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Published in J. Environ. Qual. 32:2334-2340 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Halting the Upward Trend in Soluble Phosphorus Transported from a Grassland Catchment

R. V. Smith*,a, S. D. Lennoxb and J. S. Baileya

a Agricultural and Environmental Science Division, Agriculture and Food Science Centre, Newforge Lane, Belfast BT9 5PX, United Kingdom
b Biometrics Division, Department of Agriculture and Rural Development, Agriculture and Food Science Centre, Newforge Lane, Belfast BT9 5PX, United Kingdom

* Corresponding author (roger.smith{at}dardni.gov.uk).



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
An upward trend in soluble reactive phosphorus (SRP) concentrations in Northern Ireland rivers leading to increased eutrophication has been reported for the last two decades. To identify if a similar trend could be observed in land drainage waters SRP and other P fractions were measured weekly from 1989 to 1997 in land drainage from a 9-ha grassland catchment in Northern Ireland that had a mean P surplus applied of 23.4 kg P ha-1 yr-1. Regressions of annual median concentrations of P fractions in land drainage waters against time for 1989 through to 1997 showed significant increases of SRP and soluble unreactive phosphorus (SUP) of 2.4 and 1.2 µg P L-1 yr-1, respectively. However, the annual flow-weighted concentrations and loads of all P fractions did not show significant increases with time. During the period 1998–2000 a change of management was introduced when only maintenance dressings of P were applied to the catchment according to Ministry of Agriculture, Fisheries and Food guidelines. This resulted in significant reductions in SRP concentrations in 2000 compared with 1997.

Abbreviations: AFWM, annual flow-weighted mean • ET, evapotranspiration • PP, particulate phosphorus • SRP, soluble reactive phosphorus • SUP, soluble unreactive phosphorus • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
NUTRIENT ENRICHMENT of surface waters leading to eutrophication is considered to be one of the major pollution problems facing developed countries. Eutrophication is defined by the UK Environment Agency as "the enrichment of waters by inorganic plant nutrients which results in the stimulation of an array of symptomatic changes. These include the increased production of algae and/or other aquatic plants affecting the quality of the water and disturbing the balance of organisms present within it" (Environment Agency, 1998). There is growing evidence that diffuse losses of phosphorus (P) from agricultural land to surface water in Europe and the USA have been increasing in recent years (Sharpley and Rekolainen, 1997) and that P is the key nutrient controlling the degree of eutrophication of rivers, lakes, and impounding reservoirs in both the USA (USEPA, 1996) and Europe (Foy and Bailey-Watts, 1998). These increased losses of agricultural P have been partly linked to annual P surpluses of the order 20 kg P ha-1 yr-1 on most European farms leading to soil saturation with P (Edwards and Withers, 1998). In the UK, Haygarth et al. (1998) reported that a typical intensive dairy farm had an accumulation rate of 26 kg P ha-1 yr-1 in the plant–soil system although this contrasted with only 0.28 kg P ha-1 yr-1 for a hill farm with extensive sheep production. In both grassland and cropped situations it has been shown that increasing soil P is associated with higher concentrations of P in drainage water than in low-P soils (Sharpley et al., 1996; Sims et al., 1998).

In the case of the Lough Neagh catchment area in Northern Ireland, which is predominantly a grassland catchment, the mean rate of P accumulation in soils over the last 50 years was reported by Foy and Withers (1995) to be 10 kg P ha-1 yr-1. Monitoring of rivers in this catchment area has shown that over the period 1974–1991 the soluble reactive phosphorus (SRP) loss rate increased by 15 g P ha-1 yr-1. Although this SRP loss rate to surface waters is only 0.15% of the P accumulation rate in soils, it nevertheless represents an annual increase of 1.54 ± 0.54 µg P L-1 yr-1 in rivers, which is significant in terms of lake eutrophication. Similar increases in river P loads have been reported in Scandinavia by Krug (1993) and over a 15-year period in Finland by Rekolainen (1997).

Despite the implied causal relationship between rising soil P content and increased P losses to surface waters, it is possible that changes in other factors such as farmyard practices may have been the cause of the increase in SRP loadings in the Lough Neagh river system. Direct evidence that increases in SRP transported in land drainage were a response to soil P accumulation came from temporal studies on a small rural grassland catchment that had no domestic or industrial sources of P and an absence of farm buildings. The term "land drainage" as employed in the present paper is defined as water and solute export from a subcatchment resulting from anthropogenic land drainage practices and is the term recommended by Haygarth and Sharpley (2000) for this hydrochemical transfer pathway. Smith et al. (1995) compared SRP concentration data for 1981–1982 and 1990–1991 and showed that median SRP concentrations had increased by 1.1 µg P L-1 yr-1 in land drainage in response to a soil P accumulation rate of 24 kg P ha-1 yr-1. The aims of the present study were to identify whether the concentrations of SRP and other P fractions in land drainage from the above catchment had further increased in the period 1989–1997 when there was a mean applied P surplus of 23.4 kg P ha-1 yr-1 and assess the response to a new management regime introduced during the period 1998–2000.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study Site
The land drain (sampling point: Irish Grid Reference IG 315400383600) employed in both previous (Smith et al., 1995) and present studies (see Fig. 1) lies within the property of Greenmount Agricultural and Horticultural College, Antrim, County Antrim and also within the 4453-km2 Lough Neagh catchment area. It drains 9 ha of predominantly flat grassland with a typical cambic gley soil (British Soil Survey, 1984) derived from basaltic glacial till. A pipe drainage system was installed in the early 1960s in all three fields that contribute to the catchment area of the drain. The drainage system has a herring bone layout using 10-cm main clay pipes at an 80-cm depth fed by 7.5-cm clay laterals at about a 70-cm depth and spaced about 15 m apart. The drains were backfilled with washed stones and sand to about 45 cm from the surface. The catchment has been continuously under grass since 1965 and has been used for grazing (2000–3000 cow grazing days per year) by beef cattle. The catchment supports two or three silage cuts each year and up to and including 1997 received both inorganic fertilizer and liquid animal manure. The annual rate of nitrogen application has been 300 kg N ha-1 yr-1 consisting of six applications of 50 kg N ha-1 yr-1. With regard to the application of inorganic P fertilizers the procedure employed by Greenmount Agricultural and Horticultural College for the period 1965–1997 was on a whole-farm basis, rather than on a site-specific basis to meet the grass requirements of individual fields. In this procedure, the application rate of P was that required to meet the grass requirements of the most P-deficient field on the farm as a whole, assessed on the basis of standard Ministry of Agriculture, Fisheries and Food recommendations following soil P (Olsen) tests (Her Majesty's Stationery Office, 2000). During the period 1998–2000 a new management regime was introduced. Animal manure was no longer applied to the site and only sufficient fertilizer inorganic P was applied to maintain each individual field at Index 2 according to Ministry of Agriculture, Fisheries and Food recommendations (Her Majesty's Stationery Office, 2000).



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  		Fig. 1. The study site and sampling point. Altitudes are given in meters above mean sea level at Belfast, Northern Ireland.

 
Sampling and Analysis
The catchment area of the drain lies within three fields: O'Neills Meadow (3.8 ha), Low Field (2.8 ha), and March Field (2.1 ha). Twenty soil samples (0–7.6 cm) were composited weekly along a diagonal transect of Low Field for the period January 1998 to December 2000. These samples were air-dried, sieved, and analyzed for Olsen-extractable inorganic P according to the method of Olsen et al. (1954). The drain was monitored from 1 Jan. 1989 to 31 Dec. 2000 at a manhole, where the drain entered and discharged through concrete pipes sited 1 m below ground level. Apart from a period of several weeks in the year 2000 drainage flow was monitored continuously and logged at 15-min intervals using a Warren Jones FJ 460 (GLI Int., Milwaukee, WI) flow monitor. Weekly composite samples consisting of 28 samples taken at 6-h intervals were taken by a Warren Jones Hobo Sampler.

Soluble reactive P was determined on 0.45-µm-membrane-filtered water samples by the acidic molybdate–ascorbic acid method of Murphy and Riley (1962). Total soluble phosphorus (TSP) and total phosphorus (TP) were determined on filtered and unfiltered samples, respectively, by digestion with potassium persulfate and sulfuric acid, followed by analysis of the digest as for SRP (Eisenreich et al., 1975). The difference between TSP and SRP was called soluble unreactive phosphorus (SUP) and the difference between TP and TSP was called particulate phosphorus (PP). Annual flow-weighted mean (AFWM) concentrations were calculated for each P species by Eq. [1]:

where w was week number (1–52), fw was the weekly flow through the weir, and cw was the concentration of the determinant in the weekly composite sample. Yearly rainfall and evapotranspiration (ET) data were obtained from the nearby Aldergrove Meteorological Station. An aim of the present study was to quantify annual P loads to surface waters from an agricultural catchment. However, as the drain did not intercept all the water and solute exported from the catchment, the areal annual loss rates of P from the catchment were estimated by multiplying the AFWM concentration by rainfall minus ET.

Data Analysis
Both parametric and nonparametric methods of statistical analysis of the present data were considered to measure monotic trends (no reversal of direction) over time and step trends (Hirsch et al., 1991). As all variables in all years showed a significant positive skewness together with kurtosis it was felt inappropriate to use annual mean values in linear regression analysis versus time. Instead annual median values were employed in regression analysis (See et al., 1992) because they are a superior measure of central tendency and resistant to the effects of outliers, typical of skewed data sets (Sokal and Rohlf, 1969). However, because of the lack of normality in the data a Mann–Whitney test (Hollander and Wolfe, 1999) was used to compare data for 1997 and 2000 rather than the conventional parametric two-sample t test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
To calculate a mass balance for the catchment, the amounts of P removed in land drainage, liveweight gain in grazing animals, and harvested herbage were subtracted from the total inputs in rainfall, fertilizer, and animal manure (Table 1). The catchment is predominately flat and there has been no evidence of significant losses of P in surface runoff events. For the period 1989–1997 the average net accumulation rate was 23.4 kg P ha-1 yr-1. Following the new management policy introduced during the period 1998–2000, which included curtailing animal manure inputs and providing only a maintenance dressing of P, the catchment showed an average P deficit of 15.0 kg P ha-1 yr-1. However, this P deficit was not reflected by any significant change in mean Olsen P soil test results (Table 2), which remained at about 26.0 mg P kg-1 during the 3-yr period. This was not surprising if one assumes that the deficit applies to the top 30 cm of soil and the soil has a density of 1.0 kg L-1, then the P deficit is 5.0 mg P kg-1 yr-1. However, as only about 13% of P in soils appears as Olsen available P (Johnston, 1989) then the available P decrease is in theory 0.65 mg P kg-1 yr-1. Over the 3-yr time span the predicted decrease in Olsen available P would be about 2.0 mg P kg-1 of soil. This is too small a change to expect to detect against the fluctuating soil P test results observed in this catchment, which ranged from a minimum of 14.0 mg P kg-1 to a maximum of 38.0 mg P kg-1 over the period 1998–2000 (Table 2).


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  		Table 1. Mean inputs and outputs of phosphorus for the Greenmount Agricultural and Horticultural College catchment.

 

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  		Table 2. Descriptive statistics of weekly Olsen P soil test concentrations (0–7.6 cm) in the Greenmount Agricultural and Horticultural College catchment for the period 1998–2000.

 
Mean, standard deviation, and median values of SRP, SUP, PP, and TP in land drainage waters for the years 1989–2000 are shown in Table 3. Median values of SRP, SUP, and PP in 1989 compared with those in 1997, which is the last of the years with a positive P balance in the catchment, showed increases of 29, 8, and 27 µg P L-1, respectively. Regressions of annual medians for the P fractions in land drainage waters against time showed significant increases for SRP and SUP (P < 0.01) but not for PP (Table 4). Results for SRP suggest that concentrations increased at a rate of 2.4 (SE = 0.65) µg P L-1 yr-1 for the period 1989–1997 compared with only 1.1 µg P L-1 yr-1 for the period 1981/1982–1990/1991 when there was a soil P accumulation rate of 24 kg P ha-1 yr-1 (Smith et al., 1995). Soluble unreactive P also showed an increase of 1.2 (SE = 0.32) µg P L-1 yr-1 for the period 1989–1997. However, PP did not show a significant increase during this period, which may be interpreted as a result of the marked variation in flow rates between years (Table 5). High PP concentrations tend to be associated with high flows and there was no significant upward trend in flows or rainfall minus ET over the 1989–1997 period.


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  		Table 3. Concentrations of soluble reactive phosphorus (SRP), soluble unreactive phosphorus (SUP), particulate phosphorus (PP), and total phosphorus (TP) in land drainage waters from the Greenmount Agricultural and Horticultural College catchment for the years 1989–2000.

 

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  		Table 4. Median concentrations of P fractions versus time (year 1989 = 1) regressions for land drainage waters from the Greenmount Agricultural and Horticultural College catchment for the period 1989–1997.

 

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  		Table 5. Annual flow-weighted mean (AFWM) concentrations and loads of soluble reactive phosphorus (SRP), soluble unreactive phosphorus (SUP), particulate phosphorus (PP), and total phosphorus (TP) in land drainage waters from the Greenmount Agricultural and Horticultural College catchment for the years 1989–1999.

 
By applying the SRP regression equation of SRP versus time (Table 4) one can predict that in 1998, 1999, and 2000 the median concentrations of SRP would be 50, 52, and 54 µg P L-1, respectively. However, the observed concentrations in land drainage following the application in the years 1998, 1999, and 2000 of only a maintenance dressing of fertilizer P were 40, 43, and 36 µg P L-1 yr-1, respectively (Table 3). A Mann–Whitney test was used to test the significance of the difference in SRP concentrations in 2000 compared with 1997 and indicated a significant difference (P < 0.001). This downward shift in SRP concentrations in 2000 compared with 1997 can be illustrated by a frequency histogram (Fig. 2) , which clearly shows an increase in the lower range of concentration of SRP (12–28 µg P L-1) in 2000 compared with 1997 and appears to be due to the change of P management of the catchment rather than other exogenous factors. In contrast the median value of SUP of 26 µg P L-1 yr-1 observed in 2000 is similar to the predicted value of 23 µg P L-1 yr-1 based on the SUP versus time regression equation (Table 4). The very high median concentrations of PP observed in 1999 and 2000 appear to be caused by a change of management of the catchment. In February of both years the catchment was grazed by sheep and this led to some trampling of the turf under the wet, wintry conditions.



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  		Fig. 2. Relative frequency histograms for weekly soluble reactive phosphorus (SRP) concentrations in land drainage waters from the Greenmount Agricultural and Horticultural College catchment during 1997 and 2000.

 
When flows were compared with rainfall minus ET values (Table 5) for the Greenmount site they did not show any agreement. As there were marked differences in drain efficiency between years it was decided to estimate total annual losses from the catchment by multiplying the AFWM concentrations by rainfall minus ET summed daily over the year (Table 5). It was assumed that the volume intercepted was a representative sample of soil drainage water. In contrast with median values of SRP, SUP, and TP, which showed significant upward trends for the period 1989–1997 (Table 2), AFWM concentrations and loads of all P fractions showed no significant upward trend for this period (Table 5). Soluble reactive P loads were very variable with a range from 0.08 kg P ha-1 yr-1 observed in 1989 to 0.42 kg P ha-1 yr-1 observed in 1990. Soluble unreactive P loads were also observed to show a minimum in 1989, which was 0.04 kg P ha-1 yr-1, but the maximum observed load of 0.18 kg P ha-1 yr-1 was observed in both 1992 and 1999. Of the different P fractions PP loads were the most variable between years with a minimum of 0.10 kg P ha-1 yr-1 observed in 1989 compared with a maximum of 0.92 kg P ha-1 yr-1 observed in 1999. Total P loads also showed a minimum rate in 1989, which was 0.22 kg P ha-1 yr-1 and a maximum rate of 1.32 kg P ha-1 yr-1 in 1999. Coefficients of variation (the ratio of standard deviation divided by the mean) for SRP, SUP, PP, and TP were 0.38, 0.4, 0.97, and 0.5, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Upward trends in P loads in rivers may be masked by the year to year variability of hydrological events and flows (Sharpley, 1980). Foy et al. (1995) were only able to show an upward trend in SRP loads in the major rivers entering Lough Neagh for the period 1974–1991 by using multiple regression analysis of load versus time and flow. More recently, Heaney et al. (2001) employed this approach on the 1974–1997 Lough Neagh river dataset and showed that under average flow conditions SRP concentrations were increasing by 2.5 µg P L-1 yr-1. This is an increase of about 1 µg P L-1 on the 1.54 µg P L-1 yr-1 rate reported by Foy and Withers (1995). In the present study we observed an increase of 2.4 µg P L-1 yr-1 in median concentrations of SRP in land drainage for the period 1989–1997 compared with an increase of only 1.1 µg P L-1 yr-1 for the period 1981/1982–1990/1991 (Smith et al., 1995). This increase in SRP concentration in land drainage is interpreted as reflecting an increase in equilibrium SRP concentration in soil solution resulting from the accumulation of 23.4 kg P ha-1 yr-1 of soil P over the 9-yr period. Graetz and Nair (1995) have shown that when soil P accumulates on dairy and beef farms in the Lake Okeechobee watershed there appears to be no mineral fixation of the P and a large amount is available for transport in land drainage water. In the case of overland flow (defined by Haygarth and Sharpley [2000] as "movement of water exclusively over the soil surface") there is ample evidence of a strong positive correlation between the concentration of P in overland flow and increases in soil P (Sharpley, 1995; Pote et al., 1999).

Observations from the present study confirm Lemunyon and Gilbert's (1993) generalization that soluble P is the predominant form of P exported from grassland. However, in 1999 when sheep grazing took place in the winter, PP export from the catchment (0.92 kg P ha-1 yr-1) was greater than total soluble P losses (0.41 kg P ha-1 yr-1). Total P losses ranged from 0.22 kg P ha-1 yr-1 in 1989 to 1.32 kg P ha-1 yr-1 in 1999 (Table 5) with about 60% in soluble form. Most recent published UK TP export coefficients from improved grassland are of a similar magnitude: 0.8 kg P ha-1 yr-1 from improved grassland (McGuckin et al., 1999) and a lower range of 0.1 to 0.8 kg P ha-1 yr-1 reported by Johnes and Hodgkinson (1998). These rates appear to be considerably higher than those reported from earlier studies based on observations made in the 1970s. The TP export coefficients from grassland of 0.2 kg P ha-1 yr-1 reported by Cooke (1976) and 0.2 to 0.3 kg P ha-1 yr-1 by Kolenbrander (1972) are lower than the rates reported in the present study. A similar export coefficient of 0.26 kg P ha-1 yr-1 from grassland was derived from regression analysis by Smith (1977) in the Lough Neagh catchment area. Therefore, there does seem to be evidence that the observed range of export coefficients of P from improved grassland in the UK has increased in the last 30 yr from 0.2–0.3 to 0.2–1.0 kg P ha-1 yr-1. There is abundant evidence to suggest that this increase in P enrichment of surface waters is a result of P accumulation in UK soils (Edwards and Withers, 1998).

The Ministry of Agriculture, Fisheries and Food provides guidelines for desirable soil P concentrations for agricultural and horticultural crops in the UK (Her Majesty's Stationery Office, 2000). These guidelines are based on plant-available P concentrations as determined by the Olsen method (Olsen et al., 1954). Adequate concentrations for grass production are in the range 16 to 25 mg L-1 of Olsen P and are described as Index 2. The results of a recent soil survey (Jordan et al., 2001) show that more than 80% of agricultural land in Northern Ireland has a soil P index of 2 or greater. Field trials have shown that at Index 2 the response to P fertilizer by swards cut for three crops of silage is less than 0.5 Mg dry matter ha-1 yr-1 (Bailey, 2000). In contrast, on soils that are Index 1 for K and S (the most common K and S index in Northern Ireland), the responses of silage swards to K- and S-containing fertilizers range from 3.0 to 5.0 Mg dry matter ha-1 yr-1. Indeed, the present study showed no reduction in grass yields or the P content of grass swards (>3.5 g P kg-1 dry matter) after imposing a P deficit on the Greenmount Agricultural and Horticultural College catchment. Therefore, there appears to be no justification for allowing the present average P surplus of 15 kg P ha-1 yr-1 for agricultural land in Northern Ireland to continue. The Department of Agricultural and Rural Development for the province has stated that its objective is to reduce the P surplus to 7.5 kg P ha-1 yr-1 by the year 2010 (Environment and Heritage Service, 1999). The achievement of this target will be a challenge. Beegle et al. (2000) suggest that the key to managing soil is to balance the amounts of P applied in fertilizers and manures against the actual needs of crops so that P accumulation is curtailed. If it was decided to put into action a nationwide farm nutrient management scheme under European Union or UK policy, adequate incentives must be offered to ensure that farmers comply with the operation of the scheme. In the present study such a nutrient management scheme was introduced for three years in the Greenmount Agricultural and Horticultural College catchment and led to a significant reduction in concentrations of SRP in land drainage in 2000 compared with 1997.


   ACKNOWLEDGMENTS
 
We wish to thank Phil Dinsmore's team in the Freshwater Chemistry Laboratory, AESD for their assistance in collection and analysis of samples. Linda Gogarty provided valuable assistance with data analysis.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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JEQ 2003 32: 1931-1938. [Full Text]  




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