Nitrate Movement Research Articles

Denitrification Enzyme Activity as an Indicator of Nitrate Movement through a Diversion Wetland

The Davis Pond freshwater diversion is intended to help restore Louisiana's coastal wetlands by reintroducing Mississippi River water to Barataria Basin. We hypothesized that the high NO3− concentration (2.0 mg NO3–N L−1) of the Mississippi River water would control the rate of denitrification in the receiving marsh given that the soils are saturated, anaerobic, and contain high C. Therefore, areas of high denitrification enzyme activity (DEA) in the marsh would represent soils exposed to river NO3− and actively involved in denitrification. Data from 88 soil samples (0–10 cm) collected throughout the marsh revealed significantly higher rates of DEA in a 715‐ha area adjacent to the diversion inflow. This area of generally high DEA contained >80% of all DEA observed while representing only 19% of the total marsh area at the low discharge rate of 39.5 m3 s−1 The area of high DEA coincided with the highest surface water NO3− and indicated that the marsh has a greater aerial capacity for NO3− removal than is utilized. A laboratory experiment suggested that soils loaded with external NO3− typically had higher DEA rates than soils receiving no added NO3− The DEA was strongly dependent on soil depth (92% of DEA occurred at 0–5 cm) and internal N cycling was substantial in this wetland soils. This study demonstrates the applicability of using soil DEA to map where denitrification activity is greatest, the aerial extent of soils involved in denitrification, and the general flow path of introduced nutrients in large wetlands where NO3− is the limiting factor for denitrification.

Manipulação de carga e movimento de nitrato em horizontes B de um Latossolo Vermelho

O objetivo deste trabalho foi avaliar a influência do pH e da adição de carbonatos, fosfatos e sulfatos na movimentação do nitrato, em colunas constituídas do horizonte subsuperficial de um Latossolo Vermelho Acriférrico. Os experimentos foram realizados de acordo com a técnica do deslocamento miscível. Utilizaram-se como pulso de nitrato soluções de KNO3 a 10 ou 40 mmol L-1. Em cada coluna, foram obtidas curvas de transposição do elemento. A carga líquida do solo nos diferentes tratamentos variou de - 24,0 a +35,0 mmol c L-1. Nos tratamentos que receberam adição de ácido clorídrico, ácido clorídrico + fosfato, carbonato + fosfato e sulfato, as recuperações de nitrato foram maiores do que o total aplicado no pulso. As adições de carbonatos, fosfato e sulfato favoreceram a lixiviação de nitrato, e deslocaram as curvas de transposição para a esquerda. A acidificação do solo retardou o deslocamento do nitrato, e deslocaram as curvas de transposição para a direita. A movimentação de nitrato no solo foi mais influenciada pelo pH do meio, pela presença de carbonatos, fosfatos e sulfatos, do que pela força iônica do pulso.

Movimentação de nitrato em horizonte superficial e subsuperficial de Latossolo e Nitossolo com cargas variáveis

O objetivo deste trabalho foi avaliar a movimentação de nitrato em colunas de solo considerando-se, principalmente, a quantidade total recuperada, o ajuste entre as curvas de transposição experimental e predita por um modelo matemático e os fatores de retardamento obtidos pelo modelo. Foram utilizadas amostras dos horizontes A e B de um Latossolo Vermelho acriférrico e de um Nitossolo Vermelho eutrófico (LV-A, LV-B, NV-A e NV-B). O experimento de lixiviação foi conduzido tendo em vista a teoria do deslocamento miscível, utilizando como pulso 100 mL de KNO3 10 mmol L-1. As quantidades lixiviadas de nitrato e recuperadas nas colunas, para os solos NV e LV nos horizontes A e B, variaram de 0,405 a 1,432 mmol L-1. Houve correlação significativa (p<0,05) entre as curvas experimentais e as ajustadas pelo modelo para as colunas NV-A, NV-B e LV-B. O modelo matemático utilizado é adequado para prever a lixiviação de nitrato em colunas do horizonte B dos solos Nitossolo Vermelho Eutrófico e Latossolo Vermelho Acriférrico. Nas colunas dos solos com bom ajuste entre dados experimentais e preditos, o fator de retardamento é maior do que 1, o que indica que a movimentação do nitrato é retardada em relação ao avanço da frente de molhamento do solo.

Non‐destructive monitoring of nitrate concentration in a laboratory flow experiment using time domain reflectometry (TDR)

Nitrates, when found in excess in the groundwater, are a health hazard. Nitrates can easily be transported by groundwater movement and may eventually reach aquifers located far away from the contamination location. Recent research has focused on developing innovative techniques to remediate nitrate‐contaminated soils. It is important to monitor the nitrate movement in the groundwater to predict the extent of nitrate contamination. Traditionally, nitrate movement is monitored by sampling soil pore water and analysing it in the laboratory. In this study a non‐destructive method, using time domain reflectometry (TDR) to monitor nitrate movement in laboratory flow experiments, is presented. Three flow cells (replicates) packed with a silty loam soil were subjected to a constant hydraulic gradient inducing saturated water flow through the flow cells. A source of potassium nitrate solution, containing 500 ppm NO3‐N, was connected at the inflow end, and the nitrate concentration change with time was monitored along the length of the flow cells by sampling the soil solution and analysing it in the laboratory for NO3‐N concentration. At the time of sampling, TDR wave forms were recorded using the TDR mini‐probes inserted at regular intervals along the length of the flow cells. The bulk soil electrical conductivity and the water content values extracted from the TDR wave forms were used to predict the nitrate concentrations at different locations. The nitrate concentration values predicted from the TDR‐measured bulk electrical conductivity and water content data correlated well with the nitrate concentrations obtained by soil solution sampling method.

Use of a layered double hydroxide (LDH) to buffer nitrate in soil: long-term nitrate exchange properties under cropping and fallow conditions

The potential use of a layered double hydroxide (LDH) to act as a nitrate buffer system in soil in order to reduce the movement of nitrate was investigated. Long-term plant and soil experiments were carried out under greenhouse conditions with the following objectives: (i) evaluate the nitrate adsorption capacity of the LDH during crop growth, and its influence on N uptake, (ii) study the ability of the LDH to adsorb nitrate mineralized during fallow periods, and its influence on nitrate leaching, (iii) evaluate the reversibility for nitrate exchange of the LDH under cultivation conditions, and (iv) determine the nitrate buffer capacity of the soil after LDH application. The LDH adsorbed nitrate from the soil solution during the growth period without affecting plant N uptake. As a result of the adsorption of nitrate on the LDH, the nitrate-N concentration in the soil solution at harvest was reduced by a factor of ten compared to a soil without LDH. The LDH efficiently adsorbed nitrate that was mineralized in the soil during periods without cultivation, reduced nitrate-N leaching losses by about 80%, and kept this nitrate available for a following crop. The nitrate buffer capacity of the soil after 15months increased from 0.3 (without LDH) to 2.7 with the application of 10g LDH kg−1 soil. It is concluded that the LDH has a potential to be used as a long-term nitrate exchanger to control the movement of nitrate in soil, and thereby reduce risks of nitrate leaching in crop production in sensible areas.

Effect of Five‐Year Continuous Poultry Litter Use in Cotton Production on Major Soil Nutrients

Repeated application of poultry (Gallus gallus) litter to crop lands may lead to nitrates leaching and build up of P and other elements in the soil profile, which are prone to loss from runoff and erosion. A study was conducted for 5 yr at Belle Mina, AL on a Decatur silt loam (fine, kaolinitic thermic Rhodic Paleudult) during 1994 to 1998 to determine the nitrate movement and quantify the build up of P, K, Ca, and Mg due to the application of nitrification inhibitor, carboxymethyl pyrazole (CP), treated fresh and composted poultry litter and urea in conventionally tilled cotton (Gossypium hirsutum L.). Poultry litter maintained soil pH (0–30 cm depth) where as application of urea resulted in a pH decline. The inhibitor, CP, significantly reduced the NO3−–N formation in all N sources for 41 d following application. However, over the longer period of time, very minimal changes in nitrate concentrations were observed due to change in rates or sources of N. Over the experimental period, P concentration increased significantly (by 74%) in composted litter applied plots (17.7 mg kg−1) but not in fresh litter plots (1.5 mg kg−1). Linear increase in P accumulation was observed with increase in rate of composted litter. Concentrations of K and Mg increased significantly both in fresh (93 and 25 mg kg−1, respectively) and composted litter (127 and 36 mg kg−1, respectively) applied plots by the end of 5 yr period. These results indicate that a well‐planned application of fresh poultry litter in soils that are not already overloaded with P is safe and treating litter with CP is advantageous from an environmental perspective.

EVALUATION OF A COMBINED SOIL EC AND MOISTURE SENSOR AND ITS USE TO CO-MANAGE SOIL MOISTURE AND VINE NITROGEN IN GRAPEVINES (CV. SHIRAZ) UNDER DEFICIT IRRIGATION

ISHS V International Symposium on Irrigation of Horticultural Crops EVALUATION OF A COMBINED SOIL EC AND MOISTURE SENSOR AND ITS USE TO CO-MANAGE SOIL MOISTURE AND VINE NITROGEN IN GRAPEVINES (CV. SHIRAZ) UNDER DEFICIT IRRIGATION

Source and transport controls on the movement of nitrate to public supply wells in selected principal aquifers of the United States

In 2003–2005, systematic studies in four contrasting hydrogeologic settings were undertaken to improve understanding of source and transport controls on nitrate movement to public supply wells (PSW) in principal aquifers of the United States. Chemical, isotopic, and age tracer data show that agricultural fertilizers and urban septic leachate were the primary sources of large nitrate concentrations in PSW capture zones at Modesto, California (Central Valley aquifer system) and York, Nebraska (High Plains aquifer). Urban septic leachate and fertilizer (possibly nonfarm) were the primary sources of large nitrate concentrations in PSW capture zones at Woodbury, Connecticut (glacial aquifer system), and Tampa, Florida (Floridan aquifer system), respectively. Nitrate fluxes to the water table were larger in agricultural settings than urban settings, indicating that it would be beneficial to reduce PSW capture zone areas in agricultural regions. Mixing calculations indicate that about 50 to 85% of the nitrate in water from the PSW could be from those modern anthropogenic sources, with the remainder coming from sources in old (>50 years) recharge or sources in young recharge in undisturbed settings such as forests. Excess N2 concentrations and age tracers showed that denitrification at Modesto occurred gradually (first‐order rate constant of 0.02/a) in a thick reaction zone following a ∼30‐year lag time after recharge. Denitrification generally was not an important nitrate sink at Woodbury. At York and Tampa, denitrification occurred rapidly (0.5 to 6/a) in thin reaction zones in fine‐grained sediments that separated the anoxic PSW producing zones from overlying oxic, high‐nitrate ground water. Particle tracking showed that a major pathway by which anthropogenic nitrate reached the York and Tampa PSW was by movement through long well screens crossing multiple hydrogeologic units (York) and by movement through karst features (Tampa), processes which reduced ground water residence times in the denitrifying zones. These results illustrate how PSW vulnerability to nitrate contamination depends on complex variations and interactions between contaminant sources, reaction rates, transit times, mixing, and perturbation of ground water flow in contrasting hydrogeologic settings.

Impact of Manure Application Timing in Dairy Pastures on the Migration of Nitrates to Ground Water

Manure application increases pasture crop production and minimizes manure storage costs. Excess N not absorbed by pasture vegetation can leach past the root zone and into ground water. A yearlong study was conducted on grazing pastures at 2 dairy farms to monitor potential nitrate (NO3) movement from the surface through the soil profile toward ground water. Precipitation, irrigation, manure application, and grazing were compared with NO3 movement. Both pastures were part of rotational grazing systems and receive periodic applications of manure by cattle and by typical manure application equipment. Sampling devices were located in the center of operational fields. A total of 9 sampling wells (lysimeters) were installed at depths of 1, 1.3, and 1.6m below the surface of each pasture. Each depth was replicated 3 times at each site. Water samples were taken every 2 wk at each site and tested for NO3-N. Farm A applied a total of 600kg N/ha throughout the year and farm B applied approximately 300 kg/ha. On Farm A, NO3-N concentrations averaged 15.2 ± 12.2, 6.5 ± 6.9, and 2.9 ± 3.6ppm at 1, 1.3, and 1.6 m, respectively. On Farm B, NO3-N concentrations averaged 13.8 ± 8.5, 8.5 ± 5.7, and 1.3 ± 2.9ppm at 1, 1.3, and 1.6 m, respectively. No relationship between farming activity and NO3 movement was noted. In addition, irrigation appeared to have no influence on NO3 movement. Substantial rainfall (>15cm per 2-wk period) appears to be the major factor in NO3 movement.

Efficiency of Nitrogen Fertilizer for Potato under Fertigation Utilizing a Nitrogen Tracer Technique

Efficient crop use of nitrogen (N) fertilizer is critical from economic and environmental viewpoints, especially under irrigated conditions. Nitrogen fertilizer (15N‐labeled urea) and irrigation methods (drip and furrow) were evaluated on spring and fall potato cultivars under Syrian Mediterranean climatic conditions. Field experiments were conducted in the El‐Ghab Valley near Hama in fall 2000 and spring 2001 on a heavy clay soil. Four N‐fertilizer applications (70, 140, 210, and 280 kg N/ha) were applied in five equally split treatments for both irrigation methods. Potato was irrigated when soil moisture in the specified active root depth reached 80% of the field capacity as indicated by the neutron probe. Higher marketable tuber yield of spring potato was obtained by fertigation compared to furrow irrigation; the magnitude of tuber yield increases was 4, 2, 31, and 13%, whereas for fall potato the tuber yield increases were 13, 27, 20, and 35% for N fertilizer rates of 70, 140, 210, and 280 kg N/ha, respectively. Shoot dry matter and tuber yields at the bulking stage were not good parameters to estimate marketable tuber yield. The effect of N treatments on potato yield with furrow irrigation and fertigation was limited and not significant. Drip fertigation improved tuber yield of fall potato relative to national average yield. Nitrogen uptake increased with increasing N input under both irrigation methods. Reducing N input under both irrigation methods improved N recoveries. Increasing N input significantly increased total N content in plant tissues at the bulking stage. Spring potato yields were almost double those of fall potato under both irrigation methods and all N treatments. Nitrate (NO3) movement in the soil solution for fall potato was monitored using soil solution extractors. Furrow irrigation resulted in greater movements of NO3‐N below the rooting zone than drip fertigation. Harvest index did not follow a clear trend but tended to decrease upon increasing N fertilization rates beyond 140 kg N/ha under both irrigation methods. Drip fertigation improved field water‐use efficiencies at the bulking and harvest stages. Fertigation increased specific gravity of potato tubers relative to furrow irrigation. Higher N input decreased specific gravity of potato tubers under both irrigation methods.

Field data and modelling of water and nitrate movement through deep unsaturated loess

The load and transfer time of nitrogen within the unsaturated zone must be determined to predict the evolution of nitrate concentration in groundwater. The main objectives of the study were to characterize the transfer of nitrate in deep unsaturated loess and to evaluate the capacity of the model Agriflux, coupled with a convective transfer below the first meter of soil, to simulate this transfer. Grain size, gravimetric water content, and tritium and nitrate in the pore water of solids from the unsaturated zone were determined for three profiles (15–17 m deep) collected in an agricultural plot for which detailed data on land uses and fertilization practices were available. All these data were used to identify hydrodynamic process in the unsaturated zone and to provide inputs for the modelling exercise. The transfer appeared governed by convection–dispersion processes with an estimated average transfer rate of around 20–26 cm per year that appeared very homogeneous at the plot scale. The model, received limited calibration of some nitrogen cycle parameters (humus mineralization constant, fraction of litter that passes to humus) and adjustment of some input data relative to fertilization. It subsequently generated in the vertical profiles tritium peaks that were less than 100 cm off the H3 peak observed between 8 and 9 m below soil level (bsl), and nitrate peaks less than 50 cm off the NO3 peaks observed at 2.5 and 5 m bsl. The maximum model values for NO3 are within 10% of the measured values. Furthermore, the model helped identify factors influencing nitrate concentrations along the profiles. In the hydrodynamic context of loess soils, the modelling approach used in the study is a very useful tool to evaluate impacts of variable agricultural practices on nitrate concentrations in the unsaturated zone.

Combined Effects of Rainfall Patterns and Porous Media Properties on Nitrate Leaching

Laboratory experiments were conducted to investigate the effects of water input properties on the movement of nitrate in three different porous media. A series of artificial rainfall amounts from 4.0 to 26 mm were applied every 24 h in 10 min durations over a weighing lysimeter packed with either fine sand, field soil over fine sand, or fine sand with artificial macropores. Movements of nitrate, chloride, and water in the lysimeter were monitored. Significant nitrate leaching occurred with pistonlike flow caused by the highest applications in all porous media. The nitrate movement coincided with that of chloride in a homogeneous sand medium. In a layered medium with soil cover over sand, nitrate/chloride ratios in the field soil decreased with time, suggesting the presence of degradation processes. Some parts of the soil may be serving as spots for denitrification. High nitrate concentration remained in the topsoil layer under light to intermediate rainfall applications in the two‐layered medium, causing large vertical spreading of nitrate under intermediate rains by which infiltration occurs. Double peaks in the concentration profiles were observed in the macroporous medium under 26‐mm applications, indicating that part of the nitrate was transported by preferential flow while the rest was transported by matrix flow. We estimated that heterogeneity of the porous medium resulted in large dispersion under intermediate to heavy rains and denitrification under light rains. Heterogeneity also affected pathways of leaching under heavy rainfalls. We estimated that climate change could significantly increase the chance of nitrate leaching by at least 25% in the Pacific East area of Japan through increases of heavy rain frequency and precipitation intensity and decreased time for denitrification in the surface soil.

Movement of Nitrogen and Phosphorus Downslope And Beneath a Manure and Organic Waste Composting Site

This observational study was conducted to assess the movement of nitrate and phosphorus into and through the soil profile beneath a compacted gravel compost pad. The accumulation of nitrate and phosphorus in the vegetated filter strip immediately downslope of the pad was also evaluated. Soil samples were taken from the composting site and the immediate surrounding area in two transects each for the Control (outside of compost pad), Old Pad (combined manure stack and compost area), and Extension pad (compost only area). Each transect was divided into three sampling zones: within the pad, in an intermediate area between the pad and the filter strip, and within the filter strip. Compost samples from windrows of different ages and mixes were also taken for laboratory leaching test to determine the potential of the composts as source of nitrogen and phosphorus. The NO3-N concentrations in the soil beneath the compost pad of the Old Pad and Extension transects were higher than the soil NO3-N concentrations at the same depths and locations in the control transect. These results indicate that the compacted gravel pad did not fully prevent the downward movement and accumulation of NO3-N beneath the pad. The NO3-N concentrations in the soil surface of the pad, intermediate between the pad and filter strip, and the filter strip areas of the Control and Old Pad transects were not statistically different; suggesting that there was negligible NO3-N surface movement and transport from the pad area to the filter strip. The average Mehlich3-P concentration at the soil surface in the pad area was less than in the intermediate and the filter strip areas, indicating that there was surface runoff and downslope transport of phosphorus from the compost site to the filter strip. Statistical analysis showed that there was no significant difference in the Mehlich3-P concentration between the filter strip areas of the Old Pad and Control transects; suggesting that the downslope transport of phosphorus from the compost pad to the filter strip had not yet caused significant accumulation of phosphorus in the filter strip relative to the adjacent field. Leaching tests indicated that during the composting process, mature composts pose a greater potential as a source of NO3-N leaching than the freshly-mixed composts. In contrast, the composting process and operation poses a greater potential as a source of PO4-P during the early stages of composting than with older composts.

Nitrate pollution and its distribution in the groundwater of Srikakulam district, Andhra Pradesh, India

The complex depositional pattern of clay and sand in most of the areas controlled the vertical and lateral movement of nitrate in groundwater. The variation of nitrate concentration at different groundwater levels and the lateral distribution of nitrate in the groundwater at two sites indicated the filtration of nitrate by clayey formations. A rural agricultural district located in the Vamsadhara river basin, India was selected for studying the lateral and vertical distribution of nitrate in the groundwater and the association of nitrate with other chemical constituents. The nitrate concentrations in the groundwater are observed to vary between below detectable limit and 450 mg NO3/L. The sources for nitrate are mainly point sources (poultry farms, cattleshed and leakages from septic tanks) and non-point sources (nitrogenous fertilisers). The nitrate concentrations are increased after fertiliser applications. However, very high concentrations of nitrate are derived from animal wastes. Relatively better correlations between nitrate and potassium are observed (R = 0.74 to 0.82). The better relationship between these two chemical constituents in the groundwater may be due to the release of potassium and nitrate from both point and non-point sources. The nitrate and potassium concentrations are high in the groundwater from clayey formations.

Nitrate accumulation and movement under deficit irrigation in soil receiving cattle manure and commercial fertilizer

Nitrate leaching from agricultural soils can increase groundwater nitrate concentrations. The objectives of the study were to assess the accumulation and movement of nitrate in the soil profile over a 2-year period under deficit irrigation conditions following a one time application of N in cattle feedlot manure and commercial fertilizer to corn at rates to achieve yield goals expected under conditions of full irrigation. Cattle manure and ammonium nitrate were applied in 2002 at the University of Nebraska recommended rate (1M and 1F, respectively) and cattle manure was applied at twice the recommended rate (2M) for N for the 2002 corn ( Zea mays L.) crop. The recommended rate was based on expected yields under full irrigation. The manure N treatments were applied to percolation lysimeters and adjacent plots on a Cozad silt loam soil. Ammonium nitrate was applied only to the percolation lysimeters. Leachate from the lysimeters was extracted from a depth of 2.1 m and soil samples were collected from field plots in 0.3 m depth increments to 2.1 m on a periodic basis. Water available to the crop was sufficient to meet 89 and 79% of the potential crop ET in 2002 and 2003, respectively. When averaged over the manure N treatments, reduced ET resulted in grain yields that were approximately 2.1 and 2.7 Mg ha −1 less than expected in 2002 and 2003. Under deficit water inputs there was leachate movement below the root zone. Leachate depths averaged over N treatments were, however, reduced by 15% (33 mm) in 2002 and 47% (102 mm) in 2003 compared with those reported under full irrigation. The average nitrate-N (NO 3 −-N) concentrations in leachate were higher under the 2M treatment (41 mg L −1) compared to the 1M treatment (17 mg L −1). The average NO 3 −-N concentrations in leachate from the 1F treatment (28 mg L −1) was not different than the 1 or 2M treatments. There were trends for greater NO 3 −-N mass losses in leachate averaged over all treatments in 2003 compared to 2002, indicating that NO 3 −-N derived from the 2002 application leached to at least 2.4 m below the soil surface. There were no mass loss differences in leachate due to the 2001 crop in 2002. In 2003, mass of NO 3 −-N in lysimeters cropped to soybean in 2001 were significantly higher (144 kg NO 3 −-N ha −1) than the mass in lysimeters cropped to corn in 2001 (51 kg NO 3 −-N ha −1). Nitrate-N mass increased in the 0.9–2.1 m soil depth 12–13 months after manure N treatment applications. The 2M treatment had greater soil NO 3 −-N mass than the 1M treatment for most sampling dates in the surface 0.9 m. This research shows that there can be significant nitrate losses under deficit irrigation when manure N is over applied. These losses are likely related to water initially stored below the root zone and preferential flow of water from irrigation and precipitation. Determining accurate yield expectations under deficit irrigation conditions, correct scheduling of irrigation, and the use current best management practices for N management can help minimize nitrate losses in leachate.

Towards an improved understanding of the nitrate dynamics in lowland, permeable river-systems: Applications of INCA-N

The Integrated Catchment Model of Nitrogen (INCA-N) was applied to the Lambourn and Pang river-systems to integrate current process-knowledge and available-data to test two hypotheses and thereby determine the key factors and processes controlling the movement of nitrate at the catchment-scale in lowland, permeable river-systems: (i) that the in-stream nitrate concentrations were controlled by two end-members only: groundwater and soil-water, and (ii) that the groundwater was the key store of nitrate in these river-systems. Neither hypothesis was proved true or false. Due to equifinality in the model structure and parameters at least two alternative models provided viable explanations for the observed in-stream nitrate concentrations. One model demonstrated that the seasonal-pattern in the stream-water nitrate concentrations was controlled mainly by the mixing of ground- and soil-water inputs. An alternative model demonstrated that in-stream processes were important. It is hoped further measurements of nitrate concentrations made in the catchment soil- and ground-water and in-stream may constrain the model and help determine the correct structure, though other recent studies suggest that these data may serve only to highlight the heterogeneity of the system. Thus when making model-based assessments and forecasts it is recommend that all possible models are used, and the range of forecasts compared. In this study both models suggest that cereal production contributed approximately 50% the simulated in-stream nitrate load in the two catchments, and the point-source contribution to the in-stream load was minimal.

FIELD ESTIMATION OF NITRATE LOAD FROM VEGETABLE FIELDS IN THE CARIBBEAN BASIN

The development of best management practices (BMP) and the alloca-tion of pollution among land users in a watershed (TMDL) requires an understanding of the effect of cultural practices on both yields and nutrient leaching below the root zone. `Florida 47' tomato and `La Estrella' tropical pumpkin were grown on plasticulture using combinations of UF–IFAS recommended N and irrigation rates in a 1-ha field in 2004. Average N and irrigation rates over the whole field were 100% and 80%, respectively. Nitrate movement was assessed with twenty-four 1-m-long drainage lysimeters in each plot and ten 7-m-deep wells in and around the field Lysimeters and wells were sampled every 2 and 3 weeks throughout the year, respectively. Leachate volume and concentration in the drainage lysimeters were highly variable. Except shortly after the 25-cm rain due to hurricane Jeanne, most leachate volumes were <1 L·m–1. Annual NO3-N mean treatment load ranged between 7 and 15 kg·ha–1, but these differences were not significant due to high variability (CV = 175%). Single-lysimeter annual highest load was 39 kg·ha–1 of NO3-N (17.5% of N applied). In 2004, NO3-N concentration in well water was <1, ranged between 15 and 35, ranged between 0 and 10 mg·L–1 NO3-N in the up-stream control, inside, and perimeter wells, respectively. These concentrations are in the same ranges as those observed in this field in previous years (1997–2003) and often exceeded the 10 mg/L drinking water standard. Because NO3-N discharge into the environment may occur after the growing season, BMPs should be implemented on a year round basis. The methodology used in load measurement should be improved to better account for spatial variability.

EFFECTIVE NITRATE CONTROL WITH ELECTROKINETICS IN SANDY SOIL

Nitrate contamination of surface and ground water is a problem worldwide. Nitrate in drinking water presentsa human health risk. The major source of nitrate contamination is believed to be nitrogen fertilizer from agricultural fields.Best Management Practices have been developed to guide fertilizer use and minimize nitrogen losses, but they do not addresscontrol of nitrate movement from the crop root zone. It is proposed that electrokinetics, an in-situ method, could be used tocontrol nitrate movement, retaining it near the root zone. This article reports results from a study on nitrate movement andpH changes in a vertical, partially saturated sandy soil column subjected to an electrical current. The highest measurednitrate concentration (7155 mg/L) was within 5 mm of the anode after application of a 6 h 80 mA current. The nitrateconcentration at the cathode was 1/5 of the inflow solute concentration. The pH was 11 near the cathode, 3.5 near the anode,and showed little change in intermediate layers. Studies of different nitrate concentrations and electrical current levelssuggest that 80 mA electrical current applied for 6 h duration might effectively control nitrate migration in similar sandy soil conditions.

Incorporation of Auxiliary Information in the Geostatistical Simulation of Soil Nitrate Nitrogen

In north‐central Florida the potential risk for movement of nitrate into the aquifer is high due to the large extent of well‐drained marine‐derived quartz sand overlying porous limestone material coupled with high precipitation rates. Our objective was to estimate spatio‐seasonal distributions of soil NO3–N across the Santa Fe River Watershed in north‐central Florida. We conducted spatially distributed synoptic and seasonal sampling (September 2003—wet summer/fall season, January 2004—dry winter season, May 2004—dry spring season) of soil NO3–N. Prior distributions of probability for NO3–N were inferred at each location across the watershed using ordered logistic regression. Explanatory variables included environmental spatial datasets such as land use, drainage class, and the Floridian aquifer DRASTIC index. These prior probabilities were then updated using indicator kriging, and multiple realizations of the spatial distribution of soil NO3–N were generated by sequential indicator simulation. Cross‐validation indicated that smaller prediction errors are obtained when secondary information is incorporated in the analysis and when indicator kriging is used instead of ordinary kriging to analyze these datasets characterized by the presence of extreme high values and a nonnegligible number of data below the detection limit. The NO3–N values were lowest in September 2003 as a result of excessive leaching caused by large, intense tropical storms. Overall the NO3–N values in January 2004 were high and could be attributed to fertilization of crops and pastures, low plant uptake, and low microbial transformation during the winter period. Despite seasonal trends reflected by the values of observed and estimated NO3–N, we found areas that showed consistently high soil NO3–N throughout all seasons. Those areas are prime targets to implement best management practices.

Modelling nitrate and bromide leaching from sewage sludge

A deterministic model for assessing the risk of groundwater contamination by nitrate from land-based sludge disposal was evaluated. A controlled large-lysimeter experiment was set up to monitor movement of nitrate through soil. Four large lysimeters of 900 mm length were packed with Manawatu fine sandy loam (a Dystric Fluventric Eutrochrept), on top of which 200 mm of municipal sewage sludge was applied. One of the lysimeters was planted with pasture ( Lolium perenne and Festuca arundacea), one with a willow tree ( Salix sp. ‘Tongoio’), another with a poplar tree ( Populus sp. ‘Kawa’), and one was left bare. Bromide was used as a conservative tracer. Movement of bromide and nitrate was analysed in the effluent from the base of the lysimeters. The processes of water and nutrient transport were modelled using a mechanistic scheme based on Richards’ equation for water transport and the convection–dispersion equation (CDE) for nutrient transport. These equations were both linked to a sink term for plant uptake. The model simulated well the transport of water and movement of bromide in the four different lysimeters. The agreement between measured and simulated nitrate leaching was also reasonable considering the simplified model. Uptake of nitrogen by trees reduced the quantity of nitrogen available for leaching. The model could aid development of sustainable management of land-based sewage sludge disposal in terms of nitrate leaching. The next step will be to further develop the model for heavy metal movement, as heavy metals are common co-contaminants of sewage sludge.