The P content of precipitation reflects the amount of P subject to wash- out from the atmosphere at the time of the precipitation event. The amounts of P carried in precipitation rarely exceed 1 kg/ha per year as total P (Miller, 1961; Weibel et al., 1966; Allen et al., 1968; Gore, 1968).
Weibel et al. (1966) reported that the average concentration of total acid- hydrolyzable P in precipitation falling on Cincinnati, Ohio, was 0.080 pg/ml, whereas Taylor et al. ( 197 1 ) reported an average concentration of 0.020 pg/ml for total dissolved P in precipitation collected at rural Coshocton, Ohio.
Data for the P content of precipitation should be viewed with some skepticism unless adequate precautions have been taken to guard against contamination of the collection vessel (Gore, 1968; White, 1972). White (1972) found that although rainwater collected over an extended period indicated a mean dissolved inorganic P concentration of 0.020 pg/ml, a mean concentration of 0.003 pg/ml, based on specific showers, might be a more accurate estimate.
It is difficult to evaluate the effect of P carried in precipitation on P loads in runoff and streams. Phosphorus contained in precipitation which becomes a part of any soil-water ecosystem may undergo considerable change in form, depending primarily on the chemical factors discussed pre- viously, and will become an integral part of the P forms in runoff and streams.
Surface runoff water is the carrier of not only the P initially present in precipitation but also any P which enters surface runoff water because
of chemical interactions or the energy of the water itself. Several factors affect the amount and energy of surface runoff water at any particular loca- tion and, therefore, the amount of additional P entering and carried by it. These include nature of land use, extent of vegetative cover, slope, in- tensity of rainfall, and permeability of the land surface.
The quantity of precipitation entering subsurface and groundwater runoff is inversely related to that disposed of in surface runoff and evapo- transpiration. It is consequently affected by the factors listed above for surface runoff. The major portion of P in subsurface and groundwater run- off is expected to be in dissolved forms. If subsurface runoff is accelerated by artificial drainage systems, however, soil colloids, with associated P, may appear in the water as it enters streams.
The P load carried by a stream under given flow conditions will repre- sent the relative contribution of P loads in each of the runoff components, as well as the influence of any point source of P.
A. INFLUENCE O F POINT SOURCES ON PHOSPHORUS IN STREAMS Estimates of the contribution of P to surface waters from domestic wastes in the United States range from 91 x loo to 227 x l o G kg per year with total P concentrations ranging from 3.5 to 9.0 pg/ml
(McCarty, 1967; Ferguson, 1968). Weibel et al. ( 1964) estimated that P discharged as raw sewage from combined storm sewers in Cincinnati, Ohio, amounted to 3.4 kg/ha per year as total dissolved P. In the area of Madison, Wisconsin, the per capita contribution of P to surface waters from treated domestic waste was estimated to be 0.544 kg/capita per year (Sawyer, 1947), whereas an estimate of 1 kg/capita per year was given by Metzler et al. (1958) for Chanute, Kansas. The difference between the estimates of Sawyer (1947) and Metzler et al. (1958) may reflect the increased use of P in domestic products, particularly detergents.
The impact of sewage outfall on the dissolved inorganic P concentration of streams and rivers was studied by Brink and Gustafsson (1970). Their results are summarized in Table 11. Obviously the impact of the outfall is dependent on factors which include flow rate of the receiving stream and the P content of the effluent.
Under certain agricultural management conditions animal excrement may constitute a point source of P to streams. Excrement may enter streams during surface runoff from feedlot operations or by the cleaning of milking sheds into open drains. The magnitude of these sources of P will be discussed later.
McCarty (1967) was unable to estimate the magnitude of contributions of P made to streams from industrial wastes. The amounts of P discharged
22 J . C. RYDEN, J . K. SYERS, AND R. F . HARRIS TABLE I1
Effect of Sewage Outfall on tllc Dissolved Inorganic Phosphorus Concentration of the Receiving Water"
IXssolved inorganic P concentration ( p g P/ml) Receiving water Before outfall After outfall
River 0 . 0 9 0 4%
Stream 0 . 0 5 0 . 1 8
Stream 0 . 1 1 4 . 3 0
Ditch 0 . 0 1 0 . 7 5
Data from Brink and Gustafsson (1970).
to streams will depend on the industrial process concerned and local legis- lation covering the discharge of industrial effluent. Mackenthun et al.
(1968), for example, estimated that a potato canning factory and a woollen mill contributed 3446 and 835 kg of P per year, respectively, to the East Branch of the Sebasticook River, Maine.
Domestic and many industrial wastes not only supply large amounts of total P to streams but also have a pronounced effect on the concentrations of dissolved forms of P in the receiving stream. Because domestic and in- dustrial wastes are point sources, they are easily recognized within a water- shed and are amenable to direct manipulation.
B. RUNOFF FROM FOREST WATERSHEDS
A compilation of data from several studies of the quantities of P lost in streanis from stable forest and woodland watersheds is presented in Table 111. Exports of P in streams from long-established and stable forest watersheds provide a useful datum line against which losses of P from other land-use areas may be compared. The data in Table I11 show a con- siderable degree of uniformity. Total P losses range from 0.68 to 0.02 kg/ha per year with three out of the four values being less than or equal to 0.1 kg/ha per year.
Only a few measurements have been made of the dissolved inorganic P concentration of stream water in forested watersheds. The values re- ported by Brink and Gustafson (1970) in Sweden show a mean of 0.015 pg/ml, with this fraction amounting to 33% of the total annual loss of P. The data suggest that total P and dissolved inorganic P concentrations rarely exceed 0.1 15 and 0.025 pg/ml, respectively. Two interesting points arise from the data in Table 111. From the study of a stream draining a
El z
2 8
Losses of P in Streams Draining Forest Watersheds
P concentration in streamwater (pg P/ml) P loss
Study Location Form measured (kg/ha/yr) Range Mean 2
0 c
Bormann et al. (1968) New Hampshire Total P 0 . 0 2
Dissolved inorganic P 0 . 0 2
Cooper (1969) N. Minnesota Not specified 0.1s
Jaworslii and Hetting (1970) Potomac River Basin Total P 0 . 1
Brink and Gustafsson (1970) Sweden Total P 0 . 0 6
Sylvester (1961) Washington Total P 0 . 6 8
Dissolved inorganic P 0 . 0 7 Taylor et al. (1971) Coshocton, Ohio Total soluble P 0 . 0 5
- - .4
0.008-0.053 0.048 q
0.002-0.026 0 . 0 1 5 +
0.043-0.060 0.041 3
0.024-0.115 0.069 +I
0.004-0.009 0.007 b
0 . 0 1 1 - 0 . O P O 0.015
- - v)
24 J . C. RYDEN, J . K . SYERS, A N D R . F. HARRIS
woodland area at Coshocton, Ohio, to which no fertilizer P had been ap- plied for over 30 years (Taylor et al., 1971), it would appear that the woodland is conservative of P. The average total soluble P content of rain- fall was 0.020 pg/ml, whereas that in the stream draining the watershed was 0.015 pg/ml. The extent of addition of total dissolved P to the wood- land can be calculated from precipitation data given by Taylor et al.
(1971 ) ; a value of 0.17 kg/ha per year is obtained. This value is more than three times greater than the annual P loss in the stream. The con- servative nature of forests for P is further borne out by the fact that the annual contributions of P to the land surface in precipitation, quoted previ- ously, are in most cases considerably greater than annual exports of P in streams from forest watersheds. In many cases there is an order of magni- tude difference. This hypothesis assumes that data covering the P content of precipitation are correct.
The second point of interest relates to the “background” P concentration in forest streams. The data suggest only minor seasonal fluctuations in P concentrations, particularly that of dissolved inorganic P. As a major por- tion of streamflow is considered to have a groundwater origin (Biggar and Corey, 1969; Johnson and Moldenhauer, 1970), it is conceivable that the dissolved inorganic P load in streams of forested areas is primarily due to that in groundwater runoff. If the reported mean P concentrations of forest streams are compared to those for groundwaters, a marked similarity is observed. Juday and Birge ( 193 1 ) found that the total dissolved P con- centrations of 19 wells in northern Wisconsin, an extensively forested area, ranged from 0.002 to 0.197 pg/ml, with an average of 0.018 pg/ml when the highest value is omitted. This mean value is, if anything, slightly higher than the mean concentrations for dissolved fractions of P reported in Table 111. The higher mean concentrations of total P probably arise from sus- pended inorganic and organic solids that enter streamflow due to turbu- lence, especially during high flow.
The minor fluctuations in P concentrations reported for forest streams suggest that P export is minimally affected by P input from surface runoff.
Amounts of surface runoff in forest watersheds will be low owing to the protection afforded by canopy cover and/or forest floor vegetation. The
“background” P export in forest streams is a direct reflection of the chemi- cal and physical factors that affect P concentrations in groundwater and subsurface runoff. Because larger amounts of stream flow from forest watersheds will arise from groundwater and subsurface runoff, the “chemi- cal sieving” action of the soil plays a major role in maintaining the con- sistently low dissolved inorganic P concentrations in forest streams and may also account in part for the apparent conservative nature of forest watersheds for P.
C. RUNOFF FROM AGRICULTURAL WATERSHEDS
The loss of P in streams draining agricultural (in most cases arable) watersheds is far less well defined than that for forest streams. This is prob- ably due to the fact that in studies designed to estimate this loss, little differentiation has been made with respect to the forms of runoff. Conse- quently, there are major problems in estimating P loss from agricultural watersheds using many of the data presented in the literature. Losses of P from agricultural land have not only been based on analyses of streams draining a specific watershed (Campbell and Webber, 1969; Taylor et al., 1971 ), but have also been estimated from data obtained in surface runoff studies (Timmons et al., 1968). Many previous reviews of this subject have relied on such data (Taylor, 1967).
Losses of P in streams draining various agricultural watersheds are sum- marized in Table IV. The lowest loss of total P is from rangeland in On- tario, Canada (Campbell and Webber, 1969) which had received no P fertilizer in living memory. This loss is very similar to losses of total P from forest watersheds, suggesting a minimal contribution if P from sur- face runoff. Similarly, the total P carried in the base flow, primarily at- tributable to groundwater runoff, of several streams draining arable agricul- tural watersheds in S.W. Wisconsin (Minshall et al., 1969) is also little different from total P loads in streams draining forest watersheds. Minshall et al. (1969) reported the total P loss in base flow to be less than 0.12 kg/ha per year. If stream flow during periods of surface and subsurface runoff is included, however, the estimated annual loss of total P increases by one order of magnitude, as indicated by the data of Witzel et al. ( 1 969) for the same area of S.W. Wisconsin (Table I V ) .
These studies suggest that the groundwater runoff or base-flow compo- nent of streams draining agricultural watersheds is little different from the total P load of forest streams. It is therefore necessary to estimate the ex- tent to which the P load of streams draining agricultural watersheds may be augmented by P loads of surface and subsurface runoff.
The major factors affecting the loads of P in surface runoff from agricul- tural land include time, amount, and intensity of rainfall, rates of infiltra- tion and percolation, slope, soil texture, nature and distribution of native soil P, P fertilization history, cropping practice, crop type, and crop cover density.
A selection of reported losses of P in surface runoff from arable land of various slopes and cropping practices is summarized in Table V. Losses range from the extremely high values of 67 kg/ha per year to almost zero.
Losses of P in all studies listed in Table V have been based on the collec- tion of surface runoff (water and particulates) from small experimental
TABLE I V
Losses of P in Streams Draining Agricultural Watersheds
Soil Form of P P applied P lost
Study Location texture measured Slope (%) Crop (kg/ha/yr) (kg/ha/yr)
Campbell and Fippin (1945)
Webber (1969)
Taylor d al.
(1971)
Witzel d al.
(1969)
S. Ontario, - Total P
Tennessee - Total P
Coshoeton, Silt Total dis-.
Canada Valley
Ohio loam solved P
S.W. Wisconsin Silt Total P loam
- 90% Rangeland 0 0 . 0 8
open farmland - 1.88
- Row crops, - 6 . 2 6
12-18 50% Permanent 3 . 5 0 . 0 7
pasture; 50%
winter wheat- meadow
cultivation-hay- 9 . 6 4 1.51
6-8 100% Pasture 4.08 1.000
pasture 4 . 2 7 1 . 2 0
I-r P
I-r
?c
a January through September 1967.
plots frequently no larger than 30 x 6 m, with subsequent analysis for one or more forms of P. Although this approach was originally developed to investigate soil fertility losses due to soil erosion, it is still used to estimate P loads in surface runoff as it relates to the fertility of surface waters (Tim- mons et al., 1968; Nelson and Romkens, 1969).
It is difficult to make any generalizations regarding the P loads carried in surface runoff or to draw conclusions from them in terms of how agricul- tural practices and natural variables affect P loads in streams draining agri- cultural watersheds. This is due to the differences in forms of P measured and the lack of comparative studies with respect to slope, soil texture, cropping, and climatic variables.
One of the few studies from which meaningful interpretations of P loss in surface runoff can be made in relation to degree of slope and cropping practice is that by Massey et al. (1953) in Wisconsin (Table V ) . As ex- pected, greater “available” (water-soluble plus pH 3 extractable) P losses to surface runoff occurred on the steeper slopes when cropping practice was kept constant. The introduction of two years hay into the rotation reduced the P loss by a factor of approximately four. The value of “im- proved” or “conservative” agricultural practices in reducing the magnitude of P losses is illustrated in the studies at Coshocton, Ohio (Weidner et al., 1969) and at Lafayette, Indiana (Stoltenberg and White, 1953). It should be noted, however, that although the “improved” practice reduced the total amounts of acid-hydrolyzable P lost in surface runoff at Coshoc- ton, the concentration of this fraction during surface runoff increased from 0.43 to 0.59 pg/ml.
Attempts have been made to measure the relative contributions of the aqueous and particulate fractions of surface runoff to the total loss of a measured form of P. In a plot study using simulated rainfall, Nelson and Romkens ( 1969) obtained dissolved inorganic P concentrations of 0.05, 0.30, and 0.50 pg of P per milliliter in the aqueous phase of surface runoff from fallow plots 12 days after 0, 56, and 1 1 2 kg of P per hectare, respec- tively, had been disked into the soil, with only slight decreases in concen- trations up to 75 days after fertilizer application. Although very high arti- ficial rainfall rates were employed (up to 73.5 mm/hr), indications are that high concentrations of dissolved inorganic P may be maintained in surface runoff water. Timmons et al. (1968) determined the distribution of total P loss in surface runoff between the aqueous and particulate phases from plots under natural precipitation. Although these workers did not report P concentrations, losses of total P in the aqueous phase of surface runoff arising from snowmelt far o.utweighed those in the particulate phase.
In contrast, total P loss in the aqueous phase varied in most cases between 5 and 40% of the loss in particulates in surface runoff arising from rainfall.
TABLE V
Losses of Phosphorus in Surface Runoff from Field Plots
Soil Form of P Slopc P applied P lost
Study Location texture measured (%I Crop (kg/ha/yr) (kg/ha/yr)
Knoblauch et al. New Jersey Sandy
(1942) loam
Massey et al. Wisconsin Silt
(1953) loam
Stoltenberg and Lafayette, Silt
White (1953) Indiana loam
Thomas et al. Tifton, Sandy
(1968) Georgia loam
Total P
Soluble + pH 3
extractable P (available)
0.5 M N H 4 F +
0.1 N HCI extractable P (“available”)
0.05 N HCI +
0.025 N H ~ S O I extractable P
3 . 5
3 3 11 11 20 00
0 . 5
3
Vegetables +
(i) No manure (ii) Manure (iii) Cover crop (iv) Cover crop Corn-oats Corn-oats- 2 yr hay Corn-oats Corn-oats-? yr
hay Corn-oats-4 yr
hay Oats-5 yr hay
Coma Cornh Soybeansa Soybeansb Wheat”
Wheat*
Meadow“
Meadow*
c o r n
Rye-peanuts-rye Rye-corn-oats Oats-rye
+ manure
40 06 67 07 59 66 49 65 0 91 0 24 2 91 0 73 0 75 0 13 2 86 0 86 3 82 1 93 0 84 0 48 0 99 0 74 0 02 0 07 0 05 0 02
Corn-rotation 9 9 . 1 0 . 1 Oats-rotation 3 0 . 2 0 . 0 - 0 . 1 Hay-rotation - 0 . 1 - 0 . 3
Weinder et al. Coshocton, Silt Total acid- - Cornc 4 . 8 2 10.24
(1969) Ohio loam hydrolyzable P Cornd 1 7 . 3 4 3 . 1 1
Wheat" 4 . 8 3 1 . 3 3 Wheatd 1 7 . 3 4 0 . 4 1
a Prevailing practice: moderate fertilizer levels; liming t o p H 6.0; straight row planting and cultivation.
* Conservation practice: higher fertilizer levels; liming t o p H 6.5; manure before corn; contour planting and cultivation.
Improved practice: contour tillage; high P fertilizer level; clover-alfalfa-timothy meadow mixture; liming t o p H 6.8.
0
c Prevailing practice: straight row tillage across slope; low P fertilizer level; alsike-red clover-timothy meadow mixture; liming t o p H 5.4.
2
? w C
30 J. C. RYDEN, J. K. SYERS, AND R. F. HARRIS
These observations are not unexpected because rainfall tends to loosen soil particles by drop impact, facilitating their entry into surface runoff waters. It is apparent that an appreciable dilution of dissolved P may occur when surface runoff augments base flow in streams. Taylor et al. (1971) reported a mean total dissolved P concentration of 0.022 pg/ml in a stream draining an agricultural watershed at Coshocton, Ohio; concentrations never exceeded 0.100 ,g/ml even under conditions of high stream flow when surface runoff was occurring.
It is generally considered that P is retained sufficiently strongly by soil particulates that movement out of the soil profile in percolating waters is minimal (Way, 1850; Black, 1970). Subsurface runoff from agricultural land, however, may contain significant concentrations of dissolved inor- ganic P in relation to those present in surface waters (Table V I ) . It should
be noted, however, that the data in Table VI represent losses of dissolved inorganic P in tile and irrigation return flow drains. Artificial drainage sys- tems increase the rates of infiltration and percolation, reducing contact times between the soil solution and soil components capable of sorbing inorganic P from solution. Furthermore, tile drains will remove water from surface horizons of the soil profile, diminishing the possibility for contact between percolating waters and more P-deficient subsoil material.
Not all the data in Table VI, however, indicate a net loss of P from the soil profile to subsurface runoff. In the Snake River Valley, Idaho, Carter et al. (1971) found that only 30% of the dissolved inorganic P in irrigation water left an irrigation tract by return flow. When the dis- solved inorganic P concentration in irrigation water exceeded 0.010-0.020 pg/ml, irrigation decreased the downstream P load, a useful field example of the chemical sieving action of soils. Johnston et al. (1965), however, reported a net loss of 3 % at an applied P fertilizer rate of 51.9 kg/ha on irrigated land in the San Joaquin Valley, California.
The data in Table VI indicate that a reasonable proportion of P loss to streams draining arable watersheds can be due to subsurface runoff.
Although no data are available to compare P loads due to surface and subsurface runoff, Sylvester (1961) reported that total P loss by irrigation return flow in the Yakima Valley, Washington, ranged from 3.8 to 14.3 kg/ha per year, values higher than many reported for surface runoff losses.
Under a nonirrigated farming system, Bolton et a!. (1970) observed losses of dissolved inorganic P in tile drain effluent ranging from 0.13 to 0.29 kg/ha per year at a fertilization rate of 28.9 kg of P per hectare per year.
It would appear, therefore, that losses of P in subsurface runoff can be similar or even greater than those in surface runoff. Furthermore, subsur- face runoff will occur not only during periods of surface runoff, but also when evapotranspiration is less than infiltration.