Agriculture, Ecosystems and Environment 79 2000 143–157
Variation of phosphorus loss from a small Catchment in south Devon, UK
Richard McDowell
∗ ,1
, Stephen Trudgill
Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK Received 16 June 1999; received in revised form 23 November 1999; accepted 23 November 1999
Abstract
The application of fertilizers and manure in excess of plant requirements has resulted in an accumulation in soil phosphorus P, and increased potential for P loss. To develop P-based catchment management plans, we need to be able to estimate
the impact of soil P on water draining a catchment. A 12-month investigation August 1997–July 1998 determined the temporal change of soil P forms and soluble reactive P SRP in stream discharge in a small catchment of mixed landuse
cereal crops Triticum aestivum and Hordeum sativum, root crops Solanum tuberosum and Brassica sp., grassland Phleum pratense and woodland largely Castanea sativa, in south Devon, UK. This included monthly sampling of soils for sodium
bicarbonate extractable P Olsen P, calcium chloride extractable P CaCl
2
-P on wet and air-dry soil, organic carbon and pH. Also available were weekly data for stream discharge and SRP concentration during 1987–1989 and 1994–1998, which
enabled an 8-year mean to be calculated for each month. All forms of soil P exhibited seasonal variation, with a late summer maximum and late winter minimum. Olsen P and CaCl
2
-P were related by a quantity–intensity relationship. Above a certain value of Olsen P, termed the change point, CaCl
2
-P increased more per unit Olsen P than below this point. The change point remained virtually constant throughout the year, never deviating more than 5 mg kg
− 1
from a mean value of 31 mg kg
− 1
Olsen P. Changes in stream SRP concentrations for the monthly means for 8 years data were correlated only with CaCl
2
-P from dry soil. Plots of cumulative SRP export against cumulative discharge over the 8-year data set suggested that SRP loss was limited
by the supply of SRP from the soil matrix. Olsen P for root and cropping soils was twice that needed for maximum yields. Thus, to reduce SRP loss, P fertilizer applications should be stopped to allow Olsen P to decrease below the change point.
The use of CaCl
2
-P and the change point has the potential to form the basis of simple environmental management planning at the catchment scale. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Phosphorus loss; Catchment; Change point; Soil P; SRP
∗
Corresponding author. Tel.: +44-1223-333399; fax: +44-1223-333392.
E-mail address: rwm25hermes.cam.ac.uk R. McDowell
1
Present address: USDA-ARS, Posture Systems and Watershed Management Laboratory, Building 3702, Curtin Road, University
Park, PA 16802-3702, USA.
1. Introduction
Phosphorus P is a primary factor in the nutrition of plants and the eutrophication of surface waters. Soils
with a high soil P concentration will generally give rise to high P concentrations in runoff surface and
sub-surface. Pote et al. 1996 showed that P concen- tration in surface runoff was closely correlated r≈1
to water extractable P in the topsoil. Smith et al. 1995
0167-880900 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 5 4 - 1
144 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
related the increase in soluble P in drainflow from a grassland catchment to a rise in soil Olsen P concentra-
tions. Heckrath et al. 1995 showed that the concen- tration of dissolved 0.45 mm reactive P, dissolved
organic P and total P were linearly related to Olsen P above a certain point at about 60 mg kg
− 1
Olsen P. This point, termed the ‘change point’, is also evident
when comparing molybdate reactive P MRP, essen- tially inorganic P, in 0.01M CaCl
2
extracts with Olsen P in laboratory extractions Brookes et al., 1997. This
comparison has been suggested, in the absence of data for P concentrations in drainage water, as an indicator
to predict soil P concentrations above which P losses to drainage water become environmentally significant.
Seasonal changes in inorganic P P
i
concentrations extractable from field soils under various vegetation
types is well known. A 3-fold increase in extractable P
i
during summer was found in an unfertilized plot in Scotland by Smith 1959. Concentrations of P
i
extracted using anion exchange resin were greatest during the summer in arable and grassland ley plots
at Rothamsted and Woburn, UK Garbouchev, 1966. Similar variation occurred in arable and grassland
plots sequentially extracted with resin, NaHCO
3
and NaOH from sandy soils in Denmark Magid and
Nielsen, 1992. Saunders and Metson 1971 and Goodwin et al. 1998 noted that seasonal variation
of P
i
extracted by anion exchange resin and Olsen’s reagent NaHCO
3
from improved grassland soils exhibited a similar trend to P
i
in 0.01M CaCl
2
soil extracts and soil solution. A seasonal variation in wa-
ter extractable P
i
concentrations occurred in an arable soil Kuo and Jellum, 1987. Shand et al. 1994
showed the maximum concentration of P
i
in soil so- lution of three P-deficient Cambisols in NE Scotland
occurred in summer August. Such seasonal varia- tion could affect the timing of soil sampling to assess
fertilizer requirements, the amount of P available for plant uptake and the loss of P in runoff surface or
sub-surface.
The Slapton Wood catchment in south Devon, UK is a small 0.94 km
2
second order tributary of the River Gara which drains into Slapton Ley, the largest
body of freshwater in south-west England. Since late 1969, continual monitoring of stream discharge and
weekly water sampling at this site, along with three other catchments has occurred, aimed at quantifying
inputs of water, sediments and solutes into the lake. Of the total annual runoff, only 1.15 is surface-runoff
Troake and Walling, 1973. During winter, a lag time of several days occurs between rainfall and the maxi-
mum baseflow in stream discharge. This is indicative of a catchment where sub-surface runoff is favorable
Burt et al., 1983; Burt, 1988. It is therefore likely that P concentrations in runoff may reflect soil P con-
centrations if little water bypasses the soil in macro- pore flow. However, it is recognised that a significant
amount of total P loss and stream P concentration may be as particulate P PP or desorbed from PP in sur-
face runoff during intense rainfall.
Seasonal dynamics in catchments cause changes in stream P concentrations. Many researchers have es-
tablished a linear relationship between the log of flow rates and the log of stream dissolved or soluble reac-
tive P SRP concentrations e.g., Lennox et al., 1997. Large SRP concentrations occur during periods of low
flow in summer Leinweber, 1998. However, most SRP load is lost during periods of high flow in winter.
Xue et al. 1998 used the theory of first-order kinet- ics in a plot of cumulative P export versus cumulative
flow to describe P loss in tile drainage. They suggested that the exhaustion of P from the readily leachable
soil P pool caused a decrease in cumulative P export compared to cumulative flow. Hodgkinson and With-
ers 1998 used the same type of plot to show that most P was lost during winter. During a single runoff
event, P concentrations will be affected by soil wa- ter residence times and dilution, but the potential for
phosphorus loss in runoff and for plant nutrition will always be affected by the amount of soil P, and that
which can be rapidly released into soil solution.
To develop P-based catchment management plans, we need to be able to estimate the impact of soil P
on water draining a catchment. This study aims to compare and investigate the temporal change in soil
P concentrations, an indicator for P loss the change point and P concentrations in runoff in the short term
1997–1998 and on average over 8 years 1987–1989 and 1994–1998 by measuring: 1 the concentration
of key plant available and soil solution P fractions, 2 calculating the change point between 0.01M CaCl
2
and Olsen P concentrations and 3 SRP stream con- centration and load. Accompanying measurements
thought to influence the movement of P from the soil were made of, pH, organic matter and the kinetics of
P release.
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 145
2. Materials and methods
