Leaching of lime and fertilisers from a reseeded upland pasture on a stagnogley soil in mid-Wales

Agricultural watermanagement Agricultural Water Management 28 ( 1995) 95-112

Leaching of lime and fertilisers from a reseeded upland pasture on a stagnogley soil in mid-Wales S.P. Cuttle *, A.R. James Institute ofGrassland and Environmental Research, Plas Gogerddan, Aberystwyth, Dyfed SY23 3EB, UK

Accepted 17 November 1994

Abstract

Concentrations of nutrients were measured in drainage water from a drained but otherwise unimproved area of Molinia grassland and from a similar area following pasture improvement in Year 1

of the 4-year study. Application of lime and fertilisers to the improved area and reseeding with a grass/ white clover mixture had little effect on concentrations of nitrate and ammonium-N in drainage water except for one year when nitrate concentrations were increased to a maximum of 24 mg N l- ’ for a brief period following application of nitrogen fertiliser. Concentrations of organic-N, potassium, phosphorus and calcium and the pH of water samples increased following pasture improvement and were consistently greater than corresponding values for the Moliniu area. By Year 4 almost all of the potassium fertiliser applied to the reseeded area in Years 1 and 2 had been leached. In the same period, about 12% of the phosphorus fertiliser and 24% of the lime application had been leached. Keywords: Fertilisers; Leaching: Lime; Pasture; Upland; Water quality

1. Introduction

The productivity of native hill pastures in the UK is limited by climate, low productivity of the native species, soil acidity and shortages of available nutrients (Munro and Davies, 1973; Munro et al., 1973). The development of suitable land improvement techniques involving additions of lime and fertilisers and the introduction of improved grass species and white clover, together with the provision of financial assistance to encourage farmers to increase the productivity of their hill land, led to an expansion in the area of pasture improvement in the uplands during the 1970s and early 1980s. In Wales, much of the land remaining available for improvement was represented by native grassland on peaty soils overlying impermeable subsoils. In these high rainfall areas, * Corresponding author. Tel.: 01970 828255; fax: 01970 828357 0378-3774/95/$09.50

SSDIO378-3774(95)01

0 1995 Elsevier Science B.V. All rights reserved

174-9

96

S.P. Cuttle, A.R. James/Agricultural

Water Management 28 (1995) 95-112

these soils are waterlogged for much of the year and excess water is an important factor influencing their management after improvement. Artificial drainage would be expected to reduce the risk of poaching by stock and increase the productivity and persistency of sown species. However, these sites, even after improvement, are of limited agricultural potential and information is required for assessing the cost-effectiveness of any drainage measures. The changes in agricultural management associated with pasture improvement are also of significance in the wider context of land use and water resources. The uplands of Wales are an important source area for the water supply industry, traditionally providing high quality water, largely free of contamination from industry and intensive agriculture. The expansion in the area of pasture improvement in these upland catchments raised concerns that intensification of agricultural activity would lead to increased leaching of nutrients, particularly nitrogen and phosphorus, and reductions in water quality. Conversely, pasture improvement may in some circumstances have beneficial effects on water quality where the application of lime results in increased leaching of bases and neutralisation of the acidity arising from atmospheric acid deposition. By the end of the 1980s this period of agricultural intensification had been replaced by one of increased concern about overproduction of agricultural commodities and the loss of semi-natural habitats. If future policies lead to a reduction in agricultural activity in the uplands, it is probable that inputs of lime and fertilisers to pastures on these relatively difficult-to-manage soils will be reduced. Predictions of how land-use changes may influence water quality require an understanding of rates of depletion of fertiliser nutrients from improved pastures and the contribution that leaching currently makes to nutrient loadings to surface waters in these upland areas. This paper describes an experiment that was started in 1983 to compare the cost-effectiveness of alternative drainage techniques and to measure leaching of nutrients following reseeding of an area of Molinia grassland. Results of the leaching study are presented and interpreted in terms of the effects of pasture improvement on water quality, the likely duration of these effects and fertiliser inputs required for the maintenance of reseeded swards on peaty soils. Results of the drainage comparisons are not considered in detail.

2. Materials and methods 2.1.

Site

The experiment was established on a relatively uniform area of unimproved moorland at Pwllpeiran Experimental Husbandry Farm, near Aberystwyth (SN 789756) in mid-Wales. The site was at an altitude of 400 m with a slope of about 4” and a southerly aspect. Soils were cambic stagnohumic gleys of the Wilcocks series (Rudeforth et al., 1984) with a 0. l0.3 m depth of humified peat overlying a clay loam subsoil developed from compacted drift material. At the start of the experiment, the vegetation was dominated by tussocks ofMolinia caerulea L. with less abundant Eriophorum uaginatum L., Vaccinium myrtillus L., Juncus squarrosus L., Festuca spp. and mosses. A drainage trial had been conducted at the site between 1960 and 1968 and the open ditches which survived from this earlier work defined the layout of the present experiment

S.P. Cuttle, A.R. James /Agricultural

Water Management 28 (1995) 95-112

97

u

AWS

0 0

0

0

D2

0 0 0

0

0

D3

’ 0

0

0

0

0

0

0 0

D4 0 0

0

0

0

50 m

I

Dl 0

0

Fig. I. Plan of the leaching experiment at Pwllpeiran showing the reseeded area (shaded) and Molinia area with cut-off ditch (ABC), collection ditches leading to weirs/sampling points (Wl and W2) and position of the automatic weather station ( AWS). Dipwell positions are indicated by open circles. See Table 1 for a description of the drainage treatments (DlLD4) applied to subplots.

(Fig. 1) . This provided two hydrologically isolated areas, each of approximately 1 ha. Water draining from each area was collected by separate 1 m-deep ditches along their lower sides while a 0.6-1.0 m-deep cut-off ditch along the upper boundary prevented entry of water from outside of the experimental area. At the start of the investigation the experimental area was drained as part of an associated study by the Agricultural Development and Advisory Service to compare the cost-effectiveness of alternative drainage treatments. The two halves of the area were divided into subplots with the same four drainage treatments applied to both halves (Table 1). Although Table I Drainage treatments

installed in the reseeded and Mdiniu areas in 1983-1984

Drainage treatment

Drain spacing (m)

Drain depth (mm)

Dl D2 D3

11 22 22 1 5

750 650 650 450 450

D4

Drains (plastic pipe) Drains with permeable till Drains with permeable fill + mole drains Mole drains alone, with gravel in mole channel and slot

98

S.P. Cuttle, A.R. James/Agricultural

Water Munagement 28 (1995) 95-112

this subdivision introduced undesirable complications into the interpretation of the leaching data, both of the hydrologically separated areas contained the same treatments, differing only in their relative positions, and for the purpose of the nutrient leaching studies the overall hydrological properties of the two areas were assumed to be similar. Plastic underdrains were installed in November 1983 and moling operations were carried out in October 1984. An automatic weather station adjacent to the main plots provided hourly values of rainfall which were supplemented by weekly values from a standard bulk collection gauge. The automatic station was removed in April 1987 and only the weekly rainfall measurements were continued after this date. The automatic weather station also provided data for the calculation of potential evapotranspiration using the Penman-Monteith equation for a short grass surface (Monteith, 1965). Soil moisture deficits in the rooting zone (to about 700 mm depth) in Years 1 to 3 were estimated from the difference between rainfall and evaporation, assuming that the first 50 mm of available water in the soil profile would be evaporated at the potential rate and the next 50 mm at half this rate (Smith, 1976). Water table depths were measured at weekly intervals in a series of dipwells installed in the drainage subplots and in adjoining undrained areas following the completion of moling operations in autumn 1984. 2.2. Pasture establishment

and management

The treatments included in the earlier drainage trial involved the excavation of shallow surface ditches and installation of tile drains but no other pasture improvement measures. For the present experiment, one of the areas was left as Molinia grassland and the other reseeded using a minimal cultivation technique appropriate to wet hill land conditions. In December 1983, a rotary mower was used to remove Moliniu tussocks from the area that was to be improved. Lime was applied in April 1984 (Table 2) and two weeks later the area was sown with a seed mixture of S.23 and Perma perennial ryegrass (Lolium perenne L.), S.59 red fescue (Festuca rubra L.), S.48 timothy (Phleum pratense L.) and S.184 white clover (Trifolizdm repens L.). The strip seeder that was used limited soil disturbance to the cultivation of 25 mm-deep strips affecting about 30% of the ground surface. A Rhizobium inoculum was applied to the sward in June 1984. A compound NPK fertiliser and triple superphosphate were applied at the time of sowing and further dressings of compound fertiliser were applied in June and August 1984. Details of the quantities of nutrients supplied as fertiliser are given in Table 2. In each subsequent year, ammonium nitrate fertiliser was applied to the pasture in April. A further dressing of PK fertiliser, corresponding to the recommended maintenance requirements for a 3-year period (Ministry of Agriculture, Fisheries and Food, 1973; Ministry of Agriculture, Fisheries and Food, 198 I ), was applied in August 1985. No lime or fertilisers were applied to the Molinia area. The abundance of native and sown species was assessed in autumn each year by recording the ground cover of each species within randomly positioned quadrats. The two areas were fenced and grazed as separate units but without further subdivision between drainage subplots. Both areas were grazed continuously by Welsh Mountain sheep between April and October with sheep numbers adjusted on the basis of visual assessments of the herbage available. Mature sheep were used throughout the experiment except for part

S.P. Cuttle, A.R. James /Agricultural Water Management 28 (1995) 95-112 Table 2 Quantities Application

99

of lime and fertiliser nutrients applied to the reseeded area in each year of the investigation date

Quantity applied Lime (t ha-‘)

1984 9 April 24 April 25 June 29 August I985 22 April I August 1986 30 April 1987 28 April 1988 12 April

P (kg ha-‘)

K (kg ha-‘)

29 49 25

41 11 5

12 20 10

_

65 _

40

_

65

_

65

8.8

N (kg ha-‘)

_

_ 15

_

50

of the 1986 season when the reseeded area was stocked with ewes and lambs. Average stocking rates were 15 mature sheep ha-’ on the reseeded pasture and five sheep ha-’ on the Molinia area. 2.3. Flow measurements

and water sampling

The volume of water draining from each area was measured by separate V-notch weirs and water level recorders installed on the main outlet ditches (Wl and W2 in Fig. 1). Recorder traces of water level at the weir were digitised and converted to discharges using a standard equation (British Standards Institution, 1965). The weirs did not operate satisfactorily when the water in the weir channels was frozen and discharges in these cases were estimated from the water balance for the affected period. Water samples for chemical analysis were collected from the outflow at each weir. Bulk samples of water were collected on a weekly basis via a sampling tube inserted through the weir plate 5 mm below the level of the vertex of the V-notch and leading to a collecting bin. Initially, the how through the sampling tube was controlled by a constriction which limited the volume collected during the weekly sampling periods. In September 1985 this arrangement was replaced on both weirs by a flow-proportional sampler providing a composite sample made up of sample increments collected at 30 min intervals (Cuttle and Mason, 1988). The collecting bins were protected from sunlight but no preservatives were added to their contents. Samples of rainfall were also obtained from two open-funnel collectors at the site. 2.4. Analysis

of water samples

The pH of drainage water and rain-water samples was measured immediately on return to the laboratory using a glass combination electrode. Samples were filtered through a glass

100

S.P. Cuttle. A.R. James /Agricultural

Water Management 28 (1995) 95-112

microfibre filter ( 1.2 pm particle retention) before storage at 4°C. Concentrations of nitrate + nitrite-N, ammonium-N and orthophosphate-P were measured calorimetrically (Murphy and Riley, 1962; Henriksen and Selmer-Olsen, 1970; Crooke and Simpson, 1971) . Concentrations of nitrite were assumed to be negligible and results of the nitrate + nitriteN analyses are reported as nitrate-N. Potassium and calcium concentrations were measured by llame emission and atomic absorption spectroscopy, respectively. Analyses were normally completed within 3 days of sample collection. From November 1986, additional analyses were included to determine ‘total’ contents of nitrogen, phosphorus and potassium in unfiltered water samples after digestion with a sulphuric acid/hydrogen peroxide mixture (Allen, 1989).

3. Results 3.1. Vegetation changes and establishment

of the reseeded pasture

There was little change in the botanical composition of the vegetation on the unimproved area during the experiment. Molinia remained the dominant species, representing 57-64% of the ground-cover at the time of the botanical surveys in the autumn of each year. Although there was an apparent increase in the proportion of ground occupied by mosses, from 6% prior to drainage to 19% in the final year of the investigation, this may have been a result of increased grazing pressure exposing more of the basal vegetation. The sown species established successfully on the reseeded area. Although the total proportion of the ground surface occupied by sown species remained relatively constant at between 83 and 90%, the relative proportion of individual species changed during the study. Perennial ryegrass was the most abundant initially (5 1% ground-cover) but was replaced by red fescue as the dominant species in 1986 and 1987 (45 and 36% ground-cover, respectively). Timothy remained a minor component of the sward throughout the investigation. The proportion of clover increased from 3% initially to 13% in autumn 1987. 3.2. Hydrology of the site and water-flows Results were analysed on the basis of hydrological years from April to March, referred to as Years l-4. Annual rainfall, evaporation and drainage from the two areas are summarised in Table 3. Maximum soil moisture deficits in Years l-3 were 70, 21 and 46 mm, respectively. Moisture deficits could not be calculated for Year 4 because of the absence of evaporation data. There was considerable difference in the effectiveness of the drains in lowering water tables within individual drainage subplots and poor agreement between duplicate treatments in the reseeded and Molinia blocks. In the worst cases, water tables were little different from those in adjoining undrained areas. In spite of the variation between subplots, mean water table levels over the reseeded area as a whole were similar to those for the Molinia area. In view of this similarity and the apparent independence of water tables levels and drainage treatments, it was considered valid to regard the overall hydrology of the two drained areas as being broadly similar. Fig. 2 shows the weekly variation in water table

S.P. Cuttle, A.R. James /Agricultural Water Management 28 (1995) 95-112

101

Table 3 Annual rainfall, evaporation and measured discharge from the Molinia and reseeded areas (April-March) with discharges expressed as a percentage of the flow predicted from the water balance. Evaporation was not measured in Year 4 Period

Rainfall (mm)

Year 1 1364 Year2 1950 Year3 1788 Year4 2076

Evaporation

423 407 365

(mm)

Drainage balance)

(mm) (and as % of value predicted from the water

Molinia area

Reseeded area

991 1635 1465 1783

1020 1727 1574 1768

(105) (106) (103) (-)

(108) (111) (110) (-)

level during the experiment expressed as means of the values for all dipwells within the drainage subplots. Over the full period of the experiment, the mean depth to the water table in these drained areas was 233 mm, compared with a mean of 178 mm for the adjoining undrained areas. Patterns of water flow recorded at the weirs were also similar for the Molinia and reseeded areas, Weekly discharges are shown in Fig. 2 as means of the values for the two areas. Hydrographs were characterised by rapid responses to rainfall events, generally within less than an hour of the onset or cessation of rainfall. These rapid fluctuations were superimposed on a base flow that continued through all but the driest periods. Even though Year 1 was abnormally dry, there were only 6 weeks during the summer when no flow occurred. Measured values of drainage per unit area were slightly higher for the reseeded area than for the Molinia area in Years 1 to 3 (Table 3). The relatively lower discharge from the reseeded area in Year 4 was due, at least in part, to a slight leakage of water around the weir -0

2000

-

‘Cl C mL 1500

-200

-2 .5 a, ii s

%J b 1000~ 5 v) 6 >\ 3 8 Sz

-400

$ P 0

500.

-600

0 : I-

I_

Year

3

Year

,’ ‘Ta E

-800

4

Fig. 2. Weekly drain discharge (bars) and level of the water table below the ground surface (dashed line) in Years l-4 (April-March). Drain discharges are means of values for the Molinia and reseeded areas. Water table levels are means of values from all dipwells within the drainage subplots.

102

S.P. Cut&

A.R. James /Agriculturul

Water Management 28 (1995) 95-112

during the summer. Both weirs and recorders performed satisfactorily for about 90% of the time. Except during freezing weather, there were few occasions when both weirs were nonoperational at the same time and data from one weir could generally be used to substitute for missing values from the other. The hydrological integrity of the areas was examined by comparing measured discharges for Years 1-3 with estimates of flow obtained from the water balance, determined as the difference between annual rainfall (April-March) and evaporation, assuming that differences in soil water storage between the start and end of the year were negligible (Table 3). Measured discharges from the Molinia and reseeded areas in Years l-3 were, respectively, 5 and 9% greater than those predicted from the water balance. These discrepancies are within the range of errors expected from measurements of rainfall and stream flow and provide little evidence of significant entry of water from outside the areas or of loss other than via the weirs. Fluctuations in the nutrient content of drainage water from the Molinia area were independent of fertiliser applications to the reseeded area and neighbouring areas, further indicating that the two areas were hydrologically isolated. 3.3. Solute concentrations

andpH

of drainage water

Concentrations of inorganic solutes and the pH of water samples from the Molinia and reseeded areas are shown in Fig. 3 < FIGR > 4 < /FIGR > < FIGR > 5 < /FIGR > . Reseeding and the application of lime and fertilisers increased the concentrations of all the nutrients studied. Increases in nitrate- and ammonium-N concentrations were confined to short periods following fertiliser applications (Fig. 3). These increases were generally small ( < 3 mg N 1- ’), except in Year 2 when the nitrate concentration in the bulk sample reached a peak of 23.6 mg N I ~ ’ two weeks after the fertiliser had been applied. Although this was considerably in excess of the European Community limit of 11.3 mg N 1-l for drinking water supplies (European Economic Community, 1980)) subsequent samples contained less than 4 mg N 1- ’ The nitrate peak was accompanied by a corresponding peak concentration of 3.7 mg ammonium-N I- ‘. During the course of the study, water samples from the Molinia area contained between 0.01 and 1.8 mg nitrate-N l- ’and a similar range of ammonium-N concentrations. Concentrations of soluble orthophosphate in water samples from the Molinia area and in rainfall were frequently below the analytical limit of detection ( < 0.002 mg P 1~ ‘) and at no time exceeded 0.006 mg P 1-l. Much larger concentrations were measured in samples from the reseeded area following the applications of phosphorus fertiliser, particularly after the maintenance dressing of PK fertiliser in Year 2 (Fig. 4). The sample collected in the week following this application contained 1.3 mg P 1- ‘, indicating that the moderately high rainfall that week was sufficient to provide rapid transport of dissolved fertiliser to the drains. Care was taken to avoid spreading fertiliser directly into the open ditches. During the following month, concentrations declined to less than 0.4 mg P 1-l but remained well above those from the Molinia area for the remainder of the investigation. Potassium fertiliser was applied to the reseeded area on the same dates as phosphorus and patterns of loss were similar to those described above (Fig. 4). The initial applications in Year 1 increased concentrations slightly compared to the Molinia area but the application in August of Year 2 had a much greater effect and was followed by a peak concentration of

S.P. Cuttle, A.R. James /Agricultural

Water Management 28 (1995) 95-112

103

(i) Ammonium-N

I

Year 2

I

Year 3

(ii)

Year 1

Year 2

Year 3

I

Year 4

Nitrate-N

Year 4

Fig. 3. Concentrations of (i) ammonium-N and (ii) nitrate-N determined in non-digested Molinia area (solid line) and reseeded area (dotted line) in Years l-4 (April-March).

water samples from the

26 mg K 1-l. Over the next 3 weeks, concentrations fell to less than 6 mg 1-r and then declined more gradually until by the end of Year 4 they were approaching those from the unfertilised Molinia area. Water samples from the Molinia area contained between 0.1 and 2.3 mg K l- ‘. Liming increased both the calcium content and pH of drainage water from the reseeded area. Calcium concentrations and pH values increased during the summer following the lime application and were consistently greater than those from the Molinia area for the remainder of the investigation (Fig. 5). Water samples collected from September of Year 1 onwards contained between 5.8 and 41.9 mg Ca 1-l and pH varied between 5.7 and 7.8.

S.P. Curtle. A.R. James/Agricultural Water Management 28 (1995) 95-112

104

(i) Phosphorus

.;.. :;..

i I

I

Year 1

-;30 E

Year 2

1

Year 1

I

Year 3

I

Year 4

(ii) Potassium

Year 2

Year 3

Year 4

Fig. 4. Concentrations of (i) orthophosphate-P and (ii) potassium determined in non-digested water samples from the Mdinia area (solid line) and reseeded area (dotted line) in Years 1-4 (April-March). Concentrations of phosphorus in samples from the Mdinia area were all less than 0.01 mg I-’ and do not appear on the graph.

In contrast, water samples from the Moliniu area contained between 0.4 and 5.0 mg Ca 1~ ’ and varied in pH from 3.8 to 6.8. In the case of the unlimed area, pH values greater than 5 occurred more frequently from the middle of Year 2 onwards, after installation of the flowproportional samplers, although moderately high pHs also occurred in samples collected from both areas prior to liming. In the case of samples collected from Year 2 onwards, and as observed elsewhere (e.g. Jenkins et al., 1991)) there was an inverse relationship between pH and discharge. Quantities of nutrients leached from the areas were calculated from solute concentrations in water samples and the corresponding weekly drainage volumes. Annual losses are sum-

S.P. Cuttle, A.R. James /Agricultural Water Management 28 (1995) 95-112

105

(i) Calcium

I

I Year 1

Year 3

Year 2

Year 1

Year 2

Year 4

I Year 3

Fig. 5. (i) Concentrations of calcium and (ii) pH of non-digested line) and reseeded area (dotted line) in Years l-4 (April-March).

I Year 4

water samples from the Molinia area (solid

marked in Table 4, together with similarly calculated values for the input of nutrients in rainfall. As phosphorus concentrations were unavailable for part of Years 3 and 4, the annual totals for these two years were estimated using the average of the concentrations from before and after this period in place of the missing values. Greater quantities of phosphorus, potassium and calcium were leached from the reseeded area than from the unfertilised Moliltia area. Only in Year 2 was the loss of mineral-N (nitrate + ammonium-N) from the reseeded area appreciably greater than that from the Moliniu area. Differences in the quantities leached each year largely reflected variations in nutrient concentration rather than differences in drainage volumes.

106

S. P. Cuttle, A.R. James /Agricultural

Water Management 28 (1995) 95-112

Table 4 Quantities of nutrients supplied in rainfall and leached from the Molinia and reseeded areas annually (ApriMarch), as derived from the analysis of non-digested water samples. Phosphorus losses in Years 3 and 4 include some estimated values Input in rain and quantity leached (kg ha-’ yr- ‘)

Mineral-N (NO, f NH,) Rainfall Molinia area Reseeded area Phosphorus Rainfall Molinia area Reseeded area Potassium Rainfall Molinia area Reseeded arca Calcium Rainfall Molinia area Reseeded area

3.4. Concentrations

Year 1

Year 2

Year 3

Year 4

1.9 3.8 3.5

1.2 4.1 10.0

1.7 4.1 4.6

7.8 3.0 2.7

< 0.01 < 0.02 0.33

< 0.02 < 0.02 3.15

< 0.02 ( < 0.03) ( 1.22)

< 0.02 ( < 0.04) (0.75)

< 0.07
0.3 4.6 8.6

1.4 5.0 63.6

1.8 6.5 19.3

2.7 3.9 12.2

6.2 20.0 103.7

0.4 14 117

2 12 324

of ‘total’ nitrogen, phosphorus

4 13 258

5 14 246

Total

24.5 15.0 20.7

I1 53 946

and potassium in water samples

Water from both areas, but particularly the reseeded area, was often strongly coloured, indicating that these samples contained an appreciable content of dissolved organic matter. Organic-N and -P, if present in the samples, would not be detected by the calorimetric methods used in the study although the flame spectroscopy determinations of potassium and calcium would be expected to include some organic components. Analysis of acid digests of unfiltered samples collected in the later part of the study provided a measure of ‘total’ nutrient content. As well as simple inorganic solutes, these determinations would include organic species, condensed phosphates and nutrients displaced from suspended mineral material. Concentrations of ‘total’ nitrogen, phosphorus and potassium determined after acid digestion are summarised in Table 5. In almost all cases, greater concentrations were measured in digested than in non-digested samples. However, there was no consistent pattern of variation between increases in the concentration of one nutrient and those of another, indicating that the digestion process released nutrients from different sources in each case. Concentrations of ‘total’ nitrogen in water samples from the reseeded area were appreciably greater than those from the Molinia treatment. This is in contrast to concentrations of mineral-N which were generally similar for both areas except for brief periods following application of nitrogen fertiliser. On the basis of ‘total’ contents, the quantities of nitrogen leached from the Molinia and reseeded areas in Year 4 would be approximately 15 and 30 kg N ha-‘, respectively. The increased concentrations of nitrogen measured in samples after digestion are assumed to be mainly due to organic-N. The greater ‘total’ nitrogen

S.P. Cuttle, A.R. James/Agricultural

Water Management 28 (1995) 95-112

107

Table 5 Mean concentrations of mineral-N, orthophosphate and potassium in filtered, non-digested water samples collected between November 1986 and March 1988 and ‘total’ contents determined after digestion of unfiltered samples Nutrient contents of non-digested Mean (and range) (mg 1-l)

N content - non-digested - digested P content - non-digested - digested K content - non-digested - digested

and digested water samples

Molinia area

Reseeded area

0.23 (0.06-0.73) 0.70 (0.24-3.13)

0.25 (0.05-1.10) 1.77 (0.41-6.86)

< 0.002 ( < 0.002-0.004) 0.020 ( < 0.002~.048)

0.056 (0.032-0.108) 0.146 (0.032-0.563)

0.26 (0.10-0.60) 0.74 (0.05-4.34)

0.86 (0.1 l-1.50) 1.49 (0.374.61)

content of water samples from the reseeded area, together with their stronger coloration, suggests that mineralisation of soil organic matter was increased by the lime and fertiliser applications. Hornung et al. ( 1986) reported that soil solutions from limed upland pastures contained greater concentrations of dissolved organic carbon than did samples from unimproved moorland. The largest proportional increase as a result of digestion occurred in the case of phosphorus concentrations in samples from the Moliniu area. ‘Total’ concentrations were generally more than an order of magnitude greater than concentrations measured in non-digested samples. Soluble organic phosphorus compounds may represent a significant proportion of the total dissolved phosphorus in soil solution, particularly in the case of organic soils and those of low nutrient status (Harrison, 1987; Ron Vaz et al., 1993). Digestion may have also released adsorbed phosphorus from the surface of silt and clay particles present in the unfiltered samples. The transport of phosphorus sorbed on suspended sediments has been identified as an important contributor to the loss of phosphorus from catchments (e.g. Bargh, 1978). In the case of water samples from the reseeded area, relative differences between concentrations of soluble orthophosphate and ‘total’ phosphorus were smaller but are of greater significance in terms of absolute concentrations. Estimates of fertiliser loss based on ‘total’ contents would be approximately double those in Table 4. In the case of potassium concentrations, the increases brought about by digestion were similar for samples from both areas and estimates of fertiliser loss based on ‘total’ concentrations are therefore little different from those calculated using values for non-digested samples.

4. Discussion The drained Molinia area represents a situation that is unlikely to occur in agricultural practice as the cost of draining this type of land would not be considered worthwhile except

108

S.P. Cuttle. A.R. Jumes/Agricultural Water Management 28 (1995) 95-112

as part of a comprehensive pasture improvement scheme involving reseeding and the application of fertilisers. As drainage would be expected to improve soil aeration and modify the hydrological pathways by which solutes are transported, leaching data from the Molinia treatment cannot be considered to be representative of unimproved upland pasture. The absence of equivalent undrained areas prevented a separate assessment of the effects of drainage; however, the quantities and concentrations of solutes leached from this area are similar to those reported from studies on unimproved moorland elsewhere in the UK (Crisp, 1966; Edwards et al., 1985; Hornung et al., 1985; Reynolds et al., 1989)) indicating that in this instance the effects of drainage were small. This may be a reflection of the limited disturbance of the site and the ineffectiveness of the drainage treatments over much of the area. Quantities of mineral nitrogen and orthophosphate leached from the Molinia area were similar to the input of these nutrients in rainfall (Table 4). Although this indicates that nutrient balances had not been disturbed by drainage, estimates of nitrogen and phosphorus losses based on ‘total’ concentrations are greater and, particularly in the case of phosphorus, indicate a net depletion of these elements that may be associated with increased mineralisation of soil organic matter. Quantities of potassium and calcium leached from the Molinia area both exceeded the input in rainfall, irrespective of whether losses were calculated from solute or ‘total’ concentrations. The Lower Palaeozoic rocks from which the local soils are derived are relatively rich in potassium but contain little calcium (Adams and Evans, 1990) and the net depletion of this element is less readily explained. Reynolds et al. ( 1987) also reported a net depletion of calcium from an undrained, unimproved moorland catchment on a similar lithology elsewhere in mid-Wales. The Molinia and reseeded areas both included similar drainage treatments and differences in the quantities of nutrients leached from the two areas could therefore be attributed to effects of reseeding and the applications of lime and fertilisers. The greater loss of ‘total’ nitrogen from the reseeded pasture compared with the Molinia area may be a reflection of the greater input of nitrogen or of increased mineralisation of soil organic matter as a result of liming and application of other nutrients (Shah et al., 1990). The only significant loss that could be directly attributed to the application of nitrogen fertiliser occurred in Year 2 of the study when nitrate concentrations increased following the application. In this instance, a maximum rainfall intensity of 15 mm in 10 h was recorded in the week during which the nitrate peak occurred. Though relatively modest, this was greater than the maxima recorded following nitrogen applications in other years and appeared to have been sufficient to promote rapid transport of dissolved fertiliser to the drains. The loss of nitrogen in the 5 weeks following application was equivalent to 9% of the fertiliser applied and accounted for almost half of the total quantity of mineral-N leached that year. In addition to the nitrogen applied as fertiliser, clover in the reseeded pasture would also have supplied nitrogen through symbiotic fixation of atmospheric nitrogen. Goodman ( 1988) estimated that a similar sward atthesamesitefixed 32 kgNhaa’ year-‘. Other estimates of fixation for improved pastures on peaty soils indicate larger inputs of 92-125 kg N ha- ’year- ’ (Newbould, 1982). These values indicate that in the present investigation, the use of fertilisers and introduction of clover would have increased the input of nitrogen to the reseeded area by at least 100 kg N ha-’ year- ‘; however, leaching appeared to account for only a small proportion of this increased input. On a wet site such as this, with a plentiful supply of organic carbon,

S.P. Cuttle, A.R. James /Agricultural

Water Management 28 (1995) 95-112

109

denitrification may be the dominant form of nitrogen loss. Net immobilisation may also contribute to the apparent loss of nitrogen following application of fertiliser to improved peat soils (Williams and Wheatley, 1992). A much greater proportion of the potassium fertiliser applied to the reseeded area was leached. Estimates derived from solute concentrations in undigested water samples indicate that 72% of the fertiliser applied in Years 1 and 2 had been leached by the end of the experiment. Although relatively little of the fertiliser applied in Year 1 was leached, the maintenance dressing in Year 2 appeared to exceed the capacity of the soil to retain potassium and subsequent losses were greater. This later application was intended to provide sufficient potassium to maintain the nutrient status of the soil for 3 years. In the 2.5 years between the application and the end of the investigation, the increased loss from the reseeded area accounted for almost all of the potassium supplied in the maintenance dressing. In this time, potassium concentrations had fallen to values similar to those in drainage from the Moliniu area. Although these results indicate that the effects of potassium fertiliser are likely to persist for 2-3 years and broadly confirm the accuracy of the recommendations, they also demonstrate the poor utilisation of the applied fertiliser: 64% of the potassium applied in the summer of Year 2 had been leached before the start of the following growing season. Although the application of fertiliser had a marked effect on concentrations of orthophosphate in drainage water, the quantities involved were relatively small. The greater loss from the reseeded pasture in Years l-4 compared with the Moliniu area was equivalent to only 6% of the fertiliser applied. Revised estimates based on concentrations of ‘total’ phosphorus determined in digested water samples in Years 3 and 4 indicate that the total loss was unlikely to have exceeded 12% of the phosphorus applied. The effects of phosphate applications are therefore likely to persist for much longer than those of potassium fertiliser. As little phosphorus is exported from these grazed pastures as livestock or by other means, repeating the maintenance dressing every 3 years as recommended would be expected to lead to a considerable net accumulation of phosphorus in the soil and the possibility of an increased rate of loss. Broadly similar patterns of fertiliser loss were observed by Roberts et al. ( 1989) following improvement of an upland pasture on an area of stagnohumic gleys and ironpan stagnopodzols at Plynlimon, 11 km north of Pwllpeiran. The pasture was reseeded using a minimal cultivation technique similar to that in the present study but no artificial drains were installed. As at Pwllpeiran, nitrate concentrations in drainage remained small with only occasional peaks greater than 1 mg N l- ‘. Concentrations of orthophosphate increased following reseeding and reached a peak value of 2 mg P l- ’4 months after fertiliser had been applied. The loss of phosphorus in the establishment year was estimated as 5 kg P ha-‘, similar to that measured in the present investigation. The same authors reported contrasting results from a second site at Plynlimon. In this case, pasture improvement on a drained peat soil had little effect on phosphorus and potassium concentrations in drainage but increased nitrate concentrations considerably with annual leaching losses of 14-61 kg N ha-‘. The greater loss of nitrogen appeared to result from increased mineralisation of soil organic matter following drainage and cultivation rather than from fertiliser applications. The use of disc harrows to cultivate the site would have disturbed the soil far more than the minimal cultivation technique used at Pwllpeiran. Losses of phosphorus and potassium may have been less than at Pwllpeiran because the fertilisers were worked into the soil and much of

110

S.P. Cuttle. A.R. James/Agriculturul

Water Management 28 (1995) 95-112

the phosphorus was applied as relatively insoluble basic slag. It should also be noted that losses of phosphorus and potassium were small at both sites in the reseeding year: at Pwllpeiran, it was only after the additional maintenance dressing in Year 2 of the study that appreciable losses occurred. The pH and calcium content of water samples from the reseeded area at Pwllpeiran increased following liming and showed little tendency to decline during the course of the experiment. Soil pH was increased by liming and also remained relatively constant during this period. The pH of the O-50 mm soil depth, which increased from 3.5 prior to liming to 5.6 at the start of Year 2, remained at 5.5 when measurements were repeated in Year 4. As observed in other studies on peat soils (Dampney, 1985)) effects of liming were confined to a relatively shallow depth. Soil pH between 50-100 mm depth increased from 3.8 in Year 2 to 4.5 in Year 4 but was little changed below this. Differences between the quantities of calcium leached from the two areas indicated that 24% of the lime application had been leached by the end of Year 4. This corresponds to an annual loss of about 0.5 t lime ha-‘. At this rate, the lime applied at reseeding would be fully leached in about 17 years. However, it is more likely that the leaching rate would decline as the reservoir of calcium in the soil was depleted with the result that the effects of liming would persist for longer but at a lower intensity. In a study of upland pastures at Plynlimon reported by Hornung et al. ( 1986), concentrations of calcium in soil water samples from improved pasture were greater than those from unimproved moorland, even though lime had last been applied 40 years earlier. Adams and Evans (1989) modelled rates of calcium depletion using data from a study of soil properties and drainage water chemistry from an upland pasture in mid-Wales and similarly concluded that the effects of liming would persist for 3040 years. Comparative values for rates of calcium depletion in the initial 2-3 years after liming are available from an investigation in which different liming strategies for ameliorating surface water acidification were applied to stream catchments draining into the Llyn Brianne reservoir in mid-Wales (Jenkins et al., 1991). Blanket liming of a moorland catchment at a rate of 9 t lime ha- ’, but without other pasture improvement measures, resulted in an annual loss equivalent to about 1% of the lime applied. Lime applied at 16 t ha-’ to the 5 ha hydrological source area within a 54 ha catchment resulted in an annual loss of about 12% of the total applied. This latter treatment equates most closely to the reseeded area at Pwllpeiran, both hydrologically and in terms of the rate of lime depletion, which at Pwllpeiran was equivalent to 6% per year following an application of 8.8 t lime ha-‘. Agricultural improvement schemes in the uplands often involve only part of the total catchment area and the overall impact on water quality will be less than that indicated by studies of nutrient leaching from relatively small areas of uniformly improved land. Roberts et al. ( 1989) stressed the importance of extrapolating the results from isolated areas to the catchment scale and also the difficulties associated with this process. In the simplest case, all areas of a catchment may be assumed to contribute equally to streamflow so that leaching data from improved areas may be extrapolated to the larger scale on the basis of the proportion of the total catchment that has been subjected to improvement. However, the hydrology of upland catchments is frequently complex and non-uniform. For example, in the Llyn Brianne study, a further treatment involving standard agricultural improvement of

S.P. Cuttle, A.R. James/Agricultural

Water Management 28 (1995) 95-112

111

23% of the total catchment area had no effect on stream water chemistry, possibly because this part of the catchment did not contribute to streamflow generation (Jenkins et al., 199 1).

5. Conclusions Drainage alone, in the absence of other pasture improvement measures, appeared to have little effect on nutrient losses. Pasture improvement had little effect on concentrations of nitrate and ammonium-N in drainage water samples except for brief periods following the application of nitrogen fertiliser. Increases in mineral-N concentrations at these times were usually small and were only of significance in one year of the study. However, concentrations of organic-N in water from the reseeded area were consistently greater than those from the Moliniu area. Concentrations of phosphorus, potassium and calcium and the pH of drainage water increased following pasture improvement and were consistently greater than those in water from the Moliniu area. Almost all of the potassium fertiliser applied to the reseeded area in Years 1 and 2 had been leached by the end of Year 4 indicating that effects of potassium fertiliser applications on drainage water chemistry are unlikely to persist for more than about 3 years. In contrast, about 12% of the phosphorus fertiliser and 24% of the lime had been leached by the end of the experiment and effects of liming and phosphorus fertilisation would be expected to continue well beyond the 4-year study period. Much of the phosphorus in non-digested water samples from this peaty soil, and a particularly high proportion in the case of the unimproved area, was present in forms that were not detected by the molybdate calorimetric method used for the analysis of these samples.

Acknowledgements The study was conducted as part of work commissioned by the Ministry of Agriculture, Fisheries and Food. We are grateful to Pwllpeiran Experimental Husbandry Farm for provision of the site and to B.M.S. Davies, G. Daniel and D. Jones for technical assistance during the experiment.

References Allen, SE. (Editor), 1989. Chemical Analysis of Ecological Materials. 2nd edn. Blackwell Scientific, Oxford, 368 pp. Adams, W.A. and Evans, G.M., 1989. Effects of lime applications to parts of an upland catchment on soil properties and the chemistry of drainage waters. J. Soil Sci., 40: 585-597. Adams, W.A. and Evans, G.M., 1990. Input/export relationships of major ions in west Wales catchments. Agric. Ecosyst. Environ., 32: 13-24. Bargh, B.J., 1978. Output of water, suspended sediment, and phosphorus and nitrogen forms from a small agricultural catchment. N. 2. J. Agric. Res., 21: 29-38.

112

S.P. C&e,

A.R. James /Agricultural

Water Management 28 (1995) 95-112

British Standards Institution, 1965. Methods of measurement of liquid flow in open channels. B.S.3680, Part 4A. Her Majesty’s Stationery Office, London, 91 pp. Crisp, D.T., 1966 Input and output of minerals for an area of Pennine moorland: the importance of precipitation, drainage, peat erosion and animals. J. Appl. Ecol., 3: 327-348. Crooke, W.M. and Simpson, W.E., 197 1. Determination of ammonium in Kjeldahl digests of crops by an automated procedure. J. Sci. Food Agric., 22: 9-10. Cuttle, S.P. and Mason, D.J., 1988. A flow-proportional water sampler for use in conjunction with a V-notch weir in small catchment studies. Agric. Water Manage., 13: 93-99. Dampney, P.M.R., 1985. A trial to determine the lime requirement for reseeded grassland on a peaty hill soil. Soil Use Manage., 1: 95-100. Edwards, A.C.. Creasey, .I. and Cresser, M.S., 1985. Factors influencing nitrogen inputs and outputs in two Scottish upland catchments. Soil Use Manage., 1: 83-87. European Economic Community, 1980. Council directive on the quality of water for human consumption. Official Journal No. 80/778, EEC L229. Goodman, P.J., 1988. Nitrogen fixation, transfer and turnover in upland and lowland grass-clover swards using 15N isotope dilution. Plant Soil, 112: 247-254. Harrison, A.F., 1987. Soil Organic Phosphorus. A Review of World Literature. C.A.B. International, Wallingford, UK, 257 pp. Henriksen, A. and Selmer-Olsen, A.R., 1970. Automatic methods of determining nitrate and nitrite in water and soil extracts. Analyst, 95: 5 14-5 18. Homung, M., Reynolds, B. and Hatton, A.A., 1985. Land management, geological and soil effects on streamwater chemistry in upland mid-Wales. Appl. Geog., 5: 71-80. Hornung, M., Stevens, P.A. and Reynolds, B., 1986. The impact of pasture improvement on the soil solution chemistry of some stagnopodzols in mid-Wales. Soil Use Manage., 2: 18-26. Jenkins, A., Waters, D. and Donald, A., 1991. An assessment of terrestrial liming strategies in upland Wales, J. Hydrol., 124: 243-261. Ministry of Agriculture, Fisheries and Food, 1973. Fertilizer Recommendations. MAFF Bull. 209, 106 pp. Ministry of Agriculture, Fisheries and Food, 198 1. Systems for Welsh mountain sheep. Sheep Husbandry 3. MAFF Booklet 2323, 26 pp. Monteith, J.L., 1965. Evaporation and environment. In: G.E. Fogg (Editor), The State and Movement of Water in Living Organisms. 19th Symp. Sot. Exp. Biol., Academic Press, New York, pp. 205-234. Munro, J.M.M. and Davies, D.A., 1973. Potential pasture production in the uplands of Wales. 2. Climatic limitations on production. J. Br. Grassld. Sot., 28: 161-169. Munro, J.M.M., Davies, D.A. and Thomas, T.A., 1973. Potential pasture production in the uplands of Wales. 3. Soil nutrient resources and limitations. J. Br. Grassl. Sot., 28: 247-255. Murphy, J. and Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta, 27: 31-36. Newbould, P., 1982. Biological nitrogen fixation in upland and marginal areas of the U.K. Phil. Trans. R. Sot. Lond., B 296: 4054 17. Reynolds, B., Homung, M. and Hughes, S., 1989. Chemistry of streams draining grassland and forest catchments at Plynlimon, mid-Wales. Hydrol. Sci. J., 34: 667-686. Reynolds, B., Homung, M. and Stevens, P.A., 1987. Solute budgets and denudation rate estimates for a midWales catchment. Catena, 14: 13-23. Roberts, A.M., Hudson, J.A. and Roberts, G., 1989. A comparison of nutrient losses following grassland improvement using two different techniques in an upland area of mid-Wales. Soil Use Manage., 5: 174-179. Ron Vaz, M.D., Edwards, A.C., Shand, CA. and Cresser, M.A., 1993. Phosphorus fractions in soil solution: influence of soil acidity and fertilizer additions. Plant Soil, 148: 175-183. Rudeforth, CC., Hartnup, R., Lea, J.W., Thompson, T.R.E. and Wright, P.S., 1984. Soils and their Use in Wales. Bull. 11, Soil Survey of England and Wales, Harpenden, 336 pp. Shah, Z., Adams, W.A. and Haven, C.D.V., 1990. Composition and activity of the microbial population in an acidic upland soil and effects of liming. Soil Biol. Biochem., 22: 257-263. Smith, L.P., 1976. The Agricultural Climate of England and Wales. Tech. Bull. 35, Ministry of Agriculture, Fisheries and Food. Her Majesty’s Stationery Office, London, 147 pp. Williams, B.L. and Wheatley, R.E., 1992. Mineral nitrogen dynamics in poorly drained blanket peat. Biol. Fe&l. Soils. 13: 96-101.