Sowing a winter catch crop can reduce nitrate leaching losses from winter‐applied urine under simulated forage grazing: a lysimeter study

Grazing of winter forage crops is a common management option used in the dairy industry of New Zealand, particularly in the South Island, where they are used to feed nonlactating, pregnant dairy cows prior to calving. However, there is concern that the large crop yields per hectare grazed, combined with a high stocking density of cows, lead to large amounts of urinary nitrogen (N) deposited on bare, wet soil that, in turn, could lead to large nitrate leaching losses. We report the results of a simulated winter forage grazing event using field lysimeters planted with a kale (Brassica oleracea L.) crop. The effect of sowing a ‘catch crop’ of oat (Avena sativa L.) following the simulated winter forage grazing on nitrate leaching losses from urine applied at different times throughout the winter was measured. A catch crop sown between 1 and 63 days after the urine deposition in early winter reduced N leaching losses from urine patches by ~34% on average (range: 19–49%) over the winter–spring period compared with no catch crop. Generally, the sooner the catch crop was sown following the crop harvest, the greater the uptake of N by the catch crop and the greater the reduction in nitrate leaching losses. The results indicate that sowing of a catch crop following winter crop grazing could be an effective management strategy to reduce nitrate leaching as well as increase the N‐use efficiency of dairy winter forage grazing systems.     

Computer simulation of winter leaching losses of nitrate from soils cropped with winter wheat

Abstract. A computer simulation model was used to estimate the effects of season, site, sowing date, residual-N after harvest, autumn-N and field drains on winter losses of nitrate from soils growing winter wheat. The simulations were based on weather data between 1970–71 and 1983–84 and soil data from Rothamsted and Woburn. The residual-N after harvest was predicted to have most effect on nitrate losses, followed by season and site. For the values of residual-N and autumn-applied fertilizer-N tested, the predicted average nitrate-N losses differed between seasons by up to 100 kg N ha^-1, and the nitrate-N concentrations varied between 30 and 80 mg N l^-1.     

Modellstudien zur Nitratauswaschung unter beweidetem Grünland

Modeling nitrate leaching from grazed pasture A method for estimating nitrate concentrations in seepage water under pastures using the model WASMOD was developed. Urin-N and dung-N input by grazing cattle was calculated as a function of stocking rate, length of grazing, and amounts of urin-N and dung-N excreted (data from literature). Urin-N was modeled as NH₄⁺-fertilizer, dung-N as fresh organic matter (C/N ratio 18.6:1). The model was tested using average stocking rates on pastures and mowing pastures inside the waterworks ‘Föhr-West’ catchment area and a long-term climate scenario (35 yr). The modeled average nitrate concentration (55.5 mg l⁻¹) agreed well with the average nitrate concentration measured in the public wells (59.5 mg l⁻¹). Model studies indicate that the nitrate concentrations in seepage water can be reduced by 40% if the cattle graze only 9 hours per day and no longer than until mid of September.     

农田尺度下土体硝酸盐淋失的随机模拟及其风险性评价

以田间试验为基础,对所建立的简化土壤水氮联合运移模型进行了验证。将条件模拟方法所得到的表层土壤饱和导水率(Ks)的随机场与该联合模型相结合,随机地模拟了冬小麦地1999年3月15日至6月10日这一时段的硝酸盐淋失情况。在总来水量为353.8 mm、施N量为86.25 kg hm-2的情况下,2 m土体硝酸盐淋失量最小值为N 15.04 kg hm-2,最大值为N 26.04 kg hm-2,分别占此段施肥量的17.4%和30.2%。以传统方法(Ks平均值)所得到硝酸盐淋失量作为对照,发现硝酸盐淋失量超过该值且概率大于70%的面积约占到田块总面积的20%左右。说明由于田间空间变异的作用,造成了硝酸盐淋失的巨大差异,故不能用传统的方法来代替,因为这样做就掩盖了地下水可能造成硝酸盐污染的风险性。     

Effects of grazing strategy on limiting nitrate leaching in grazed grass‐clover pastures on coarse sandy soil

Urinations of ruminants on grazed pastures increase the risk of nitrate leaching. The study investigated the effect of reducing the length of the grazing season on nitrate leaching from a coarse sandy, irrigated soil during 2006–2007 and 2007–2008. In both years, precipitation was above the long‐term mean. The experiment was initiated in a 4‐yr‐old grass‐clover sward in south Denmark. Three treatments were as follows grazing only (G), spring cut followed by grazing (CG) and both spring and autumn cuts with summer grazing (CGC). Nitrate leaching was calculated by extracting water isolates from 80 cm depth using ceramic suction cups. Because of considerable variation in measured nitrate concentrations, the 32 installed suction cups per treatment were insufficient to reveal differences between treatments. However, weighted nitrate leaching estimations for G, CG and CGC showed estimated mean nitrate N concentrations of 23, 19 and 13 mg/L for an estimated proportion area occupied by urine patches of 0.33, 0.26 and 0.16, respectively. Thus, N concentrations in G and CG exceeded the EU limit of 11.3 mg N/L. Under the prevailing conditions, the time of urination did not appear important. The estimated background leaching calculated from suction cups presumably not situated under urine patches resulted in mean nitrate N concentrations of 2.6 mg/L.     

Effects of the nitrification inhibitor dicyandiamide on potassium, magnesium and calcium leaching in grazed grassland

Abstract. Leaching of calcium (Ca), potassium (K) and magnesium (Mg) from urine patches in grazed grassland represents a significant loss of valuable nutrients. We studied the effect on cation loss of treating the soil with a nitrification inhibitor, dicyandiamide (DCD), which was used to reduce nitrate loss by leaching. The soil was a free-draining Lismore stony silt loam (Udic Haplustept loamy skeletal) and the pasture was a mixture of perennial ryegrass ( Lolium perenne ) and white clover ( Trifolium repens ). The treatment of the soil with DCD reduced Ca²⁺ leaching by the equivalent of 50%, from 213 to 107 kg Ca ha⁻¹ yr⁻¹ on a field scale. Potassium leaching was reduced by 65%, from 48 to 17 kg K ha⁻¹ yr⁻¹. Magnesium leaching was reduced by 52%, from 17 to 8 kg Mg ha⁻¹ yr⁻¹. We postulate that the reduced leaching loss of these cations was due to the decreased leaching loss of nitrate under the urine patches, and follows from their reduced requirement as counter ions in the drainage water. The treatment of grazed grassland with DCD thus not only decreases nitrate leaching and nitrous oxide emissions as reported previously, but also decreases the leaching loss of cation nutrients such as Ca²⁺, K⁺ and Mg²⁺.     

Does variable rate irrigation decrease nutrient leaching losses from grazed dairy farming?

The irrigation of grazed dairy pastures can be highly profitable, but can lead to the enhanced leaching of nitrogen (N) and phosphorus (P) and impairment of freshwater quality. A six‐year study was conducted to determine whether the use of variable rate irrigation (VRI), compared to uniform rate irrigation (URI), could decrease N and P leaching losses from a 143‐ha area under intensively grazed dairy cattle that had been partly hydrologically isolated by the installation of artificial drainage pipes. Median concentrations in drainage of NH₄‐N, NO₃‐N and total N were enriched after passing through the irrigated area under both URI and VRI. However, median concentrations of N species and filterable reactive P and total P in drainage downstream of the irrigated area were much less under three years of VRI than under three years of URI. After accounting for potential differences in flow, annual load estimates of N and P species at the downstream site under VRI were about 80–85% less than that lost under URI. Wider adoption of VRI technology could therefore decrease farm leaching losses and nutrient concentrations in receiving waterbodies compared to areas with widespread URI or flood irrigation.     

Effects of the nitrification inhibitor dicyandiamide on soil mineral N, pasture yield, nutrient uptake and pasture quality in a grazed pasture system

Recent lysimeter studies have demonstrated that the nitrification inhibitor, dicyandiamide (DCD), can reduce nitrate (NO) leaching losses from cow urine patches in grazed pasture systems. The objective of this study was to quantify the effects of fine particle suspension (FPS) DCD on soil mineral N components, pasture yield, nutrient uptake and pasture quality under grazed pasture conditions. A field study was conducted on the Lincoln University dairy farm, Canterbury, New Zealand, from 2002 to 2006. FPS DCD was applied to grazed pasture plots at 10 kg ha^?1 in early May in addition to applied cow urine patches at a nitrogen (N) loading rate of 1000 kg N ha^?1, with DCD reapplied in early August. Soil mineral N levels in the urine patches were monitored. Pasture yield, N and cation concentrations and uptake were measured in treatment urine patches and inter‐urine areas of the pasture. Comparisons were made with control plots which did not receive DCD. NO levels under the DCD‐treated urine patches (0–7.5 cm) were in the order of 10 kg N ha^?1 compared with 40–80 kg N ha^?1 under untreated patches, and soil ammonium (NH) levels were consistently higher under the DCD‐treated patches. The DCD significantly and consistently increased pasture yield in both the urine patches, and inter‐urine areas of the pasture in all 4 years of the trial. Mean annual dry matter (DM) yields over 4 years were inter‐urine areas, 10.3; inter‐urine + DCD, 12.4; urine, 12.4 and urine +DCD 16.0 t DM ha^?1, representing an average DM yield increase of 20 and 29% in inter‐urine and urine patch areas, respectively. On a whole paddock basis, the increase in annual DM yield resulting from DCD application was estimated to be 21%. N, calcium (Ca), magnesium (Mg) and potassium (K) concentrations in pasture were unaffected by treatment with DCD; however, total annual uptake of these nutrients by pasture was significantly higher in all years where DCD had been applied. Pasture DM, protein, carbohydrate, metabolizable energy and fibre levels and sward clover content were not affected by treatment with DCD. The results demonstrate the agronomic value of the DCD treatment in addition to the environmental benefits in a grazed pasture system.     

Computer‐based evaluation of methods to sample nitrate leached from grazed pasture

Scientists, land managers and environmental regulators all want to quantify the amount of nitrate leached into groundwater to better understand and manage the risk of environmental contamination from agricultural land, including grazed pasture. The ability of current technology to adequately measure the nitrate leached from a grazed paddock is tested in a stochastic simulation study. Results show that impractical numbers of samplers are needed to achieve estimates accurate to within ±20% of the true value. Rather than trying to directly measure paddock‐scale leaching under grazing, we suggest that further consideration be given to wider application of controlled application of nitrogen onto a few lysimeters and then extrapolate from the resulting measurements of leached nitrate to the paddock scale and beyond based on urine patch coverage.     

基于三维Copula函数的滴灌硝态氮淋失风险评估方法

硝态氮淋失是滴灌系统设计和运行管理需要考虑的重要因素。该研究构建了滴灌条件下的水氮运移模型,利用HYDRUS-2D软件进行了求解,模拟分析了田间尺度砂壤土饱和导水率和初始含水率空间变异对NO₃^--N淋失率的影响,并利用三维Gumbel-Hougaard Copula函数构建了土壤饱和导水率、初始含水率和NO₃^--N淋失率的联合分布函数,分析了给定土壤饱和导水率和初始含水率条件下NO₃^--N淋失率超过某一阈值的条件概率。结果表明,NO₃^--N淋失率概率密度函数可用指数函数表示;土壤饱和导水率和初始含水率的空间变异会明显增加NO₃^--N淋失风险;NO₃^--N淋失率超过给定阈值（6.4%,均质土壤条件下的NO₃^--N淋失率）的条件概率基本随土壤饱和导水率和初始含水率的增大而增大。构建田间尺度土壤特性参数（如饱和导水率、初始含水率等）与NO₃^--N淋失率的联合分布函数为研究多变量空间变异条件下NO₃^--N淋失风险评估提供了参考。     

Evaluating and predicting agricultural management effects under tile drainage using modified APSIM

An accurate and management sensitive simulation model for tile-drained Midwestern soils is needed to optimize the use of agricultural management practices (e.g., winter cover crops) to reduce nitrate leaching without adversely affecting corn yield. Our objectives were to enhance the Agricultural Production Systems Simulator (APSIM) for tile drainage, test the modified model for several management scenarios, and then predict nitrate leaching with and without winter wheat cover crop. Twelve years of data (1990-2001) from northeast Iowa were used for model testing. Management scenarios included continuous corn and corn-soybean rotations with single or split N applications. For 38 of 44 observations, yearly drain flow was simulated within 50 mm of observed for low drainage (< 100 mm) or within 30% of observed for high drain flow. Corn yield was simulated within 1500 kg/ha for 12 of 24 observations. For 30 of 45 observations yearly nitrate-N loss in tile drains was simulated within 10 kg N/ha for low nitrate-N loss (< 20 kg N/ha) or within 30% of observed for high nitrate-N loss. Several of the poor yield and nitrate-N loss predictions appear related to poor N-uptake simulations. The model accurately predicted greater corn yield under split application (140-190 kg N/ha) compared to single 110 kg N/ha application and higher drainage and nitrate-N loss under continuous corn compared to corn/soybean rotations. A winter wheat cover crop was predicted to reduce nitrate-N loss 38% (341 vs. 537 kg N/ha with and without cover) under 41-years of corn-soybean rotations and 150 kg N/ha applied to corn. These results suggest that the modified APSIM model is a promising tool to help estimate the relative effect of alternative management practices under fluctuating high water tables.     

Nitrate leaching from grazed grassland lysimeters: effects of fertilizer input, field drainage, age of sward and patterns of weather

Drained and undrained grassland lysimeter plots were established in 1982 on a clay loam of the Hallsworth series at a long-term experimental site in south-west England. The plots were continuously grazed by beef cattle, and received fertilizer at either 200 or 400 kg N ha^-1 per annum to the existing permanent sward, or at 400 kg N ha^-1 to a new sward, reseeded to perennial ryegrass following cultivation. Drainage water was monitored at V-notch weirs and sampled daily for the analysis of nitrate-N. Seven years of data are presented (five years for the reseeded swards). On the drained plots a large proportion of the rainfall was routed preferentially down large pores to the mole drains, whilst on the undrained plots, drainage was mainly by surface runoff. The average quantities of nitrate N leached per year were 38.5, 133.8 and 55.7 kg ha^-1 from the old sward that received 200 and 400 kg N ha^-1, and from the reseed that received 400 kg N ha^-1 fertilizer, respectively. Ploughing and reseeding resulted in a two-fold reduction in leaching, except during the first winter after ploughing, and twice as much leaching occurred after a hot, dry summer as after a cool, wet one. Nitrate concentrations in drainage from either drained or undrained plots were rather insensitive to rainfall intensity, such that concentration was a good predictor of nitrate load for a given drainage volume. The drainage volume determined the proportion of the leachable N that remained in the soil after the winter drainage period. Initial (peak) concentrations of nitrate N ranged, on average, from 55 mg dm^-3 for the drained old sward that received 400 kg N ha^-1 fertilizer, to 12 mg dm^-3 for the undrained sward at 200 kg N ha^-1 fertilizer input. Concentrations of nitrate N in drainage from similar, unfertilized plots rarely exceeded 1 mg dm^-3. The results suggest that manipulating the nitrate supply can lessen leaching and that the route of water through soil to the watercourse determines the maximum nitrate concentration for a given load.     

Modelling and measurement of the nitrogen cycle in a vegetable field in Switzerland II. Validation of the nitrogen model with field data

Field experiments were conducted over a period of two years on a commercial vegetable farm for calibration and validation of the nitrogen model (tables 1 and 2). Yield and nitrogen uptake of the crops were measured at least five times during crop growth. Soil water tension and nitrate concentration in the soil solution were assessed by frequent sampling of different soil layers. The volumetric soil water content was calculated from continuously measured water potential by means of a field desorption curve. The model was calibrated using a transfer factor for rapid and slow water transport corresponding to changes in water content observed during a leaching experiment in winter 1987/88. Other required input parameters were taken from existing knowledge about the growth of vegetable crops, an evaluation of the soil profile to assess water transport in the soil, and the results of an incubation experiment for the estimation of the net nitrogen-mineralisation. Yield and nitrate uptake of different vegetables are predicted with an accuracy of about 15% for yield and of about 20 kg N/ha for nitrate uptake (table 4, figure 1). Measurement of water and nitrate transport under fennel and a fallow plot (figure 2) reveals that the simulated net nitrogen-mineralisation is overestimated by about 30 percent, and that simulation of rooting depth and evapotranspiration should be improved. Nitrate leaching during summer takes place only after periods of very heavy rain. The accuracy of the agreement between simulated and measured data is within 10% for the water content, and within 25% for the nitrate concentration in the soil solution, while measured nitrate leaching exceeded the simulated values by up to 80 kg N/ha. The precision of the model seems to be satisfactory in view of the large amount of nitrogen and water cycled in a vegetable field (table 5). However with respect to water management improvements are necessary. Further validation of the model on other sites is required before a predictive model for practical purposes can be elaborated.     

Methanotroph abundance not affected by applications of animal urine and a nitrification inhibitor,dicyandiamide, in six grazed grassland soils

Purpose  

Methanotrophs are an important group of methane (CH₄)-oxidizing bacteria in the soil, which act as a major sink for the greenhouse gas, CH₄. In grazed grassland, one of the ecologically most sensitive areas is the animal urine patch soil, which is a major source of both nitrate (NO₃ ⁻) leaching and nitrous oxide (N₂O) emissions. Nitrification inhibitors, such as dicyandiamide (DCD), have been used to mitigate NO₃ ⁻ leaching and N₂O emissions in grazed pastures. However, it is not clear if the high nitrogen loading rate in the animal urine patch soil and the use of nitrification inhibitors would have an impact on the abundance of methanotrophs in grazed grassland soils. The purpose of this study was to determine the effect of animal urine and DCD on methanotroph abundance in grazed grassland soils.     

Nitrogen cycling in organic farming systems with rotational grass–clover and arable crops

Organic farming is considered an effective means of reducing nitrogen losses compared with more intensive conventional farming systems. However, under certain conditions, organic farming may also be susceptible to large nitrogen (N) losses. This is especially the case for organic dairy farms on sandy soils that use grazed grass–clover in rotation with cereals. A study was conducted on two commercial organic farms on sand and loamy sand soils in Denmark. On each farm, a 3‐year‐old grass–clover field was selected. Half of the field was ploughed the first year and the other half was ploughed the following year. Spring barley (Hordeum vulgare L.) was sown after ploughing in spring. Measurements showed moderate N leaching during the pasture period (9–64 kg N ha^?1 year^?1) but large amounts of leaching in the first (63–216 kg N ha^?1) and second (61–235 kg N ha^?1) year after ploughing. There was a small yield response to manure application on the sandy soil in both the first and second year after ploughing. To investigate the underlying processes affecting the residual effects of pasture and N leaching, the dynamic whole farm model farm assessment tool (FASSET) was used to simulate the treatments on both farms. The simulations agreed with the observed barley N‐uptake. However, for the sandy soil, the simulation of nitrate leaching and mineral nitrogen in the soil deviated considerably from the measurements. Three scenarios with changes in model parameters were constructed to investigate this discrepancy. These scenarios suggested that the organic matter turnover model should include an intermediate pool with a half‐life of about 2–3 years. There might also be a need to include effects of soil disturbance (tillage) on the soil organic matter turnover.     

The spatial variability of soil nitrates in arable and pasture landscapes: implications for the development of geographical information system models of nitrate leaching

Abstract. In response to the European Community Nitrate Directive (91/676) a catchment scale Geographical Information System (GIS) model of nitrate leaching has been developed to map nitrate vulnerability and predict average weekly fluxes of nitrate from agricultural land units to surface water. This paper presents a pilot study which investigated the spatial variability of soil nitrates in order to: (1) define an appropriate pixel size for modelling N leaching; (2) quantify the within-unit variability of soil nitrate concentrations for pasture and arable fields; and (3) assist in the design of an efficient sampling strategy for estimating mean nitrate concentrations. Soil samples, taken from two 800 m transects in early September 1994, were analysed for water soluble nitrate. The arable soils had a mean nitrate-nitrogen concentration of 0.693 μg/g (S.E. 0.054 μg/g) and the pasture soils had a higher mean nitrate-nitrogen concentration of 0.86 μg/g (S.E. 0.085 μg/g). Spatial variability was investigated using variograms. The pasture data had a weak spatial relationship, whereas the arable data exhibited a strong spatial relationship which fitted a spherical variogram model (r² 0.87), with a range of 40 m. A pixel size of 40 m is suggested for nitrate modelling within the GIS based on the arable variogram and an improved sampling strategy for model validation is suggested, involving bulking sub-samples over a 40 m grid for estimating mean nitrate concentrations in combined land use and soil units.     

华北平原农田裂缝对硝态氮淋溶的影响

土壤干缩开裂是常见的自然现象。目前关于土壤干缩开裂的研究主要集中于裂缝的最终形态特征,并且以室内试验为主。本研究通过室外大田试验,结合动态计算机图像分析及水氮运移模拟软件WHCNS,研究土壤干缩开裂的动力学过程、特征及其对农田水氮运移的影响。利用原位熔化石蜡浇筑得到了裂缝三维结构形态,借助三维激光扫描仪量化裂缝的几何特征,发现每平米裂缝平均长度为4.58m,裂缝上表面平均宽度为5.72 mm,平均深度为9.06 cm。基于三维扫描仪提取得到的裂缝几何参数,通过WHCNS仿真模拟,发现相较于无裂隙情况,裂隙的存在分别增加了传统施肥和优化施肥情况下97.40%和256.43%的硝态氮淋失量;与优化施肥模式相比,传统施肥模式更容易造成硝态氮的淋失风险。在模拟灌溉模式对硝态氮淋洗情况的影响时,其差异不明显;强降雨的设置同样增加了硝态氮的淋失风险,导致硝态氮的年均淋洗量增加83.61%。裂缝的存在严重影响农田作物对肥料的吸收和利用,通过优化施肥量、更改灌溉模式以及避免强降雨前施肥都可以减少肥料的损失。     

Factors influencing nitrogen saturation in Sitka spruce stands in Wales,UK

Preliminary empirical nitrogen critical load exceedance maps for the UK have identified large areas of Wales where nitrogen deposition exceeds the nitrogen critical load, indicating that some ecosystems are at risk from eutrophication. This paper synthesises the monitoring and experimental work which have been carried out to collect evidence for exceedance in spruce plantations in the uplands and to investigate the implications for acidity and eutrophication in these areas. The results have conclusively demonstrated that current nitrogen deposition to mature Sitka spruce stands, planted on freely draining acid soils, is in excess of ecosystem requirements and results in elevated nitrate leaching losses. In contrast, stands with large biological sinks such as aggrading stands or stands with high denitrification potential have low ntirate leaching losses. The controls on the magnitude of leaching losses in the mature stands on this soil type are discussed within the context of two categories: (i) “nitrate saturated” stands which exhibit no retention of incoming nitrate-N but retain incoming ammonium-N as demonstrated in the Welsh NITREX experimental site and (ii) more nitrogen-rich stands which are saturated for both nitrate-N and ammonium-N and respond directly to incoming ammonium-N with immediate increases in nitrate production and thus nitrate leaching losses. There is little evidence for any adverse effects on tree growth or health in response to excess nitrogen deposition, however, tree growth in the most mature stands is now limited by phosphorus and potassium deficiency. The risk of a reduction in soil and stream water quality in acid sensitive areas of Wales due to the link between nitrate leaching and aluminium concentrations has also been confirmed. The results are discussed within the framework proposed by Aber et al. 1989 for the sequence of changes in ecosystem function which occur following long-term chronic nitrogen deposition.     

不同施肥条件下农田硝态氮迁移的试验研究

NO^-₃-N的淋失是旱地农田氮素损失的重要途径之一，也是引起地下水污染的一个主要原因。在黄土高原地区，夏玉米生长正逢雨季，是NO^-₃-N淋失的主要时期。该研究基于阻水层理论和黄土高原地区传统的垄作习惯，在手工模拟机具成垄压实施肥的基础上研究了该施肥法与传统的平地施肥、垄沟施肥(成垄不压实)条件下土壤NO^-₃-N的迁移动态，结果表明，在供水量相同条件下，由于平地和垄沟条件下水分分布的差异，导致平地土壤中的NO^-₃-N较垄沟耕作易于迁移。在生育前期，由于作物根系对NO^-₃-N的吸收和拦截，成垄压实与成垄不压实施肥对阻止NO^-₃-N随水下移差异不大；生育后期，当作物需肥量减小时，成垄压实施肥能够阻止NO^-₃-N向深层土壤迁移累积。玉米收获后，3种施肥方式下土壤NO^-₃-N迁移深度为平地(>60 cm)>垄沟施肥(>45 cm)>成垄压实施肥(＜35 cm)。     

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha^–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha^–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone. Received: 15 April 1996