Leaching and crop uptake of nitrogen and phosphorus from pig slurry as affected by different application rates

The influence of increasing pig slurry applications on leaching and crop uptake of N and P by cereals was evaluated in a 3-yr study of lysimeters filled with a sandy soil. The slurry was applied at N rates of 50 (S50), 100 (S100), 150 (S150), and 200 (S200) kg ha(-1) during 2 of the 3 yr. The P rates applied with slurry were: 40 (S50), 80 (S100), 120 (S150), and 160 (S200) kg ha(-1) yr(-1). Simultaneously, NH4NO3 and Ca(H2PO4)2 were applied at rates of 100 kg N ha(-1) and 50 kg P ha(-1), respectively, to additional lysimeters (F100), while others were left unfertilized (F0). During the 3-yr period, the leaching load of total N tended to increase with increasing slurry application to, on average, 139 kg ha(-1) at the highest application rate (S200). The corresponding N leaching loads (kg ha(-1)) in the other treatments were: 75 (F0), 103 (F100), 93 (S50), 120 (S100), and 128 (S150). The loads of slurry-derived N in the S100, S150, and S200 treatments were significantly larger (P < 0.05) than those of fertilizer-derived N. In contrast, P leaching tended to decrease with increasing input of slurry, and it was lower in all treatments that received P at or above 50 kg P ha(-1) yr(-1) with slurry or fertilizer than in the unfertilized treatment. The crop use efficiency of added N and P was clearly higher when NH4NO3 and Ca(H2PO4)2 were used rather than slurry (60 vs. 35% for N, 38 vs. 6-9% for P), irrespective of slurry application rate. Therefore, from both a production and water quality point of view, inorganic fertilizers seem to have environmental benefits over pig slurry when used on sandy soils.     

Phosphorus leaching at cold temperatures as affected by wastewater application and soil phosphorus levels

Land application of wastewater in the northern-tier United States during winter months has been suggested as a means to reduce cost of building storage lagoons. A study was initiated in 1996 to assess land application of potato-processing wastewater on a 120-ha field at Park Rapids, MN. One objective of this study was to evaluate the effects of soil P levels and temperature on P leaching in soil columns. In this paper, we report the P sorption, desorption, and leaching characteristics of a high-P (>200 mg kg(-1)) and a low-P (<25 mg kg(-1)) surface soil from the wastewater irrigation site. The leaching experiment was done with wastewater at 4 +/- 2 or 10 +/- 2 degrees C. The high-P soil resulted in an equilibrium P concentration of 8.0 mg L(-1) compared with 0.14 mg L(-1) for the low-P soil. When low-P wastewater was applied to the high-P soil, the soil acted as a P source, and the total phosphorus (TP) concentration in the leachate was 3.5 times higher than the input TP concentration (C0). When high-P wastewater was applied to the high-P soil, the soil acted as a P sink retarding the TP concentration in the leachate by 80%. Phosphorus desorption was higher at 10 degrees C compared with 4 degrees C. The results showed that depending on P levels of the soil and the wastewater, reduction or increase in leachate P will occur below the surface soil. However, further mobility of this P under field conditions will depend on the volume and rate of percolating water as well as the sorption-desorption characteristics of the subsoil.     

Colloidal and dissolved phosphorus in sandy soils as affected by phosphorus saturation

Fertilization exceeding crop requirements causes an accumulation of phosphorus (P) in soils, which might increase concentrations of dissolved and colloidal P in drainage. We sampled soils classified as Typic Haplorthods from four fertilization experiments to test (i) whether increasing degrees of phosphorus saturation (DPS) increase concentrations of dissolved and colloidal P, and (ii) if critical DPS levels can be defined for P release from these soils. Oxalate-extractable concentrations of P, iron (Fe), and aluminum (Al) were quantified to characterize DPS. Turbidity, zeta potential, dissolved P, and colloidal P, Fe, Al, and carbon (C) concentrations were determined in water and KCl extracts. While concentrations of dissolved P decreased with increasing depth, concentrations of water-extractable colloidal P remained constant. In topsoils 28 +/- 17% and in subsoils 94 +/- 8% of water-extractable P was bound to colloids. Concentrations of dissolved P increased sharply for DPS > 0.1. Colloidal P concentrations increased with increasing DPS because of an additional mobilization of colloids and due to an increase of the colloids P contents. In addition to DPS, ionic strength and Ca(2+) affected the release of colloidal P. Hence, using KCl for extraction improved the relationship between DPS and colloidal P compared with water extraction. Accumulation of P in soils increases not only concentrations of dissolved P but also the risk of colloidal P mobilization. Leaching of colloidal P is potentially important for inputs of P into water bodies because colloidal P as the dominant water-extractable P fraction in subsoils was released from soils with relatively low DPS.     

Cost-effectiveness of tillage methods to reduce phosphorus loss from agricultural land

In this paper, we estimate the cost-effectiveness of tillage methods as a measure to reduce phosphorus loss. The study was based on real-world information on costs. Data on phosphorus loss for different soil tillage methods were modelled. The cost-effectiveness of various soil tillage methods were related to autumn ploughing. The results showed large variation in cost-effectiveness related to erosion risk. Furthermore, spring harrowing was the most cost-effective method to reduce phosphorus loss, followed by autumn harrowing and spring ploughing in spring cereals. Implementation of changed tillage methods showed lower costs for spring cereals compared to winter wheat. The differences in costs between areas were most evident for spring tillage due to differences in yields and agronomic management. Cost-effectiveness is an important criterion in selecting mitigation methods, but due to large variations in the effect of changed tillage, these should be locally adapted to the high risk areas of erosion.     

Effect of tomato packinghouse wastewater properties on phosphorus and cation leaching in a spodosol

Land application of wastewater is a common practice. However, coarse-textured soils and shallow groundwater in Florida present favorable conditions for leaching of wastewater-applied constituents. Our objective in this study was to determine phosphorus (P) and associated cations (Ca, Mg, K, Na) leaching in a Spodosol irrigated with tomato packinghouse wastewater. We packed 12 polyvinyl chloride soil columns (30 cm internal diameter × 50 cm length) with two soil horizons (Ap and A/E) and conducted 30 sequential leaching events by irrigating with wastewater at low (0.84 cm d), medium (1.68 cm d), and high (2.51 cm d) rates. The control treatment received deionized water at 1.68 cm d Leachate pH was lower (6.4-6.5) and electrical conductivity (EC) was higher in the wastewater-treated columns (0.85-1.78 dS m) than in the control treatment (pH 6.9; EC, 0.12 dS m) due to the low pH (6.2) and high EC (2.16 dS m) of applied wastewater. Mean leachate P concentrations were greatest in the control treatment (0.70 mg L), followed by the high (0.60 mg L) and low and medium wastewater-treated columns (0.28-0.33 mg L). Leachate concentrations of Na, Ca, Mg, and K were significantly ( < 0.05) greater in wastewater-treated columns than in the control. Concentrations of P, Na, and K in leachate remained lower than the concentrations in the applied wastewater, indicating their retention in the soil profile. In contrast, leachate Ca and Mg concentrations were greater than in applied wastewater during several leaching events, suggesting that additional Ca and Mg were leached from the soil. Our results suggest that tomato packinghouse wastewater can be beneficially land-applied at 1.68 cm d in Florida's Spodosols without significant P and cation leaching.     

Phosphorus release from a biosolids-amended sandy spodosol

Most regulations governing biosolids land application do not consider that phosphorus (P) solubility can vary widely among biosolids. Current regulations typically group all biosolids together in one category or group biosolids together with manures. Research has shown that not all biosolids have the same potential to affect the environment when land applied, but the database is limited. The purpose of this study was to characterize P release from several biosolids produced and/or marketed in Florida. A small soil column dynamic laboratory incubation was conducted to assess P release potential. Eleven biosolids and a mineral fertilizer (triple super phosphate) were individually mixed with a typical low-P sorbing Florida soil (Immokalee fine sand) at 56 and 224 kg P ha(-1). Columns were periodically leached over 5.5 mo to attain 60 mL (1/2 pore volume) of drainage in each leaching. Soluble reactive P was determined and summed over the eight leachings to represent total P source release. Cumulative P release (as a percentage of P applied) was greatest from biological P removal (BPR) and BPR-like biosolids and least from heat-dried materials. Phosphorus release from biosolids depends on biosolids treatment type (digestion) and P chemistry, suggesting that biosolids regulations must account for differences in P lability to accurately gauge environmental risk.     

Nitrate loss in subsurface drainage as affected by nitrogen fertilizer rate

The relationships between N fertilizer rate, yield, and NO3 leaching need to be quantified to develop soil and crop management practices that are economically and environmentally sustainable. From 1996 through 1999, we measured yield and NO3 loss from a subsurface drained field in central Iowa at three N fertilizer rates: a low (L) rate of 67 kg ha(-1) in 1996 and 57 kg ha(-1) in 1998, a medium (M) rate of 135 kg ha(-1) in 1996 and 114 kg ha(-1) in 1998, and a high (H) rate of 202 kg ha(-1) in 1996 and 172 kg ha(-1) in 1998. Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown in rotation with N fertilizer applied in the spring to corn only. For the L treatment, NO3 concentrations in the drainage water exceeded the 10 mg N L(-1) maximum contaminant level (MCL) established by the USEPA for drinking water only during the years that corn was grown. For the M and H treatments, NO3 concentrations exceeded the MCL in all years, regardless of crop grown. For all years, the NO3 mass loss in tile drainage water from the H treatment (48 kg N ha(-1)) was significantly greater than the mass losses from the M (35 kg N ha(-1)) and L (29 kg N ha(-1)) treatments, which were not significantly different. The economically optimum N fertilizer rate for corn was between 67 and 135 kg ha(-1) in 1996 and 114 and 172 kg ha(-1) in 1998, but the net N mass balance indicated that N was being mined from the soil at these N fertilizer levels and that the system would not be sustainable.     

Manure phosphorus extractability as affected by aluminum- and iron by-products and aerobic composting

Shifts in manure phosphorus (P) chemical forms and pool sizes induced by water treatment residuals and industrial mineral by-products are largely undefined. We conducted a manure P fractionation study to determine mechanisms of reduction of dissolved reactive phosphorus (DRP) in poultry manure upon mineral by-product additions. The effects of composting on the P immobilization efficacy of the by-products were determined using laboratory self-heating composting simulators. The mineral by-products included an aluminum-water treatment residual (Al-WTR) and an iron-rich titanium-processing by-product. The noncomposted manure averaged 0.11 g g(-1) of total P as DRP forms. The by-products significantly reduced manure DRP, by an average of 39 and 48% in the Al- and the Fe-treated manure, respectively. The by-products also reduced the 0.5 M NH4F-extractable phosphorus (FEP) fraction. Shifts in P forms between FEP and 0.1 M NaOH-extractable phosphorus (SHEP) depended upon the Al and Fe contents of the by-products while the combined FEP + SHEP pool remained constant. Phosphate sorption measurements supported the observations that the Fe-rich by-product was more effective at reducing manure DRP and enhancing the formation of SHEP forms at the expense of FEP than the Al-WTR. Composting had no effect on the efficacy of either by-product to reduce DRP. Potential mechanisms of enhanced P stabilization in treated manure upon composting included chemical shifts from the DRP and FEP fractions to the citrate-bicarbonate-dithionite extractable P fraction. Thus, the choice of P immobilization agents affected the stability of immobilized P forms and should be taken into consideration in developing manure processing and nutrient stabilization methods.     

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 release from a manure-impacted spodosol: effects of a water treatment residual

Long-term depositions of animal manures affect P dynamics in soils and can pose environmental risks associated with P losses. Laboratory studies were done on P solubility characteristics in a manure-impacted Immokalee soil (sandy, siliceous, hyperthermic Arenic Alaquod) and the effectiveness of water treatment residual (WTR) in controlling P leaching. Soil samples with contrasting initial total P concentrations were prepared by mixing samples of a manure-impacted surface A horizon and a minimally P-impacted E horizon. Effects of mixing various ratios of A and E horizons, WTR rates (0, 25, 50, and 100 g kg(-1)), and depths of WTR incorporation (mixed throughout the soil column or partially incorporated) on P leaching were determined. Between 62 and 77% of total P was released from the soil mixes by successive water extractions, suggesting a considerable buffering capacity of this manure-impacted soil to resupply P into solution. Between 224 and 408 mg kg(-1) P were leached during the 36-wk leaching period in the absence of WTR. Mixing WTRs with soil reduced soluble P concentration in leachates by as much as 99.8% compared with samples without WTR. Thoroughly mixing WTR with the entire soil column (15 cm) was much more efficient than mixing WTR with only the top 7.5 cm of soil. Calcium- and Mg-P forms appear to control P release in soils without WTR, whereas sorption-desorption reactions probably determine P leaching in WTR-treated samples. Soil P distribution in various chemical forms was affected by WTR additions. Data suggest that WTR-immobilized P is stable in the long term.     

Nitrate losses in subsurface drainage from a corn-soybean rotation as affected by time of nitrogen application and use of nitrapyrin

Subsurface drainage, a water management practice used to remove excess water from poorly drained soils, can transport substantial amounts of NO3 from agricultural crop production systems to surface waters. A field study was conducted from the fall of 1986 through 1994 on a tile-drained Canisteo clay loam soil (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll) to determine the influence of time of N application and use of nitrapyrin [NP; 2-chloro-6-(trichloromethyl) pyridine] on NO3 losses from a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Four anhydrous ammonia treatments [fall N, fall N + NP, spring preplant N, and split N (40% preplant and 60% sidedress)] were replicated four times and applied at 150 kg N ha(-1) for corn on individual drainage plots. Sixty-two percent of the annual drainage and 69% of the annual NO3 loss occurred in April, May, and June. Flow-weighted NO3-N concentrations in the drainage water were two to three times greater in the two years following the three-year dry period compared with preceding and succeeding years. Nitrate N concentrations and losses in the drainage from corn were greatest for fall N with little difference among the other three N treatments. Nitrate losses from soybean were affected more by residual soil NO3 following corn than by the N treatments per se. Averaged across the four rotation cycles, flow-normalized NO3-N losses ranked in the order: fall N > split N > spring N = fall N + NP. Under these conditions NO3 losses from a corn-soybean rotation into subsurface drainage can be reduced by 13 to 18% by either applying N in the spring or using NP with late fall-applied ammonia.     

Nitrate losses in subsurface drainage from a corn-soybean rotation as affected by fall and spring application of nitrogen and nitrapyrin

Substantial amounts of NO3 from agricultural crop production systems on poorly drained soils can be transported to surface water via subsurface drainage. A field study was conducted from the fall of 1993 through 2000 on a tile-drained Canisteo clay loam soil (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll) to determine the influence of fall vs. spring application of N and nitrapyrin [NP; 2-chloro-6-(trichloromethyl) pyridine] on NO3 losses from a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Four anhydrous ammonia treatments (fall N, fall N + NP, spring preplant N, and spring N + NP) were replicated four times and applied at 135 kg N ha(-1) for corn on individual drainage plots. Drainage occurred in all seven years. Seventy-one percent of the annual drainage and 75% of the annual NO3 loss occurred in April, May, and June. Fifty-four percent of the NO3 lost in the drainage occurred during the corn phase and 46% during the soybean phase. Annual flow-weighted NO3-N concentrations for the fall, fall + NP, spring, and spring + NP treatments averaged 14.3, 11.5, 10.7, and 11.3 mg L(-1) during the corn phase but annual NO3-N concentrations were still > or =10 mg L(-1) in three of six years for the spring preplant treatment. Averaged across the six rotation cycles, flow-normalized NO3-N losses ranked in the order: fall N > spring N + NP > fall N + NP > spring N. Under these conditions, NO3 losses in subsurface drainage from a corn-soybean rotation can be reduced 14% by spring N and 10% by late fall N + NP compared with fall-applied N. Nitrate losses were not appreciably reduced by adding NP to spring preplant N.     

Nitrite formation and nitrous oxide emissions as affected by reclaimed effluent application

The effect of irrigation with reclaimed effluent (RE) (after secondary treatment) on the mechanisms and rates of nitrite formation, N2O emissions, and N mineralization is not well known. Grumosol (Chromoxerert) soil was incubated for 10 to 14 d with fresh water (FW) and RE treated with 15NO3- and 15NH4+ to provide a better insight on N transformations in RE-irrigated soil. Nitrite levels in RE-irrigated soil were one order of magnitude higher than in FW- irrigated soil and ranged between 15 to 30 mg N kg(-1) soil. Higher levels of NO2- were observed at a moisture content of 60% than at 70% and 40% w/w. Nitrite levels were also higher when RE was applied to a relatively dry Grumosol (20% w/w) than at subsequent applications of RE to soil at 40% w/w. Isotopic labeling indicated that the majority of NO2 was formed via nitrification. The amount of N2O emitted from RE-treated Grumosol was double the amount emitted from FW treatments at 60% w/w. Nitrification was responsible for about 42% of the emissions. The N20 emission from the RE-treated bulk soil (passing a 9.5-mm sieve) was more than double the amount formed in large aggregates (4.76-9.5 mm in diameter). No dinitrogen was detected under the experimental conditions. Results indicate that irrigation with secondary RE stimulates nitrification, which may enhance NO3 leaching losses. This could possibly be a consequence of long-term exposure of the nitrifier population to RE irrigation. Average gross nitrification rate estimates were 11.3 and 15.8 mg N kg(-1) soil d(-1) for FW- and RE-irrigated bulk soils, respectively. Average gross mineralization rate estimates were about 3 mg N kg(-1) soil d(-1) for the two water types.     

Potential loss of phosphorus from a rice field in Taihu Lake basin

Nonpoint-source pollution by phosphorus (P) poses a threat to waters in the Taihu Lake basin in China. The potential transfer of P in rice (Oryza sativa L.) fields through surface drainage and subsurface flow was investigated under simulated conventional irrigation-drainage management. Surface drainage events were conducted to avoid overflow across the plots after heavy rainfall and for rice harvest, at which time P losses were also investigated. This study was conducted in 2001 in a long-term rice field experiment. The experimental plots were treated with 0, 26, or 52 kg P ha(-1) as superphosphate or 26 kg P ha(-1) with equal parts of P supplied as superphosphate and pig manure. Phosphorus concentrations and loads in field floodwater on plots receiving P rapidly declined in a nonlinear manner before the first drainage, three weeks after fertilizer application. The combined application of fertilizer and manure P resulted in higher P transfer potential in field floodwater than with fertilizer P alone one week after P application. Phosphorus concentrations in interflow water sampled by Teflon suction cups inserted at a depth of 150 to 200 mm gradually increased within two weeks after P application, then declined. The concentration of P in interflow water was related to soil P buildup from long-term P application, as well as recently applied P. The 26 kg P ha(-1) treatment (the conventional P rate in this region) resulted in a loss of 0.74 kg total phosphorus (TP) ha(-1) and a drainage-weighted average concentration of 0.25 mg TP L(-1) from the three surface drainage events. Results indicate that avoiding overflow drainage after P input and extending the time between P application and drainage may reduce P losses from rice paddies.     

Impact of manure phosphorus fractions on phosphorus loss from manured soils after incubation

The risk of P loss from manured soils is more related to P fractions than total P concentration in manure. This study examined the impact of manure P fractions on P losses from liquid swine manure- (LSM), solid cattle manure- (SCM), and monoammonium phosphate- (MAP) treated soils. Manure or fertilizer was applied at 50 mg P kg soil, mixed, and incubated at 20°C for 6 wk to simulate the interaction between applied P and soil when P is applied well in advance of a high risk period for runoff. Phosphorus fractions in manure were determined using the modified Hedley fractionation scheme. We used simulated rainfall (75 mm h?1 for 1 h) to quantify P losses in runoff from two soils (sand and clay loam). The proportion of total labile P (total P in water+NaHCO fractions) in manure was significantly greater in LSM (70%) than SCM (44%). Mean dissolved reactive P (DRP) load in runoff over 60 min was greatest from MAP-treated soil (18.1 mg tray?1), followed by LSM- (14.0 mg tray?1) and SCM- (11.0 mg tray?1) treated soils, all of which were greater than mean DRP load from the check (5.2 mg tray?1). Total labile P (water+NaHCO) in manure was a more accurate predictor of runoff DRP loads than water extractable P, alone, for these two soils. Therefore, NaHCO extraction of manure P may be a useful tool for managing the risk of manure P runoff losses when manure is applied outside a high risk period for runoff loss.     

Results of a program to control phosphorus discharges from dairy operations in south-central Florida,USA

During 1987–1992, a mandatory program to control phosphorus discharges was implemented at dairy operations located to the north of Lake Okeechobee, Florida, USA. Thirty of 48 dairies participated in this program and implemented best management practices (BMPs), which included the construction of intensive animal waste management systems. Eighteen dairies closed their milkproducing operations under a government-funded buyout program. In this paper, we compare trends in runoff total phosphorus (TP) concentrations among the dairies that remained active and implemented BMPs. A central feature of the dairy waste management system is the high intensity area (HIA), defined as the milking barn and adjacent vegetation-free land, encircled by a drainage ditch and dike. Animal waste from the HIA is diverted into anaerobic lagoons and storage ponds, from which water is periodically removed and used for irrigation of field crops. The impacts of BMP construction on runoff TP concentrations were immediate and, in most cases, dramatic. Average TP concentrations declined significantly (P < 0.001), from 9.0 to 1.2 mg TP liter^–1 at dairies in one basin (Lower Kissimmee River), and from 2.6 to 1.0 mg TP liter^–1 in another (Taylor Creek/Nubbin Slough). Some sites experienced greater declines in TP than others. To elucidate possible causes for the difference in response, a multivariate statistical model was utilized. Independent variables included soil pH, soil drainage characteristics, spodic horizon depth, and the areas of different BMP components (pasture, HIA, spray fields). The analysis significantly separated dairies with the highest and lowest runoff TP concentrations. Lowest TP occurred at dairies having particular soil characteristic (shallow spodic horizon) and certain BMP features (large HIA and small heard pastures).     

Simple models for phosphorus loss from manure during rainfall

Mechanistic, predictive equations for phosphorus (P) transport in runoff from manure-applied fields constitute a critical knowledge gap for developing nonpoint-source pollution models. We derived two simple equations to describe the P release from animal manure during a rainfall event-one based on first-order P desorption kinetics and one based on second-order kinetics. The manure characteristics needed in the two kinetic equations are the maximum amount of water-extractable phosphorus (WEP) and a characteristic desorption time. Water-extractable P can be measured directly but currently the characteristic time can only be obtained by fitting experimental data. In addition, we evaluated two models usually used to estimate P loss from soil, the Elovitch equation and power function, both of which relate P loss to time. The models were tested against previously published data of P release from different manures under laboratory conditions. All equations fit the data well. Of the two kinetic equations, the second-order model showed better agreement with the data than the first-order model; for example, maximum relative differences between the model results and measured data were 2.6 and 4.7%, respectively. The characteristic times varied between 20 min for dairy manure and almost 100 min for poultry manure. The characteristic time did not appear to change with flow rate but decreased with smaller manure aggregates. The parameters for power-function relationships could not be related to measured manure characteristics. These results provide the first step to process-based approximations for predicting P release from manure with time during rainfall shortly after land application, when P losses are the greatest.     

Gaseous nitrogen emissions and mineral nitrogen transformations as affected by reclaimed effluent application

Irrigation with reclaimed effluent (RE) is essential in arid and semiarid regions. Reclaimed effluent has the potential to stimulate gaseous N losses and affect other soil N processes. No direct measurements of the N2 and N2O emissions from Mediterranean soils have been conducted so far. We used the 15N gas flux method in a field and a laboratory experiment to study the effect of RE irrigation on gaseous N losses and other N transformations in a Grumosol (Chromoxerert) soil. The fluxes of N2, N2O, and NH3 were measured from six Grumosol lysimeters following application of either fresh water or RE. The N fertilizer was applied either as 15NH4 or 15NO3. Only up to 0.3% from the applied N fertilizer was lost as N2O + NH3. Reclaimed effluent enhanced the losses of NH3, but did not affect those of N2O. Nitrification and denitrification were equally important to N2O production. Laboratory incubations were performed to both confirm the influence of the irrigation water type and to test the effect of moisture content. Significant quantities of N2 and N2O (up to 3.1% of the applied fertilizer) were emitted from saturated soils. Reclaimed effluent application did not induce higher N2O emissions, yet significantly more (approximately 33%) N2 was emitted from RE-irrigated soils. Denitrification contributed up to 75% of the N2O amounts emitted from saturated soils. Reclaimed effluent application inhibited nitrification in the Grumosol by 15 to 25% and induced NO2 accumulation in soils incubated at a field-capacity moisture content.     

Evaluating aeration techniques for decreasing phosphorus export from grasslands receiving manure

Because surface-applied manures can contribute to phosphorus (P) in runoff, we examined mechanical aeration of grasslands for reducing P transport by increasing infiltration of rainfall and binding of P with soil minerals. The effects of three aeration treatments and a control (aeration with cores, continuous-furrow "no-till" disk aeration perpendicular to the slope, slit aeration with tines, and no aeration treatment) on the export of total suspended solids, total Kjeldahl P (TKP), total dissolved P (TDP), dissolved reactive P (DRP), and bioavailable P (BAP) in runoff from grasslands with three manure treatments (broiler litter, dairy slurry, and no manure) were examined before and after simulated compaction by cattle. Plots (0.75 x 2 m) were established on a Cecil soil series with mixed tall fescue (Festuca arundinacea Schreb.)-bermudagrass [Cynodon dactylon (L.) Pers.] vegetation on 8 to 12% slopes. Manures were applied at a target rate of 30 kg P ha(-1), and simulated rainfall was applied at a rate of 85 mm h(-1). Although the impact of aeration type on P export varied before and after simulated compaction, overall results indicated that core aeration has the greatest potential for reducing P losses. Export of TKP was reduced by 55%, TDP by 62%, DRP by 61%, total BAP by 54%, and dissolved BAP by 57% on core-aerated plots with applied broiler litter as compared with the control (p < 0.05). Core and no-till disk aeration also showed potential for reducing P export from applied dairy slurry (p < 0.10). Given that Cecil soil is common in pastures receiving broiler litter in the Southern Piedmont, our results indicate that pairing core aeration of these pastures with litter application could have a widespread impact on surface water quality.     

Evaluating colloidal phosphorus delivery to surface waters from diffuse agricultural sources

Colloid-facilitated phosphorus (P) delivery from agricultural soils in different hydrological pathways was investigated using a series of laboratory and field experiments. A soil colloidal P test was developed that yields information on the propensity of different soils to release P attached to soil colloids. The relationship between turbidity of soil extracts and total phosphorus (TP) was significant (r2 = 0.996, p < 0.001) across a range of agricultural soils, and a strong positive relationship (r2 = 0.86, p < 0.001) was found between "colloidal P" (H2O-CaCl2 extracts) and turbidity. Linear regression of the proportion of fine clay (<2 microm) for each soil type evaluated against the (H2O-CaCl2) colloidal P fraction gave a weak but positive relationship (r2 = 0.38, p = 0.082). The relative contribution of different particle-size fractions in transporting P in agricultural runoff from grassland soils was evaluated using a randomized plot experiment. A significant difference (p = 0.05) in both TP and reactive phosphorus (RP) in subsurface flow was recorded for different particle-size fractions, with most TP transferred either in association with the 2-microm fraction or with the 0.001-microm or smaller fractions. Total P concentrations in runoff were higher from plots receiving P amendments compared with the zero-P plots; however, these differences were only significant for the >0.45-microm particle-size fractions (p = 0.05), and may be evidence of surface applications of organic and inorganic fertilizers being transferred through the soil either as intact organic colloids or attached to mineral particles. Our results highlight the potential for drainage water to mobilize colloids and associated P during rainfall events.