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.     

Impact of glyphosate-tolerant soybean and glufosinate-tolerant corn production on herbicide losses in surface runoff

Residual herbicides used in the production of soybean [Glycine max (L.) Merr] and corn (Zea mays L.) are often detected in surface runoff at concentrations exceeding their maximum contaminant levels (MCL) or health advisory levels (HAL). With the advent of transgenic, glyphosate-tolerant soybean and glufosinate-tolerant corn this concern might be reduced by replacing some of the residual herbicides with short half-life, strongly sorbed, contact herbicides. We applied both herbicide types to two chiseled and two no-till watersheds in a 2-yr corn-soybean rotation and at half rates to three disked watersheds in a 3-yr corn/soybean/wheat (Triticum aestivum L.)-red clover (Trifolium pratense L.) rotation and monitored herbicide losses in runoff water for four crop years. In soybean years, average glyphosate loss (0.07%) was approximately 1/7 that of metribuzin (0.48%) and about one-half that of alachlor (0.12%), residual herbicides it can replace. Maximum, annual, flow-weighted concentration of glyphosate (9.2 microg L(-1)) was well below its 700 microg L(-1) MCL and metribuzin (9.5 microg L(-1)) was well below its 200 microg L(-1) HAL, whereas alachlor (44.5 microg L(-1)) was well above its 2 microg L(-1) MCL. In corn years, average glufosinate loss (0.10%) was similar to losses of alachlor (0.07%) and linuron (0.15%), but about one-fourth that of atrazine (0.37%). Maximum, annual, flow-weighted concentration of glufosinate (no MCL) was 3.5 microg L(-1), whereas atrazine (31.5 microg L(-1)) and alachlor (9.8 microg L(-1)) substantially exceeded their MCLs of 3 and 2 microg L(-1), respectively. Regardless of tillage system, flow-weighted atrazine and alachlor concentrations exceeded their MCLs in at least one crop year. Replacing these herbicides with glyphosate and glufosinate can reduce the occurrence of dissolved herbicide concentrations in runoff exceeding drinking water standards.     

Bioavailable phosphorus in runoff from alfalfa, smooth bromegrass, and alfalfa-smooth bromegrass

Runoff from sloping landscapes cropped with established alfalfa (Medicago sativa L.) may contain bioavailable P (BAP) which accelerates eutrophication of surface water bodies. Such BAP exists as either dissolved reactive P (DRP) or bioavailable reactive particulate P (BPP). We hypothesized that before and after harvest, sod-forming smooth bromegrass (Bromus inermis Leyss.) or alfalfa-smooth bromegrass mixtures would have less BAP, DRP, and BPP runoff losses than taprooted alfalfa. Swards established in 1992 near Lancaster, WI were subjected to a 72 mm simulated rainfall applied for 1 h in 1993 and 1994 to forage regrowth at 4 and 6 wk after first harvest and immediately (0 wk) after second harvest. Hourly BAP losses for all sward types were 82% less when 1.5 Mg ha(-1) of forage dry matter was present. High DRP losses (>0.050 kg ha(-1)) were associated with high DRP concentrations (>7.1 micromol L(-1)) and high surface soil P concentrations (>59 mg kg(-1)) resulting from broadcast maintenance P fertilizer. High BPP losses (>0.035 kg ha(-1)) were associated with high runoff volumes (>24 mm) and sediment concentrations (>2 g L(-1)). Summed over all 6 rainfall simulations, total BAP loss was only 0.07 kg ha(-1) at the 6 wk stage of regrowth compared with 0.35 at 4 wk, and 0.41 at 0 wk. Moreover, there was no significant difference between sward types for DRP concentration, DRP loss, or BAP loss. We conclude that avoiding excessive defoliation was more effective at reducing BAP losses than specific forage species selection.     

Leaching of three imidazolinone herbicides during sprinkler irrigation

Some imidazolinone herbicides have been shown to be mobile in soil, raising concern about their possible movement to ground water. Three imidazolinone herbicides (imazamethabenz-methyl, 497 g ha(-1); imazethapyr, 14.7 g ha(-1); and imazamox, 14.7 g ha(-1)) commonly used in crop production on the Canadian prairies were applied to a tile-drained field to assess their susceptibility to leach when subjected to sprinkler irrigation using a center pivot. Tile-drain flow began when the water table rose above tile-drain depth, and peak flow rates corresponded to the greatest depths of ground water above the tile drains. Interception of irrigation water by the tile drains in each quadrant of the field varied from ～11 to 20% of the water applied. Under a worst-case scenario in which irrigation began the day after herbicide application and irrigation water was applied at 25 mm d(-1) for 12 d, there was evidence of preferential flow of all three herbicides and hydrolysis of imazamethabenz-methyl to imazamethabenz in the initial samples of tile-drain effluent. In subsequent samples, concentrations (analysis by LC-MS-MS) of the summation of imazamethabenz-methyl (25-24,000 ng L(-1)) plus its hydrolysis product imazamethabenz (63-26,500 ng L(-1)) greatly exceeded those of imazethapyr (<13-1260 ng L) and imazamox (19-599 ng L(-1)), thus reflecting relative application rates. In contrast, estimates of total transport of each herbicide from the root zone, which varied in each quadrant and ranged from 0.06 to 2.3% for imazamethabenz-methyl plus imazamethabenz, 0.71 to 3.1% for imazethapyr, and 0.61 to 2.8% for imazamox, did not reflect application rates. In shallow ground water (piezometer samples), there was inconsistent and infrequent detection all four compounds. With the frequency and amount of rainfall typically encountered in the prairie region of Canada, contamination of shallow ground water with detectable concentrations of the three imidazolinone herbicides would be unlikely.     

Residue level and manure application timing effects on runoff and sediment losses

There is growing interest in evaluating the effects of corn silage harvesting methods on erosion control. Increasing the silage cutting height will increase residue cover and could conceivably minimize off-site migration of sediments compared with conventional silage harvesting. The effects of residue level and manure application timing on runoff and sediment losses from no-till corn were examined. Treatments included conventional corn grain (G) and silage (SL) and nonconventional, high-cut (60-65 cm) silage (SH). Corn harvesting treatments were subjected to different manure application regimes: no manure (N) or surface application in fall (F) or spring (S). Simulated rainfall (76 mm/h; 1 h) was applied in spring and fall for two years (2002-2003), runoff from 2.0- x 1.5-m plots collected, and a subsample analyzed for sediment concentration and aggregate size distribution. Runoff volume was inversely related to residue cover. Manure addition to silage plots reduced spring runoff by 71 to 88%, attributable to an increase in soil organic matter content, compared with SH-N and SL-N. Differences in sediment concentration between SH and SL were not significant. For silage plots, spring-applied manure had the greatest influence on sediment export reducing it by 84 to 93% in spring runoff compared with corresponding N plots. Sediment loads were also 85 to 97% lower from SH-S compared with SL-N in all four seasons. Except for spring 2003, sediment export was lower from G compared with SL. The combination of manure and higher residue associated with high-cut silage often lowered sediment export compared with low-cut silage. Nearly identical aggregate size distributions were observed in sediments from SH and SL plots. High residue levels combined with spring-applied manure led to enrichment in the clay-sized fraction of runoff sediment. Recently applied manure and higher residue levels achieved by high-cutting silage can substantially lower sediment losses in spring runoff when soil is most susceptible to erosion.     

Phosphorus and nitrogen runoff from a forested watershed fertilized with biosolids

Municipal biosolids are typically not used on the steepest of forested slopes in the U.S. Pacific Northwest. The primary concern in using biosolids on steep slopes is movement of biosolids particles and soluble nutrients to surface waters during runoff events. We examined the pattern and extent of P and N runoff from a perennial stream draining a small, forested 21.4-ha watershed in western Washington before and after biosolids application. In this study, we applied biosolids at a rate of 13.5 Mg ha(-1) (700 kg N ha(-1) and 500 kg P ha(-1)) to 40% of the watershed following nearly 1.5 years of pre-application water sampling and 1.5 years thereafter. There was no evidence of direct runoff of P or N from biosolids into surface water. Elevated surface water discharge did not change the concentration of PO4-P, biologically available phosphorus (BAP), bioavailable particulate phosphorus (BPP), or total P nor did it affect the concentration-discharge relationship. Some instances of total P concentrations exceeding the USEPA surface water standard of 0.1 mg L(-1) were observed following biosolids application. However, total P in 27 Creek was predominately in particulate form and not labile, suggesting that detritus moving into the main creek channel and ephemeral drainage courses may be the principal P source. Ammonium N concentrations in runoff water were consistent before and after biosolids application, ranging from below detection limits (0.01 mg L(-1)) to 0.1 mg L(-1); no concentration-discharge relationship existed. Biosolids application changed the 27 Creek concentration-discharge relationship for NO3(-)-N. Before application, no relationship existed. Beginning nine months after biosolids application, increases in discharge were positively related to increases in NO3(-)-N concentrations. Nitrate concentrations in runoff following biosolids application were approximately 10 times less than the USEPA drinking water standard of 10 mg L(-1).     

Movement of diuron and hexazinone in clay soil and infiltrated pond water

Pre-emergence herbicide residues were detected in domestic wells sampled near Tracy, CA. This study sought to determine the source of contamination by comparing soil distribution of diuron [N'-(3,4-dichlorophenyl)-N,N-dimethylurea] and hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] in an agricultural field where the soil was a cracking clay to infiltration of residues in water captured by an adjacent holding pond. Diuron and hexazinone were applied in December to a 3-yr-old alfalfa (Medicago sativa L.) crop. Water content of soil taken after major rainfall but before irrigation at 106 d after application was elevated at the lowest depth sampled centered at 953 mm, indicating water was available for percolation. Herbicide residues (reporting limit 8 microg kg(-1)) were confined above the 152 mm soil depth, even after subsequent application of two border-check surface irrigations. The pattern of distribution and concentration of residues in the soil were similar to results obtained from the LEACHM model, suggesting that macropore flow was limited to a shallow depth of soil. Herbicide residues were measured in runoff water at the first irrigation at 20 microg L(-1) for diuron and 1 microg L(-1) for hexazinone. Runoff water captured in the pond rapidly infiltrated into the subsurface soil, causing a concomitant rise in ground water elevation near the pond. Herbicide residues were also detected in the sampled ground water. We concluded that the pond was the predominant source for movement to ground water. Since addition of a surfactant to the spray mixture did not reduce concentrations in runoff water, mitigation methods will focus on minimizing infiltration of water from the pond.     

Managing biosolids runoff phosphorus using buffer strips enhanced with drinking water treatment residuals

Vegetated buffers strips typically have limited ability to reduce delivery of dissolved phosphorus (DP) from agricultural fields to surface waters. A field study was conducted to evaluate the ability of buffer strips enhanced with drinking water treatment residuals (WTRs) to control runoff P losses from surface-applied biosolids characterized by high water-extractable P (4 g kg(-)(1)). Simulated rainfall (62.4 mm h(-1)) was applied to grassed plots (3 m x 10.7 m including a 2.67 m downslope buffer) surface-amended with biosolids at 102 kg P ha(-1) until 30 min of runoff was collected. With buffer strips top-dressed with WTR (20 Mg ha(-1)), runoff total P (TP = 2.5 mg L(-1)) and total DP (TDP = 1.9 mg L(-1)) were not statistically lower (alpha = 0.05) compared to plots with unamended grass buffers (TP = 2.7 mg L(-1); TDP = 2.6 mg L(-1)). Although the applied WTR had excess capacity (Langmuir P maxima of 25 g P kg(-1)) to sorb all runoff P, kinetic experiments suggest that sheet flow travel time across the buffers ( approximately 30 s) was insufficient for significant P reduction. Effective interception of dissolved P in runoff water by WTR-enhanced buffer strips requires rapid P sorption kinetics and hydrologic flow behavior ensuring sufficient runoff residence time and WTR contact in the buffer. Substantial phosphate-adsorbent contact opportunity may be more easily achieved by incorporating WTRs into P-enriched soils or blending WTRs with applied P sources.     

Dissolved organic carbon fluxes under bare soil

The flux of dissolved organic carbon (DOC) in soil facilitates transport of nutrients and contaminants in soil. There is little information on DOC fluxes and the relationship between DOC concentration and water flux in agricultural soils. The DOC fluxes and concentrations were measured during 2.5 yr using 30 automatic equilibrium tension plate lysimeters (AETPLs) at 0.4 m and 30 AETPLs at 1.20-m depth in a bare luvisol, previously used as an arable soil. Average annual DOC fluxes of the 30 AETPLS were 4.9 g C m(-2) y(-1) at 0.4 m and 2.4 g C m(-2) y(-1) at 1.2 m depth. The average leachate DOC concentrations were 17 mg C L(-1) (0.4 m) and 9 mg C L(-1) (1.2 m). The DOC concentrations were unrelated to soil moisture content or average temperature and rarely dropped below 9 mg C L(-1) (0.4 m) and 5 mg C L(-1) (1.2 m). The variability in cumulative DOC fluxes among the plates was positively related to leachate volume and not to average DOC concentrations at both depths. This suggests that water fluxes are the main determinants of spatial variability in DOC fluxes. However, the largest DOC concentrations were inversely proportional to the mean water velocity between succeeding sampling periods, suggesting that the maximal net DOC mobilization rate in the topsoil is limited. Elevated DOC concentrations, up to 90 mg C L(-1), were only observed at low water velocities, reducing the risks of DOC-facilitated transport of contaminants to groundwater. The study emphasizes that water flux and velocity are important parameters for DOC fluxes and concentrations.     

Impact of preferential flow at varying irrigation rates by quantifying mass fluxes

Solute concentration and soluble dye studies inferring that preferential flow accelerates field-scale contaminant transport are common but flux measurements quantifying its impact are essentially nonexistent. A tile-drain facility was used to determine the influence of matrix and preferential flow processes on the flux of mobile tracers subjected to different irrigation regimes (4.4 and 0.89 mm h(-1)) in a silt loam soil. After tile outflow reached steady state either bromide (Br; 280 kg ha(-1)) or pentafluorobenzoic acid (PFBA; 121 kg ha(-1)) was applied through the irrigation system inside a shed (3.5 x 24 m). Bromide fluxes were monitored at an irrigation rate of 4.4 mm h(-1) while PFBA fluxes were monitored at an irrigation rate of 0.89 mm h(-1). At 4.4 mm h(-1) nearly one-third of the surface-applied Br was recovered in the tile line after only 124 mm of irrigation and was poorly fit by the one-dimensional convective-dispersive equation (CDE). On the other hand, the one-dimensional CDE fit the main PFBA breakthrough pattern almost perfectly, suggesting the PFBA transport was dominated by matrix flow. Furthermore, after 225 mm of water had been applied, less than 2% of the applied PFBA had been leached through the soil compared with more than 59% of the applied Br. This study demonstrates that the methodology of applying a narrow strip of chemical to a tile drain facility is appropriate for quantifying chemical fluxes at the small-field scale and also suggests that there may be a critical input flux whereby preferential flow is initiated.     

Surface and subsurface phosphorus losses from fertilized pasture systems in Ohio

Phosphorus is an essential plant nutrient and critical to agricultural production, but it is also a problem when excessive amounts enter surface waters. Summer rotational grazing and winter feeding beef pasture systems at two fertility levels (56 and 28 kg available P ha(-1)) were studied to evaluate the P losses from these systems via surface runoff and subsurface flow using eight small (0.3-1.1 ha), instrumented watersheds and spring developments. Runoff events from a 14-yr period (1974-1988) were evaluated to determine the relationships between event size in mm, total dissolved reactive phosphorous (TDRP) concentration, and TDRP transport. Most of the TDRP transported was via surface runoff. There were strong correlations (r2 = 0.45-0.66) between TDRP transport and event size for all watersheds, but no significant (P = 0.05) correlations between TDRP concentration and event size. Flow-weighted average TDRP concentrations from the pasture watersheds for the 14-yr period ranged from 0.64 to 1.85 mg L(-1) with a few individual event concentrations as high as 85.7 mg L(-1). The highest concentrations were in events that occurred soon after P fertilizer application. Average seasonal flow-weighted TDRP concentrations for subsurface flow were < 0.05 mg L(-1). Applying P fertilizer to pastures in response to soil tests should keep TDRP concentrations in subsurface flow at environmentally acceptable levels. Management to reduce runoff and avoidance of P fertilizer application when runoff producing rainfall is anticipated in the next few days will help reduce the surface losses of P.     

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.     

Fluometuron and pendimethalin runoff from strip and conventionally tilled cotton in the southern atlantic coastal plain

In the Atlantic Coastal Plain region of southern Georgia (USA), cotton (Gossypium hirsutum L.) acreage increased threefold in the past decade. To more effectively protect water quality in the region, best management practices are needed that reduce pesticide runoff from fields in cotton production. This study compared runoff of two herbicides, fluometuron [N,N-dimethyl-N'-[3-(trifluoromethyl)-phenyl]-urea] and pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitro-benzenamine], from plots in strip-tillage (ST) and conventional-tillage (CT) management near Tifton, GA. Rainfall simulations were conducted one day after preemergence herbicide applications to 0.0006-ha plots and runoff from 0.15-ha plots due to natural rainfall following preemergence pendimethalin and fluometuron and postemergence fluometuron use was monitored. Pendimethalin runoff was greater under CT than ST due to strong pendimethalin soil sorption and higher erosion and runoff under CT. The highest losses, 1.3% of applied in CT and 0.22% of applied in ST, were observed during rainfall simulations conducted 1 DAT. Fluometuron runoff from natural rainfall was substantially lower from ST than from CT plots but the trend was reversed in rainfall simulations. In all studies, fluometuron runoff was also relatively low (<1% of applied), and on plots under natural rainfall, desmethylfluometuron (DMF) represented about 50% of total fluometuron runoff. Fluometuron's relatively low runoff rate appeared linked to its rapid leaching, and high DMF detection rates in runoff support DMF inclusion in fluometuron risk assessments. Results showed that ST has the potential to reduce runoff of both herbicides, but fluometuron leaching may be a ground water quality concern.     

Dissolved Organic Carbon and Estrogen Transport in Surface Runoff From Agricultural Land Receiving Poultry Litter1

Dutta, Sudarshan, Shreeram Inamdar, Jerry Tso, Diana S. Aga, and J. Tom Sims, 2012. Dissolved Organic Carbon and Estrogen Transport in Surface Runoff from Agricultural Land Receiving Poultry Litter. Journal of the American Water Resources Association (JAWRA) 48(3): 558-569. DOI: 10.1111/j.1752-1688.2011.00634.x Abstract: Dissolved organic carbon (DOC) provides a reactive substrate for the transport of organic contaminants with runoff. Very few studies have investigated the export of DOC from agricultural land, especially those receiving manure applications. We investigated exports of DOC in surface runoff from agricultural fields receiving various treatments of poultry litter (raw vs. pelletized). In addition, we also investigated how estrogens in runoff were associated with DOC. Different forms of estrogens studied were: estrone, 17β-estradiol, estriol, and their conjugates. Experimental agricultural plots were 12 m × 5 m long and had reduced tillage and no-till management practices. The aromatic content of DOC was characterized using specific ultraviolet absorbance (SUVA). Flow-weighted concentrations of DOC and SUVA in surface runoff from plots with poultry litter were significantly (p ≤ 0.10) greater than the control (no litter) plots. Compared to pelletized poultry litter, reduced-tillage plots with raw litter yielded higher DOC concentrations and SUVA values. No significant differences (p ≥ 0.10) in DOC and SUVA were observed between litter treatments for plots with no-till. Total estrogen concentrations (including all forms) were positively and significantly (p ≤ 0.10) correlated with DOC. These results can help select and guide agricultural management practices that can reduce the exports of DOC and associated contaminant from agricultural land receiving manure applications.     

Runoff losses of sediment and phosphorus from no-till and cultivated soils receiving dairy manure

Managing manure in no-till systems is a water quality concern because surface application of manure can enrich runoff with dissolved phosphorus (P), and incorporation by tillage increases particulate P loss. This study compared runoff from well-drained and somewhat poorly drained soils under corn (Zea mays, L.) production that had been in no-till for more than 10 yr. Dairy cattle (Bos taurus L.) manure was broadcast into a fall planted cover crop before no-till corn planting or incorporated by chisel/disk tillage in the absence of a cover crop. Rainfall simulations (60 mm h(-1)) were performed after planting, mid-season, and post-harvest in 2007 and 2008. In both years and on both soils, no-till yielded significantly less sediment than did chisel/disking. Relative effects of tillage on runoff and P loss differed with soil. On the well-drained soil, runoff depths from no-till were much lower than with chisel/disking, producing significantly lower total P loads (22-50% less). On the somewhat poorly drained soil, there was little to no reduction in runoff depth with no-till, and total P loads were significantly greater than with chisel/disking (40-47% greater). Particulate P losses outweighed dissolved P losses as the major concern on the well-drained soil, whereas dissolved P from surface applied manure was more important on the somewhat poorly drained soil. This study confirms the benefit of no-till to erosion and total P runoff control on well-drained soils but highlights trade-offs in no-till management on somewhat poorly drained soils where the absence of manure incorporation can exacerbate total P losses.     

Role of rainfall intensity and hydrology in nutrient transport via surface runoff

Loss of soil nutrients in runoff accelerates eutrophication of surface waters. This study evaluated P and N in surface runoff in relation to rainfall intensity and hydrology for two soils along a single hillslope. Experiments were initiated on 1- by 2-m plots at foot-slope (6%) and mid-slope (30%) positions within an alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.) field. Rain simulations (2.9 and 7.0 cm h(-1)) were conducted under wet (spring) and dry (late-summer) conditions. Elevated, antecedent soil moisture at the foot-slope during the spring resulted in less rain required to generate runoff and greater runoff volumes, compared with runoff from the well-drained mid-slope in spring and at both landscape positions in late summer. Phosphorus in runoff was primarily in dissolved reactive form (DRP averaged 71% of total P), with DRP concentrations from the two soils corresponding with soil test P levels. Nitrogen in runoff was mainly nitrate (NO3-N averaged 77% of total N). Site hydrology, not chemistry, was primarily responsible for variations in mass N and P losses with landscape position. Larger runoff volumes from the foot-slope produced higher losses of total P (0.08 kg ha(-1)) and N (1.35 kg ha(-1)) than did runoff from the mid-slope (0.05 total P kg ha(-1); 0.48 kg N ha(-1)), particularly under wet, spring-time conditions. Nutrient losses were significantly greater under the high intensity rainfall due to larger runoff volumes. Results affirm the critical source area concept for both N and P: both nutrient availability and hydrology in combination control nutrient loss.     

Effect of manure application timing, crop, and soil type on phosphorus leaching

Phosphorus (P) leaching losses from manure applications may be of concern when artificial drainage systems allow for hydrologic short-cuts to surface waters. This study quantified P leaching losses from liquid manure applications on two soil textural extremes, a clay loam and loamy sand soil, as affected by cropping system and timing of application. For each soil type, manure was applied at an annual rate of 93 800 L ha(-1) on replicated drained plots under maize (Zea mays L.) in early fall, late fall, early spring, and as a split application in early and late spring. Manure was applied on orchardgrass (Dactylis glomerata L.) in split applications in early fall and late spring, and early and late spring. Drain water was sampled at least weekly when lines were flowing, and outflow rate and total P content were determined. High P leaching losses were measured in the clay loam as soon as drain lines initiated flow after manure application. Flow-weighted mean P leaching losses on clay loam plots averaged 39 times higher (0.504 mg L(-1)) than those on loamy sand plots (0.013 mg L(-1)), and were above the USEPA level of concern of 0.1 mg L(-1). Phosphorus losses varied among application seasons on the clay loam soil, with highest losses generally measured for early fall applications. Phosphorus leaching patterns in clay loam showed short-term spikes and high losses were associated with high drain outflow rates, suggesting preferential flow as the main transport mechanism. Phosphorus leaching from manure applications on loamy sand soils does not pose environmental concerns as long as soil P levels remain below the saturation level.     

Spatial and Seasonal Variation of Dissolved Organic Carbon (DOC) Concentrations in Irish Streams: Importance of Soil and Topography Characteristics

Dissolved organic carbon (DOC) concentrations have increased in many sites in Europe and North America in recent decades. High DOC concentrations can damage the structure and functions of aquatic ecosystems by influencing water chemistry. This study investigated the spatial and seasonal variation of DOC concentrations in Irish streams across 55 sites at seven time occasions over 1 year (2006/2007). The DOC concentrations ranged from 0.9 to 25.9 mg/L with a mean value of 6.8 and a median value of 5.7 mg/L and varied significantly over the course of the year. The DOC concentrations from late winter (February: 5.2 ± 3.0 mg/L across 55 sites) and early spring (April: 4.5 ± 3.5 mg/L) had significantly lower DOC concentrations than autumn (October: mean 8.3 ± 5.6 mg/L) and early winter (December: 8.3 ± 5.1 mg/L). The DOC production sources (e.g., litterfall) or the accumulation of DOC over dry periods might be the driving factor of seasonal change in Irish stream DOC concentrations. Analysis of data using stepwise multiple linear regression techniques identified the topographic index (TI, an indication of saturation-excess runoff potential) and soil conditions (organic carbon content and soil drainage characteristics) as key factors in controlling DOC spatial variation in different seasons. The TI and soil carbon content (e.g., soil organic carbon; peat occurrence) are positively related to DOC concentrations, while well-drained soils are negatively related to DOC concentrations. The knowledge of spatial and seasonal variation of DOC concentrations in streams and their drivers are essential for optimum riverine water resources management.     

Determining sources of dissolved organic carbon and disinfection byproduct precursors to the McKenzie River, Oregon

This study was conducted to determine the main sources of dissolved organic carbon (DOC) and disinfection byproduct (DBP) precursors to the McKenzie River, Oregon (USA). Water samples collected from the mainstem, tributaries, and reservoir outflows were analyzed for DOC concentration and DBP formation potentials (trihalomethanes [THMFPs] and haloacetic acids [HAAFPs]). In addition, optical properties (absorbance and fluorescence) of dissolved organic matter (DOM) were measured to provide insight into DOM composition and assess whether optical properties are useful proxies for DOC and DBP precursor concentrations. Optical properties indicative of composition suggest that DOM in the McKenzie River mainstem was primarily allochthonous--derived from soils and plant material in the upstream watershed. Downstream tributaries had higher DOC concentrations than mainstem sites (1.6 +/- 0.4 vs. 0.7 +/- 0.3 mg L(-1)) but comprised < 5% ofmainstem flows and had minimal effect on overall DBP precursor loads. Water exiting two large upstream reservoirs also had higher DOC concentrations than the mainstem site upstream of the reservoirs, but optical data did not support in situ algal production as a source of the added DOC during the study. Results suggest that the first major rain event in the fall contributes DOM with high DBP precursor content. Although there was interference in the absorbance spectra in downstream tributary samples, fluorescence data were strongly correlated to DOC concentration (R2 = 0.98), THMFP (R2 = 0.98), and HAAFP (R2 = 0.96). These results highlight the value of using optical measurements for identifying the concentration and sources of DBP precursors in watersheds, which will help drinking water utilities improve source water monitoring and management programs.     

Nitrogen removal and nitrate leaching for forage systems receiving dairy effluent

Florida dairies need year-round forage systems that prevent loss of N to ground water from waste effluent sprayfields. Our purpose was to quantify forage N removal and monitor nitrate N (NO3(-)-N) concentrations in soil water below the rooting zone for two forage systems during four 12-mo cycles (1996-2000). Soil in the sprayfield is an excessively drained Kershaw sand (thermic, uncoated Typic Quartzipsamment). Over four cycles, average loading rates of effluent N were 500, 690, and 910 kg ha(-1) per cycle. Nitrogen removed by the bermudagrass (Cynodon spp.)-rye (Secale cereale L.) system (BR) during the first three cycles was 465 kg ha(-1) per cycle for the low loading rate, 528 kg ha(-1) for the medium rate, and 585 kg ha(-1) for the high. For the corn (Zea mays L.)-forage sorghum [Sorghum bicolor (L.) Moench]-rye system (CSR), N removals were 320 kg ha(-1) per cycle for the low rate, 327 kg ha(-1) for the medium, and 378 kg ha(-1) for the high. The higher N removals for BR were attributed to higher N concentration in bermudagrass (18.1-24.2 g kg(-1)) than in corn and forage sorghum (10.3-14.7 g kg(-1)). Dry matter yield declined in the fourth cycle for bermudagrass but N removal continued to be higher for BR than CSR. The BR system was much more effective at preventing NO3(-)-N leaching. For CSR, NO3(-)-N levels in soil water (1.5 m below surface) increased steeply during the period between the harvest of one forage and canopy dosure of the next. Overall, the BR system was better than CSR at removing N from the soil and maintaining low NO3(-)-N concentrations below the rooting zone.