC 4 -N

Oh

Eag

Oh

Eag

Oh

DON

Eag

Oh

Eag

%C

0.39 0.13

0.59 0.02

0.11 0.68

0.41 0.12

0.14 0.61

0.52 0.04

0.62 0.01

ÿ0.23 0.40

%N

0.63 0.01

0.60 0.01

0.10 0.72

0.14 0.60

0.22 0.42

0.50 0.05

0.43* 0.10

ÿ0.16* 0.56

C:N

ÿ0.65 0.01

ÿ0.59 0.02

ÿ0.06 0.82

ÿ0.16 0.55

ÿ0.20 0.46

ÿ0.50 0.05

ÿ0.49* 0.05

0.18* 0.52

Cell contents: Correlation coefficient (upper), P-value (lower). Bold type indicates a significant correlation at P ! 0.05. Spearman rank correlation coefficient on rank-transformed data was used except where asterisks indicate Pearson correlation coefficient.

is evidence to suggest that high molecular weight polyphenol-bound N of insoluble proteins and peptides, released from microbial biomass, are resistant to breakdown and the primary factor limiting mineralization in agricultural grassland soils (Jones et al., 2004). From this ericaceous system, a considerably higher degree of recalcitrance in such proteinaceous material would be expected (Ponge, 2003), which, if strongly sorbed onto ericaceous peat would be prevented from leaching and thus retained. An acceleration of ammonium leaching from the Oh horizon between the N inputs of the middle and high treatments (Fig. 1b) may be interpreted as a saturation threshold above microbial and plant requirements (Aber et al., 1989), exceeding the capacity for biological transformation to organic N. A similar explanation was given by Tietema (1998) in NITREX forest sites to explain lower percentage retention of labelled N inputs in the organic layer at high N inputs (30–80 kg N haÿ1 yrÿ1) than at low N inputs (0–30 kg N haÿ1 yrÿ1). Saturation of cation exchange sites would also account for this pattern (Gundersen and Rasmussen, 1995; Tietema, 1998); however, after passage through the underlying mineral Eag horizon, ammonium flux was reduced to almost zero under all treatments, suggesting that the saturation threshold of cation exchange sites in this horizon had not been exceeded, even at high N inputs. Both mechanisms may have been in play, thus cation

exchange sites may have been to some extent protected and maintained by microbial immobilisation. 4.2. Chemistry of the soil solution Indeed the protection and maintenance of cation exchange sites by transformation of N inputs to organic N species (above) may have played a major role in regulating acidifying effects of deposition (Yesmin et al., 1995) and in preventing effects of N treatment on base cations in the Oh layer. Nevertheless, increases in acidity (Table 4) showed that protection was not complete. Treatment-related changes in potential proton generation as derived from N budgets (Table 2) suggested that the increasing part of the curve shown in Fig. 2a was due to a dominance of ammonium over nitrate deposition to this layer; the decreasing part due to an increase in ammonium fluxing from the layer (Van Breemen et al., 1984). The presence of organic acids would explain greater measured than predicted acidity in the Oh horizon leachate, while the steeper increase in measured acidity between the control and the low treatment may have been the result of higher root uptake rates of ammonium with production of hydrogen ions under these treatments (INDITE, 1994). Further control of soil pH by atmospheric inputs was shown by linear relations between temporal changes in hydrogen ion flux 12

(a)

10

Total N in. (kg ha-1 yr-1)

NO3--N (kg ha-1 yr-1)

12

8 6 4 2 0

(b)

10 8 6 4 2 0 -2

-2 28

30

32

34

36

38

28

30

32

34

36

38

C:N ratio Fig. 5. Relationship between C:N ratio and leaching losses of (a) nitrate-N and (b) total inorganic N from Oh (filled circles, solid line) and Eag horizons (open circles, dotted line). Trend lines were fitted by linear regression (P ! 0.05) on non-transformed full dataset, to complement linear correlation statistics given in Table 6.

38

M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40

Gundersen et al., 1998 and Emmett et al., 1998) than at Ruabon moor and two UK N addition lowland heath sites; Budworth Heath and Thursley Common (data taken from Power et al. 2005), gave a dichotomous distribution of data points (Fig. 6a). The only two Scandinavian NITREX forest sites (Ga˚rdsjo¨n and Klosterhede) with relatively low rates of nitrate leaching and relatively low responses to N additions of 35 kg N haÿ1 yrÿ1 (Fig. 6a, G2; K2) were likely to have developed a mor humus layer (Ponge, 2003). Ga˚rdsjo¨n is a natural forest with an understorey rich in Vaccinium sp., mosses and areas of Calluna as ground vegetation (Gundersen et al., 1998; Kjønaas et al., 1998). Mosses, particularly Sphagnum, in addition to Calluna, create nutrient poor, acidic humic layers rich in phenols and highly resistant to decomposition (Van Breemen, 1995). Klosterhede was also reported as having mosses in addition to a 7 cm organic mor humus layer (Gundersen and Rasmussen, 1995). Although NITREX forest sites with higher N leaching rates, such as Aber, Speuld and Ysselsteyn had ericaceous vegetation or mosses as understorey, it is likely that ploughing or other means of development of forest sites on former moorland (e.g. Aber) or heathland (Speuld and Ysselsteyn) had rendered this humic layer inactive or subject to disintegration by aeration and mixing with other mineral layers. Nitrate leaching from moorlands or forests with moss or ericaceous understorey vegetation and litter forming an intact mor humus, may therefore remain relatively small and unresponsive to N inputs, at least in the shorter term. A characteristic of such mor humus is a litter layer with high C:N ratios, maintained by chemical resistance to decomposition and mineralization and providing a historical perspective to current N deposition effects. When used instead of total N inputs as the determinant for N leaching, and the linear relationship with total inorganic N (Table 6 and Fig. 5b) was compared with NITREX forest data (Fig. 6b), there were consistently lower losses from the moorland/heathland treatment plots in addition to the same two forest sites (Ga˚rdsjo¨n

inputs in rain and flux outputs from both the Oh and Eag layers (Pilkington, 2003), a finding also reported for the LFH horizons of Calluna moorland podsols by White et al. (1996). For potential proton generation in the Eag horizon solution, the decreasing part of the curve (Fig. 2b) was due to decreases in the input of ammonium leaching from the organic layer and increases in the flux input of nitrate (Van Breemen et al., 1984). The increasing part was due to the relatively greater increase in ammonium inputs leaching from the organic layer of the high treatment plots. There was a close match between this and the observed response of acidity to increasing N treatment and fewer significant effects in this horizon compared to the organic could be attributable at least partly to base cation replenishment from weathering of the Eag horizon (Cresser et al., 2002). Overall, these changes suggested an inability of the organic horizon to immobilise the rates of N applied at the highest treatment; the responses of ammonium and nitrate to increasing treatment suggested a threshold of w100 kg N haÿ1 yrÿ1, due to either saturation or toxicity, with a high proportion of ammonium subsequently retained in the underlying Eag horizon. In this regard, Johnson et al. (1998) also found that a treatmentrelated increase in the rate of microbial activity appeared to plateau at this rate of input some years previously in the Oh layer of the same plots. In the underlying Eag horizon, curvilinear responses shown in Fig. 3, of calcium (due to cation exchange with ammonium (White and Cresser, 1999)), of aluminium (due to increases of acidity in Oh leachate (NEGTAP, 2001)) and of Ca:Al showed similar thresholds to those of ammonium and hydrogen ions fluxing from the Oh horizon. 4.3. Controls on N leaching from moorlands, heaths and forests Relatively higher rates of nitrate leaching in European N addition NITREX forest sites (data taken from Total N in. (kg ha-1 yr-1)

55

55

(a)

45

NITREX forests

35

NITREX forests Ruabon moor

35

Thursley common

25

(b)

45

Ruabon moor

Thursley common

25

Budworth heath

15

Budworth heath

15

G2 K2

5

Etherow

5

-5

-5 0

50

100

Total N input (kg ha-1 yr-1)

150

20

25

30

35

Litter C:N ratio

Fig. 6. Effects of total N inputs on total inorganic N leaching losses (a) and relations between litter C:N ratio and total inorganic N leaching losses (b) from N manipulated plots in Ruabon, two UK heathland sites (Budworth Heath and Thursley Common (open symbols, data taken from Power et al. 2005 and NITREX forests (closed squares, data taken from Gundersen et al. (1998) and Emmett et al. (1998)). G2 and K2 refer to effect of N additions at Ga˚rdsjo¨n and Klosterhede (see text). In (b), leaching loss from a runoff study for the Etherow river in the English Peak Dsitrict was included (cross symbol, data taken from Curtis et al. (2004)).

M.G. Pilkington et al. / Environmental Pollution 135 (2005) 29–40

and Klosterhede) which could now be explained by surface layer C:N ratios below a critical value of 25, supporting the use of C:N ratio as a universal predictor for leaching. Leaching losses from a runoff study in the Etherow in the English Peak District, one of the most nitrogen polluted regions in the country and the only runoff site dominated by Calluna, in common with Ruabon may have been higher than predicted by C:N ratio due to the toxic effect of 30–40 yr of high N, S and heavy metal deposition on microbiological and immobilisation capacity, and direct throughflow of high N inputs, failing to lower the C:N ratio (Curtis et al., 2004). High N losses at Etherow have also been attributed to the stimulation of nitrate leaching following use of burning as a management tool (Cresser, pers com). Furthermore, areas of bare peat, for which this area is known, may have contributed to particulate loss of eroded peat particles in suspension (Chapman and Edwards, 1999). This is particularly so following severe burning (Maltby, 1980) and delayed vegetative recovery under conditions of high N deposition (Pilkington, 2003). However, comparison between the two datasets is complicated by the different methodologies involved, in particular the greater length of time between source and sampling for runoff data and transfer through riparian zones, allowing mineralisation of organic forms of N.

5. Conclusions Although N additions increased nitrate leaching and saturation of the system was apparent in response to total N inputs greater than 96.4 kg haÿ1 yrÿ1, relatively high rates of retention were found at all rates of N input. There were linear relations between litter C:N ratios and nitrate leaching from both the Oh and Eag horizons but C:N ratio values were higher than a threshold below which there were substantially greater leaching losses from forests of the NITREX survey. High C:N ratios in moors, heaths and forests appeared to be related to the development of a mor humus layer.

Acknowledgements This study was part of a CASE PhD studentship within the Environmental Diagnostics Thematic Programme, in association with CEH Bangor and funded by the Natural Environment Research Council. Thanks are also due to Laurence Jones for advice and help with the design of equipment, to Val Kennedy and Jude Parrington of CEH Merlewood, to Ian Eastwood of MMU Cheshire for provision of facilities, along with Ian Drew, Jonathan Howell, Sandra Sandham, Leigh Cawley, Mark Hills and Deirdre Wilson for assistance.

39

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