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TECHNICAL REPORTS

Landscape and Watershed Processes

Phosphorus Transport through Subsurface Drainage and Surface Runoff from a Flat Watershed in East Central Illinois, USA

^a King Abdulaziz City for Science and Technology, GDRGP, P. O. Box 6086, Riyadh 11442, Saudi Arabia
^b Dep. of Agricultural and Biological Engineering, Univ. of Illinois at Urbana-Champaign, 1304 W. Pennsylvania Avenue, Urbana, IL 61801 USA
^c Univ. of Illinois Extension, Springfield Center, P.O. Box 8199, Springfield, IL 62791 USA

^* Corresponding author (pkalita{at}uiuc.edu)

Received for publication April 23, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS SUMMARY AND CONCLUSIONS REFERENCES

 
A long-term water quality monitoring program was established to evaluate the effects of agricultural management practices on water quality in the Little Vermilion River (LVR) watershed, IL. This watershed has intensive random and irregular subsurface drainage systems. The objective of this study was to assess the fate and transport of soluble phosphorus (soluble P) through subsurface drainage and surface runoff. Four sites (sites A, B, C, and E) that had subsurface and surface monitoring programs were selected for this study. Three of the four study sites had corn (Zea mays L.) and soybeans (Glycine max L.) planted in rotations and the other site had seed corn and soybeans. Subsurface drainage and surface runoff across all sites removed an average of 16.1 and 2.6% of rainfall, respectively. Annual flow-weighted soluble P concentrations fluctuated with the precipitation, while concentrations tended to increase with high precipitation coupled with high application rates. The long-term average flow-weighted soluble P concentrations in subsurface flow were 102, 99, 194, and 86 µg L^–1 for sites A, B, C, and E, respectively. In contrast, the long-term average flow-weighted soluble P concentrations in surface runoff were 270, 253, 534, and 572 µg L^–1 for sites As, Bs, Cs, and Es, respectively. These values were substantially greater than the critical values that promote eutrophication. Statistical analysis indicated that the effects of crop, discharge, and the interactions between site and discharge and crop and discharge on soluble P concentrations in subsurface flow were significant ( = 0.05). Soluble P mass loads in surface runoff responded to discharge more consistently than in the subsurface flow. Subsurface flow had substantially greater annual average soluble P mass loads than surface runoff due to greater flow volume.

Abbreviations: TP, total phosphorus • RT, reduced tillage • NT, no-till • LVR, Little Vermilion River watershed • TDP, total dissolved phosphate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS SUMMARY AND CONCLUSIONS REFERENCES

 
PHOSPHORUS (P) is an essential element of plant growth and very important in the production of profitable crops. The main sources of P to crops are inorganic commercial fertilizer and animal manure (Daniel et al., 1994). Farmers add P as commercial fertilizer or animal manure to cropland to increase the level of available P to plants (Hansen et al., 2002). In the last few decades, there has been a growing concern regarding the role of P from agricultural nonpoint sources of pollution on eutrophication of surface water (Sharpley et al., 1994; Carpenter et al., 1998; Hansen et al., 2002). The critical P concentration to promote eutrophication is 10 to 20 µg P L^–1 (Daniel et al., 1998; Heathwaite and Dils, 2000). Sharpley et al. (1993) indicated that particulate P represents 75 to 90% of total P transported in runoff from agricultural land, but it is not immediately available for algal uptake. In contrast, dissolved P represents a small fraction of total P transport in runoff, but is immediately available for algal uptake, and is considered the primary source for accelerating eutrophication. In a study conducted in South Australia, Dougherty et al. (2004) found that P export in runoff is primarily in dissolved and colloidal forms from intensive pasture systems. In another study, Dougherty et al. (2006) reported that dissolved P accounted for 86% of the P in runoff.

A significant amount of agricultural lands in the midwestern United States are artificially drained. Subsurface drainage was installed to convert millions of hectares of swamplands into highly productive agricultural lands by removing excess soil water and providing a suitable environment for crop productions. Generally, subsurface drainage increases infiltration, which subsequently decreases surface runoff and sediment losses (Skaggs et al., 1994; Kladivko et al., 2001). Despite the obvious benefits to crop production of artificial subsurface drainage, some researchers have indicated that it might have negative environmental impacts on surface and ground water (Skaggs et al., 1994; Fausey et al., 1995; Shirmohammadi et al., 1995; Kladivko et al., 2001).

Many reports have shown that P transport occurs predominantly in surface runoff. Phosphorus transport through subsurface drainage is small or negligible because of its low mobility in soils (Baker et al., 1975; Sharpley et al., 1993; Sims et al., 1998; Heathwaite and Dils, 2000; Hansen et al., 2002). Because of this physical characteristic (high P fixing capacity of subsoil), most research has focused on the export of P in runoff water and has not considered P transport through subsurface drainage as an important source of P movement to surface water bodies (Sims et al., 1998; Hansen et al., 2002). Few reports have shown that P transport could occur in subsurface drainage, especially if P levels in the soil exceeded the P level needed for optimum crop production (Culley et al., 1983; Gaynor and Findlay, 1995; Beauchemin et al., 1998; Smith et al., 2001). Other factors that could contribute to P movement via subsurface drainage are soil type and structure, preferential flow paths, low P sorption capacity, and organic matter content (Sharpley et al., 1993; Sims et al., 1998; Gilliam et al., 1999; Hansen et al., 2002). In their review, Sims et al. (1998) indicated that P losses increased and could be an immediate concern in watersheds with artificial drainage systems, in regions where soil P concentrations are already high, and in areas with low soil sorption capacity. Smith et al. (2001) found that the maximum total phosphorus (TP) loss in runoff following a high rate of manure application was only 2 kg ha^–1, whereas the peak flow-weighted TP concentration was 30000 µg L^–1. They argued that TP loss might be insignificant in agronomic terms, but the high TP concentration could significantly accelerate eutrophication, especially if it coupled with high nitrate N concentrations. They concluded that P losses through subsurface flow were consistently lower (about 10% of surface runoff losses) than the losses in surface runoff. However, Gilliam et al. (1999) in their review of agrichemicals transport in humid regions stated that P transport occurs in both subsurface drainage and surface runoff in humid regions with the majority of P transport through subsurface drainage in sandy loam soils. In Quebec, Canada, Beauchemin et al. (1998) found TP concentrations in drainage water to exceed the local standard of 30 µg L^–1 for surface water in 14 out of 27 and 6 out of 25 samples in 1994 and 1995, respectively, in clayey and coarse soils. Culley et al. (1983) showed that more than 50% of the TP losses might have occurred through subsurface drainage water in flat plots in Ontario, Canada. As high as 18200 µg L^–1 and 36.8 kg ha^–1 of dissolved ortho-P concentration and loss from subsurface drainage water in an organic soil were reported by Miller (1979). In a study conducted in an agricultural watershed in Pennsylvania, Sharpley et al. (1999) reported that 52% of soil samples had concentrations of soil P above the levels sufficient for optimum crop yield. Gaynor and Findlay (1995) reported ortho-P concentrations and losses in drain flow from a clay loam soil averaged 240 µg L^–1 and 0.38 kg ha^–1 yr^–1, which represented approximately 3% of the total P fertilizer applied.

The factors that could have an influence on P transport in runoff and subsurface drainage are P application rate, time and method of application, and rainfall timing and intensity, especially if rainfall occurs soon after fertilizer application (Culley et al., 1983; Edwards and Daniel, 1993; Sharpley et al., 1993, 1994; Daniel et al., 1994; Randall et al., 2000; Tabbara, 2003; Dougherty et al., 2004). Other factors that could affect P loss from agricultural land include watershed hydrology, amount and form of P and its availability, tillage systems, soil P level, drainage intensity, and vegetative cover.

The Department of Agricultural and Biological Engineering at the University of Illinois at Urbana-Champaign has been conducting research to investigate water quality problems and define solutions to these problems (Mitchell et al., 1998, 2000, 2003). This study (often referred to as the Little Vermilion River [LVR] project) has been performed to evaluate the impacts of agricultural practices on water quality in the LVR watershed that has intensive random and irregular subsurface drainage systems. A monitoring program was established to assess the ability of different agriculture management systems in reducing the effect of nutrients and several commonly used pesticides on surface and subsurface flow. This study is unique in the sense that it investigates P transport through subsurface drains in an intensively tile-drained watershed with random or irregular subsurface drain patterns and compares the results with that in surface runoff.

The overall objectives of this study were to evaluate the effects of best management practices on soluble P transport in subsurface flow and surface runoff within the LVR watershed, Illinois. Most of the previously published reports have discussed P levels in surface runoff and indicated that P exports were greater in surface runoff in comparison with losses in subsurface flow. This paper emphasizes P transport through subsurface drain lines in a flat midwestern watershed and compared those P levels with that in surface runoff from the same area in a watershed scale. The specific objectives of this study were to determine the effects of different agricultural management practices on the concentrations and mass losses of soluble phosphorus (soluble P) in subsurface flow and surface runoff.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS SUMMARY AND CONCLUSIONS REFERENCES

 
A water quality monitoring program was established in 1991 to assess different agricultural management practices for reducing the effects of soluble P on surface and subsurface flow in the LVR watershed. However, the data presented in this manuscript is from 1994 to 2000, because analysis for P did not start until 1994.

Site Location and Description
The study site was located in the LVR watershed in east-central Illinois, USA (Fig. 1). The watershed is approximately 489 km² and is located in sections of Champaign, Edger, and Vermilion Counties with the majority of the watershed situated in Vermilion County, Illinois. The topography of the watershed is generally flat with a slope of approximately 1% or less (Mitchell et al., 2000). The site represents an example of a watershed with altered hydrology with subsurface drainage systems. When the Upper Midwest was settled, pioneers found vast areas of tall grass prairie between river valleys and these prairies were swamps most of the year. Ditches were dug first in the depressional areas and connected to existing river valleys for drainage. Most depressional areas, however, are lower than the adjacent ditch banks; in many cases, the lowest point in the depression is 50 to 100 m from the ditch bank. Eventually, subsurface drains (tile drains) were installed in nearly all depressional areas using the ditches as outlets (Mitchell et al., 2003). The drained areas have irregularly spaced (random) subsurface drainage systems. In 1936, the LVR was impounded at the Village of Georgetown to create the 18.6-ha Georgetown Reservoir. The reservoir, until recently, served as a drinking water supply for approximately 10000 people living in Georgetown, Illinois (Mitchell et al., 2000). The subsurface drain lines most often discharge into numerous man-made drainage channels, which ultimately drain to the river and end up at the reservoir. Surface runoff seldom occurs in the watershed due to nearly level topography (Mitchell et al., 2000, 2003). Nevertheless, surface runoff represents a portion of the total water movement; it occasionally could be large depending on several factors such as precipitation, crop type, and the antecedent moisture condition of the soil (Mitchell et al., 2003).

View larger version (22K): [in this window] [in a new window]   Fig. 1. The Little Vermilion River (LVR) watershed and water quality monitoring stations for each site in the LVR watershed, IL.

Procedures
Subsurface drainage flow was continuously monitored using 203-mm Palmer Bowlus flumes for all stations. The flumes were installed in access manholes near each subsurface drain system outlet. Data loggers were used to record time stage data by increments of stage from stage recorders. Water samples were obtained automatically by water samplers activated by the data logger at specified flow volumes. Additional water samples (grab samples) were taken at least biweekly (Mitchell et al., 2000, 2003). These samples were taken either as a backup or during low flow periods.

Surface runoff stations were established in conjunction with all four subsurface monitoring stations (A, B, C, and E). The surface runoff drainage areas were not of the same size as the subsurface drained areas (subsurface stations) because of the nature of topography of the lands, but these stations were installed in the same field. Flumes or weirs, stage recorders, and data loggers were used to monitor surface runoff stations. Stations As and Bs were equipped with 305-mm culvert flumes; Cs used a 380-mm culvert flume; and station Es had a 5:1 sharp-edged weir. Water samples from the surface runoff stations were taken with automatic water samplers. Details of the study site and characteristics of the monitoring stations were provided by Mitchell et al. (2000).

Precipitation was recorded using tipping bucket rain gages installed at every subsurface station, except station B, which was adjacent to station A. These two stations shared one rain gage. All the rain gages were connected to data loggers.

Management Practices
For all four sites, the landowners made their decisions about crop and fertilizer management practices. Fields at sites A, B, and E were in a corn–soybean rotation, while that at site C had a seed corn–soybean rotation. Information on tillage practices, P application rates, and application timing were communicated to the research team by the landowners. Fields at sites A and B were chisel-plowed and disked or field-cultivated every year. Tillage practices at site C included disking and field-cultivation and site E had NT throughout the study period. Phosphorus was applied after crop harvest at sites A, B, and C, and was applied before planting at site E. Average P application rates for the study sites were 74, 47, 56, and 44 kg ha^–1 for sites A, B, C, and E, respectively.

Soluble Phosphorus Concentrations Analysis
Soluble P was analyzed at the water quality laboratory in the Department of Biological and Agricultural Engineering at the University of Illinois at Urbana-Champaign. Samples were collected in the field, preserved with concentrated sulfuric acid (2 mL L^–1), and stored at 4°C until analysis. Soluble P concentrations were determined using a continuous flow Technicon Autoanalyzer II (Technicon Corp., Terrytown, NY). The procedure is based on the colorimetric method in which a blue color is formed by the reaction of soluble P, molybdate ion, and antimony ion followed by a reduction with ascorbic acid at an acidic pH. The reduced blue phosphomolybdenum complex is read at 660 nm. The detection limit for soluble P was 1.00 µg L^–1.

Soluble Phosphorus Load Calculation
Soluble P loads were calculated using soluble P concentrations and computed subsurface flow and surface runoff. Soluble P concentrations were not measured every day that the flow occurred; the data collected contained more flow rate measurements than soluble P concentrations. Soluble P concentration for a sample collected at a specific time was multiplied by half the flow volume from the concentration measurement plus half the flow volume from concentration measurement to the next concentration measurement to calculate soluble P load for that time period. This procedure continued from start to the end of record for each station. The sampling volume was constant except that intermediate grab samples altered that volume depending on timing. Soluble P losses for each sampling period were summed to give the total annual losses. Mass load at each station was calculated by dividing the soluble P load by the area draining to the station and reported in grams per hectare (g ha^–1).

Statistical Analysis
Statistical analysis was performed to test the effects of several variables on the soluble P concentrations in both subsurface flow and surface runoff. Due to the nature of the experiments of this project, the collected data were labeled as unbalanced data (variable number of observations per time period and among stations), and therefore, the general linear method procedure (GLM) was used as described by Little et al. (2002) to analyze the data. The GLM procedure uses the least squares method to relate dependent variable (soluble P concentrations) to independent variables (discharge, site, and crop). The GLM procedure was performed to do the analysis of variance of the data to evaluate the effects of independent variables (site, crop, and discharge) on the concentrations of soluble P using SAS (SAS Institute, 2001). Because of the nature of the data (unbalanced data), we have used type III error to test the null hypotheses (factors significantly affecting the chemical concentrations). Concentrations of soluble P in subsurface flow and surface runoff were not normally distributed and were transformed using the natural logarithms before analysis. Subsurface and surface station data were transformed using the natural logarithm of concentration + 0.001 and concentration + 0.1, respectively, to eliminate zero data points from the concentrations. Transformations using the natural logarithm resulted in a better distribution of the data than that with the common logarithm (base 10) and exponents (0.5 and 2).

	RESULTS AND DISCUSSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS SUMMARY AND CONCLUSIONS REFERENCES

 
Flow and Rainfall Analysis
Precipitation, subsurface flow, and surface runoff for each station are summarized in Table 2. The long-term average rainfall for the watershed at Danville, IL was 1040 mm yr^–1 (Illinois State Climatologist Office, 2006). Precipitation patterns over the study period for all stations were highly variable. The annual precipitation during 1994–2000 for each site was lower than the long-term average precipitation for the watershed except for the year 1998, in which the annual precipitation exceeded the long-term average value. The average rainfall over the study period for all sites was below the long-term average of 1040 mm yr^–1. Site C had the lowest average precipitation of all the stations with an average of 786 mm yr^–1 (254 mm below the long-term average). The above long-term average precipitation in 1998 for sites A, B, and E resulted in almost continuous drain flow in the growing seasons in these years. Drain flow occurred sporadically during the growing seasons most of the other years, but it was more pronounced in 1995, 1997, and 1999, when precipitation was below long-term average for the watershed. Subsurface drains did not flow after harvest until January for most of the years due to low or no precipitations, or frozen soil conditions.

View larger version (54K): [in this window] [in a new window]   Fig. 2. Rainfall, subsurface flow, and surface runoff for each site in the Little Vermilion River (LVR) watershed, IL.

View larger version (35K): [in this window] [in a new window]   Fig. 3. Monthly average flow volume for both subsurface and surface runoff and rainfall for the study sites at Little Vermilion River (LVR) watershed, IL.

The presence of the artificial drainage in the LVR coupled with the nearly flat topography caused reduction of surface runoff in this watershed. Depending on several factors such as soil type and condition, weather and vegetative conditions, and topography, the existence of subsurface drainage could reduce surface runoff up to 65% (Kladivko et al., 2001).

The flow as a percentage of rainfall increased as precipitation increased, and decreased when precipitation decreased. For example, at sites A and B, rainfall in 1996 and 1998 were 1006 and 1266 mm, respectively, while surface runoff and subsurface flow in those years were the greatest recorded flows during the study period. In 1998, station As runoff was above average reaching 55.5 mm (4.4%), which is 38.4 mm above the average surface runoff. This might be the result of very high rainfall intensity and high amount of rainfall occurring in a short period of time, which prevented water from taking its time to percolate through the soil profile. Furthermore, partitioning of rainfall water between surface and subsurface was not consistent over the years. For instance, at station Bs, greater surface runoff was observed in 1996 than in 1998, even though 1998 recorded higher rainfall amount than that in 1996. This might be due to high rainfall in 1996 occurring in the month of May (280 mm) right before planting or at an early stage of planting, while in 1998, the greatest amount of rain occurred in the month of June (263 mm) at the advanced stage of plant growth and higher temperature. Higher ET, which causes a drier soil profile before rainfall during summer months, might have lowered surface runoff during 1998 relative to 1996. Surface runoff (Table 2 and Fig. 2) was high from late winter to spring periods corresponding to low plant uptake and no ET during that time. Surface runoff results of this study are also comparable to other results from similar conditions in the region. Andraski et al. (2003) reported that the amount of runoff ranged from 42 to 48 mm in field studies that had well-drained silty loam soils with average slopes of 3 and 6% at Madison and Lancaster, respectively, in Wisconsin, USA. Higher runoff amount in their study compared with our result could be attributed to steeper slopes in their fields than the nearly flat fields in the LVR watershed. Smith et al. (2001) showed that surface runoff as a percentage of rainfall accounted for about 0.2 to 11.3% for a 4-yr study in an irrigated filed in UK. The average surface flow over a 4-yr study was 3.75 cm in a clay loam soil with less than 3% slope in Minnesota (Thoma et al., 2005).

Soil type and tillage system showed some effect on annual subsurface flow and surface runoff. Sites A, B, and C have Drummer and Flanagan silty clay loam, while site E has Sabina and Xenia silty loam. Sites A, B, and C have RT, and site E has NT. Site A, which has Drummer silty clay loam and RT had the lower average annual subsurface flow than that in site E, which had Sabina and Xenia silty loam and NT. Sites A and B had the lowest and second to highest average surface runoff, respectively, even though they have the same soil type and tillage system. Site E has a different soil type and tillage system than the other three sites and had the second to last average surface runoff. This indicated that Sabina and Xenia silty loam soil and NT system increased subsurface flow, which in turn reduced surface runoff. Site E has uniform tile spacing and a relatively intensive drainage system than that for the other sites. Also, preferential flow associated with the NT system at site E might have caused increased subsurface drain flows.

Phosphorus Application Rate
Result of the data analysis for P application rates in the study sites are summarized in Table 3. Inorganic P fertilizer was used throughout the study period at all sites. Phosphorus at site A was applied mainly when soybeans were harvested except in 1999, when 15 kg ha^–1 of P was applied following the harvest of corn. Phosphorus was applied at sites A, B, and C after crops were harvested from September to November. In contrast, P was broadcasted at site E right before planting when corn was grown. Phosphorus was applied at site B in all years and for both crops. The average annual P application rates range from as low as 44 kg ha^–1 at site E to as high as 74 kg ha^–1 at site A (Table 3). Site B had the second lowest annual average P application rate (47 kg ha^–1) even though fertilizer was applied in all years and for both crops. The average annual P application rate for the state of Illinois was 82 kg ha^–1 from 1994 to 2000 (USDA–National Agricultural Statistics Service, 2006). All sites had noticeably lower average P application rates than the average application rate for Illinois.

Soluble Phosphorus Concentrations
Analysis for soluble P did not start until 1994 for all sites. Annual flow-weighted soluble P concentrations in both subsurface flow and surface runoff tended to fluctuate with the rainfall and the amount of subsurface flow and surface runoff (Tables 2 and 4, and Fig. 4 and 5). At site A for instance, in 1996 when rainfall was above long-term average (which lead to high subsurface flow), flow-weighted soluble P concentration in the subsurface flow was at its highest level. However, the concentrations of soluble P did not respond to rainfall and drain flow consistently. Annual flow-weighted soluble P concentration in the subsurface flow was 212 µg L^–1 in 1996 for site A when rainfall (1006 mm) was a little below the long-term average for the watershed and higher than the average for the site. The following year, which had lower annual rainfall than the previous year, 85 µg L^–1 of soluble P was recorded despite below average precipitation. Similarly, the concentrations did not respond consistently to P application. Frequently, higher soluble P concentrations were detected in years when there was no application of P. Nevertheless, concentrations generally tended to increase with high precipitation coupled with high application rates. The detection of soluble P in years without fertilization might have occurred from high soil P concentration built up over the years.

View larger version (32K): [in this window] [in a new window]   Fig. 4. Soluble phosphorus concentrations in subsurface flow and surface runoff for all stations at the Little Vermilion River (LVR) watershed, IL.

View larger version (28K): [in this window] [in a new window]   Fig. 5. Soluble phosphorus concentrations and discharge in subsurface flow and surface runoff for all stations at the Little Vermilion River (LVR) watershed, IL.

Phosphorus is less soluble than nitrate N in water, but it strongly adsorbs to soil particles and is transported with eroded soil with the surface flow. All of the stations had substantially greater overall average and annual flow-weighted soluble P concentrations in surface runoff than in subsurface flow (Table 4 and Fig. 4). The overall averages for the annual flow-weighted soluble P concentrations in surface runoff for all of the sites were approximately two to six times greater than the annual average flow-weighted soluble P concentrations in subsurface flow, even though the annual average surface runoff was much lower than the annual average subsurface flow. These results are consistent with findings that other researchers have reported (Baker et al., 1975; Sharpley et al., 1993; Daniel et al., 1994; Brookes et al., 1997; Sims et al., 1998; Hansen et al., 2002) regarding high P concentrations in surface runoff. Smith et al. (2001) found that the peak flow-weighted TP concentration was 30000 µg L^–1 in subsurface flow. As high as 18200 µg L^–1 of dissolved ortho-P concentration from subsurface drainage water in an organic soil was reported by Miller (1979). In Iowa, Baker et al. (1975) reported that the concentrations of soluble P ranged from 0 to 38 µg L^–1 during a 3 yr of study period. Gaynor and Findlay (1995) estimated that the average ortho-P concentration in drain flow from a clay loam soil was 240 µg L^–1. During a 2-yr study, subsurface flow-weighted molybdate-reactive P ranged from 160 to 380 µg L^–1 (Hooda et al., 1999). In a 4-yr study in Waseca, MN, Randall et al. (2000) reported that the concentration of molybdate-reactive P averaged 13 µg L^–1 for plots that had urea application and 16 µg L^–1 for plots that had manure application. In a field with corn planted in rotation with rye near Belle Mina, AL, Wood et al. (1999) reported that flow-weighted concentrations of dissolve-P averaged 80 and 22 µg L^–1 for corn and rye, respectively, in surface runoff using commercial fertilizer. Daverede et al. (2003) concluded that dissolved P in surface runoff averaged 400 and 240 µg L^–1 under NT and chisel-plow, respectively, in a silty clay loam soil with 5.5% slope in Monmouth, IL. Over a 4-yr study period, the average flow-weighted soluble P in surface runoff in a clay soil (<3% slope) was about 330 µg L^–1 (Thoma et al., 2005).

Station E had the greatest average annual flow-weighted soluble P concentration in subsurface flow followed by stations A, B, and E, respectively (Table 4). On the other hand, long-term average flow-weighted soluble P concentrations in surface flow were greater in station Es followed by stations Cs, As, and Bs, respectively. The greatest average annual flow-weighted soluble P concentration measured in subsurface flow and surface runoff was 194 µg L^–1 at station C and 572 µg L^–1 at station Es, respectively. Annual flow-weighted concentrations and the overall annual average concentrations were much greater than the critical P concentration required to promote eutrophication (10–20 µg P L^–1) for both subsurface flow and surface runoff at all sites. This may pose an environmental concern, especially if P transport continues at this rate.

Monthly average flow-weighted soluble P concentrations in subsurface flow and surface runoff are presented in Table 5. Flow-weighted soluble P concentrations peaked from February until July each year for all stations. From August to December, soluble P concentrations were small in both subsurface effluents and surface runoff. Sites C and E had detectable soluble P all year round in subsurface flow. Soluble P was detected more often at low discharge, especially in subsurface drain flow (Fig. 5). In surface runoff stations, nevertheless, soluble P was detected at moderate to elevated flows. This is the result of the dilution and depletion effect.

Soluble Phosphorus Mass Loads
Annual soluble P mass loads followed the same pattern as annual flow-weighted soluble P concentrations. Annual soluble P mass loads tended to fluctuate with subsurface flow and surface runoff. Soluble P mass loads in surface runoff responded to discharge more clearly and consistently than in the subsurface flow. Soluble P mass loads fluctuated from year to year and from station to station for both subsurface and surface stations. Subsurface flow had substantially greater average annual soluble P mass loads than that in surface runoff. This was the result of greater flow volume in subsurface drains than surface runoff. In surface stations the average soluble P mass losses were the greatest from station Es followed by Cs, Bs, and As, respectively (Table 4). On the other hand, average soluble P losses in subsurface effluents were the greatest from station C followed by stations A, E, and B, respectively. Soluble P losses represented approximately 0.29 and 0.17% of applied P across all sites in subsurface flow and surface runoff, respectively. Although the amount of soluble P mass loads for both subsurface flow and surface runoff might appear to be low or negligible, it poses some concern considering the elevated soluble P concentrations, especially in subsurface flow. Similar results have been reported in the literature. Smith et al. (2001) found that the maximum TP loss in runoff following high rate manure application was only 2 kg ha^–1. Miller (1979) reported that dissolved ortho-P losses from subsurface drainage water in an organic soil were as high as 36.8 kg ha^–1. Gaynor and Findlay (1995) indicated that the average ortho-P loss in drain flow from a clay loam soil was 0.38 kg ha^–1 yr^–1. Hooda et al. (1999) reported that the molybdate-reactive P losses in subsurface flow from grass-clover and grass ranged from 1.98 to 2.03 and 1.27 to 1.34 kg P ha^–1 yr^–1, respectively. Wood et al. (1999) reported that dissolved P losses averaged 90 and 170 g ha^–1 for corn and rye, respectively, in surface runoff using commercial fertilizer. In Minnesota, soluble P losses in subsurface flow resulting from commercial application were 0.1 and 11.8 g ha^–1 under moldboard plow and ridge till treatments, respectively, in a clay loam soil with less than 2% slope (Zhao et al., 2001). On the other hand, soluble P losses in surface runoff were 54.7 and 79.7 g ha^–1 under moldboard plow and ridge till treatments, respectively. Daverede et al. (2003) reported that the dissolved P in surface runoff averaged 50 and 20 g ha^–1 under NT and chisel-plow, respectively, in a silty clay loam soil with 5.5% slope in Monmouth, IL. The average soluble P loss in surface flow was about 125 g ha^–1 from nearly flat (<3% slope) plots in Minnesota (Thoma et al., 2005). In Australia, Fleming and Cox (2001) studied P losses in runoff from two grazed dairy pastures and found that up to 2.3 kg ha^–1 of P was lost in runoff, and approximately 45% of this P was in the dissolved form.

In the LVR watershed, most of the soluble P losses occurred during the growing months with both subsurface flow and surface runoff (Fig. 6). In contrast, almost no soluble P was transported from July to December each year in all stations because of very little flow during those months.

View larger version (23K): [in this window] [in a new window]   Fig. 6. Monthly average soluble phosphorus mass loads in subsurface flow and surface runoff in all study sites at Little Vermilion River (LVR) watershed, IL.

Several other factors were not measured or considered in this study that might have added to the variability in soluble P losses. These factors include spatial variability of soil properties, antecedent soil water contents, and water table levels in the fields.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS SUMMARY AND CONCLUSIONS REFERENCES

 
Soluble P concentrations in subsurface drainage and surface runoff flows were monitored at several sites in the LVR watershed in east central Illinois, USA. The effects of several management practices on soluble P transport were evaluated for 7 yr of study period. Subsurface drainage and surface runoff across all stations removed an average of 16.1 and 2.6% of rainfall, respectively, over the four sites. Annual flow-weighted soluble P concentrations in both subsurface effluents and surface runoff fluctuated from station to station and from year to year over the study period. Soluble P concentrations were greater in surface runoff than in subsurface flow for all stations. Long-term average flow-weighted soluble P concentrations in subsurface flow were the highest at station C (194 µg L^–1) followed by stations A (102 µg L^–1), B (99 µg L^–1), and E (86 µg L^–1). In surface runoff, however, the average flow-weighted soluble P concentrations were 270, 253, 534, and 572 µg L^–1 at stations As, Bs, Cs, and Es, respectively. Soil type and tillage system showed some effect on soluble P transport in subsurface flow and surface runoff. Site E, which had Sabina and Xenia silty loam and NT and also had relatively intensive and uniform subsurface drain spacing in comparison with the other sites, had the lowest average annual flow-weighted soluble P concentration and second to the lowest in average mass load in subsurface flow than the other sites. Crop type affected soluble P transport in both subsurface flow and surface runoff. When seed corn was planted in rotation with soybeans (site C), annual flow-weighted soluble P concentrations and losses were the highest in subsurface flow and second to highest in surface runoff. Rate, timing, and method of P application affected soluble P transport in both subsurface flow and surface runoff. Greater application rate coupled with application timing tended to affect soluble P concentrations and losses in subsurface flow and surface runoff. Sites with higher application rate and applying P after crop harvest had the higher soluble P transport in subsurface flow. On the other hand, the site with lower P rate and applying fertilizer just before planting had higher soluble P transport in surface runoff.

Annual flow-weighted soluble P concentrations in both subsurface flow and surface runoff fluctuated with the rainfall and the amount of subsurface flow and surface runoff. However, the concentrations did not respond to rainfall and drain flow consistently. Similarly, the concentrations did not respond consistently to P application rates. Frequently, higher soluble P concentrations were detected in years when there was no application of P. Nevertheless, concentrations increased with high precipitation coupled with high application rates. The detection of soluble P in years without fertilization might have occurred from high soil P concentration built up over the years. The relationship between annual rainfall, tile flow, and runoff; and annual flow-weighted soluble P concentrations and mass loads were more apparent in surface runoff stations than in subsurface stations, although inconsistency between them existed. All of the stations had substantially greater overall average and annual flow-weighted soluble P concentrations in surface runoff than in subsurface flow. The overall average of the annual flow-weighted soluble P concentrations in surface runoff for all of the sites are about two to six times greater than that in subsurface flow, even though annual average surface runoff was much lower than the annual average subsurface flow. Flow-weighted soluble P concentrations peaked from February until August each year for all stations. During the dormant season, soluble P was almost undetectable. Statistically, the effects of crop, discharge, the interaction between site and discharge, and crop and discharge were significant ( = 0.05) on soluble P concentrations in subsurface flow. In surface runoff stations, the effects of site, discharge, and the interactions between site and discharge, site and crop, and crop and discharge were significant ( = 0.05) on soluble P concentrations.

Annual soluble P mass loads fluctuated with subsurface flow and surface runoff. Soluble P mass loads in surface runoff responded to discharge more clearly than in the subsurface flow. Subsurface flow had substantially greater annual average soluble P mass loads than that in surface runoff. Average soluble P mass loads were approximately 0.29 and 0.17% of applied P across all sites in subsurface flow and surface runoff, respectively. Most of the soluble P losses occurred during the growing months for both subsurface flow and surface runoff. Negligible amount of soluble P was lost during the dormant season relative to growing season as a result of low flow for all of the sites during the study period.

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