Denitrification Rates In Groundwater Research Articles

Importance of different factors for modeling nitrate transport and retention in a tile-drained agricultural catchment with distance-based generalized sensitivity analysis

Modeling of nitrate transport and retention in agricultural land use areas provides useful information to support water quality assessment and management. The accuracy and precision of model simulations are highly dependent on model input factors for which the appropriate values are generally difficult to determine and from which various uncertainties are induced into the modeling procedure. In this study, we applied a Distance-based Generalized Sensitivity Analysis (DGSA) to a high-resolution (25 × 25 m) nitrate transport and retention model for a tile-drained agricultural catchment (4.4 km2) to investigate the extent to which model input factors affect the spatially distributed nitrate retention. The input factors included the nitrate leaching from the root zone, the partitioning of nitrate into tile drainage and groundwater flux, the groundwater flux out of the catchment, the hydrogeological properties, and the denitrification rates in groundwater. The DGSA results were examined in both spatially lumped and distributed perspective. We found that the partitioning of nitrate into tile drainage and groundwater flux was the most important factor for modeling nitrate retention while the hydrogeological properties were secondary but also important. Conversely, the nitrate leaching from the root zone and denitrification rates in groundwater were noninfluential. By increasing the resolution of the DGSA analysis from catchment to model pixel, we found that input factors noninfluential on catchment scale were influential on pixel scale in discrete areas, and, as a general take-home-message, input factors influential on nitrate retention in at least 25 % of the model pixels were sensitive on catchment scale as well. Improved understanding of sensitivity of modelling nitrate retention may help the modelers and water managers to decide which input factors to prioritize in the modelling and data collection to improve the accuracy and precision of the model responses.

Reference ion method: A simple and fast method for quantitatively identifying the source of nitrate and denitrification rate in groundwater

The effective control and management of nitrate (NO3−) pollution requires the identification of the sources of NO3− pollution in groundwater and quantification of their contribution rates. In this study, the molar concentration ratio of NO3−/Cl− (n(NO3−)/n(Cl−)) and the molar concentration of Cl− (n(Cl−)) (reference ion method; RIM) was first used to identify the NO3− sources and estimate their contribution rates in groundwater. The relationship between the Cl− concentration and NO3− concentration (reference ion method; RIM) was used to judge whether denitrification had occurred and to estimate the denitrification rate in groundwater. It was proved that homology analysis was the prerequisite for applying the RIM. The main NO3− sources included chemical fertilizers (CF), sewage/manure (M&S) and soil nitrogen (SN). The contribution rate of CF in the vegetable planting area (upstream regions) (69.12%) was significantly higher than that in the grain planting area (midstream regions and downstream regions) (14.29% and 14.29%). The difference in the contribution rates of NO3− in the grain planting area was greater than that in the vegetable planting area. The results indicated that denitrification rate in the grain planting area was higher than that in the vegetable planting area, while the temporal variations in the denitrification rate in the vegetable planting area were consistent with in the grain planting area. The RIM offers a useful and simple way to quantify the contribution rates of NO3− sources and denitrification rates in groundwater.

Regional oxygen reduction and denitrification rates in groundwater from multi-model residence time distributions, San Joaquin Valley, USA

Summary Rates of oxygen and nitrate reduction are key factors in determining the chemical evolution of groundwater. Little is known about how these rates vary and covary in regional groundwater settings, as few studies have focused on regional datasets with multiple tracers and methods of analysis that account for effects of mixed residence times on apparent reaction rates. This study provides insight into the characteristics of residence times and rates of O 2 reduction and denitrification (NO 3 − reduction) by comparing reaction rates using multi-model analytical residence time distributions (RTDs) applied to a data set of atmospheric tracers of groundwater age and geochemical data from 141 well samples in the Central Eastern San Joaquin Valley, CA. The RTD approach accounts for mixtures of residence times in a single sample to provide estimates of in-situ rates. Tracers included SF 6 , CFCs, 3 H, He from 3 H (tritiogenic He), 14 C, and terrigenic He. Parameter estimation and multi-model averaging were used to establish RTDs with lower error variances than those produced by individual RTD models. The set of multi-model RTDs was used in combination with NO 3 − and dissolved gas data to estimate zero order and first order rates of O 2 reduction and denitrification. Results indicated that O 2 reduction and denitrification rates followed approximately log-normal distributions. Rates of O 2 and NO 3 − reduction were correlated and, on an electron milliequivalent basis, denitrification rates tended to exceed O 2 reduction rates. Estimated historical NO 3 − trends were similar to historical measurements. Results show that the multi-model approach can improve estimation of age distributions, and that relatively easily measured O 2 rates can provide information about trends in denitrification rates, which are more difficult to estimate.

Shallow groundwater denitrification in riparian zones of a headwater agricultural landscape.

Riparian zones adjacent to cropped lands are effective at reducing nitrate (NO) loads to receiving water bodies primarily through plant assimilation and denitrification. Denitrification represents a permanent removal pathway and a greenhouse gas source, converting NO to inert N gas or nitrous oxide (NO), and has been the subject of many studies in agricultural landscapes. Despite the prevailing notion that riparian zones can be areas of enhanced denitrification, there is a lack of in situ denitrification measurements from these areas that buffer streams and rivers from NO originating in upland cropped soils, especially over time scales that capture seasonal dynamics. We measured in situ groundwater denitrification rates in two riparian zones of an intensive dairy farm located in the headwaters of the Susquehanna River. Denitrification rates determined monthly over a 1-yr period with the N-NO push-pull method ranged from 0 to 4177 μg N kg soil d (mean, 830 ± 193 μg N kg soil d). Denitrification showed a distinct seasonal pattern, with highest rates observed in the spring and summer, concomitant with warmer temperatures and decreasing dissolved oxygen. We estimate an annual N loss of 470 ± 116 kg yr ha of riparian zone via denitrification in the shallow saturated zone, with the potential for >20% of this amount occurring as NO. Total denitrification from shallow groundwater in the riparian zone was equivalent to 32% of manure N spread on the adjacent upland field, confirming the importance of riparian zones in agricultural landscapes at controlling N loads entering downstream waters.

Quantification of In Situ Denitrification Rates in Groundwater Below an Arable and a Grassland System

Understanding denitrification rates in groundwater ecosystems can help predict where agricultural reactive nitrogen (N) contributes to environmental degradation. In situ groundwater denitrification rates were determined in subsoil, at the bedrock interface and in bedrock at two sites, grassland and arable, using an in situ ‘push–pull’ method with 15N-labelled nitrate (NO3 −–N). Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N kg−1 day−1. Exceptionally high denitrification rates observed at the bedrock interface at grassland site (470 ± 152 μg N kg−1 day−1; SE, standard error) suggest that deep groundwater can serve as substantial hotspots for NO3 −–N removal. However, denitrification rates at the other locations were low and may not substantially reduce NO3 −–N delivery to surface waters. Denitrification rates were negatively correlated with ambient dissolved oxygen, redox potential, k s and NO3 − (all p values, p < 0.01) and positively correlated with SO4 2− (p < 0.05). Higher mean N2O/(N2O + N2) ratios at an arable (0.28) site than the grassland (0.10) revealed that the arable site has higher potential to indirect N2O emissions. Identification of areas with high and low denitrification and related site parameters can be a tool to manage agricultural N to safeguard the environment.

Integrating principles of nitrogen dynamics in a method to estimate leachable nitrogen under agricultural systems

Surplus nitrogen (N) in ground and surface water is of concern in intensive agricultural regions. Surplus N leaches during lengthy periods where annual crop systems are used in temperate regions. This paper presents a model to estimate the surplus N available for leaching to ground water beneath agricultural systems and applies the model to watersheds in an intensive maize and soybean production system. The model utilizes commonly available georeferenced data on soils, crops, and livestock, making it applicable to watersheds in many regions. The model links stocks of N in soil, crops, livestock, fertilizer and the atmosphere. Nitrogen flow centers on exchange between the soil N stocks. Nitrogen mineralization rates are defined for three soil organic matter pools, crop residue, and manure based on carbon:N ratios. Nitrogen exports from the system are harvested crops, livestock and losses to the atmosphere. Application of the model in 26 Iowa watersheds finds surpluses of 18 to 43 kg-N/ha. Surpluses exceeded measured annual nitrate-N loads in regional streams by amounts equivalent to denitrification rates in groundwater. Deficits in soil N were sufficiently small to suggest that the system is in equilibrium with soils of the region.

Shallow Groundwater Denitrification Rate Measurement by Acetylene Block

AbstractWe field tested a short term, in situ method for measuring highly variable shallow groundwater denitrification rates using a methodology developed in an earlier, laboratory phase of this study. A pulse of tracer solution was injected and subsequently sampled within well clusters 50 cm in radius. The injectate contained dissolved acetylene (C2H2) to block denitrification at the nitrous oxide (N2O) stage of reduction. Six well clusters were established at two depths along a riparian area transect through field, grass, and woodland ecotypes. Bromide tracer, C2H2, and accumulated N2O were monitored at sample wells downgradient from each injection well. Adequate C2H2 concentrations (0.43 mM) to block N2O reduction between injection and sample wells were attained in five of the six well clusters tested. Acetylene peak concentrations moved laterally an average of 19% more slowly than the Br peak, N2O peak concentrations an average of 8% after C2H2 peaks. Highest denitrification rates of 2.8 and 3.3 mg N L−1 d−1 were observed near the surface of the water table within the grass buffer and riparian forest. Shallow groundwater denitrification rates in the adjoining field and 2 m deep into the saturated zone at all three ecotypes were &lt;25% of the highest rates. Acetylene block procedures were thus used to determine in situ denitrification rates within a few days time with little disruption of soil structure or associated microbial communities. Portable equipment allows application at remote sites independent of line power and road access.

Microbial Nitrate Processing in Shallow Groundwater in a Riparian Forest

We measured denitrification, immobilization, and respiration in microcosms that simulated groundwater conditions in a riparian forest in Rhode Island. Measured rates were compared with rates of NO−3 removal measured in a companion study using a groundwater monitoring well network and 10-mo injection of NO−3 and a bromide tracer to groundwater in the same riparian forest. Limiting factors for denitrification were assessed, and microbial and root biomass and potential net N mineralization and nitrification were compared in aquifer material exposed to the 10 mo of NO−3 dosing and unexposed control material. While there was significant variation in water table levels, dissolved oxygen, dissolved organic C, and total C within the aquifer in the riparian forest, there was little spatial variation in denitrification or respiration rates. Rates were higher in summer than in winter and were nearly always limited by a lack of organic C. Limited evidence suggested that immobilization was not a significant sink for NO−3. Measured groundwater denitrification rates were high enough to eliminate 27% of an incoming NO−3 concentration of 10 mg N L−1 over a riparian zone width of 60 m, with a travel time of 1600 d, and equaled 6.0 kg N ha−1 yr−1. The measured denitrification rates were much lower than rates of groundwater NO−3 removal directly measured in the companion hydrologic study at the same time (120 kg N ha−1 yr−1), suggesting that there is still considerable uncertainty about the mechanisms of NO−3 removal from groundwater in riparian forests.