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
4.1. Seasonal variation of P forms Fluctuations in the concentration of soil P are
caused by several factors. Increases arise from the addition of inorganic fertilizers or organic manures
and the decomposition of plant material and organic matter by microbial activity. Decreases are caused by
the uptake of P by crops, by leaching and the immobi- lization of P in microbes and soil constituents. These
in-turn are affected by climatic conditions such as temperature and rainfall and soil characteristics such
as pH and variation in soil mineral constitution.
The application of superphosphate during March was reflected in a rise in wet CaCl
2
-P, followed by a drop in April. Blakemore 1966 showed that the appli-
cation of fertilizer to a silty-clay loam soil at Rotham- sted Harpenden, UK reacted quickly and only raised
CaCl
2
extractable P above pre-application concentra- tions for 1 or 2 months. Increasing the moisture con-
tent of field moist soils wetting-up, and extracting the soils within a few days of collection is expected to
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 153
Fig. 7. Cumulative SRP export against cumulative discharge for the mean of 8 years data A, the 1994–95 and 1997–98 water years B.
minimize any extreme changes in soil solution chem- istry Qian and Wolt, 1990. The least disturbance to
soil solution chemistry would be expected in winter and spring months when soil moisture content is great-
est Fig. 3.
Air-drying soil is known to affect the release of P. The death of microorganisms has been reported to
cause an increase in NaHCO
3
-extractable P Brookes
Table 4 Correlation coefficients for either mean monthly stream P discharge of 8 years of the 1997–1998 year and Olsen P and wet and dry
CaCl
2
-P for each landuse Landuse
Olsen P mg kg
− 1
Dry CaCl
2
-P mg l
− 1
Wet CaCl
2
-P mg l
− 1
8 year mean monthly stream P discharge Cereal
0.36 0.66
∗
0.64
∗
Root 0.49
0.68
∗
0.63
∗
Grassland 0.15
0.60
∗
0.59 Woodland
0.11 0.64
∗
0.32 1997–1998 mean monthly stream P discharge
Cereal 0.30
0.09 0.30
Root 0.09
0.12 0.27
Grassland 0.04
0.05 0.31
Woodland 0.05
0.20 0.12
∗
Indicates significant at the p0.05 level.
et al., 1982. Sparling et al. 1987 showed that a con- siderable proportion of Olsen P might be adsorbed P
released from the microbial biomass during air-drying. Re-wetting air-dried soil also stimulates microbial ac-
tivity Hunt et al., 1989 and results in the mineral- ization of organic N Larsen and Widdowson, 1968.
However, P behaves differently, and released P may be taken up by microorganisms and released later
154 R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157
Birch, 1964. Grassland soils contained more organic C and extractable microbial biomass P than either the
cereal or root soils Table 1. However, in general CaCl
2
-P from dry soils was less than CaCl
2
-P from wet soils. Dehydration of soil has been shown to in-
crease P adsorption Haynes and Swift, 1985; Bram- ley et al., 1992; Baskaran et al., 1994. Olsen and
Court 1983 suggested that re-wetting exposes new surfaces containing native P and unreacted adsorption
sites. Marked increases in CaCl
2
-P are therefore not expected, unless unreacted sites near saturation.
In the field, soil moisture content changes slowly, especially in those landuses with a crop canopy or root
mat. This would allow the microbial biomass time to adapt. Indeed, Tate et al. 1991 has shown that no
seasonal variation occurred in the microbial biomass of two pasture soils of different P fertility. Magid and
Nielsen 1992 proposed that physiochemical changes associated with soil moisture might mask any bio-
logical cycling. Several authors have noted a winter minimum and summer maximum in P concentrations
occur in coarse textured soils where a large part of the soil volume dries out during the summer Smith,
1959; Weaver et al., 1988; Magid and Nielsen, 1992. A winter maximum and summer minimum in soil P
concentrations have been noted in fine textured soils, where P concentrations may be controlled by the re-
duction and release of P from ferric hydroxides during wet months Jensen et al., 1998. The soils at Slapton
Wood are weekly structured, loose and friable, but also contain well-marked biopores and are free draining.
Both preferential flow and soil moisture deficits are known to occur at Slapton Wood Coles and Trudg-
ill, 1985. These physical effects coupled with slow plant growth or even death during periods of low soil
moisture in warmer months will cause P to become concentrated in the soil solution.
4.2. Seasonal variation in P runoff Soluble reactive P in stream runoff is a direct
function of the concentration of available soil P for leaching and water flow surface and sub-surface. If
neither is present then SRP loss will not occur. Fig. 7 shows that loss of SRP into the stream is directly
affected by discharge and Table 4 shows that the con- centration of SRP in the stream from an 8 year period
is correlated to the concentration of CaCl
2
-P extracted from dry soil and to a lesser extent CaCl
2
-P extracted from cereal and root wet soils. Neither, Olsen P or
CaCl
2
-P extracted from wet grassland or woodland soils were significantly correlated to the concentra-
tion of SRP in stream discharge during 1997–1998 or the 8 year mean, nor was CaCl
2
-P extracted from wet or dry soil correlated to the concentration of SRP in
stream discharge from 1997–1998 Table 3. When a soil is enriched with P fertilizer or leached of P, there
are changes in both the concentration of P in solution intensity and the supporting labile P pool quantity.
However, the amount of change in either differs by the slope of the sorption isotherm or QI relationship
between the two, which is an expression of the soils P buffer capacity Bache and Williams, 1971. Olsen
P represents a quantity measurement of P associated with Al and Fe hydroxides and Ca phases Schoenau
and Karamanos, 1993, whereas CaCl
2
-P represents the concentration of P immediately available for plant
uptake or lost by leaching. Since soil extracted with CaCl
2
-P measures the con- centration of P in the soil solution, we may then expect
this to be sensitive to short term changes in soil condi- tions and therefore better correlated to the 1997–1998
year than the mean of the 8 years SRP stream data. This was not the case. Rainfall during the 1997–1998
period was unusual, characterized by very low rain- fall during February and higher than average rainfall
in November and January. This was reflected in dis- charge Fig. 6. However, concentrations of CaCl
2
-P were similar to SRP in stream discharge for the mean
of 8 years. This implies that readily leachable soil P has maintained a memory of soil physical conditions
analogous to the mean of 8 years. Discharge from the Slapton Wood catchment is characterized by a large
amount of base flow, supplied with P by the soil ma- trix. Physical conditions in the soil matrix control-
ling P concentration such as soil moisture will change slowly especially in those soils with a root mat or crop
canopy Fig. 3.
During the summer months more soils were above the change point and consequently at a much higher
CaCl
2
-P concentration than during winter. At this time differences between CaCl
2
extractions of wet and dry soil is lowest. The kinetics of P release Table 2, Fig. 2
shows that soils under root cropping were quickest to release P. This landuse also has the greatest correlation
R. McDowell, S. Trudgill Agriculture, Ecosystems and Environment 79 2000 143–157 155
to SRP concentrations in stream discharge. However, while this suggests that soils under root cropping are
most closely linked to SRP in stream discharge, with- out knowing the proportion of water flowing through
root soils and the influence of grassland soils nearer the stream Fig. 1, we cannot make any prediction for
the amount of SRP supplied by them.
Plots of cumulative SRP export against cumu- lative flow show two contrasting situations for the
1994–1995 water year and the 1997–1998 water year compared to the mean of 8 years Fig. 7. The shape
of the plot for the 1994–1995 water year is best de- scribed by linking two or three first order kinetic
curves, where the rapid loss of SRP is stemmed by SRP supply and the curve begins to plateau. The pool
of readily leachable P then begins to be replenished by desorption, mineralization and fertilization ready for
the next period of rapid loss. The major period of loss during the 1994–1995 year and the mean of 8 years
occur during late autumn, when soil P concentrations and rainfall are high. Compared to 1994–1995 and
the mean of 8 years data, the plot for the 1997–1998 water year is nearly linear, especially during winter
when most SRP is lost. This suggests that SRP export from the catchment in 1997–1998 is not related to the
amount of SRP that was in the catchment.
4.3. Management implications It is possible to present correlations between
changes in landuse, climate and management on the amount and concentration of SRP in streams Smith
et al., 1995. However, if we are to create predictive models or policy tools then the causal links, which
depend upon the hydro-chemical processes within the soil, must be understood. For practical reasons
there is also a need to maximize land area described by the least number of soil samples, but within an
acceptable standard of error. This is critical if the results are to be used in a farm scale environmental
management tool such as a ‘P index’ Gburek et al., 1996, or used at larger scales within a GIS system.
The results presented here show that for cereal and root landuses there is a common change point, above
which P intensity increases greatly compared to plant available Olsen P, than if below. There is some ev-
idence to show that a change point also exists for grassland soils, which is affected by pH McDowell
and Condron, 1999. Any predictive tool that is based on plant-available Olsen P is presented with a prob-
lem, namely ‘what happens once the change point is exceeded?’ Data presented here shows that while
the increase or decrease in P intensity CaCl
2
-P and quantity Olsen P varies according to season, there
is a constant change point. Further work is required to assess the variability of
P intensity with topography and the sensitivity of P losses. Clearly we need to understand where within
a catchment a soil sample of high P intensity is of concern. The data from this site suggests that on av-
erage over 8 years, P transport within the catchment is limited by SRP supply rather than by water flow
Fig. 6.. Soils, which are ‘high risk’ in terms of sup- plying P, are those above the change point with rapid
P release kinetics. In general, the root and some of the cereal soils were well above the Olsen P recom-
mended for maximum yield for potatoes Solanum tuberosum 25 mg kg
− 1
Olsen P, sugar beet Beta vulgaris 20 mg kg
− 1
Olsen P and winter wheat Triticum aestivum 20 mg kg
− 1
Olsen P Johnston et al., 1986. Consequently, the first step to mitigat-
ing P losses within this catchment would be to allow Olsen P to decrease to half their present values by
stopping P fertilizer application.
5. Conclusions
