Characterizing sources of nitrate leaching from an irrigated dairy farm in Merced County, California

Dairy farms comprise a complex landscape of groundwater pollution sources. The objective of our work is to develop a method to quantify nitrate leaching to shallow groundwater from different management units at dairy farms. Total nitrate loads are determined by the sequential calibration of a sub-regional scale and a farm-scale three-dimensional groundwater flow and transport model using observations at different spatial scales. These observations include local measurements of groundwater heads and nitrate concentrations in an extensive monitoring well network, providing data at a scale of a few meters and measurements of discharge rates and nitrate concentrations in a tile-drain network, providing data integrated across multiple farms. The various measurement scales are different from the spatial scales of the calibration parameters, which are the recharge and nitrogen leaching rates from individual management units. The calibration procedure offers a conceptual framework for using field measurements at different spatial scales to estimate recharge N concentrations at the management unit scale. It provides a map of spatially varying dairy farming impact on groundwater nitrogen. The method is applied to a dairy farm located in a relatively vulnerable hydrogeologic region in California. Potential sources within the dairy farm are divided into three categories, representing different manure management units: animal exercise yards and feeding areas (corrals), liquid manure holding ponds, and manure irrigated forage fields. Estimated average nitrogen leaching is 872 kg/ha/year, 807 kg/ha/year and 486 kg/ha/year for corrals, ponds and fields respectively. Results are applied to evaluate the accuracy of nitrogen mass balances often used by regulatory agencies to assess groundwater impacts. Calibrated leaching rates compare favorably to field and farm scale nitrogen mass balances. These data and interpretations provide a basis for developing improved management strategies.     

Evaluation of nitrification inhibitor 3,4-dimethyl pyrazole phosphate on nitrogen leaching in undisturbed soil columns

The application of nitrogen fertilizers leads to various ecological problems such as nitrate leaching. The use of nitrification inhibitors (NI) as nitrate leaching retardants is a proposal that has been suggested for inclusion in regulations in many countries. In this study, the efficacy of the new NI, 3,4-dimethyl pyrazole phosphate (DMPP), was tested under simulated high-risk leaching situations in two types of undisturbed soil columns. The results showed that the accumulative leaching losses of soil nitrate under treatment of urea with 1.0% DMPP, from columns of silt loam soil and heavy clay soil, were 66.8% and 69.5% lower than those soil columns tested with regular urea application within the 60 days observation, respectively. However, the losses of ammonium leaching were reversely increased 9.7% and 6.7% under the former treatment than the latter one. Application of regular urea with 1.0% DMPP addition can reduce about 59.3%-63.1% of total losses of inorganic nitrogen via leaching. The application of DMPP to urea had stimulated the inhibition effects of DMPP on the ammonium nitrification process in the soil up to 60 days. It is proposed that the DMPP could be used as an effective NI to control inorganic N leaching losses, minimizing the risk of nitrate pollution in shallow groundwater.     

NLEAP model simulation of climate and management effects on N leaching for corn grown on sandy soil

The Nitrate Leaching and Economic Analysis Package (NLEAP) model was used to evaluate effects of climate and N fertility on nitrate leaching from a 3-yr field experiment of continuous corn (Zea mays L.). Half of the plots were randomly chosen to be either nonirrigated or irrigated (based upon calculated potential evapotranspiration). Three replications of nitrogen (N) fertility (56, 112 and 224 kg ha⁻¹) were used. Soil was a Hecla sandy loam to loamy sand (Pachic Udic Haploboroll). Soil and climate data were from the upper Midwest U.S.A. database for NLEAP. On-site data were used in the model when available.This study shows that NLEAP is capable of integrating data collected for nonirrigated and irrigated conditions on sandy soil for a wide range of N treatments and predicting the nitrate available for leaching (NAL). Precipitation distribution and amount were different in each year. Calculated NAL provided an excellent indicator of potential nitrate leaching hazard. NLEAP output showed that leaching of residual N on this sandy soil is very sensitive to early-spring precipitation. The NLEAP model provided valuable insights concerning effects of climate and N and irrigation management on N leaching. To obtain optimum yields while minimizing nitrate leaching, this study indicates the need to use soil and plant-tissue testing, post-emergence N-fertilizer application, and modem irrigation-scheduling technology. Also, use of the NLEAP model along with field-plot experiments provide additional important information concerning timing of N-leaching events relative to climate and an additional assessment of the effectiveness of fertilizer-N management decisions.     

Influence of the DMPP (3,4-dimethyl pyrazole phosphate) on nitrogen transformation and leaching in multi-layer soil columns

The application of nitrogen fertilizers leads to various ecological problems such as nitrate leaching. The use of nitrification inhibitors as nitrate leaching retardants is a proposal that has been suggested for inclusion in regulations in many countries. In this study, using a multi-layer soil column device, the influence of new nitrification inhibitor DMPP (3,4-dimethyl pyrazole phosphate) was studied for understanding the nitrogen vertical transformation and lowering the nitrate leaching at different soil profile depths. The results indicated that, within 60 d of experiment, the regular urea added 1.0% DMPP can effectively inhibit the ammonium oxidation in the soil, and improve the ammonium concentration in soil solution over the 20cm depths of soil profile, while decline the concentrations of nitrate and nitrite. No obvious difference was found on ammonium concentrations in soil solution collected from deep profile under 20cm depths between regular urea and the urea added 1.0% DMPP. There was also no significant difference for the nitrate, ammonium and nitrite concentrations in the soil solution under 40cm depths of soil profile with the increasing nitrogen application level, among the treatments of urea added 1.0% DMPP within 60 d. It is proposed that DMPP could be used as an effective nitrification inhibitor in some region to control ammonium oxidation and decline the ion-nitrogen leaching, minimizing the shallow groundwater pollution risk and being beneficial for the ecological environment.     

Preferential flow, nitrogen transformations and N balance under urine-affected areas of irrigated and non-irrigated clover-based pastures

Urine-affected areas can lead to considerable losses of N by leaching, ammonia volatilisation and denitrification from dairy pastures in the southeast of South Australia. Potable groundwater supplies are considered to have become contaminated by nitrate as a result of leaching from these leguminous pastures. Dairy cow urine, labelled with 15N urea, was applied to micro-plots and mini-lysimeters installed in two adjacent irrigated (white clover-rye grass) and non-irrigated (subterranean clover-annual grasses) paddocks of a dairy farm on four occasions representing different seasonal conditions. These experiments allowed measurement of nitrogen transformations, recovery of ¹⁵N in the pasture and soil, and leaching below various depths. Gaseous losses were calculated from the nitrogen balance.The results of the four experiments showed that within a day of urine application up to 40% of the applied urinary-N was leached below a depth of 150 mm as a result of macropore flow in the irrigated paddock, and up to 24% in the non-irrigated one. After application to the irrigated paddock 17% of the urinary-N moved immediately below 300 mm but only 2% below the 450-mm depth.The urinary-N remaining in the soil was converted from urea to ammonium within a day regardless of season. Within the first 7 days of application six times more nitrate was produced in summer than in winter. This has obvious implications for leaching potential.Leaching of 15N from the top 150 mm of soil, following urine applications in all seasons, was between 41% and 62% of the applied 15N in the irrigated paddock and 25–51% in the non-irrigated paddock. However, leaching losses measured at depths of 300 or 450 mm were smaller by a factor of 2–4. The leaching loss of ¹⁵N applied in spring in both paddocks was 41% below 150 mm and 12% below 450 mm. Recovery of ¹⁵N from the soil-plant system in the 450-nm deep lysimeters was 60% of that applied.Estimated ammonia was 9% of applied ¹⁵N with no paddock or season effect. No denitrification was evident in summer nor in the non-irrigated paddock in winter but 12% of the applied ¹⁵N was lost due denitrification following winter application to the irrigated paddock. Estimated ¹⁵N loss due to denitrification from urine applied in spring was 13% of that applied and no difference was found between paddocks. The combination of mini-lysimeters, micro-plots and ¹⁵N measurements enabled the nitrogen budget to be determined during four periods throughout the year.     

Assessment of groundwater contamination from fertilizers in the Delhi area based on 0, N03 and K composition

Increasing application of nitrogen fertilizers in the irrigated lands of the studied area is likely to create a blanket non-point source of nitrate. Groundwater contamination from fertilizers, in this context, has been reported as derived from N0₃⁻, K⁺ and ¹⁸0 composition of groundwater. The data suggest both point and non-point sources of groundwater pollution. Thirty-three percent of the groundwater samples showed nitrate contents exceeding the general acceptable limit of 20 p.p.m. and 15% of the samples crossed the maximum permissible limit of 45 p.p.m. High nitrate levels are associated with high δ¹⁸O values, clearly indicating that significant quantities of evaporated (isotopically enriched) irrigation water infiltrate along with fertilizer nitrate to the groundwater system. Different δ¹⁸O---N0₃⁻ trends suggest isotopically distinct, non-point source origins which vary spatially and temporally, due to different degrees of evaporation/recharge and amounts of fertilizer applied. A scatter diagram of N0₃⁻ vs K⁺ suggests a common source of these ions when the concentration is less than 40 p.p.m. The investigation indicates that a combination of isotope (¹⁸0) and hydrochemical data can clearly characterize the impact of fertilizer on groundwater. Application of high nitrate, high potassium groundwater irrigation can minimize the requirement for inorganic fertilizers and bring down the cost of cultivation considerably, through appropriate management of fertilizer and water and modifications in agronomic practices and strategies on crops grown. Such practices will help protect groundwater from further degradation.     

Environmental fate of pesticides used in Australian viticulture: Behaviour of dithianon and vinclozolin in the soils of the South Australian Riverland

The red calcareous earth soils of the South Australian Riverland produce more than one-third of the grapes used in Australian winemaking. As part of on-going investigations into pesticide transport in Australian vineyard soils, the movement of the fungicides dithianon and vinclozolin through such strongly alkaline soils was investigated. Small, undisturbed soil cores were extracted from the inter-row topsoil of a vineyard adjacent to the River Murray, approximately 10 km S.W. of Overland Corner, South Australia. The vines were grown in a deep (1 – 4 m) reddish brown, strongly alkaline, sandy loam with a low organic carbon content (1 – 2 %). Surface fluxes of pesticide were applied at the maximum recommended application rates to the surface of the cores, which were then irrigated, and pesticide residues in the leachate determined by HPLC. No leaching of either dithianon or vinclozolin occurred. Dithianon was immobilised in the top 2 cm of the soil. Dithianon concentrations were low ( 0 – 37 % applied dose) suggesting that rapid degradation of this compound occurs in these soils (63 – 100 % degradation in 10 days). Extremely low concentrations of vinclozolin were found throughout the soil core profiles (0.05 – 1.4 % applied dose) suggesting that this fungicide was somewhat mobile, but also that it too was unstable in such alkaline soils (> 98 % degradation in 10 days). These results suggest that the irrigated vineyard soils of this region are unlikely to be prone to leaching of dithianon or vinclozolin, and therefore that groundwater supplies in this area are unlikely to be at any significant risk of contamination through viticultural use of these compounds.     

Microbiological characterization of the biological treatment of aircraft paint stripping wastewater

Leaching rates of the herbicide dichlorprop [(+/--2-(2,4-dichlorophenoxy)propanoic acid] and nitrate were measured together in field lysimeters containing undisturbed clay and peat soils. The purpose of the study was to investigate the leaching pattern of the two solutes in structured soils under different precipitation regimes. Spring barley (Hordeum distichum L.) was sown on each monolith and fertilized with 100 kg N ha(-1). Dichlorprop was applied at a rate of 1.6 kg active ingredient (a.i.) ha(-1). Each soil type received supplemental irrigation at two levels ('average' and 'worst-case'), giving total water inputs (irrigation and precipitation) of 664 and 749 mm year(-1), respectively. The larger water input approximately doubled the nitrate loads, from, on average, 11.6 to 21.8 kg N ha(-1) year(-1) in the clay soil and from 37.6 to 65.4 kg N ha(-1) year(-1) in the peat soil. In contrast, dichlorprop leaching was reduced by more than one order of magnitude when the water input was increased, from average amounts of 3.22 to 0.26 g a.i. ha(-1) during an S-month period in the clay and from 28.9 to 2.67 g a.i. ha(-1) in the peat. This leaching pattern of dichlorprop was explained in terms of preferential flow. The dried-out topsoil of 'average' watered monoliths may have allowed water flow in cracks, thus moving some of the herbicide rapidly through the topsoil to the subsoil. Once the compound reached the subsoil, degradation rates would be reduced and the herbicide residues would be stored for later leaching. Nitrate was presumably more evenly distributed in the soil matrix; therefore, water rapidly moving through macropores would not carry significant amounts of nitrate. In contrast, leaching would occur more evenly through the soil matrix, causing larger nitrate loads in the 'worst-case' watered monoliths. These results show that wet years may constitute a worst case scenario in terms of nitrate leaching, but not pesticide leaching, if macropore flow exerts a significant influence on leaching.     

Field Measurements of Natural and Fertilizer-Induced N2O Release Rates from Soils

The influence of the soil on atmospheric N₂O was studied by In-situ measurements in 1976–1979 at several field stations near Mainz, Germany, where different soil types were located. Measurements were carried out using the closed chamber method and applying stainless steel capillaries allowing soil air sampling down to 60 cm depth. The N2O In soil was found to be produced and consumed simultaneously In the uppermost soil layer resulting In a net flux of N₂O with release rates of 0.5–16 μg N₂O–Nm^?2h^?1 on unfertilized natural as well as agriculturally used soils. After fertilization with mineral fertilizers the N₂O release rates increased to values ≤43 μg N₂O–Nm^?2h^?1. The total amount of fertilizer-N released Into the atmosphere as N₂O was determined to be 0.01–0.05% for nitrate and 0.03–0.09 % for ammonium fertilizer.     

黄浦江上游蔬菜田氮素流失规律测坑研究

通过测坑试验,研究了黄浦江上游蔬菜田渗漏水中氮素的变化动态和流失规律。结果表明,蔬菜田渗漏水中氮素以NO3^--N为主,NO3^--N作为氮素在土壤中流失的主要形态将成为施用氮肥造成地下水污染的重要来源;施用精制有机肥或粗猪粪代替部分化学氮肥有利于减少蔬菜田渗漏水中氮素的流失。     

Use of dolomite phosphate rock (DPR) fertilizers to reduce phosphorus leaching from sandy soil

There is increasing concern over P leaching from sandy soils applied with water-soluble P fertilizers. Laboratory column leaching experiments were conducted to evaluate P leaching from a typical acidic sandy soil in Florida amended with DPR fertilizers developed from dolomite phosphate rock (DPR) and N-Viro soil. Ten leaching events were carried out at an interval of 7 days, with a total leaching volume of 1,183 mm equivalent to the mean annual rainfall of this region during the period of 2001-2003. Leachates were collected and analyzed for total P and inorganic P. Phosphorus in the leachate was dominantly reactive, accounting for 67.7-99.9% of total P leached. Phosphorus leaching loss mainly occurred in the first three leaching events, accounting for 62.0-98.8% of the total P leached over the whole period. The percentage of P leached (in the total P added) from the soil amended with water-soluble P fertilizer was higher than those receiving the DPR fertilizers. The former was up to 96.6%, whereas the latter ranged from 0.3% to 3.8%. These results indicate that the use of N-Viro-based DPR fertilizers can reduce P leaching from sandy soils.     

污泥堆肥对几种花卉的生长响应研究

施用污泥堆肥后，木槿、月季、美人蕉及旱荷花的开花量增加，花期延长，金枣、龟背竹与五叶地锦的各生长参数也明显增大；土壤理化性质有明显改善；当污泥堆肥施用量小于９ｋｇ／ｍ＾２时，无因盐分、ＮＯ３＾－－Ｎ淋溶及地面径流而造成的环境问题。     

Area-differentiated modelling of excess nutrient and contaminant leaching risks at the river-basin scale using a hydrological approach in India

Application of excess nutrients, such as fertilizers, is a significant and sometimes even major component of groundwater pollution. Diffuse inputs of nutrients and contaminants to the groundwater are related to runoff generated from precipitation on a catchment. This implies that the analysis of diffuse fluxes from the land surface to the groundwater requires an analysis of water fluxes for a catchment. This requires the simulation and modelling of total runoff, groundwater recharge, and plant-available water as a function of the regional interaction of the climate, soil, hydrogeology, topography and land-use conditions in the river basin. A model has been developed for large river basins in India, and has been applied to the Upper Yamuna basin, to quantify the exchange probability of plant-available soil water, which can be taken as a measure to determine the nutrient and contaminant leaching risk of a site. It was found that, with the available large-scale databases and methods, regional patterns of the total runoff could be simulated successfully. In this way, about 75001km² of the total 121000 km² of the Upper Yamuna basin was classified as an area sensitive to nutrient and contaminant leaching.     

Atrazine sorption and fate in a Ultisol from humid tropical Brazil

This study combined laboratory based microcosm systems as well as field experiments to evaluate the mobility of atrazine on a Ultisol under humid tropical conditions in Brazil. Results from sorption experiments fit to the Freundlich isotherm model [K(f) 0.99 mg kg(-1)/(mg l(-1))(1/n)], and indicate a low sorption capacity for atrazine in this soil and consequently large potential for movement by leaching and runoff. Microcosm systems using (14)C-atrazine to trace the fate of the applied herbicide, showed that 0.33% of the atrazine was volatilized, 0.25% mineralized and 6.89% was recorded in the leachate. After 60 d in the microcosms, 75% of the (14)C remained in the upper 5 cm soil layer indicating atrazine or its metabolites remained close to the soil surface. In field experiments, after 60 d, only 5% of the atrazine applied was recovered in the upper soil layers. In the field experiments atrazine was detected at a depth of 50 cm indicating leaching. Simulating tropical rain in field experiments resulted in 2.1% loss of atrazine in runoff of which 0.5% was adsorbed onto transported soil particles and 1.6% was in solution. Atrazine runoff was greatest two days after herbicide application and decreased 10 fold after 15 d. The use of atrazine on Ultisols, in the humid tropics, constitutes a threat to water quality, causing surface water and ground water pollution.     

Dolomite phosphate rock (DPR) application in acidic sandy soil in reducing leaching of phosphorus and heavy metals—a column leaching study

A column leaching study was designed to investigate the leaching potential of phosphorus (P) and heavy metals from acidic sandy soils applied with dolomite phosphate rock (DPR) fertilizers containing varying amounts of DPR material and N-Viro soils. DPR fertilizers were made from DPR materials mixing with N-Viro soils at the ratios of 30, 40, 50, 60, and 70 %, and applied in acidic sandy soils at the level of 100 mg available P per kilogram soil. A control and a soluble P chemical fertilizer were also included. The amended soils were incubated at room temperature with 70 % field water holding capacity for 21 days before packed into a soil column and subjected to leaching. Seven leaching events were conducted at days 1, 3, 7, 14, 28, 56, and 70, respectively, and 258.9 mL of deionized water was applied at each leaching events. The leachate was collected for the analyses of pH, electrical conductivity (EC), dissolved organic carbon (DOC), major elements, and heavy metals. DPR fertilizer application resulted in elevations up to 1 unit in pH, 7–10 times in EC, and 20–40 times in K and Ca concentrations, but 3–10 times reduction in P concentration in the leachate as compared with the chemical fertilizer or the control. After seven leaching events, DPR fertilizers with adequate DPR materials significantly reduced cumulative leaching losses of Fe, P, Mn, Cu, and Zn by 20, 55, 3.7, 2.7, and 2.5 times than chemical fertilizer or control. Even though higher cumulative losses of Pb, Co, and Ni were observed after DPR fertilizer application, the loss of Pb, Co, and Ni in leachate was <0.10 mg (in total 1,812 mL leachate). Significant correlations of pH (negative) and DOC (positive) with Cu, Pb, and Zn (P?<?0.01) in leachate were observed. The results indicated that DPR fertilizers had a great advantage over the soluble chemical fertilizer in reducing P loss from the acidic sandy soil with minimal likelihood of heavy metal risk to the water environment. pH elevation and high dissolved organic carbon concentration in soils after DPR fertilizer application are two influential factors.     

Time aggregation of nitrogen leaching in relation to critical threshold values

Leaching of nitrate contributes to the deterioration of groundwater and can consequently have a negative influence on the quality of our drinking water. Critical threshold values for nitrogen leaching are established to preserve groundwater quality. A critical threshold value for nitrate leaching of 50 mg 1⁻¹ (11.3 mg N 1⁻¹), similar to the drinking water standard, serves as a threshold value for European countries. However, the temporal aggregation scale on which this threshold value should be considered is unknown. A well tested simulation model was used to evaluate the exceedance of the threshold value at different time aggregation levels, ranging from one day till 30 yr. For three different soil structure types within one soil type and a selected fertilisation regime, the aggregated nitrogen leaching over 30 yr was 11.4, 19.2 and 10.6 mg 1⁻¹. Considering an aggregation level of one day, the critical threshold value of 11.3 mg N 1⁻¹ was exceeded 2973, 5801 and 2556 times, respectively, for the three structure types during 30 yr. By considering other time aggregation levels, a clear relation resulted between time aggregation level and the number of time elements during which the critical level was exceeded. Results strongly indicate that a critical threshold value for leaching should include an associated time-aggregation level.     

Mobility of adsorbed arsenic in two calcareous soils as influenced by water extract of compost

The mobility of arsenic (As) in soil affects both the bio-toxicity of As and the groundwater quality, which in turn indirectly affects the quality of edible part of crops and human health, if the crops were irrigated with As contaminated groundwater. A vertical soil column simulates the depth of a soil profile in a real soil environment. Thus soil column experiment is much more pertinent to soil environment than a batch experiment to simulate solute movement as well as leaching through soil profile. A laboratory soil column experiment was conducted to determine the extent of As leaching from soil percolated with influent that contained organic substances. The water extract of compost (WEC) was used as the source of organic substances. The results of As breakthrough curves (BTCs) showed that less pore volumes of influents were required to reach the relative concentration ratio of 1 (C/C0=1) for the two calcareous soils treated with influent that contained WEC. The concentrations of As in the column effluents of soils percolated with 0.01M KCl in WEC were significantly higher than those percolated with 0.01M KCl with the same volumes of effluents collected. This clearly indicates that dissolved as well as deprotonated organic substances which are negatively charged have higher potential for competing the adsorption sites with As on soils, leading to increasing mobility of As in soil and associated environments. Further, it is observed that the characteristics of soil components related to As adsorption affected the adsorption as well as desorption processes and subsequent mobility of As in soil environment as influenced by organic substances.     

The importance of ammonium mobility in nitrogen-impacted unfertilized grasslands: A critical reassessment

The physico-chemical absorption characteristics of ammonium-N for 10 soils from 5 profiles in York, UK, show its high potential mobility in N deposition-impacted, unfertilized, permanent grassland soils. Substantial proportions of ammonium-N inputs were retained in the solution phase, indicating that ammonium translocation plays an important role in the N cycling in, and losses from, such soils. This conclusion was further supported by measuring the ammonium-N leaching from intact plant/soil microcosms. The ammonium-N absorption characteristics apparently varied with soil pH, depth and soil texture. It was concluded for the most acid soils especially that ammonium-N leached from litter horizons could be seriously limiting the capacity of underlying soils to retain ammonium. Contrary to common opinion, more attention therefore needs to be paid to ammonium leaching and its potential role in biogeochemical N cycling in semi-natural soil systems subject to atmospheric pollution.     

The fate of nitrogen in a moderately alkaline and calcareous soil amended with biosolids and urea

The determination of nitrogen (N) based loading rates for land application of biosolids is challenging and site specific. Over loading may contribute to environmental, agricultural, or human health problems. The objective of this study was to monitor N mineralization and losses in a moderately alkaline and calcareous desert soil amended with either anaerobically digested (AN) or lime-stabilized (LS) biosolids, and irrigated with and without urea enriched water. For Experiment 1, N inputs, leaching and residuals in soil were evaluated in an open soil column system. For Experiment 2, ammonia (NH₃) emissions were evaluated in a closed soil column system. In Experiment 1, AN and LS biosolids increased soil ON (organic N) by three and two fold, respectively. Respective net N mineralization of ON from biosolids alone was 90% and 62% without urea, and 71% and 77%, respectively with added urea. Nitrogen leaching losses and residuals in amended soil did not account for all N inputs into the soil/biosolids system. In Experiment 2, NH₃ emissions were not significantly different among treated soils with or without added urea, except LS amended soil receiving urea. Ammonia losses did not account for unaccounted N in Experiment 1. We concluded that deep placement and rapid mineralization of AN biosolids promoted anaerobic soil conditions and denitrification, in addition to the high denitrification potential of desert soil. LS biosolids showed greater potential than AN biosolids for safe and beneficial land application to desert soils regardless of biosolids placement and the inclusion of N rich irrigation water.     

Long-term nitrate increases in two oligotrophic lakes, due to the leaching of atmospherically-deposited N from moorland ranker soils

During the last 50 years nitrate concentrations in Buttermere and Wastwater (Cumbria, UK) have risen significantly, by 70 and 100%, respectively. By estimating contemporary nitrate fluxes in the lakes' catchments and in sub-catchments and comparing them with the fractional areas of different soil types, it is deduced that the surface water nitrate is derived almost entirely from organic-rich ranker soils that have a limited ability to retain atmospherically-deposited nitrogen. Little or no nitrate leaches from the other major soil type, a brown podzol, despite it having a lower C:N ratio (12.0 g g(-1)) than the ranker (17.0 g g(-1)), nor is there much contribution from the small areas of improved (chemically fertilised) grassland within the catchments. Although some nitrate leaching is occurring, total N losses are appreciably smaller than atmospheric inputs, so the catchment soils are currently accumulating between 3 and 4 g N m(-2) a(-1).