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Nitrate from artificial drainage pipes (tiles) underlying agricultural fields is a major source of reactive N, especially NO, in surface waters. A novel approach for reducing NO loss is to intercept a field tile where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer. This practice is called a saturated riparian buffer (SRB), and although it is promising, little data on the performance of the practice is available. This research investigated the effectiveness of SRBs in removing NO at six sites installed across Iowa, resulting in a total of 17 site-years. Water flow and NO in the tile outlets, diverted into the buffers, and NO concentration changes within the buffers were monitored throughout the year at each site. Results showed that all the SRBs were effective in removing NO from the tile outlet, with the average annual NO load removal ranging from 13 to 179 kg N for drainage areas ranging from 3.4 to 40.5 ha. This is NO that would have otherwise discharged directly into the adjoining streams. The annual removal effectiveness, which is the total NO removed in the SRB divided by the total NO draining from the field, ranged from 8 to 84%. This corresponds to an average removal rate of 0.040 g N m d with a range of 0.004 to 0.164 g N m d. Assuming a 40-yr life expectancy for the structure and a 4% discount rate, we computed a mean equal annual cost for SRBs of US$213.83. Given the average annual removal of 73 kg for all site-years, this cost equates to $2.94 kg N removed, which is very competitive with other field-edge practices such as denitrification bioreactors and constructed wetlands. Thus, SRBs continue to be a promising practice for NO removal in tile-drained landscapes.

Excess NO leaching from the agricultural Midwest via tile drainage water has contributed to both local drinking water and national Gulf of Mexico benthic hypoxia concerns. Both in-field and edge-of-field practices have been designed to help mitigate NO flux to surface waters. Edge-of-field practices focus on maximizing microbial denitrification, the conversion of NO to N gas. This study assessed denitrification rates from two saturated riparian buffers (SRBs) for 2 yr and a third SRB for 1 yr, for a total of five sample years. These SRBs were created by diverting NO-rich tile drainage water into riparian buffers soils. The SRBs in this study removed between 27 and 96% of the total diverted NO load. Measured cumulative average denitrification rate for each SRB sample year accounted for between 3.7 and 77.3% of the total NO removed. Both the cumulative maximum and 90% confidence interval denitrification rates accounted for all of the NO removed by the SRBs in three of the five sample years, indicating that denitrification can be a dominant NO removal mechanism in this edge-of-field practice. When adding the top 20 cm of each core to the cumulative denitrification rates for each SRB, denitrification accounted for between 33 and over 100% of the total NO removed. Buffer age (time since establishment) was speculated to enhance denitrification rates, and there was a trend of the soil closer to the surface making up the majority of the total denitrification rate. Finally, both NO and C could limit denitrification in these SRBs.

There is a general understanding in the scientific community as to how denitrifying bioreactors operate, but we lack a quantitative understanding of the details of the denitrification process acting within them and comprehensive models for simulating their performance. We hypothesized that nitrate transport through woodchip bioreactors would be best described by a dual-porosity transport model where the bioreactor water is divided into a mobile domain (i.e., the water between the woodchips where it is free to flow and solute movement is by advection and dispersion) and an immobile domain of water (i.e., the water mostly within the woodchips that is stagnant and where solute movement is by diffusion alone). We calibrated the dual-porosity model contained in the HYDRUS model for a woodchip bioreactor using the results of a Br breakthrough experiment where we treated Br as a conservative nonadsorbing tracer. We then used the resulting model parameters to describe 2 yr of NO transport and denitrification within a bioreactor supplied by tile drainage. The only model parameters fitted to the NO data were either the zero- or first-order denitrification rate and its temperature dependence. The bioreactor denitrified 2.23 kg N (38%) of the NO entering it in 2013 and 3.73 kg N (49%) of the NO that entered it in 2014. The dual-porosity model fit the NO data very well, with fitted zero-order reaction rates of 8.7 and 6.8 mg N L d in 2013 and 2014, respectively, and corresponding first-order reaction rates of 0.99 and 1.02 d. For the 2-yr data set, both reaction rate models fit the data equally well. Consistent model parameters fitted for the 2 yr indicated that the model used was robust and a promising approach for modeling fate and transport of NO in woodchip bioreactors.

Improved understanding of year-to-year late-spring soil nitrate test (LSNT) variability could help make it more attractive to producers. We test the ability of the Root Zone Water Quality Model (RZWQM) to simulate watershed-scale variability due to the LSNT, and we use the optimized model to simulate long-term field N dynamics under related conditions. Autoregressive techniques and the automatic parameter calibration program PEST were used to show that RZWQM simulates significantly lower nitrate concentration in discharge from LSNT treatments compared with areas receiving fall N fertilizer applications within the tile-drained Walnut Creek, Iowa, watershed (>5 mg NL(-1) difference for the third year of the treatment, 1999). This result is similar to field-measured data from a paired watershed experiment. A statistical model we developed using RZWQM simulations from 1970 to 2005 shows that early-season precipitation and early-season temperature account for 90% of the interannual variation in LSNT-based fertilizer N rates. Long-term simulations with similar average N application rates for corn (Zea mays L.) (151 kg N ha(-1)) show annual average N loss in tile flow of 20.4, 22.2, and 27.3 kg N ha(-1) for LSNT, single spring, and single fall N applications. These results suggest that (i) RZWQM is a promising tool to accurately estimate the water quality effects of LSNT; (ii) the majority of N loss difference between LSNT and fall applications is because more N remains in the root zone for crop uptake; and (iii) year-to-year LSNT-based N rate differences are mainly due to variation in early-season precipitation and temperature.

Excessive nitrate leaching from the U.S. Corn Belt has created serious water quality problems and contributed to the expansion of the hypoxic zone in the Gulf of Mexico. We evaluated the effect of implementing the late spring nitrate test (LSNT) for corn (Zea mays L.) grown within a 400-ha, tile-drained subbasin in central Iowa. Surface water discharge and NO3 concentrations from the treated subbasin and two adjacent subbasins receiving primarily fall-applied, anhydrous ammonia were compared. In two of four years, the LSNT method significantly reduced N fertilizer applications compared with the farmers' standard practices. Average corn yield from LSNT fields and nonlimiting N fertilizer check strips was not significantly different. Autoregressive (AR) models using weekly time series in surface water NO3 concentration differences between the LSNT and control subbasins indicated no consistent significant differences during the pre-LSNT (1992-1996) period. However, by the second year (1998) of the treatment period (1997-2000), NO3 concentrations in surface water from the treated subbasin were significantly lower than the concentrations coming from both control basins. Annual average flow-weighted NO3 concentrations for the last two years (1999-2000) were 11.3 mg N L(-1) for the LSNT and subbasin and 16.0 mg N L(-1) for the control subbasins. Based on these values and the AR models, widespread adoption of the LSNT program for managing N fertilizer where fall N application is typically practiced could result in a > or = 30% decrease for NO3 concentrations in surface water.

The relationships between N fertilizer rate, yield, and NO 3 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 NO 3 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, NO 3 concentrations in the drainage water exceeded the 10 mg N L 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, NO, concentrations exceeded the MCL in all years, regardless of crop grown. For all years, the NO, 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.

Most phosphate mineral solubility data in use were measured more than 20 yr ago when instrumentation and methods were generally less sophisticated. The objective of this study was to measure calcium phosphate mineral solubilities using modern analytical equipment and techniques. Natural and synthetic Ca phosphate minerals (

There is a lack of quantitative information describing the impact of farming on water quality at the watershed scale. This study documents the surface water quality of Walnut Creek-a 5130-ha watershed with about 86% of the land used for crop production. Starting in 1990, flow and concentrations of NO 3 -N and four herbicides-atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], metribuzin [4-amino-6-(1, 1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-on ], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-methoxy-1-methylethyl)acetamide] were measured at eight locations. Nitrate-N concentrations often exceeded 10 mg L -1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha -1 yr -1 and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 μg L -1 in about half of all water samples, while alachlor and metribuzin were seldom detected. Median concentrations for atrazine and metolachlor were below 1 μg L -1 for all locations within the watershed. During runoff events, herbicide concentrations in the stream increased while NO 3 -N concentrations decreased. Yearly losses from the watershed ranged from 0.2 to 7.5 g ha -1 for atrazine and from 0.3 to 6.7 g ha -1 for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year.

Nonpoint-source pollution has been linked to agricultural practices; however, there is a need for quantitative information describing the effect of specific farming practices on ground and surface water quality. Lack of information at the watershed scale limits our ability to make decisions about the effect of potential changes in either farming practices or landscape management that would enhance water quality. A multidisciplinary study was designed to evaluate the effect of farming practices on subsurface drainage, surface runoff, stream discharge, groundwater, volatilization, and soil processes that influence water quality. Walnut Creek watershed is a 5130-ha intensively cropped area in central Iowa on the Des Moines Lobe landform region. Soils within the watershed are in the Clarion-Nicollet-Webster (Typic Hapludoll-Aquic Hapludoll-Typic Haplaquoll) soil association, and the underlying surficial material is glacial till. Land use is predominantly corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Fertilizer use, herbicide application, tillage practices, and crop selection were obtained through surveys of each field operator. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine [2-[ 4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl amino]-2-methylpropanenitrile], EPTC [S-ethyl dipropyl carbamothioate], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] are the primary herbicides used within the watershed at rates similar to those for the state. Nitrogen fertilizer was applied as anhydrous ammonia on 60% of the corn fields at an average rate of 153 kg ha -1 for the 1991-1994 period, but the frequency of corn fields receiving

Herbicide transport in subsurface drainage can result in unacceptable levels of contamination in surface waters. This study assessed the extent of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3-,5-triazine-2,4-diamine] and metribuzin [4-amino-6-(1,1-dimethyethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one transport to subsurface drainage and shallow groundwater. A corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation was used with corn receiving banded atrazine applications of 459 g ha -1 in 1992 and 561 g ha -1 in 1994. Soybean were treated with metribuzin at 420 g ha -1 in 1993 and 1995. Monthly flow-weighted average concentrations of atrazine in drainage water did not exceed 3 μg L -1 and annual losses ranged from 0.02 to 2.16 g ha -1 during the 4-yr study. Less than 3% of the groundwater samples contained atrazine concentrations exceeding the 3 μg L -1 maximum contaminant level for drinking water (MCL). Atrazine was detected more frequently in groundwater beneath the lowest parts of the field, despite greater than average sorption to soils in that area. Metribuzin was also found in groundwater, but only half as frequently as atrazine. The patterns observed in subsurface drainage and groundwater reflected the persistence of atrazine and metribuzin in soil. Atrazine was detected in >90% of surface soil samples up to 23 mo after application, whereas metribuzin was rarely detected during the second year following application. Atrazine was found far more commonly than metribuzin in soil below 30 cm depth.

Nonpoint source contamination of surface and groundwater resources with nitrate-N (NO 3 -N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extent of NO 3 -N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, mid-summer, and after harvest for each of the 4 yr. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrification potential among subsurface sediments was observed for deep unoxidized loess. Soil profile NO 3 -N concentrations decreased with depth and were the same below 30 cm for fertilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L -1 for 12 mo and were between 6 and 9 mg L -1 for 32 mo during the 4-yr study. The temporal pattern of NO 3 -N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO 3 -N losses to subsurface drains were greatest in 1993 (51.3 kg ha 1 ) and least in 1994 (4.9 kg ha -1 ). Most of the subsurface drainage water NO 3 -N was lost when crop plants were not present (November-May), except in 1993. Our results indicate that NO 3 -N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of NO 3 -N in the soil and the presence of large quantities of potentially mineralizable N in the soil organic matter.

A potential methods for reducing pesticide leaching is to base application rates on the leaching potential of a specific chemical and soil combination. However, leaching is determined in part by the partitioning of the chemical between the soil and soil solution, which varies across a field. Standard methods of measuring the pesticide-soil partitioning coefficient (K d ) are too expensive and slow for routine field mapping. Therefore, alternative methods for mapping K d must be found if variable application methods are to be successful. We investigated the use of noncontacting electromagnetic induction measurements as surrogate measures of K d . We measured the partition coefficient for atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), apparent electrical conductivity by electromagnetic induction (E m ), and mass fraction of soil organic carbon (f ) on a 250 by 250 m grid with a 25 m spacing. Both K d and f were lognormally distributed, while E m was poorly described by either a normal or lognormal distribution. Maps of the measured parameters showed similar spatial patterns, having low values on well-drained soils and high values on poorly drained soils. Correlation coeffients between K d and E m and K d and f were 0.575 and 0.686, and showed distinct spatial patterns. Spatial structure as indicated by correlograms indicated that each parameter was spatially dependent to distances of about 80 m. Simple relationships of K d =176 f and K d =exp(0.00336 E m ) were found between the data. Maps of K d estimated from f or E m were similar to measured K d , but more diffuse. Electromagnetic induction measurements failed to predict the observed high K d values. The advantage of using E m measurements to map K d that it is a rapid, easy, and inexpensive method once it has been calibrated

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

Nitrate in water removed from fields by subsurface drain ('tile') systems is often at concentrations exceeding the 10 mg N L(-1) maximum contaminant level (MCL) set by the USEPA for drinking water and has been implicated in contributing to the hypoxia problem within the northern Gulf of Mexico. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO(3) concentrations in subsurface drainage below the MCL, additional approaches are needed. In this field study, we compared the NO(3) losses in tile drainage from a conventional drainage system (CN) consisting of a free-flowing pipe installed 1.2 m below the soil surface to losses in tile drainage from two alternative drainage designs. The alternative treatments were a deep tile (DT), where the tile drain was installed 0.6 m deeper than the conventional tile depth, but with the outlet maintained at 1.2 m, and a denitrification wall (DW), where trenches excavated parallel to the tile and filled with woodchips serve as additional carbon sources to increase denitrification. Four replicate 30.5- by 42.7-m field plots were installed for each treatment in 1999 and a corn-soybean rotation initiated in 2000. Over 5 yr (2001-2005) the tile flow from the DW treatment had annual average NO(3) concentrations significantly lower than the CN treatment (8.8 vs. 22.1 mg N L(-1)). This represented an annual reduction in NO(3) mass loss of 29 kg N ha(-1) or a 55% reduction in nitrate mass lost in tile drainage for the DW treatment. The DT treatment did not consistently lower NO(3) concentrations, nor reduce the annual NO(3) mass loss in drainage. The DT treatment did exhibit lower NO(3) concentrations in tile drainage than the CN treatment during late summer when tile flow rates were minimal. There was no difference in crop yields for any of the treatments. Thus, denitrification walls are able to substantially reduce NO(3) concentrations in tile drainage for at least 5 yr.

A significant portion of the NO 3 from agricultural fields that contaminates surface waters in the Midwest Corn Belt is transported to streams or rivers by subsurface drainage systems or "tiles." Previous research has shown that N fertilizer management alone is not sufficient for reducing NO 3 concentrations in subsurface drainage to acceptable levels; therefore, additional approaches need to be devised. We compared two cropping system modifications for NO 3 concentration and load in subsurface drainage water for a no-till corn (Zea mays L.)-soybean (Glycine max [L.] Merr.) management system. In one treatment, eastern gamagrass (Tripsacum dactyloides L.) was grown in permanent 3.05-m-wide strips above the tiles. For the second treatment, a rye (Secale cereale L.) winter cover crop was seeded over the entire plot area each year near harvest and chemically killed before planting the following spring. Twelve 30.5 x 42.7-m subsurface-drained field plots were established in 1999 with an automated system for measuring tile flow and collecting flow-weighted samples. Both treatments and a control were initiated in 2000 and replicated four times. Full establishment of both treatments did not occur until fall 2001 because of dry conditions. Treatment comparisons were conducted from 2002 through 2005. The rye cover crop treatment significantly reduced subsurface drainage water flow-weighted NO 3 concentrations and NO 3 loads in all 4 yr. The rye cover crop treatment did not significantly reduce cumulative annual drainage. Averaged over 4 yr, the rye cover crop reduced flow-weighted NO 3 concentrations by 59% and loads by 61%. The gamagrass strips did not significantly reduce cumulative drainage, the average annual flow-weighted NO 3 concentrations, or cumulative NO 3 loads averaged over the 4 yr. Rye winter cover crops grown after corn and soybean have the potential to reduce the NO 3 concentrations and loads delivered to surface waters by subsurface drainage systems.

Nitrate in water from tile drained corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields in the U.S. Midwest contributes to nitrate contamination of surface waters. Denitrification-based biofilters are a promising strategy for reducing nitrate concentrations, but these systems require an external carbon supply to sustain denitrification. The ability of four organic materials to serve as carbon substrates for denitrification biofilters was evaluated in this laboratory study. Wood chips, wood chips amended with soybean oil, cornstalks, and cardboard fibers were mixed with subsoil (oxidized till) and incubated anaerobically for 180 d. Periodically, 15NO3-N was added to maintain nitrate N concentrations between 10 and 100 mg L-1. All of the materials stimulated NO3-N removal and the degree of removal from highest to lowest was: cornstalks, cardboard fibers, wood chips with oil, and wood chips alone. Analysis of 15N showed that immobilization and dissimilatory nitrate reduction to ammonium accounted for

Subsurface drainage is a beneficial water management practice in poorly drained soils but may also contribute substantial nitrate N loads to surface waters. This paper summarizes results from a 15-yr drainage study in Indiana that includes three drain spacings (5, 10, and 20 m) managed for 10 yr with chisel tillage in monoculture corn (Zea mays L.) and currently managed under a no-till corn-soybean [Glycine max (L.) Merr.] rotation. In general, drainflow and nitrate N losses per unit area were greater for narrower drain spacings. Drainflow removed between 8 and 26% of annual rainfall, depending on year and drain spacing. Nitrate N concentrations in drainflow did not vary with spacing, but concentrations have significantly decreased from the beginning to the end of the experiment. Flow-weighted mean concentrations decreased from 28 mg L(-1) in the 1986-1988 period to 8 mg L(-1) in the 1997-1999 period. The reduction in concentration was due to both a reduction in fertilizer N rates over the study period and to the addition of a winter cover crop as a "trap crop" after corn in the corn-soybean rotation. Annual nitrate N loads decreased from 38 kg ha(-1) in the 1986-1988 period to 15 kg ha(-1) in the 1997-1999 period. Most of the nitrate N losses occurred during the fallow season, when most of the drainage occurred. Results of this study underscore the necessity of long-term research on different soil types and in different climatic zones, to develop appropriate management strategies for both economic crop production and protection of environmental quality.

Excessive nitrate leaching from the U.S. Corn Belt has created serious water quality problems and contributed to the expansion of the hypoxic zone in the Gulf of Mexico. We evaluated the effect of implementing the late spring nitrate test (LSNT) for corn (Zea mays L.) grown within a 400-ha, tile-drained subbasin in central Iowa. Surface water discharge and NO3 concentrations from the treated subbasin and two adjacent subbasins receiving primarily fall-applied, anhydrous ammonia were compared. In two of four years, the LSNT method significantly reduced N fertilizer applications compared with the farmers' standard practices. Average corn yield from LSNT fields and nonlimiting N fertilizer check strips was not significantly different. Autoregressive (AR) models using weekly time series in surface water NO3 concentration differences between the LSNT and control subbasins indicated no consistent significant differences during the pre-LSNT (1992-1996) period. However, by the second year (1998) of the treatment period (1997-2000), NO3 concentrations in surface water from the treated subbasin were significantly lower than the concentrations coming from both control basins. Annual average flow-weighted NO3 concentrations for the last two years (1999-2000) were 11.3 mg N L(-1) for the LSNT and subbasin and 16.0 mg N L(-1) for the control subbasins. Based on these values and the AR models, widespread adoption of the LSNT program for managing N fertilizer where fall N application is typically practiced could result in aor = 30% decrease for NO3 concentrations in surface water.

In a semiarid environment, climate is a critical factor in the decomposition of surface-applied biosolids. This study examined the effect of 2- to 7-yr exposure times on the composition of single applications of New York, NY biosolids in western Texas. Exposure time effects on organic matter, N, P, S, Cu, Cr, Pb, Hg, and Zn were studied near Sierra Blanca, TX. Due to organic matter decomposition, total organic C decreased from 340 g kg(-1) in fresh biosolids to 180 g kg(-1) in biosolids after 82 mo of exposure, whereas the inorganic ash content of the biosolids increased from 339 to 600 g kg(-1). Total N decreased from 50 to 10 g N kg(-1) and total S decreased from 12 to 6 g S kg(-1). Bicarbonate-available P in the biosolids decreased from 0.9 to 0.2 g kg(-1). Successive H2O extractions yielded soluble P concentrations consistent with dicalcium phosphate (dical) for fresh biosolids and tricalcium phosphate (trical) for biosolids exposed for 59 months or more. Sparingly soluble phosphates, such as dical and trical, potentially yield > 0.5 mg P L(-1) in runoff waters for extended periods after biosolids applications, especially after multiple applications. Selective dissolution of the biosolids indicated that as much as 66 to 78% of P exists as iron phosphates, 16 to 21% as Fe oxides, and 5 to 12% as insoluble Ca phosphates. Chemical analyses of ash samples suggest that Cu and Zn have been lost from biosolids through leaching or runoff and no losses of Pb, Cr, or Hg have occurred since application.

Nitrate N fluxes from tile-drained watersheds have been implicated in water quality studies of the Mississippi River basin, but actual NO 3 -N loads from small watersheds during long periods are poorly documented. We evaluated discharge and NO 3 -N fluxes passing the outlet of an Iowa watershed (5134 ha) and two of its tile-drained subbasins (493 and 863 ha) from mid-1992 through 2000. The cumulative NO 3 -N load from the catchment was 168 kg ha -1 , and 176 and 229 kg ha -1 from the subbasins. The outlet had greater total discharge (1831 mm) and smaller flow-weighted mean NO 3 -N concentration (9.2 mg L -1 ) than the subbasins, while the larger subbasin had greater discharge (1712 vs. 1559 mm) and mean NO 3 -N concentration (13.4 vs. 11.3 mg L -1 ) than the smaller subbasin. Concentrations exceeding 10 mg L -1 were common, but least frequent at the outlet. Nitrate N was generally not diluted by large flows, except during 1993 flooding. The outlet showed smaller NO 3 -N concentrations at low flows. Relationships between discharge and NO 3 -N flux showed log-log slopes near 1.0 for the subbasins, and 1.2 for the outlet, considering autocorrelation and measurement-error effects. We estimated denitrification of subbasin NO 3 -N fluxes in a hypothetical wetland using published data. Assuming that temperature and NO 3 -N supply could limit denitrification, then about 20% of the NO 3 -N would have been denitrified by a wetland constructed to meet USDA-approved criteria. The low efficiency results from the seasonal timing and NO 3 -N content of large flows. Therefore, agricultural and wetland best management practices (BMPs) are needed to achieve water quality goals in tile-drained watersheds.