Soil and surface runoff phosphorus relationships for five typical USA midwest soils

Excessively high soil P can increase P loss with surface runoff. This study used indoor rainfall simulations to characterize soil and runoff P relationships for five Midwest soils (Argiudoll, Calciaquaoll, Hapludalf, and two Hapludolls). Topsoil (15-cm depth, 241-289 g clay kg(-1) and pH 6.0-8.0) was incubated with five NH4H2PO4 rates (0-600 mg P kg(-1)) for 30 d. Total soil P (TPS) and soil-test P (STP) measured with Bray-P1 (BP), Mehlich-3 (M3P), Olsen (OP), Fe-oxide-impregnated paper (FeP), and water (WP) tests were 370 to 1360, 3 to 530, 10 to 675, 4 to 640, 7 to 507, and 2 to 568 mg P kg(-1), respectively. Degree of soil P saturation (DPS) was estimated by indices based on P sorption index (PSI) and STP (DPSSTP) and P, Fe, and Al extracted by ammonium oxalate (DPSox) or Mehlich-3 (DPSM3). Soil was packed to 1.1 g cm(-3) bulk density in triplicate boxes set at 4% slope. Surface runoff was collected during 75 min of 6.5 cm h(-1) rain. Runoff bioavailable P (BAP) and dissolved reactive P (DRP) increased linearly with increased P rate, STP, DPSox, and DPSM3 but curvilinearly with DPSSTP. Correlations between DRP or BAP and soil tests or saturation indices across soils were greatest (r > or = 0.95) for FeP, OP, and WP and poorest for BP and TPS (r = 0.83-0.88). Excluding the calcareous soil (Calciaquoll) significantly improved correlations only for BP. Differences in relationships between runoff P and the soil tests were small or nonexistent among the noncalcareous soils. Routine soil P tests can estimate relationships between runoff P concentration and P application or soil P, although estimates would be improved by separate calibrations for calcareous and noncalcareous soils.     

Estimating dissolved reactive phosphorus concentration in surface runoff water from major Ontario soils

Phosphorus (P) loss from agricultural land in surface runoff can contribute to eutrophication of surface water. This study was conducted to evaluate a range of environmental and agronomic soil P tests as indicators of potential soil surface runoff dissolved reactive P (DRP) losses from Ontario soils. The soil samples (0- to 20-cm depth) were collected from six soil series in Ontario, with 10 sites each to provide a wide range of soil test P (STP) values. Rainfall simulation studies were conducted following the USEPA National P Research Project protocol. The average DRP concentration (DRP30) in runoff water collected over 30 min after the start of runoff increased (p < 0.001) in either a linear or curvilinear manner with increases in levels of various STPs and estimates of degree of soil P saturation (DPS). Among the 16 measurements of STPs and DPSs assessed, DPS(M3) 2 (Mehlich-3 P/[Mehlich-3 Al + Fe]) (r2 = 0.90), DPS(M3)-3 (Mehlich-3 P/Mehlich-3 Al) (r2 = 0.89), and water-extractable P (WEP) (r2 = 0.89) had the strongest overall relationship with runoff DRP30 across all six soil series. The DPS(M3)-2 and DPS(M3)-3 were equally accurate in predicting runoff DRP30 loss. However, DPS(M3)-3 was preferred as its prediction of DRP30 was soil pH insensitive and simpler in analytical procedure, ifa DPS approach is adopted.     

Particulate phosphorus transport within stream flow of an agricultural catchment

There is interest in quantifying phosphorus (P) loss from intensively grazed dairy landscapes to identify key pathways and target remediation methods. The Bog Burn drains a dairying catchment in Southland, New Zealand, and has been monitored at fortnightly intervals over a 12-mo period at four sites for suspended sediment (SS), dissolved reactive phosphorus (DRP), and total phosphorus (TP). Time-integrated samplers, deployed at 0.6 median water depth at each site (calculated from previous year's flow data), collected sediment samples, which were analyzed for SS, bioavailable phosphorus (BAP), and TP. Mean concentrations of DRP and TP in stream flow and BAP and TP in sediment were generally highest in summer or autumn (0.043 mg DRP L(-1), 0.160 mg TP L(-1), 173 mg BAP kg(-1), 2228 mg TP kg(-1)) and lowest in winter or spring (0.012 mg DRP L(-1), 0.034 mg TP L(-1), 6 mg BAP kg(-1), 711 mg TP kg(-1)), while loads were highest in winter. Analysis of (137)Cs concentrations in trapped sediment, topsoil, subsoil, and stream bed and bank sediment indicated that trapped sediment was derived from topsoil and entered the stream either through tile drainage or, to a lesser extent, overland flow. Because concentrations of DRP and TP in stream flow are in excess of recommended limits for good water quality (>0.01 mg DRP L(-1), 0.033 mg TP L(-1)), management should focus on the topsoil and specifically on decreasing P loss via tile drainage. This is best achieved by decreasing soil Olsen P concentrations, especially because, on average, Olsen P concentrations in the catchment were above the agronomic optimum.     

Soil testing to predict phosphorus leaching

Subsurface pathways can play an important role in agricultural phosphorus (P) losses that can decrease surface water quality. This study evaluated agronomic and environmental soil tests for predicting P losses in water leaching from undisturbed soils. Intact soil columns were collected for five soil types that a wide range in soil test P. The columns were leached with deionized water, the leachate analyzed for dissolved reactive phosphorus (DRP), and the soils analyzed for water-soluble phosphorus (WSP), 0.01 M CaCl2 P (CaCl2-P), iron-strip phosphorus (FeO-P), and Mehlich-1 and Mehlich-3 extractable P, Al, and Fe. The Mehlich-3 P saturation ratio (M3-PSR) was calculated as the molar ratio of Mehlich-3 extractable P/[Al + Fe]. Leachate DRP was frequently above concentrations associated with eutrophication. For the relationship between DRP in leachate and all of the soil tests used, a change point was determined, below which leachate DRP increased slowly per unit increase in soil test P, and above which leachate DRP increased rapidly. Environmental soil tests (WSP, CaCl2-P, and FeO-P) were slightly better at predicting leachate DRP than agronomic soil tests (Mehlich-1 P, Mehlich-3 P, and the M3-PSR), although the M3-PSR was as good as the environmental soil tests if two outliers were omitted. Our results support the development of Mehlich-3 P and M3-PSR categories for profitable agriculture and environmental protection; however, to most accurately characterize the risk of P loss from soil to water by leaching, soil P testing must be fully integrated with other site properties and P management practices.     

Management practice effects on phosphorus losses in runoff in corn production systems

Phosphorus losses in runoff from cropland can contribute to nonpoint-source pollution of surface waters. Management practices in corn (Zea mays L.) production systems may influence P losses. Field experiments with treatments including differing soil test P levels, tillage and manure application combinations, and manure and biosolids application histories were used to assess these management practice effects on P losses. Runoff from simulated rainfall (76 mm h(-1)) was collected from 0.83-m2 areas for 1 h after rainfall initiation and analyzed for dissolved reactive P (DRP), bioavailable P, total P (TP), and sediment. In no-till corn, both DRP concentration and load increased as Bray P1 soil test (STP) increased from 8 to 62 mg kg(-1). A 5-yr history of manure or biosolids application greatly increased STP and DRP concentrations in runoff. The 5-yr manure treatment had higher DRP concentration but lower DRP load than the 5-yr biosolids treatment, probably due to residue accumulation and lower runoff in the manure treatment. Studies of tillage and manure application effects on P losses showed that tillage to incorporate manure generally lowered runoff DRP concentration but increased TP concentration and loads due to increased sediment loss. Management practices have a major influence on P losses in runoff in corn production systems that may overshadow the effects of STP alone. Results from this work, showing that some practices may have opposite effects on DRP vs. TP losses, emphasize the need to design management recommendations to minimize losses of those P forms with the greatest pollution potential.     

Degree of phosphorus saturation thresholds in manure-amended soils of alberta

The risk of P losses from agricultural land to surface and ground water generally increases as the degree of soil P saturation increases. A single-point soil P sorption index (PSI) was validated with adsorption isotherm data for determination of the P sorption status of Alberta soils. Soil P thresholds (change points) were then examined for two agricultural soils after eight annual applications of different rates of cattle manure and for three agricultural soils after one application of different rates of cattle manure. Linear relationships were found between soil-test P (STP) levels up to 1000 mg kg(-1) and desorbed P in the five Alberta soils. Weak linear relationships were also found between STP and runoff dissolved reactive phosphorus (DRP) in three of these soils. Change points for the degree of P saturation (DPS) were detected in four of the five soils at 3 to 44% for water-extractable P (WEP) and at 11 to 51% for CaCl(2)-extractable P (CaCl(2)-P). Change points were not found for DPS or runoff DRP. Overall DPS thresholds for the five soils combined were 27% for WEP and 44% for CaCl(2)-P at a critical desorbable-P value of 1 mg L(-1). The corresponding STP levels (44 mg kg(-1) for WEP and 71 mg kg(-1) for CaCl(2)-P) are similar to agronomic thresholds for crops grown on Alberta soils. Soluble P losses in overland flow and leaching may be greater in soils with DPS values that exceed these thresholds than in soils with lower DPS values.     

Approximating phosphorus release from soils to surface runoff and subsurface drainage

Phosphorus application in excess of crop needs has increased the concentration of P in surface soil and runoff and led many states to develop P-based nutrient management strategies. However, insufficient data are available relating P in surface soil, surface runoff, and subsurface drainage to develop sound guidelines. Thus, we investigated P release from the surface (0-5 cm depth) of a Denbigh silt loam from Devon, U.K. (30-160 mg kg-1 Olsen P) and Alvin, Berks, Calvin, and Watson soils from Pennsylvania (10-763 mg kg-1 Mehlich-3 P) in relation to the concentration of P in surface runoff and subsurface drainage. A change point, where the slopes of two linear relationships between water- or CaCl2-extractable soil P and soil test phosphorus (STP) (Olsen or Mehlich-3) meet, was evident for the Denbigh at 33 to 36 mg kg-1 Olsen P, and the Alvin and Berks soils at 185 to 190 mg Mehlich-3 P kg-1. Similar change points were also observed when STP was related to the P concentration of surface runoff (185 mg kg-1) and subsurface drainage (193 mg kg-1). The use of water and CaCl2 extraction of surface soil is suggested to estimate surface runoff P (r2 of 0.92 for UK and 0.86 for PA soils) and subsurface drainage P (r2 of 0.82 for UK and 0.88 for PA soils), and to determine a change point in STP, which may be used in support of agricultural and environmental P management.     

Relationships between soil and runoff phosphorus in small Alberta watersheds

Field-scale relationships between soil test phosphorus (STP) and flow-weighted mean concentrations (FWMCs) of dissolved reactive phosphorus (DRP) and total phosphorus (TP) in runoff are essential for modeling phosphorus losses, but are lacking. The objectives of this study were (i) to determine the relationships between soil phosphorus (STP and degree of phosphorus saturation (DPS)) and runoff phosphorus (TP and DRP) from field-sized catchments under spring snowmelt and summer rainfall conditions, and (ii) to determine whether a variety of depths and spatial representations of STP improved the prediction of phosphorus losses. Runoff was monitored from eight field-scale microwatersheds (2 to 248 ha) for 3 yr. Soil test phosphorus was determined for three layers (0 to 2.5 cm, 0 to 5 cm, and 0 to 15 cm) in spring and fall and the DPS was determined for the surface layer. Average STP (0 to 15 cm) ranged from 3 to 512 mg kg(-1), and DPS (0 to 2.5 cm) ranged from 5 to 91%. Seasonal FWMCs ranged from 0.01 to 7.4 mg L(-1) DRP and from 0.1 to 8.0 mg L(-1) TP. Strong linear relationships (r2=0.87 to 0.89) were found between the site mean STP and the FWMCs of DRP and TP. The relationships had similar extraction coefficients, intercepts, and predictive power among all three soil layers. Extraction coefficients (0.013 to 0.014) were similar to those reported for other Alberta studies, but were greater than those reported for rainfall simulation studies. The curvilinear DPS relationship showed similar predictive ability to STP. The field-scale STP relationships derived from natural conditions in this study should provide the basis for modeling phosphorus in Alberta.     

Use of drinking water treatment residuals as a potential best management practice to reduce phosphorus risk index scores

The P risk index system has been developed to identify agricultural fields vulnerable to P loss as a step toward protecting surface water. Because of their high Langmuir phosphorus adsorption maxima (P(max)), use of drinking water treatment residuals (WTRs) should be considered as a best management practice (BMP) to lower P risk index scores. This work discusses three WTR application methods that can be used to reduce P risk scores: (i) enhanced buffer strip, (ii) incorporation into a high soil test phosphorus (STP) soil, and (iii) co-blending with manure or biosolids. The relationship between WTR P(max) and reduction in P extractability and runoff P was investigated. In a simulated rainfall experiment, using a buffer strip enhanced with 20 Mg WTR ha(-1), runoff P was reduced by from 66.8 to 86.2% and reductions were related to the WTR P(max). When 25 g kg(-1) WTR was incorporated into a high STP soil of 315 mg kg(-1) determined using Mehlich-3 extraction, 0.01 M calcium chloride-extractable phosphorus (CaCl(2)-P) reductions ranged from 60.9 to 96.0% and were strongly (P < 0.01) related to WTR P(max). At a 100 g kg(-1) WTR addition, Mehlich 3-extractable P reductions ranged from 41.1 to 86.7% and were strongly (P < 0.01) related to WTR P(max). Co-blending WTR at 250 g kg(-1) to manure or biosolids reduced CaCl(2)-P by >75%. The WTR P(max) normalized across WTR application rates (P(max) x WTR application) was significantly related to reductions in CaCl(2)-P or STP. Using WTR as a P risk index modifying factor will promote effective use of WTR as a BMP to reduce P loss from agricultural land.     

Relationship between soil test phosphorus and phosphorus in runoff: effects of soil series variability

Phosphorus loss in runoff from agricultural fields has been identified as an important contributor to eutrophication. The objective of this research was to determine the relationship between phosphorus (P) in runoff from a benchmark soil (Cecil sandy loam; fine, kaolinitic, thermic Typic Kanhapludult) and Mehlich III-, deionized water-, and Fe(2)O(3)-extractable soil P, and degree of phosphorus saturation (DPS). Additionally, the value of including other soil properties in P loss prediction equations was evaluated. Simulated rainfall was applied (75 mm h(-1)) to 54 1-m(2) plots installed on six fields with different soil test phosphorus (STP) levels. Runoff was collected in its entirety for 30 min and analyzed for total P and dissolved reactive phosphorus (DRP). Soil samples were collected from 0- to 2-, 0- to 5-, and 0- to 10-cm depths. The strongest correlation for total P and DRP occurred with DPS (r(2) = 0.72). Normalizing DRP by runoff depth resulted in improved correlation with deionized water-extractable P for the 0- to 10-cm sampling depth (r(2) = 0.81). The STP levels were not different among sampling depths and analysis of the regression equations revealed that soil sampling depth had no effect on the relationship between STP and P in runoff. For all forms of P in runoff and STP measures, the relationship between STP and runoff P was much stronger when the data were split into groups based on the ratio of oxalate-extractable Fe to Al. For all forms of P in runoff and all STP methods, R(2) increased with the inclusion of oxalate-extractable Al and Fe in the regression equation. The results of this study indicate that inclusion of site-specific information about soil Al and Fe content can improve the relationship between STP and runoff P.     

The reduction of internal phosphorus loading using alum in Spring Lake, Michigan

The release of P from lake sediments, which occurs as a part of internal loading, may contribute a significant portion of the total P load to a lake. Phosphorus release rates from sediments in Spring Lake, Michigan, and the degree to which alum reduces P release from these sediments, were investigated during the summer of 2003. Triplicate sediment cores were sampled from four sites in the lake, and exposed to one of four treatments in the laboratory: (i) aerobic water column/alum, (ii) aerobic water column/no alum, (iii) anaerobic water column/alum, or (iv) anaerobic water column/no alum. Total P (TP) release rates were virtually undetectable in the alum treatments (both aerobic and anaerobic). Low, but detectable, release rates were measured in the aerobic/no alum treatment. The highest release rates were measured in the anaerobic/no alum treatments, and ranged from 1.6 to 29.5 mg P m(-2) d(-1) depending on how the calculations were derived. These fluxes translated to mean internal loads that ranged between 2.7 (low range) and 6.4 (high range) Mg yr(-1) when extrapolated to a whole-lake basis. Internal P loads accounted for between 55 and 65% of the total P load to Spring Lake. Although alum is a potentially effective means of reducing the sediment source of P, there is considerable uncertainty in how long an alum treatment would remain effective in this system given the current rates of external loading and the lack of information on wind-wave action and bioturbation in Spring Lake.     

Soil characteristics and phosphorus level effect on phosphorus loss in runoff

The loss of phosphorus (P) in runoff from agricultural soils may accelerate eutrophication in lakes and streams as well as degrade surface water quality. Limited soil specific data exist on the relationship between runoff P and soil P. This study investigated the relationship between runoff dissolved reactive phosphorus (DRP) and soil P for three Oklahoma benchmark soils: Richfield (fine, smectitic, mesic Aridic Argiustoll), Dennis (fine, mixed, active, thermic Aquic Argiudoll), and Kirkland (fine, mixed, superactive, thermic Udertic Paleustoll) series. These soils were selected to represent the most important agricultural soils in Oklahoma across three major land resource areas. Surface soil (0-15 cm) was collected from three designated locations, treated with diammonium phosphate (18-46-0) to establish a wide range of water-soluble phosphorus (WSP) (3.15-230 mg kg(-1)) and Mehlich-3 phosphorus (M3P) (27.8-925 mg kg(-1)). Amended soils were allowed to reach a steady state 210 d before simulated rainfall (75 mm h(-1)). Runoff was collected for 30 min from bare soil boxes (1.0 x 0.42 m and 5% slope) and analyzed for DRP and total P. Soil samples collected immediately before rainfall simulation were analyzed for the following: M3P, WSP, ammonium oxalate P saturation index (PSI(ox)), water-soluble phosphorus saturation index (PSI(WSP)), and phosphorus saturation index calculated from M3P and phosphorus sorption maxima (P(sat)). The DRP in runoff was highly related (p < 0.001) to M3P for individual soil series (r2 > 0.92). Highly significant relationships (p < 0.001) were found between runoff DRP and soil WSP for the individual soil series (r2 > 0.88). Highly significant relationships (p < 0.001) existed between DRP and different P saturation indexes. Significant differences (p < 0.05) among the slopes of the regressions for the DRP-M3P, DRP-WSP, DRP-PSI(ox), DRP-PSI(WSP), and DRP-P(sat) relationships indicate that the relationships are soil specific and phosphorus management decisions should consider soil characteristics.     

Swine manure injection with low-disturbance applicator and cover crops reduce phosphorus losses

Injection of liquid swine manure disturbs surface soil so that runoff from treated lands can transport sediment and nutrients to surface waters. We determined the effect of two manure application methods on P fate in a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] production system, with and without a winter rye (Secale cereale L.)-oat (Avena sativa L.) cover crop. Treatments included: (i) no manure; (ii) knife injection; and (iii) low-disturbance injection, each with and without the cover crop. Simulated rainfall runoff was analyzed for dissolved reactive P (DRP) and total P (TP). Rainfall was applied 8 d after manure application (early November) and again in May after emergence of the corn crop. Manure application increased soil bioavailable P in the 20- to 30-cm layer following knife injection and in the 5- to 20-cm layer following low-disturbance injection. The low-disturbance system caused less damage to the cover crop, so that P uptake was more than threefold greater. Losses of DRP were greater in both fall and spring following low-disturbance injection; however, application method had no effect on TP loads in runoff in either season. The cover crop reduced fall TP losses from plots with manure applied by either method. In spring, DRP losses were significantly higher from plots with the recently killed cover crop, but TP losses were not affected. Low-disturbance injection of swine manure into a standing cover crop can minimize plant damage and P losses in surface runoff while providing optimum P availability to a subsequent agronomic crop.     

Corn residue level and manure application timing effects on phosphorus losses in runoff

Growing interest in corn (Zea mays L.) silage utilization on Wisconsin dairy farms may have implications for nutrient losses from agricultural lands. Increasing the silage cutting height will increase residue cover and could reduce off-site migration of sediments and associated constituents compared with conventional silage harvesting. We examined the effects of residue level and manure application timing on phosphorus (P) losses in runoff from no-till corn. Treatments included conventional corn grain (G) and silage (SL; 10- to 15-cm cutting height) and nonconventional, high-cut (60-65 cm) silage (SH) subjected to different manure application regimes: no manure (N) or surface application in fall (F) or spring (S). Simulated rainfall (76 mm h(-1); 1 h) was applied in spring and fall for two years (2002-2003), runoff from 2.0- x 1.5-m plots was collected, and subsamples were analyzed for dissolved reactive phosphorus (DRP), total phosphorus (TP), and P mass distribution in four particle size classes. Total P and DRP loads were inversely related to percent residue cover, but both TP and DRP concentrations were unaffected by residue level. Manure application increased DRP concentrations in spring runoff by two to five times but did not significantly affect DRP loads, since higher concentrations were offset by lower runoff volumes. Spring manure application reduced TP loads in spring runoff by 77 to 90% compared with plots receiving no manure, with the extent of reductions being greatest at the lower residue levels (<24%). The TP concentration in sediments increased as particle size decreased. Manure application increased the TP concentration of the 0- to 2-microm fraction by 79 to 125%, but elevated the 2- to 10- and 10- to 50-microm fractions to a lesser extent. Recent manure additions were most influential in enriching transported sediments with P. By itself, higher residue cover achieved by high-cutting silage was often insufficient to lower P losses; however, the combination of manure application and higher residue levels significantly reduced P losses from corn fields harvested for silage.     

Sources of sediment and phosphorus in stream flow of a highly productive dairy farmed catchment

Both sediment and phosphorus (P) are important contaminants for surface water quality. Knowing the main sources of sediment and P loss within agricultural catchments enables mitigation practices to be better targeted. With this in mind seasonal loads of suspended sediment (SS), dissolved reactive P (DRP), total P (TP), and bioavailable P (BAP) were measured in a low gradient stream draining an intensively farmed New Zealand dairying catchment. Integrating in situ samplers were deployed to collect samples and the results merged with continuous flow data to calculate seasonal loads during 2005 through 2006. Flow rate, SS, and TP concentrations peaked in winter-spring and were lowest in summer-autumn. Concentrations of BAP in trapped sediment were greatest in autumn, contrasting with winter and spring when greater amounts of sediment were trapped, but with lower P enrichment. Analysis of (137)Cs and mixing model output showed that a major source of sediment and associated P in winter and spring was stream banks. Possible causes for this include trampling and destabilization by stock, channel straightening and sediment removal, and removal of riparian trees that stabilize banks. Modelling indicated that overland flow probably from topsoil (but could include sediment from lanes) contributed most sediment during summer and autumn. Remediation aimed at decreasing particulate P inputs to streams should focus on riparian protection measures, such as permanent stock exclusion and planting with shrubs and trees, ensuring runoff from lanes is minimized, and decreasing Olsen P to nearer optimum agronomic levels.     

Phosphorus runoff: effect of tillage and soil phosphorus levels

Continued inputs of fertilizer and manure in excess of crop requirements have led to a build-up of soil phosphorus (P) levels and increased P runoff from agricultural soils. The objectives of this study were to determine the effects of two tillage practices (no-till and chisel plow) and a range of soil P levels on the concentration and loads of dissolved reactive phosphorus (DRP), algal-available phosphorus (AAP), and total phosphorus (TP) losses in runoff, and to evaluate the P loss immediately following tillage in the fall, and after six months, in the spring. Rain simulations were conducted on a Typic Argiudoll under a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Elapsed time after tillage (fall vs. spring) was not related to any form of P in runoff. No-till runoff averaged 0.40 mg L(-1) and 0.05 kg ha(-1) DRP and chisel-plow plots averaged 0.24 mg L(-1) and 0.02 kg ha(-1) DRP concentration and loads, respectively. The relationship between DRP and Bray P1 extraction values was approximated by a logistic function (S-shaped curve) for no-till plots and by a linear function for tilled plots. No significant differences were observed between tillage systems for TP and AAP in runoff. Bray P1 soil extraction values and sediment concentration in runoff were significantly related to the concentrations and amounts of AAP and TP in runoff. These results suggest that soil Bray P1 extraction values and runoff sediment concentration are two easily measured variables for adequate prediction of P runoff from agricultural fields.     

Controlling runoff from subtropical pastures has differential effects on nitrogen and phosphorus loads

A 4-yr (2005-2008) study was conducted to evaluate the potential of pasture water management for controlling nutrient losses in surface runoff in the Northern Everglades. Two pasture water management treatments were investigated on Bahia grass ( Flüggé) pastures: reduced flow and unobstructed flow. The reduced flow treatment was applied to four of eight 20.23-ha pastures by installing water control structures in pasture drainage ditches with flashboards set at a predetermined height. Four other pastures received the unobstructed-flow treatment, in which surface runoff exited pastures unimpeded. Automated instruments measured runoff volume and collected surface water samples for nutrient analysis. In analyzing data for before-after treatment analysis, the 2005 results were removed because of structural failure in water control structures and the 2007 results were removed because of drought conditions. Pasture water retention significantly reduced annual total nitrogen (TN) loads, which were 11.28 kg ha and 6.28 kg ha, respectively, in pastures with unobstructed and reduced flow. Total phosphorus (TP) loads were 27% lower in pastures with reduced flow than in pastures with unobstructed flow, but this difference was not statistically significant. Concentrations of available soil P were significantly greater in pastures with reduced flow. Pasture water retention appears to be an effective approach for reducing runoff volume and TN loads from cattle pastures in the Northern Everglades, but the potential to reduce TP loads may be diminished if higher water table conditions cause increased P release from soils, which could result in higher P concentration in surface runoff.     

Runoff phosphorus losses as related to phosphorus source, application method, and application rate on a Piedmont soil

Land application of animal manures and fertilizers has resulted in an increased potential for excessive P losses in runoff to nutrient-sensitive surface waters. The purpose of this research was to measure P losses in runoff from a bare Piedmont soil in the southeastern United States receiving broiler litter or inorganic P fertilizer either incorporated or surface-applied at varying P application rates (inorganic P, 0-110 kg P ha(-1); broiler litter, 0-82 kg P ha(-1)). Rainfall simulation was applied at a rate of 76 mm h(-1). Runoff samples were collected at 5-min intervals for 30 min and analyzed for reactive phosphorus (RP), algal-available phosphorus (AAP), and total phosphorus (TP). Incorporation of both P sources resulted in P losses not significantly different than the unfertilized control at all application rates. Incorporation of broiler litter decreased flow-weighted concentration of RP in runoff by 97% and mass loss of TP in runoff by 88% compared with surface application. Surface application of broiler litter resulted in runoff containing between 2.3 and 21.8 mg RP L(-1) for application rates of 8 to 82 kg P ha(-1), respectively. Mass loss of TP in runoff from surface-applied broiler litter ranged from 1.3 to 8.5 kg P ha(-1) over the same application rates. Flow-weighted concentrations of RP and mass losses of TP in runoff were not related to application rate when inorganic P fertilizer was applied to the soil surface. Results for this study can be used by P loss assessment tools to fine-tune P source, application rate, and application method site factors, and to estimate extreme-case P loss from cropland receiving broiler litter and inorganic P fertilizers.     

Phosphorus runoff during four years following composted manure application

Repeated manure application can lead to excessive soil test P (STP) levels and increased P concentration in runoff, but also to improved water infiltration and reduced runoff. Research was conducted to evaluate soil P tests in prediction of P concentration in runoff and to determine the residual effects of composted manure on runoff P loss and leaching of P. The research was conducted from 2001 to 2004 under natural runoff events with plots of 11-m length. Low-P and high-P compost had been applied during the previous 3 yr, resulting in total applications of 750 and 1150 kg P ha(-1). Bray-P1 in the surface 5 cm of soil was increased from 16 to 780 mg kg(-1) with application of high-P compost. Runoff and sediment losses were 69 and 120% greater with no compost than with residual compost treatments. Runoff P concentration increased as STP increased, but much P loss occurred with the no-compost treatment as well. Agronomic soil tests were predictive of mean runoff P concentration, but increases in STP resulted in relatively small increases in runoff P concentration. Downward movement of P was not detected below 0.3 m. In conclusion, agronomic soil tests are useful in predicting long-term runoff P concentration, and risk of P loss may be of concern even at moderate soil P levels. The residual effect of compost application in reducing sediment and runoff loss was evident more than 3 yr after application and should be considered in P indices.