146 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
Walling and Webb 1982 of calculating loads. How- ever, given the poor nature of rating curves between
SRP and flow which could be used to estimate load by extrapolation r
2
generally 0.10, data not given and the frequency of sampling, this method was seen
as most appropriate. However, it must be recognized that without more intensive sampling, some amount
of error is to be expected.
2.3. Soil sampling and analyses Six soil samples of the 0–7.5 cm depth of 10 fields
three grassland Phleum pratense, cereal cropping Triticum aestivum and Hordeum sativum, root crop-
ping fields Solanum tuberosum and Brassica sp. and one woodland field largely Castanea sativa
were taken at monthly intervals from August 1997 to July 1998, excluding January 1998. Each sample was
sieved to 2 mm. SRP was determined in filtered 0.01M CaCl
2
extracts of wet and air-dried soils CaCl
2
-P and 0.5M NaHCO
3
adjusted to pH 8.5 with NaOH [Olsen P] extracts of air-dried soils. Soils had been
shaken for 30 min using soil:solution ratio of 1:20 for Olsen P Olsen et al., 1954 and 1:5 for CaCl
2
-P Schofield, 1955. Soils were extracted field moist
with 0.01M CaCl
2
in order to minimize disturbance of soil solution chemistry. Results for 0.01M CaCl
2
extracts of wet soils are presented on an air-dry basis following a gravimetric moisture determination. Mi-
crobial biomass P was determined on moist soils of the cereal, root and grassland landuses from the June
sampling not enough woodland soil was available, using the method of Brookes et al. 1982. Organic
carbon C was determined on oven dry soil Grewal et al., 1991 and soil pH in water using a 1:2.5 soil:
solution ratio. The kinetics of SRP release in 0.01M CaCl
2
extracts of air-dry soils at either low and high Olsen P for each landuse from the June sampling was
determined on filtered extracts after 2, 10, 30, 120, 300 and 1440 min and fitted to an expanded Elovich
equation Polyzopoulos et al., 1986:
Q = [lnab + lnt + c]
b 2
where Q is the amount mg kg
− 1
of released P at time t, t is the desorption time min and a, b and c are
constants. The equation was fitted using nonlinear regression
SPSS v6.0 and the fit assessed through a linear plot of observed versus predicted values giving an r
2
value. The r
2
value is given, since nonlinear regression does not yield true r
2
values. A preliminary analysis of variance using Minitab
v8.0 showed that four groups of data could separate according to their landuse. The management of each
field has remained relatively unchanged during the 8 years of stream data. Data from each landuse were
pooled three fields each for soils under cereal crops, root crops and grassland, resulting in an acceptable
standard error of the mean commonly less than 5 from 18 samples covering an area of 1.2 ha. Samples
from the woodland landuse commonly had a standard error less than 9.
3. Results
3.1. Soil characteristics Table 1 gives the mean, standard error of the mean
and range of pH and organic C g kg
− 1
for each lan- duse during the study period and microbial biomass P
mg kg
− 1
Olsen P for the cereal, root and grassland soils from the June sampling. No systematic or sig-
nificant variation in pH of organic C between months was noted. Microbial biomass P was greatest in the
grassland soils and greater in the root soils than the cereal soils. During the study period, pH ranged from
3.47 in the woodland soil to 7.10 in soil under cere- als. Organic C ranged from 0.26 g kg
− 1
in a grassland soil to 1.87 g kg
− 1
in a woodland soil. On average, soils under root and cereal cropping had a significantly
higher pH p0.01 and lower organic C than soils under grassland or woodland. One field that had been
used for cropping 3 years before being sown to grass caused low values of organic C in grassland.
Curve fits and data for the kinetics of P desorption is shown in Fig. 2. The kinetics of P release was de-
scribed by the expanded Elovich equation. While it is believed that no one equation can describe the desorp-
tion kinetics of all soils, the expanded Elovich equa- tion has been found to fit desorption rates well over a
wide range of soil types Raven and Hossner, 1994. The r
2
values for any of the fits were never less than
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 147
Table 1 Mean ± standard error of the mean, with range given below, for topsoil 0–7.5 cm pH in water 1:2.5 ratio, organic C and microbial
biomass P for each landuse Landuse
pH Organic C g kg
− 1
Microbial biomass P mg kg
− 1
Olsen P Cereals
6.23 0.08 0.41 0.20
26.9 2.9 5.22–7.10
0.28–0.52 18.7–39.5
Root 6.01 0.06
0.38 0.17 32.7 2.4
5.12–6.97 0.29–0.51
28.5–44.5 Grassland
5.68 0.04 0.52 0.20
42.5 5.4 4.83–6.03
0.26–0.85 38.3–55.1
Woodland 4.63 0.12
1.08 0.58 n.d.
a
3.47–5.17 0.74–1.87
a
n.d.=not determined.
0.951. The quantity of desorbed P and the rate of P desorbed was assessed through the quantities and rates
of P released after 30 and 1440 min Table 2. These data show that more P is released and at a faster rate
with increasing Olsen P.
Fig. 2. Concentration of CaCl
2
-P with time for low and high Olsen P grassland, cereal and root soils symbols fitted to the expanded Elovich equation line.
3.2. Soil moisture and temperature Soil moisture, temperature and rainfall for each
month during the study period are shown in Fig. 3. A total of 1175 mm of rain fell on the catchment,
148 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
Table 2 The initial and final quantity and rate of desorbed P for soils of each landuse at one or two Olsen P concentrations
Landuse r
2
value Olsen P
Desorbed soil P Soil P desorption rates
mg kg
− 1
mg kg
− 1
h
− 1
mg kg
− 1
h
− 1
Initial
a
Final
b
Initial Final
Grassland 0.972
∗∗∗
20 0.376
0.940 0.752
0.039 0.969
∗∗∗
39 0.865
1.729 1.316
0.054 Cereal
0.989
∗∗∗
23 0.418
0.964 0.836
0.040 0.951
∗∗∗
47 1.234
1.512 2.468
0.063 Root
0.990
∗∗∗
26 0.451
0.977 0.902
0.040 0.982
∗∗∗
55 1.429
1.767 2.857
0.074 Woodland
0.997
∗∗∗
8 0.376
0.602 0.752
0.025
a
Initial=after 30 min.
b
Final=after 1440 min.
∗∗∗
Indicates significant at the p0.01 level.
compared to a long-term mean of 1051 mm Ratsey, 1975. Despite the greater than average annual rain-
fall, only 9.4 mm fell during February compared to a long-term average of 86 mm. This caused low soil
Fig. 3. Monthly ± standard error of the mean SEM I soil moisture for each landuse, rainfall and soil temperature at 25 cm.
moisture content during February in the cereal and root landuses. No cultivation and the presence of a
permanent root mat or canopy in the grassland and woodland soils imparts a resilience to low rainfall by
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 149
decreasing evaporative soil moisture loss. In contrast to February, greater than average rainfall 120 mm
for November and 122 mm for January fell during November and January affecting soil moisture espe-
cially in the grassland landuse. The seasonal pattern in soil temperature at 25 cm was normal, except for a
lower than average temperature for April. Soil tem- perature was greatest in August and lowest in January.
3.3. Soil P forms The monthly concentrations and variation in Olsen
P and wet and dry CaCl
2
-P for each landuse, is shown in Fig. 4. Mean values of Olsen P within
each landuse generally decreased in order from rootcerealgrasslandwoodland for each month.
Within each landuse there is a clear seasonal pattern in Olsen P concentration, which has been noted pre-
viously, and related to the control and release of P by the microbial biomass Seeling and Zasoski, 1993.
The maximum concentration of Olsen P occurred in either July or August, while the lowest concentration
occurred in winter.
Phosphorus extracted from wet and dry soil by 0.01M CaCl
2
was an order of magnitude lower than Olsen P. Mean values of wet and dry CaCl
2
-P within each landuse generally decreased in order from
rootcerealgrasslandwoodland for each month. Both wet and dry CaCl
2
-P concentrations exhibited a seasonal variation, with a summer maximum and
a winter minimum. The exception was the wood- land soil that had no obvious pattern. The variation
and difficulty in detecting the low concentrations ex- tracted may have caused this. There was a rise in wet
CaCl
2
-P in the root, cereal and grassland soils, which corresponded to the application of superphosphate
fertilizer in March. However, the seasonal variation and summer maxima in dry CaCl
2
-P and Olsen P concentrations for root, cereal and grassland soils is
probably not caused by a spring fertilizer application since neither rose during March or April.
3.4. Change points From Fig. 4 it is evident that an approximate 6-fold
increase in Olsen P between woodland and root soils for August is paralleled by a 6-fold increase in dry
CaCl
2
-P. However, an approximate 5-fold increase in Olsen P between woodland and grassland soils in
February only gives an approximate 3-fold increase in dry CaCl
2
-P. This is caused by the non-linear quantity Olsen P–intensity CaCl
2
-P QI relationship that exists between the two measurements McDowell and
Condron, 1999. This relationship can be described by sorption isotherms e.g. Freundlich, but a split-line
model has also been used Brookes et al., 1997 which gives a change point above which environmen-
tally significant levels of P may be lost. This change point is attractive as a management tool for farmers
and policy makers alike. Fitting a split-line model to monthly dry CaCl
2
-P data shows that the change point has a mean value of 31 mg kg
− 1
Olsen P and a range of 26–36 mg kg
− 1
Olsen P Table 3. There is no apparent seasonal variation in the value of the
change point. However, the number of soil samples that lie above the change point varies according to
season. For example, Fig. 5 shows the split line model fitted to data from February and June. The summer
maximum associated with Olsen P and dry CaCl
2
-P results in more soil samples above the change point,
and consequently at risk of accelerated P loss but requiring moving water to be lost, than soils above
the change point in winter. However, high flow rates in winter means that more P is lost than in summer
months Figs. 4, 6 and 7. Although following the same pattern as dry CaCl
2
-P, much more variation was associated with fitting the split-line model to cal-
culate change points for wet CaCl
2
-P against Olsen P data not shown.
3.5. Seasonal variation in stream P concentration and load
Monthly SRP concentration in stream discharge for the 1997–1998 water year beginning in August and
for the mean of 8 years 1987–1989 and 1994–1998, along with the P load for the mean of 8 years is seen
in Fig. 4. Cumulative discharge and P load for the Slapton Wood stream for the mean of 8 years and for
the 1994–1995 and 1997–1998 water year beginning 1 August is seen in Fig. 6. These data show that P load
parallels discharge and that the major loss of P occurs during winter when discharge is at its maximum. In
the 1994–1995 water year, high discharge rates during
150 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
Fig. 4. Monthly concentration ± standard error of the mean SEM I and variation in Olsen P and wet and dry CaCl
2
-P for each landuse, along with the monthly SRP concentration and load for the mean of 8 years stream discharge and monthly SRP concentrations for the
1997–1998 water year.
January and February generated 40 of the annual load. In contrast, low discharge rates during the same
months in 1998 generated 19 of the annual P load Fig. 6. The mean annual loss of P over 8 years is
equivalent to about 7.5 of the P fertilizer applied to the catchment.
Temporal variation in discharge P concentration is seen in Fig. 4 along with soil P concentration. Along
with the large variation in mean values, there appears to be a late summer maximum and late winter min-
imum in discharge P concentration. The correlation between discharge SRP concentration and P in soil
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 151
Fig. 5. Split line model fitted to CaCl
2
-P vs Olsen P for the February and June soil sampling. Numbers indicate the change point for each month and the dotted lines the total range of change points over the 12-month sampling period.
Table 3 Monthly calculated change points for the QI relationship of Olsen
P vs dry CaCl
2
-P Month
Change point value mg kg
− 1
Olsen P August 1997
33 September 1997
31 October 1997
26 November 1997
26 December 1997
36 January 1998
n.d.
a
February 1998 28
March 1998 33
April 1998 32
May 1998 30
June 1998 34
July 1998 33
a
n.d.=not determined.
extracts is given in Table 4. These data show that al- though the cause and effects relationships are not clear,
and that P in soil extracts do not necessary represent the catchment as a whole, they suggest that changes
in mean stream SRP concentration over an 8 year pe- riod were related to changes in CaCl
2
-P concentra- tion. Correlations of P in soil extracts with stream SRP
concentrations during the August 1997–July 1998 pe- riod were low and may have been caused by a combi-
nation of unusually low rainfall during February and high variation in stream SRP concentration associated
with the low number of samples n=47 compared to the 8 year period n=300.
Variation in P export for the mean of 8 years and for the water year of 1994–1995 and 1997–1998 is
shown in Fig. 7 as a plot of cumulative SRP export
152 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
Fig. 6. Cumulative SRP export and flow for the mean of 8 years, the 1994–95 and 1997–98 water years.
against cumulative discharge. Variation from a linear relationship occurs twice around late autumn and late
winter. Cumulative SRP export plateaus where P sup- ply is becoming exhausted. This effect is not as dis-
tinct during the 1997–1998 water year compared to the 1994–1995 water year possibly due to unusually
low rainfall and discharge in February 1998.
4. Discussion
