Predicting nitrate leaching under potato crops using transfer functions

Nitrate leaching is a major issue in many cultivated soils. Models that predict the major processes involved at the field scale could be used to test and improve management practices. This study aims to evaluate a simple transfer function approach to predict nitrate leaching in sandy soils. A convective lognormal transfer (CLT) function is convoluted with functional equations simulating N mineralization, plant N uptake, N fertilizer dissolution, and nitrification at the soil surface to predict solute concentrations under potato (Solanum tuberosum L.) and barley (Hordeum vulgare L.) fields as a function of drainage water. Using this approach, nitrate flux concentrations measured in drainable lysimeters (1-m soil depth) were reasonably predicted from 29 Apr. 1996 to 3 Dec. 1996. With average application rates of 16.9 g m(-2) of N fertilizer in potato crops, mean nitrate-leaching losses measured under potato were 8.5 g N m(-2). Tuber N uptake averaged 9.7 g N m(-2) and soil mineral N at start (spring) and end (fall) of N mass balance averaged 1.7 and 4.5 g N m(-2), respectively. Soil N mineralization was estimated by difference (4.3 g N m(-2) on average) and was small compared with N fertilization. Small nitrate flux concentrations at the beginning of the cropping season (May) resulted mainly from initial soil nitrate concentrations. Measured and predicted nitrate flux concentrations significantly increased at mid-season (July-August) following important drainage events coupled with complete dissolution and nitrification of N fertilizers, and declining N uptake by potato plants. Decreases in nitrate concentrations before the end of year (November-December) underlined the predominant effect of N fertilizers applied for the most part at planting acting as a pulse input of solute.     

Perennial filter strips reduce nitrate levels in soil and shallow groundwater after grassland-to-cropland conversion

Many croplands planted to perennial grasses under the Conservation Reserve Program are being returned to crop production, and with potential consequences for water quality. The objective of this study was to quantify the impact of grassland-to-cropland conversion on nitrate-nitrogen (NO3-N) concentrations in soil and shallow groundwater and to assess the potential for perennial filter strips (PFS) to mitigate increases in NO3-N levels. The study, conducted at the Neal Smith National Wildlife Refuge (NSNWR) in central Iowa, consisted of a balanced incomplete block design with 12 watersheds and four watershed-scale treatments having different proportions and topographic positions of PFS planted in native prairie grasses: 100% rowcrop, 10% PFS (toeslope position), 10% PFS (distributed on toe and as contour strips), and 20 PFS (distributed on toe and as contour strips). All treatments were established in fall 2006 on watersheds that were under bromegrass (Bromus L.) cover for at least 10 yr. Nonperennial areas were maintained under a no-till 2-yr corn (Zea mays L.)--soybean [Glycine max. (L.) Merr.] rotation since spring 2007. Suction lysimeter and shallow groundwater wells located at upslope and toeslope positions were sampled monthly during the growing season to determine NO3-N concentration from 2005 to 2008. The results indicated significant increases in NO3-N concentration in soil and groundwater following grassland-to-cropland conversion. Nitrate-nitrogen levels in the vadose zone and groundwater under PFS were lower compared with 100% cropland, with the most significant differences occurring at the toeslope position. During the years following conversion, PFS mitigated increases in subsurface nitrate, but long-term monitoring is needed to observe and understand the full response to land-use conversion.     

Bromide and nitrate movement through undisturbed soil columns

Field experiments often assume that Br-, 14NO3(-)-N, and 15NO3(-)-N have similar leaching kinetics. This study tested this assumption. Twenty-four undisturbed soil columns (15-cm diameter) were collected from summit-shoulder, backslope, and footslope positions of a no-tillage field with a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Each of the landscape positions had a different soil series. After conditioning the columns with 4 L of 0.01 M CaCl2 (2 pore volumes), 15N-labeled Ca(NO3)2 and KBr were applied to the soil surface and leached with 4 L of 0.01 M CaCl2. Leachate was collected, weighed, and analyzed for NO3(-)-N, NH4(+)-N, 15N, 14N, and Br-. The total amount of 15NO3(-)-N and 14NO3(-)-N collected in 1000, 2000, and 3000 mL of leachate was similar. These data suggest that 15N discrimination during leaching did not occur. Bromide leached faster through the columns than NO3(-)-N. The more rapid transport of Br- than NO3(-)-N was attributed to lower Br- (0.002 +/- 0.036 mg kg(-1)) than NO3(-)-N (0.17 +/- 0.03 mg kg(-1)) sorption. Results from this study suggest that (i) if Br- is used to estimate NO3(-)-N leaching loss, then NO3(-)-N leaching losses may be overestimated by 25%; (ii) the potential exists for landscape position to influence anion retention and movement in soil; and (iii) 15N discrimination was not detected during the leaching process.     

Economically optimal nitrogen rate reduces soil residual nitrate

Post-harvest residual soil NO(3)-N (RSN) is susceptible to transfer to water resources. Practices that minimize RSN levels can reduce N loss to the environment. Our objectives were (i) to determine if the RSN after corn (Zea mays L.) harvest can be reduced if N fertilizer is applied at the economically optimal N rate (EONR) as compared to current producer practices in the midwestern USA and (ii) to compare RSN levels for N fertilizer rates below, at, and above the EONR. Six experiments were conducted in producer fields in three major soil areas (Mississippi Delta alluvial, deep loess, claypan) in Missouri over 2 yr. Predominant soil great groups were Albaqualfs, Argiudolls, Haplaquolls, and Fluvaquents. At four transects in each field, six treatment N rates from 0 to 280 kg N ha(-1) were applied, the EONR was determined, and the RSN was measured to a 0.9-m depth from five treatment plots. The EONR at sampling sites varied from 49 to 228 kg N ha(-1) depending on site and year. Estimated average RSN at the EONR was 33 kg N ha(-1) in the 0.9-m profile. This was at least 12 kg N ha(-1) lower than RSN at the producers' N rates. The RSN increased with increasing Delta EONR (total N applied - EONR). This relationship was best modeled by a plateau-linear function, with a low RSN plateau at N rates well below the EONR. A linear increase in RSN began anywhere from 65 kg N ha(-1) below the EONR to 20 kg N ha(-1) above the EONR at the three sites with good data resolution near the EONR. Applying N rates in excess of the EONR produced elevated RSN values in all six experiments. Our results suggest that applying the EONR will produce environmental benefits in an economically sound manner, and that continued attempts to develop methods for accurately predicting EONR are justified.     

Identifying riparian sinks for watershed nitrate using soil surveys

The capacity of riparian zones to serve as critical control locations for watershed nitrogen flux varies with site characteristics. Without a means to stratify riparian zones into different levels of ground water nitrate removal capacity, this variability will confound spatially explicit source-sink models of watershed nitrate flux and limit efforts to target riparian restoration and management. We examined the capability of SSURGO (1:15 840 Soil Survey Geographic database) map classifications (slope class, geomorphology, and/or hydric soil designation) to identify riparian sites with high capacity for ground water nitrate removal. The study focused on 100 randomly selected riparian locations in a variety of forested and glaciated settings within Rhode Island. Geomorphic settings included till, outwash, and organic/alluvial deposits. We defined riparian zones with "high ground water nitrate removal capacity" as field sites possessing both >10 m of hydric soil width and an absence of ground water surface seeps. SSURGO classification based on a combination of geomorphology and hydric soil status created two functionally distinct sets of riparian sites. More than 75% of riparian sites classified by SSURGO as organic/alluviumhydric or as outwash-hydric had field attributes that suggest a high capacity for ground water nitrate removal. In contrast, >85% of all till sites and nonhydric outwash sites had field characteristics that minimize the capacity for ground water nitrate removal. Comparing the STATSGO and SSURGO databases for a 64000-ha watershed, STATSGO grossly under-represented critical riparian features. We conclude that the SSURGO database can provide modelers and managers with important insights into riparian zone nitrogen removal potential.     

Use of beryllium-7 to document soil redistribution following forest harvest operations

Rapid and reliable methods for documenting soil erosion associated with forest harvest operations are needed to support the development of best management practices for soil and water conservation. To address this need, the potential for using 7Be measurements to estimate patterns and amounts of soil redistribution associated with individual post-harvest events was explored. The 7Be technique, which was originally developed for use on agricultural land, was employed to estimate soil redistribution associated with a period of heavy rainfall within a harvested forest area located in the Lake Region of Chile (39 degrees 44'7' S, 73 degrees 10'39' W; 22% slope; and mean annual rainfall 2300 mm yr(-1)). The results provided by the 7Be technique were validated against direct measurements of soil gain or loss during the same period obtained using erosion pins. The information produced by the two approaches was similar. The results of this study demonstrate the potential for using 7Be measurements to document event-based erosion in recently harvested forest areas.     

Fertilizer use efficiency and nitrate leaching in a tropical sandy soil

Maize (Zea mays L.) production in the smallholder farming areas of Zimbabwe is based on both organic and mineral nutrient sources. A study was conducted to determine the effect of composted cattle manure, mineral N fertilizer, and their combinations on NO3 concentrations in leachate leaving the root zone and to establish N fertilization rates that minimize leaching. Maize was grown for three seasons (1996-1997, 1997-1998, and 1998-1999) in field lysimeters repacked with a coarse-grained sandy soil (Typic Kandiustalf). Leachate volumes ranged from 480 to 509 mm yr(-1) (1395 mm rainfall) in 1996-1997, 296 to 335 mm yr(-1) (840 mm rainfall) in 1997-1998, and 606 to 635 mm yr(-1) (1387 mm rainfall) in 1998-1999. Mineral N fertilizer, especially the high rate (120 kg N ha(-1)), and manure plus mineral N fertilizer combinations resulted in high NO3 leachate concentrations (up to 34 mg N L(-1)) and NO3 losses (up to 56 kg N ha(-1) yr(-1)) in 1996-1997, which represent both environmental and economic concerns. Although the leaching losses were relatively small in the other seasons, they are still of great significance in African smallholder farming where fertilizer is unaffordable for most farmers. Nitrate leaching from sole manure treatments was relatively low (average of less than 20 kg N ha(-1) yr(-1)), whereas the crop uptake efficiency of mineral N fertilizer was enhanced by up to 26% when manure and mineral N fertilizer were applied in combination. The low manure (12.5 Mg ha(-1)) plus 60 kg N ha(-1) fertilizer treatment was best in terms of maintaining dry matter yield and minimizing N leaching losses.     

Fertilizer management effects on nitrate leaching and indirect nitrous oxide emissions in irrigated potato production

Potato ( L.) is a N-intensive crop, with high potential for nitrate (NO) leaching, which can contribute to both water contamination and indirect nitrous oxide (NO) emissions. Two approaches that have been considered for reducing N losses include conventional split application (CSA) of soluble fertilizers and single application of polymer-coated urea (PCU). The objectives of this study were to: (i) compare NO leaching using CSA and two PCUs (PCU-1 and PCU-2), which differed in their polymer formulations, and (ii) use measured NO leaching rates and published emissions factors to estimate indirect NO emissions. Averaged over three growing seasons (2007-2009), NO leaching rates were not significantly different among the three fertilizer treatments. Using previously reported direct NO emissions data from the same experiment, total direct plus indirect growing season NO emissions with PCU-1 were estimated to be 30 to 40% less than with CSA. However, PCU-1 also resulted in greater residual soil N after harvest in 2007 and greater soil-water NO in the spring following the 2008 growing season. These results provide evidence that single PCU applications for irrigated potato production do not increase growing season NO leaching compared with multiple split applications of soluble fertilizers, but have the potential to increase N losses after the growing season and into the following year. Estimates of indirect NO emissions ranged from 0.8 to 64% of direct emissions, depending on what value was assumed for the emission factor describing off-site conversion of NO to NO. Thus, our results also demonstrate how more robust models are needed to account for off-site conversion of NO to NO, since current emission factor models have an enormous degree of uncertainty.     

Recycling soil nitrate nitrogen by amending agricultural lands with oily food waste

With current agricultural practices the amounts of fertilizer N applied are frequently more than the amounts removed by the crop. Excessive N application may result in short-term accumulation of nitrate nitrogen (NO3-N) in soil, which can easily be leached from the root zone and into the ground water. A management practice suggested for conserving accumulated NO3-N is the application of oily food waste (FOG; fat + oil + greases) to agricultural soils. A two-year field study (1995-1996 and 1996-1997) was conducted at Elora Research Center (43 degrees 38' N, 80 degrees W; 346 m above mean sea level), University of Guelph, Ontario, Canada to determine the effect of FOG application in fall and spring on soil NO3-N contents and apparent N immobilization-mineralization of soil N in the 0- to 60-cm soil layer. The experiment was planned under a randomized complete block design with four replications. An unamended control and a reference treatment [winter wheat (Triticum aestivum L.) cover crop] were included in the experiment to compare the effects of fall and spring treatment of oily food waste on soil NO3-N contents and apparent N immobilization-mineralization. Oily food waste application at 10 Mg ha(-1) in the fall decreased soil NO3-N by immobilization and conserved 47 to 56 kg NO3-N ha(-1), which would otherwise be subject to leaching. Nitrogen immobilized due to FOG application in the fall was subsequently remineralized by the time of fertilizer N sidedress, whereas no net mineralization was observed in spring-amended plots at the same time.     

SOIL-SOILN simulations of water drainage and nitrate nitrogen transport from soil core lysimeters

Water resources protection from nitrate nitrogen (NO3-N) contamination is an important public concern and a major national environmental issue. The abilities of the SOIL-SOILN model to simulate water drainage and nitrate N fluxes from orchardgrass (Dactylis glomerata L.) were evaluated using data from a 3-yr field experiment. The soil is classified as a Hagerstown silt loam soil (fine, mixed, semiactive, mesic Typic Hapludalf). Nitrate losses below the 1-m depth from N-fertilized grazed orchardgrass were measured with intact soil core lysimeters. Five N-fertilizer treatments consisted of a control, urine application in the spring, urine application in the summer, urine application in the fall, and feces application in the summer. The SOIL-SOILN models were evaluated using water drainage and nitrate flux data for 1993-1994, 1994-1995, and 1995-1996. The N rate constants from a similar experiment with inorganic fertilizer and manure treatments under corn (Zea mays L.) were used to evaluate the SOILN model under orchardgrass sod. Results indicated that the SOIL model accurately simulated water drainage for all three years. The SOILN model adequately predicted nitrate losses for three urine treatments in each year and a control treatment in 1994-1995. However, it failed to produce accurate simulations for two control treatments in 1993-1994 and 1995-1996, and feces treatments in all three years. The inaccuracy in the simulation results for the control and feces treatments seems to be related to an inadequate modeling of N transformation processes. In general, the results demonstrate the potential of the SOILN model to predict NO3-N fluxes under pasture conditions using N transformation rate constants determined through the calibration process from corn fields on similar soils.     

Influence of sugarcane bagasse-derived biochar application on nitrate leaching in calcaric dark red soil

Application of biochar has been suggested to improve water- and fertilizer-retaining capacity of agricultural soil. The objective of this study was to evaluate the effects of bagasse charcoal (sugarcane [ L.] bagasse-derived biochar) on nitrate (NO) leaching from Shimajiri Maji soil, which has low water- and fertilizer-retaining capacity. The nitrate adsorption properties of bagasse charcoal formed at five pyrolysis temperatures (400-800° C) were investigated to select the most suitable bagasse charcoal for NO adsorption. Nitrate was able to adsorb onto the bagasse charcoal formed at pyrolysis temperatures of 700 to 800° C. Nitrate adsorption by bagasse charcoal (formed at 800° C) that passed through a 2-mm sieve was in a state of nonequilibrium even at 20 h after the addition of 20 mg N L KNO solution. Measurements suggested that the saturated and unsaturated hydraulic conductivity of bagasse charcoal (800° C)-amended soils are affected by changes in soil tortuosity and porosity and the presence of meso- and micropores in the bagasse charcoal, which did not contribute to soil water transfer. In NO leaching studies using bagasse charcoal (800° C)-amended soils with different charcoal contents (0-10% [w/w]), the maximum concentration of NO in effluents from bagasse charcoal-amended soil columns was approximately 5% less than that from a nonamended soil column because of NO adsorption by bagasse charcoal (800° C). We conclude that application of bagasse charcoal (800°C) to the soil will increase the residence time of NO in the root zone of crops and provide greater opportunity for crops to absorb NO.     

Soil nitrate nitrogen dynamics after biosolids application in a tobosagrass desert grassland

Dormant-season application of biosolids increases desert grass production more than growing season application in the first growing season after application. Differential patterns of NO3-N (plant available N) release following seasonal biosolids application may explain this response. Experiments were conducted to determine soil nitrate nitrogen dynamics following application of biosolids during two seasons in a tobosagrass [Hilaria mutica (Buckl.) Benth.] Chihuahuan Desert grassland. Biosolids were applied either in the dormant (early April) or growing (early July) season at 0, 18, or 34 dry Mg ha(-1). A polyester-nylon mulch was also applied to serve as a control that approximated the same physical effects on the soil surface as the biosolids but without any chemical effects. Supplemental irrigation was applied to half of the plots. Soil NO3-N was measured at two depths (0-5 and 5-15 cm) underneath biosolids (or mulch) and in interspace positions relative to surface location of biosolids (or mulch). Dormant-season biosolids application significantly increased soil NO3-N during the first growing season, and also increased soil NO3-N throughout the first growing season compared to growing-season biosolids application in a year of higher-than-average spring precipitation. In a year of lower-than-average spring precipitation, season of application did not affect soil NO3-N. Soil NO3-N was higher at both biosolids rates for both seasons of application than in the control treatment. Biosolids increased soil NO3-N compared to the inert mulch. Irrigation did not significantly affect soil NO3-N. Soil NO3-N was not significantly different underneath biosolids and in interspace positions. Surface soil NO3-N was higher during the first year of biosolids application, and subsurface soil NO3-N increased during the second year. Results showed that biosolids rate and season of application affected soil NO3-N measured during the growing season. Under dry spring-normal summer precipitation conditions, season of application did not affect soil NO3-N; in contrast, dormant season application increased soil NO3-N more than growing season application under wet spring-dry summer conditions.     

Fate of nitrate and bromide in an unsaturated zone of a sandy soil under citrus production

Understanding water and nutrient transport through the soil profile is important for efficient irrigation and nutrient management to minimize excess nutrient leaching below the rootzone. We applied four rates of N (28, 56, 84, and 112 kg N ha(-1); equivalent to one-fourth of annual N rates being evaluated in this study for bearing citrus trees), and 80 kg Br- ha(-1) to a sandy Entisol with >25-yr-old citrus trees to (i) determine the temporal changes in NO3-N and Br- distribution down the soil profile (2.4 m), and (ii) evaluate the measured concentrations of NO3-N and Br- at various depths with those predicted by the Leaching Estimation and Chemistry Model (LEACHM). Nitrate N and Br concentrations approached the background levels by 42 and 214 d, respectively. Model-predicted volumetric water content and concentrations of NO3-N and Br- at various depths within the entire soil profile were very close to measured values. The LEACHM data showed that 21 to 36% of applied fertilizer N leached below the root zone, while tree uptake accounted for 40 to 53%. Results of this study enhance our understanding of N dynamics in these sandy soils, and provide better evaluation of N and irrigation management to improve uptake efficiency, reduce N losses, and minimize the risk of ground water nitrate contamination from soils highly vulnerable to nutrient leaching.     

Modeling liquid distribution in soil after pocket injection

Information on liquid distribution after injection of liquid manure is important to the evaluation of injection methods and the design of injection tools. A two-dimensional numerical model was developed to predict liquid distribution in the soil around a soil pocket. Model outputs are the lateral spread (L), vertical thickness (T), and cross-sectional area (A) of the liquid infiltration zone. Values of L do not vary with injection depth, whereas T and A increase with the injection depth. These model outputs are sensitive to effective saturated soil content (the difference between saturated and initial water contents). At greater effective saturated water content, values of L, T, and A are lower. Values of L vary from 0.13 to 0.23 m, values ofT vary from 0.09 to 0.19 m for injection depths from 0.05 to 0.15 m, and values of A vary from 0.009 to 0.030 m2. The model results were compared with field measurements taken from six forage fields with sandy loam and clay soils. The model better predicts L than T and A. The relative average error between the predictions and field measurements vary from 10.3 to 101.9%.     

Comparison of triticonazole dissipation after seed or soil treatment

This study examined the long-term fate of the fungicide triticonazole (TTZ; 5-[(4-chlorophenyl)methylene]-2,2-dimethyl-1-(1H-1,2,4-triazol-1-ylmethyl)cydopentanol) applied at a normal field dose (8.9 g ha(-1)) via seed treatment, which is the normal alternative in practice. The TTZ was applied to wheat (Triticum aestivum L.) grains as a disinfectant before sowing or spraying on bare soil for comparison and reference to the seed treatment. The seeds were germinated and grown in pots in a greenhouse at 22 +/- 3 degrees C. The dissipation of TTZ was studied by gas chromatography-mass spectrometry (GC-MS) analysis of the residues every fourth week until no TTZ could be detected. The recovery for analysis of TTZ in soil was between 98 and 131%, and the quantification level was 0.002 mg kg(-1). After 56 d of incubation, 20 and 28% of the TTZ applied remained in the soil and seed treatments, respectively, with corresponding half-lives of 27 and 29 d. The microbial biomass initially decreased in the soil treatment but had recovered after 56 d. The active part of the biomass was not changed during the experimental time. Thus, with respect to dissipation of TTZ and its effect on the soil microbial biomass and activity, no long-lasting difference between soil and seed treatments could be found.     

Nitrogen- vs. phosphorus-based dairy manure applications to field crops: nitrate and phosphorus leaching and soil phosphorus accumulation

Management of animal manures to provide nutrients for crop growth has generally been based on crop N needs. However, because manures have a lower N/P ratio than most harvested crops, N-based manure management often oversupplies the crop-soil system with P, which can be lost into the environment and contribute to eutrophication of water bodies. We examined the effects of N- vs. P-based manure applications on N and P uptake by alfalfa (Medicago sativa L.), corn (Zea mays L.) for silage, and orchardgrass (Dactylis glomerata L.), leaching below the root zone, and accumulation of P in soil. Treatments included N- and P-based manure rates, with no nutrient input controls and inorganically fertilized plots for comparison. Nitrate concentrations in leachate from inorganic fertilizer or manure treatments averaged 14 mg NO(3)-N L(-1), and did not differ by nutrient treatment. Average annual total P losses in leachate did not exceed 1 kg ha(-1). In the top 5 cm of soil in plots receiving the N-based manure treatment, soil test P increased by 47%, from 85 to 125 mg kg(-1). Nitrogen- and P-based manure applications did not differ in ability to supply nutrients for crop growth, or in losses of nitrate and total P in leachate. However, the N-based manure led to significantly greater accumulation of soil test P in the surface 5 cm of soil. Surface soil P accumulation has implications for increased risk of off-field P movement.     

Trace element changes in soil after long-term cattle manure applications

Manure application supplies plant nutrients, but also leads to trace element accumulation in soil. This study investigated total and EDTA-extractable B, Cd, Co, Cu and Zn in soil after 25 annual manure applications. The residual effect of 14 annual manure applications followed by 11 yr with no applications was also investigated. Manure was applied at 0, 30, 60 and 90 Mg ha(-1) yr(-1) (wet weight) under rainfed (treatments Mr0, Mr30, Mr60, and Mr90) and at 0, 60, 120 and 180 Mg ha(-1) yr(-1) under irrigated conditions (Mi0, Mi60, Mi120, and Mi180). The manure applications had no significant effect on soil B, Cd and Co content under both rainfed and irrigated conditions, but significantly increased total Cu and Zn content under irrigated conditions with Zn in Mi120 and Mi180 reaching the lower maximum concentration (MAC) level set by the European Community. Manure application also significantly increased EDTA-extractable Cd and Zn content in soil. Up to 27% of the total Cd (0.156 mg kg(-1)) and 21% of total Zn (38 mg kg(-1)) are found in EDTA-extractable form (Mi180 at 0-15 cm). EDTA-extractable Cd and Zn content was also significantly elevated in the irrigated residual plots due to the higher manure rates used. Thus, the impacts of cattle manure application on trace elements in soil are long lasting. Elevated Cd and Zn are a concern as other studies have linked them with certain types of cancers and human illnesses.     

Bioremediation of residual fertilizer nitrate: II. Soil redox potential and soluble iron as indicators of soil health during treatment

The prospect of using wastewater containing high loads of soluble organic matter (OM) for removing residual agricultural chemicals (fertilizer, pesticide, or herbicide) in farm soil, although promising, could have adverse effects on soil agricultural quality as a result of development of redoximorphic features in the soil profile. In this study, the effect of organic carbon supplement for bioremediation of residual fertilizer nitrate on soil properties, redox potential (Eh), pH, and metal ion mobilization was studied using sandy soils packed in columns. The study was included in a general project, described elsewhere (Ugwuegbu et al., 2000), undertaken to evaluate use of controlled water table management (WTM) systems to supply organic carbon for creating a reduced environment conducive to denitrification of residual fertilizer nitrate leaching from the farm to subsurface water. The columns were subjected to subirrigation with water containing soluble organic carbon in the form of glucose. The work was carried out in two experimental setups and the long-term effect of a range of glucose concentrations on the Eh, pH, and soluble levels of Fe and Mn was investigated. From the results obtained, it could be concluded that excessive organic carbon supplement to soil can have adverse effects on soil quality and that Eh and soluble Fe are the two most practical parameters for monitoring soil health during treatment of farm chemicals.     

Response of a stream in disequilibrium to timber harvest

Timber was harvested on South Fork of Thomas Creek, White Mountains of Arizona, USA, for the first time in 1978–1979. This caused significant increases in annual flow volumes and annual instantaneous peak flows. North Fork remained untouched, but both streams were in disequilibrium before harvest time. Due to wetter years during the postharvest period, North Fork also experienced some flow increases, but the difference was not significant. Flow increases cause increased erosion in disequilibrium channels. While in South Fork channel cross sections enlarged by 10% since preharvest time, those in North Fork enlarged by only 2.5%. The number of knickpoints tripled in South Fork, which was about double that in North Fork. Knickpoint development resulted in destruction of the natural control structures (log steps and transverse gravel bars) in South Fork (47%), while in North Fork they increased by 23%. Knickpoints are scarps on the channel bed that have the appearance of gully headcuts. The tripling of the number of knickpoints signifies that adjustment processes of the bed profile are intensified drastically in South Fork. The geomorphic changes signify that due to increases in discharge, the extent of disequilibrium is exacerbated in South Fork. Yet, volumes of erosion are relatively small, as will be sediment volumes leaving the watershed at a given time, because of the stepwise sediment transport occurring in this ephemeral stream.     

Using the late spring nitrate test to reduce nitrate loss within a watershed

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.