Soil Water NO3 Research Articles

Nitrate variability in hydrological flow paths for three mid‐Appalachian forested watersheds following a large‐scale defoliation

Nitrate (NO3−) leakage from forested watersheds due to disturbance is a well documented but not well understood process that can contribute to the degradation of receiving waters through eutrophication. Several studies have shown that large‐scale defoliation and deforestation events in small forested watersheds in the eastern United States cause immediate and dramatic increases in NO3− flux to steams, with large differences in recovery time. Water quality and discharge data collected from 1992 to 2004 following a large‐scale gypsy moth defoliation were used to investigate hydrological controls on long‐term NO3− leakage from three forested watersheds in Shenandoah National Park, Virginia. During storm events, a conventional two‐component hydrograph separation in conjunction with an inverse solution technique was employed to determine the concentrations of NO3− in groundwater and soil water. Following defoliation, groundwater NO3− concentrations declined exponentially with a distinct seasonal pattern. A rank‐order relationship between the rate constants associated with the exponential declines in groundwater NO3− concentrations and groundwater recession constants indicates a hydrological control on long‐term watershed recovery for these defoliated systems. Comparisons to deforested systems in Hubbard Brook, New Hampshire, and Coweeta, North Carolina, indicate hydrological controls are similarly present. Biogeochemical differences, however, need to be considered to account for the more attenuated recovery observed in defoliated systems. No long‐term trend was found in the model‐derived soil water NO3− concentrations, which suggests the presence of some form of rate limitation on the transformation of the nitrogen pool introduced during the disturbance and/or reduced nutrient uptake due to tree mortality.

The influence of basswood (Tilia americana) and soil chemistry on soil nitrate concentrations in a northern hardwood forest

In the Arbutus Lake Watershed in the Adirondack Mountains, New York, two nearly adjacent catchments (14 and 15) varied significantly in volume-weighted stream water nitrate (NO3) export (54 and 17 μequiv.·L–1, respectively; P < 0.001). The most notable differences between the catchments were that Catchment 14 had significantly higher soil Ca concentrations and patches of basswood ( Tilia americana L.). We evaluated the possible contributions of basswood and soil Ca concentrations to soil water NO3 concentrations. Among the major overstory tree species, basswood leaf litter had the lowest C:N ratios, highest Ca concentrations, and among the lowest lignin:N ratios. Basswood basal area was significantly related to soil water NO3 concentrations (R = 0.46, P = 0.01). Forest floor and mineral soil Ca concentrations were positively correlated with basswood basal area and negatively correlated with American beech ( Fagus grandifolia Ehrh.) basal area. Our results suggest that a relatively low-density, calciphilic species such as basswood may create, given the proper soil conditions, hotspots with elevated soil water NO3 concentrations. These hotspots result from the convergence of high soil Ca concentrations, due mostly to soil geology, with relatively labile litter substrate available for N mineralization and nitrification.

Seasonal variations of nitrous oxide emission in relation to nitrogen fertilization and energy crop types in sandy soil

Nitrous oxide (N 2O) is a greenhouse gas and agricultural soils are major sources of atmospheric N 2O. Its emissions from soils make up the largest part in the global N 2O budget. Research was carried out at the experimental fields of the Leibniz-Institute of Agricultural Engineering Potsdam-Bornim (ATB). Different types (mineral and wood ash) and levels (0, 75 and 150 kg N ha −1) of fertilization were applied to annual (rape, rye, triticale and hemp) and perennial (poplar and willow) plants every year. N 2O flux measurements were performed 4 times a week by means of gas flux chambers and an automated gas chromatograph between 2003 and 2005. Soil samples were also taken close to the corresponding measuring rings. Soil nitrate and ammonium were measured in soil extracts. N 2O emissions had a peak after N fertilization in spring, after plant harvest in summer and during the freezing–thawing periods in winter. Both fertilization and plant types significantly altered N 2O emission. The maximum N 2O emission rate detected was 1081 μg N 2O m −2 h −1 in 2004. The mean annual N 2O emissions from the annual plants were more than twofold greater than those of perennial plants (4.3 kg ha −1 vs. 1.9 kg ha −1). During January, N 2O fluxes considerably increased in all treatments due to freezing–thawing cycles. Fertilization together with annual cropping doubled the N 2O emissions compared to perennial crops indicating that N use efficiency was greater for perennial plants. Fertilizer-derived N 2O fluxes constituted about 32% (willow) to 67% (rape/rye) of total soil N 2O flux. Concurrent measurements of soil water content, NO 3 and NH 4 support the conclusion that nitrification is main source of N 2O loss from the study soils. The mean soil NO 3-N values of soils during the study for fertilized soils were 1.6 and 0.9 mg NO 3-N kg −1 for 150 and 75 kg N ha −1 fertilization, respectively. This value reduced to 0.5 mg NO 3-N kg −1 for non-fertilized soils.

Estimating the Influence of Nitrogen Transformations on Nitrate Leaching in Soils

It is challenging to estimate N leaching in soils attributable to complicated physical, chemical, and biological processes. In this study, a transfer function model was developed to simulate the outflow concentration of NO3 in the field, considering the influence of transient water flow, input of applied N, initial residual N in the soil, and main N transformations on the NO3–N leaching process. The N transformations in the model included immobilization, mineralization, volatilization, and plant uptake. In the probability density function of NO3–N, a weighting factor was introduced to quantify the leaching contributions from applied and residual N. A field experiment was conducted for 196 d during the growing seasons of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). Soil water potential and NO3 concentrations were measured during the study period along two soil profiles to a depth of 2 m: at 0.10‐m intervals from the soil surface to the 1‐m depth, and at 0.20‐m intervals from 1 to 2 m. A comparison between the experimental data and simulated results with the transfer function showed that the model provided reasonable predictions of the N leaching process as well as the total amount leached at the 2‐m depth. Results also indicated that by considering the transient water condition and N transformations, the transfer function significantly increased the estimation accuracy. Compared with the measured data, relative errors of the estimated total N leached were 1 and 20% with and without considering the transient water condition and the N transformations, respectively. The transfer function with the weighting factor can be useful to estimate the contributions from the applied and residual N to the leaching process in the field.

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO 3 − ), aluminum (Al), and calcium (Ca2+) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO 3 − concentration was about 20 μmol l−1 for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l−1 about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Alim) export increased by 4−fold during the first year after the harvest. Alim mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca2+ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Alim and NO 3 − concentrations were strongly correlated in B-horizon soil water after the clearcut (r 2 = 0.96), especially at NO 3 − concentrations greater than 100 μmol l−1. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO 3 − which mixed with acidic, high Alim soil water and decreased the concentration of Alim in streamwater after the harvest. Five years after the harvest soil water NO 3 − concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO 3 − concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO 3 − concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO 3 − and Alim concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.

Nodulating and Non‐Nodulating Soybean Rotation Influence on Soil Nitrate‐Nitrogen and Water, and Sorghum Yield

Soybean [Glycine max (L.) Merr.] rotation has been shown to enhance grain sorghum [Sorghum bicolor (L.) Moench] growth and yield due in part to N contribution. Sorghum grain and stover yield, yield components, soil water and soil NO3–N were measured in a long‐term rotation study in 2003 and 2004 on a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll). The objectives were to separate biologically fixed N from other rotation effects on sorghum grain and stover yields, and to relate yield to yield components, soil NO3–N and water contents. The cropping sequences were continuous grain sorghum, and sorghum rotated with non‐nodulating or nodulating soybean. Soil amendment treatments consisted of control (zero), manure (17–25 Mg dry matter ha−1 yr−1), and N (41 kg ha−1 for soybean and 84 kg ha−1 yr−1 for sorghum). Cropping sequence × soil amendment interaction effects were found for most parameters measured. High soil NO3–N following soybean rotation and from amendment application promoted plant growth leading to low soil water content at anthesis, and increased kernel weight, grain and stover yield. Rotation with non‐nodulating soybean without soil amendment increased grain yield by 2.6 to 3.0 Mg ha−1 and stover yield by 1.5 to 1.8 Mg ha−1 over continuous sorghum without soil amendment. Rotation with nodulating soybean further increased grain yield by 1.7 to 1.8 Mg ha−1 and stover yield by 0.6 to 0.9 Mg ha−1. Biologically fixed N effects accounted for only 35 to 41% of enhanced sorghum yield due to crop rotation with soybean. Soil NO3–N during vegetative growth, plant height, soil water content at anthesis and kernel weight were the most important parameters related to sorghum grain yield across cropping sequences and soil amendments.

Isotopic assessment of NO3− and SO42− mobility during winter in two adjacent watersheds in the Adirondack Mountains, New York

Biogeochemical cycling of N and S was examined at two watersheds in the Adirondack Mountains, New York, to better understand the retention and loss of these elements during winter and spring snowmelt. We analyzed stable isotope compositions of NO3− (δ15N‐NO3−, δ18O‐NO3−) and SO42− (δ34S‐SO42−, δ18O‐SO42−), along with concentrations and fluxes of NO3− and SO42−, in precipitation, throughfall, snowpack, snowmelt, soil water, groundwater, and stream water. Isotopic results showed no evidence of NO3− and SO42− transformations in the forest canopy and snowpack; however, markedly decreased δ18O values of NO3− and SO42− in forest floor water suggest that microbial processing occurred in organic soil horizons. Similarly low δ18O values of NO3− and SO42− were observed in forest floor and mineral soil leachates, groundwater, and streams. Over the winter observation period, most of the NO3− and SO42− in stream water was from a watershed‐derived source, whereas atmospheric contributions were relatively minor. Despite differences in soil water NO3− concentrations between watersheds, the isotopic composition of NO3− (δ15N‐NO3−, δ18O‐NO3−) was similar, and indicated that in both watersheds most of the NO3− was produced by nitrification in the forest soils. Although there was likely some contribution of SO42− from microbial oxidation of carbon‐bonded sulfur, most of the stream water SO42− appeared to be derived from weathering of S‐containing bedrock or parent material. The decreased δ18O values of NO3− and SO42− in upper soil horizons indicate that atmospheric deposition of N and S was not directly linked with stream water losses, even during winter and spring snowmelt.

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K+, Na+, Ca2+, Mg2+, NO3 −-N, NH4 +-N, PO4 3−-P, and SO4 2− were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO3 −-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO4 2−, Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3 −-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3 −-N increased four fold, and streamwater NO3 −-N doubled. Significant changes also occurred in soilwater pH, DOC, SO4 2−, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO3 −-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams.

A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.

Major factors controlling nitrous oxide emission and methane uptake from forest soil

Soil samples were taken from depth of 0-12cm in virgin broad-leaved Korean pine mixed forest, in Changbai Mountain in July 2000. The effects of temperature, soil water content, pH, NH4+ and NO3+ on N2O emission and CH4 uptake of a forest soil were studied in laboratory by the method of orthogonal design. It was observed under laboratory conditions in this study that there were significant correlations between N2O emission rate, CH4 oxidation rate, soil pH and temperature. Nevertheless, N2O emission rate also showed a significant positive correlation with CH4 oxidation rate. The results suggested that pH and temperature were important factors controlling N2O emission and CH4 oxidation under this experiment conditions.

GEOSTATISTICAL BASED MONITORING OF SOIL WATER NO3 −-N: A POTENTIAL NONPOINT SOURCE OF GROUND WATER CONTAMINATION

Soil water NO3 −-N concentrations were monitored for an alfalfa-oat-bean rotation and an alfalfa–bean–bean rotation in the Idaho Snake River Plain as part of the USEPA Section 319 National Monitoring Program. This monitoring study was conducted to evaluate potential beneficial impacts of a USDA recommended crop rotation on subsurface NO3 −-N concentrations at a 4.9 hectare (ha) farm test field. Soil water monitoring was conducted in cooperation with the USDA Snake River Plain Water Quality Demonstration Project. Geostatistical and statistical analyses of NO3 −-N data collected from a network of soil water solution samplers (lysimeters) coupled with hydrogeological characterization of the field indicated that alfalfa followed by oats reduced soil water NO3 −-N concentrations at least temporarily compared to alfalfa followed by beans which is the traditional practice in the area. Soil water NO3 −-N sample data showed a unimodal distribution, through the first two months of the split field rotations, that changed to a distinct bimodal distribution three months into the rotations. Development of the bimodal distribution of soil water NO3 −-N appeared to correspond directly to the rotational split of the field. The median soil water NO3 −-N value calculated from the sample data was approximately 50 mg L−1 greater in the field half planted in beans as compared to the field half planted in oats. Geostatistical spatial mapping results using sequential Gaussian simulations (SGS) supported these findings. SGS results suggested that elevated concentrations of NO3 −-N in the soil water were related to both stratigraphic factors as well as the rotational split.

Impacts of fertigation via sprinkler irrigation on nitrate leaching and corn yield in an acid–sulphate soil in Thailand

A study was conducted in an acid–sulphate soil in the central region of Thailand, in 1999, and 2000 to assess the influence of different rates of N fertigation on corn ( Zea mays L.) yield and nitrate leaching. Nitrogen source was urea and there were four N fertigation treatments which include 0 (control), 100, 150 and 200 kg N ha −1, each having three replications arranged in a randomized complete block design (RCBD). Soil was irrigated to field capacity at 50% available soil moisture depletion regime throughout the season. Leaching losses were calculated from measured daily fluxes of water percolation, soil water NO 3–N concentrations extracted by soil water sampler and from a seasonal N mass balance. The average maximum corn grain yield of 3.52 t ha −1 was obtained at 200 kg N ha −1 in 1999 and 5.42 t ha −1 was obtained at 150 kg N ha −1 in 2000. But the statistical analysis did not show any significant differences in grain yield between N200 and N150 treatments in either year. The leaching measurements showed that the highest values were obtained in N200 treatments in both years with 23 and 5.3 kg N ha −1 in 1999 and 2000, respectively. The lowest yield of 0.55 and 0.98 t ha −1 were obtained at 0 kg N ha −1 in 1999 and 2000, respectively. Also the results showed that a fertigation system can be an efficient practice and an advanced way to give water and plant nutrients together to the crop.

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24-year-old, treated with (NH4)2SO4 since 1989 at the rate of 35 kg N·ha–1·year–1). Net nitrification was 141, 114, and 115 kg N·ha–1·year–1, for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO3– of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO3– in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.

Nitrogen biogeochemistry of three hardwood ecosystems in the Adirondack Region of New York

The biogeochemistry of nitrogen (N) was evaluated for three forest ecosystems (Woods Lake (WL), Pancake-Hall Creek (PHC) and Huntington Forest (HF)) in the Adiron- dack region of New York, U.S.A. to evaluate the response of a range of N atmospheric inputs and experimental N additions. Bulk N deposition was higher at sites in the west than those in the central and eastern Adirondacks. These higher atmospheric N inputs were reflected in higher bulk throughfall fluxes of N (WL and PHC, 10.1 and 12.0 kg N ha −1 yr −1 , respec- tively) in the western Adirondacks than at HF (4.6 kg N ha −1 yr −1 ) in the central Adirondacks. Nitrogen was added to plots as (NH4)2SO4 at 14 and 28 kg Nh a −1 yr −1 or as HNO3 at 14 kg N ha −1 yr −1 . Litter decomposition rates of Fagus grandifolia and Acer rubrum were substantially higher at WL and PHC compared to HF but were not affected by experimental N additions. Results using mineral soil bags showed no effects of N addition on N and C concentrations in soil organic matter, but C and N concentration increases were less at WL and PHC compared to HF. Soil solution nitrate (NO − ) concentrations at 15-cm depth in the reference plots were higher at PHC than at WL and HF while at 50-cm concentrations were higher at PHC and WL than at HF. The reference plots at the two sites (WL and PHC) with the highest atmospheric inputs of N exhibited lower N retention (53 and 33%, respectively) than HF (68%) in reference plots. The greatest increase in NO − loss in response to the exper- imental treatments occurred at HF where the HNO3 additions resulted in the highest NO − concentrations and lowest N retentions. In contrast, at WL and PHC increases in soil water NO − were not evident in response to experimental N additions. The results suggest that the two sites (WL and PHC) in the western Adirondacks did not respond to additional N inputs although they have experienced elevated atmospheric N inputs and higher N drainage losses in reference plots than the HF site in the central Adirondacks. Some of these differences in site response may have also been a function of stand age of WL and PHC that were younger (24 and 33 years, respectively) than the HF (age ∼ 70). Highest NO − fluxes in the reference

Crop Management and Corn Nitrogen Rate Effects on Nitrate Leaching

AbstractExcess N use in crop production is often identified as a major contributor to NO3 enrichment of ground water. Little information is available to show the specific relationships between crop management systems and N fertilizer use on the amounts of NO3 lost by leaching. This study determined the effect of several cropping systems and N rates, providing a range of N availability to corn (Zea mays L.), on soil water NO3 concentrations and leaching below the root zone. Four cropping‐manure management systems were established in 1993 and 1994 (8‐site years) at Arlington, WI, on a Plano silt loam (fine‐silty, mixed, superactive, mesic Typic Argiudoll). Ammonium nitrate (0 to 204 kg N ha−1 in 34‐kg increments) was broadcast at the time of corn planting. Economic optimum N rates (EONR) for corn ranged from 0 to 150 kg ha−1 depending on site‐year. Soil water NO3 concentrations were determined for 18‐mo using porous‐cup samplers installed at a 120‐cm depth in the 0 and 204 kg N ha−1 treatments. Nitrate N concentrations in the samplers increased as the amount of N applied in excess of the observed EONR increased. Predicted soil water NO3−N concentration at EONR was 18 mg L−1. Average NO3−N concentrations were <10 mg L−1 where fertilizer N rates were >50 kg N ha−1 below the EONR and >20 mg L−1 where fertilizer N rates were >50 kg N ha−1 above the EONR. Total NO3−N leaching estimates based on water budget data and soil water NO3 concentrations for the 18‐mo study period ranged from 3 to 88 kg ha−1 depending on crop and manure management system, N fertilizer rate, amount of water drainage, and time of drainage event relative to treatment establishment. An end‐of‐season soil NO3 test appears to be capable of evaluating corn N management practices and indicating the amount of excess N fertilizer applied that may be leached from the root zone. These results illustrate the direct relationship between NO3 loss by leaching and N application rates that exceed crop needs.

Nutrient Removal by Grasses Irrigated with Wastewater and Nitrogen Balance for Reed Canarygrass

AbstractTo develop complementary wastewater treatment systems that increase nutrient reduction and recycling, an experiment was conducted to evaluate the efficiency of three grass species as catch crops for N, P, and K at Surahammar wastewater treatment plant (WWTP) in the southern part of Sweden. Another objective was also to assess soil accumulation of N, P, and K and the risk of N leaching by drainage. Three grasses—reed canarygrass (Phalaris arundinacea L.), meadow foxtail (Alopecurus pratensis L.), and smooth bromegrass (Bromus inermis Leyss.)—were irrigated with a mixture of treated effluent and supernatant at two levels of intensity [optimum level (equal to evapotranspiration) and over‐optimal level] and at two nutrient levels, approximately 150 and 300 kg N ha−1. There were small differences in dry matter (DM) yield between grass species and no difference in N removal among species. The amount of N removed in harvested biomass to N applied was 0.58 in 1995 and 0.63 in 1996. The amount of N removed increased with increased nutrient load. Applied amounts of P were the same as P in harvested biomass. All species removed K amounts several times greater than applied amounts. Increased nutrient load increased overall K removal. The low amount of mineral N and especially NO−3‐N in the soil profile in autumn samplings indicate the risk for leaching is small. Soil water NO−3 contents were also low, <2.5 mg NO−3‐N L−1 during the growing season, with a mean value of <1 mg NO−3‐N L−1.

Evaluation of the root zone water quality model for predicting water and NO 3–N movement in an Iowa soil

Evaluation of computer models with field data is required before they can be effectively used for predicting agricultural management systems. A study was conducted to evaluate tillage effects on the movement of water and nitrate–nitrogen (NO 3–N) in the root zone under continuous corn ( Zea mays L.) production. Four tillage treatments considered were: chisel plow (CP), moldboard plow (MP), no-tillage (NT), and ridge-tillage (RT). The root zone water quality model (RZWQM: V.3.25) was used to conduct these simulations. Three years (1990–1992) of field observed data on soil water contents and NO 3–N concentrations in the soil profile were used to evaluate the performance of the model. The RZWQM usually predicted higher soil water contents compared with the observed soil water contents. The model predicted higher NO 3–N concentrations in the soil profile for MP and NT treatments in comparison with CP and RT treatments, but the magnitude of simulated NO 3–N peak concentrations in the soil profile were substantially different from those of the observed peaks. The average NO 3–N concentrations for the entire soil profile predicted by the model were close to the observed concentrations except for ridge tillage (percent difference for CP=+5.1%, MP=+12.8%, NT=+18.4%, RT=−44.8%). Discrepancies between the simulated and observed water contents and NO 3–N concentrations in the soil profile indicated a need for the calibration of plant growth component of the model further for different soil and climatic conditions to improve the N-uptake predictions of the RZWQM.

Number of Samples Required for the Estimation of Residual Soil Nitrate-Nitrogen: a Risk Based Analysis

A field experiment was conducted to determine the number of soil samples required to estimate the average residual soil nitrate (NO3 -) in a given field under no-till and conventional tillage conditions. Four soil sampling devices (a 20.3-cm power earth auger, a 5.1-cm hand earth auger, a 3.2-cm soil probe, and a 1.9-cm soil probe) were used to collect soil samples from 35 locations each within the conventionally tilled and no-till fields. Soil samples were analyzed for soil water contents and NO3 - concentrations in the soil for various depths. Simple graphs and tables were constructed which could be used by farmers and other professionals for estimating the average residual soil NO3 - contents at a given confidence level or with a certain degree of risk. The confidence interval was taken as the difference between the highest and lowest value of the quartile range of the observed data. The results of this study indicated that the number of soil samples required to estimate the average residual soil NO3 - contents increased as the degree of risk decreased. This study also indicated that the number of soil samples required for making a reasonable estimate of the residual soil NO3 - contents were greater for a no-till field compared with the conventional tillage field.

Change in snowpack, soil water, and streamwater chemistry with elevation during 1990, Fraser Experimental Forest, Colorado

Every 7–10 days during the winter of 1989–1990, we measured precipitation ion input, snowpack water equivalent (SWE) and ion content, soil water and streamwater chemistry along a subalpine to alpine elevation gradient in the Fraser Experimental Forest (FEF), Colorado. The objective was to quantify the effect of elevation and aspect on snowpack ion accumulation and loss, and relate these processes to subsequent change in soil water and first-order stream chemistry during snowmelt. SWE increased with elevation ( P < 0.001) and was greatest on NE aspects ( P < 0.001). Snowpack ion concentration increased with elevation, with NH 4 + and NO 3 − showing the most consistent ( P < 0.05) increase throughout winter. Snowpack base cation (C B) and S0 4 2− concentrations were significantly higher on NE aspect transects. In most weeks, the snowpack NH 4 + and NO 3 − content was highest ( P < 0.05) in the alpine zone. Peak snowpack NH 4 + and NO 3 − content totalled more than 1 kg inorganic N ha −1. Alpine soil water NO 3 − concentration during and after snowmelt was greater ( P < 0.001) than at lower elevations. However, there was no significant correlation between weekly snowpack ion loss and soil water chemistry. At the FEF, it seems that most snowmelt is routed through the watershed as subsurface flow. Soil processes such as ion exchange, biological uptake, overwinter nitrification and mineralization are probably major factors modifying melt water chemistry. There was no correlation between soil water chemistry and streamwater chemistry, which probably reflected the high variation in distance and time required for soil water to reach the stream.

A relative-flux-correction scheme for analyzing three dimensional data of a tile-drained agricultural plot

The use of simple geostatistical tools is often constrained by data trend (nonstationarity) to characterize the spatial variability of soil properties in the subsurface environment influenced by any site-specific feature(s). Adaptive approaches, such as site-specific robust-resistant schemes, median polishing, trend analysis, etc., are thus used to preprocess the spatial data before analyzing for their spatial structures. Soil water nitrate–nitrogen (NO 3–N) concentration (mg l −1) and soil moisture content (cm) data collected jointly from 175 sites arranged on a 5×7×5 three-dimensional (3-D) grid network of 7.6 m×7.6 m×0.3 m spacings in a tile-drained agricultural plot were analyzed for their three-dimensional spatial distribution and for possible coregionalization. We propose a physical process-based correction scheme to preprocess the nonstationary spatial data of soil NO 3–N concentration and soil moisture content. Using the subsurface-drain flow phenomenon, we developed a relative-Darcy-flux-based correction scheme to remove any tile drainage-induced nonstationarity in the spatial data of soil NO 3–N concentration and soil moisture content prior to conducting the spatial analysis in the 3-D soil volume. 3-D composite semivariograms of relative-flux-corrected NO 3–N concentration and relative-flux-corrected moisture content showed anisotropic linear structures in three principal directions. Linear models characterized by steep slopes were found in the directions perpendicular to tile line as opposed to nugget models found in the direction parallel to the tile line. Good spatial correlation between the relative-flux-corrected NO 3–N concentration and relative-flux-corrected soil moisture content and their anisotropic linear semivariograms produced anisotropic linear cross semivariograms in 3-D. The 3-D composite cross semivariogram will be useful in predicting the more expensive variable, (relative-flux-corrected) soil water NO 3–N concentration, at unsampled locations in the soil profile with a cheaper surrogate, the measured (relative-flux-corrected) soil moisture content.