A usable mechanistic model of nitrate leaching II. Application

Simulations produced by a mechanistic model are compared with field observations of nitrate leaching from 0.35 ha hydrologically isolated experimental plots. The parameters used in the model are obtained in two ways. First by fitting the model to field observations in one year. These parameters are then used to simulate leaching in other years. Second, model parameters are obtained by fitting eluant profiles from pulse inputs of solute to undisturbed cores in the laboratory. When used with the field-derived parameters, the model simulates total leaching losses well in other years, although the pattern of loss is only approximately reproduced. The simulation suggests that water and solute flow in drained, structured soils is complex; preferential flow in the upper horizons resulting in 20% of the water-filled pore space carrying most of the solute flow, and by-pass flow in the subsoil causing rapid movement of water and solute to the drains. The result is that much of the nitrate in the upper horizons appears to be protected from leaching. When used with laboratory-derived parameters, the model was a poor predictor of both the pattern and quantity of nitrate leached.     

A regression model for nitrate leaching in Northern Ireland

Abstract. A model has been developed based on multiple regression which explains 95% of the variation in nitrate loading of the major rivers in the 4453 km² Lough Neagh catchment for the years 1971–1987. The model relates loading of nitrate in the hydrological year to fertilizer usage, previous summer rainfall, summer temperature of the current year and December-May flow. It indicates that there is an increase in nitrate loading associated with fertilizer usage, and that the equivalent of 13% of nitrogen fertilizer that is lost as leachate comprises 50% of the river loadings.     

A stochastic-empirical approach to modelling nitrate leaching

Abstract. Techniques for determining the probability density function (pdf) of travel times of solute molecules through a defined volume of soil, following a pulse or step-change input to the soil surface, are described. A stochastic transfer function model (TFM) based on the pdf of nitrate travel times works satisfactorily when the nitrate originates from a pulse input of soluble fertilizer to the soil surface. However, a TFM based on the pdf of a surface-applied tracer, such as chloride or tritiated water, is less satisfactory for simulating the leaching of indigenous soil nitrate. The main problems seem to be the difficulty of estimating mean nitrate concentrations because of the spatial variability of nitrate in field soils, accounting for denitrification during leaching, and the uncertain reproducibility of the soil's transport characteristics, as embodied in its operationally defined fractional transport volume, θ _st , Nevertheless, for many practical applications, a simplified empirical model which treats the soil's transport volume as a well mixed reactor of average initial concentration C, can provide satisfactory predictions of the quantity of nitrogen leached over extended periods. Irrespective of which model is used, a comprehensive treatment of nitrate leaching, particularly for soil generated nitrate, requires a detailed knowledge of transfers of labile nitrogen within the transport volume, and across its boundaries other than those monitored at the input and output surfaces.     

An amended functional leaching model applicable to structured soils. I. Model description

A functional model designed to simulate the transport of non-interactive solutes through macroporous soil is described. The concept of mobile and immobile water is used but the pore volume available for mobile water is partitioned to allow for flow through smaller water-filled pores and rapid preferential flow through larger macropores and fissures. The general performance of the model under steady infiltration following an injection of solute is presented. The sensitivity of the output to variations in the model parameters is also discussed. A second paper compares the model with bromide and chloride leaching data on two texturally contrasting soils.     

Uncertainty in simulated nitrate leaching due to uncertainty in input data. A case study

Abstract. Deterministic leaching models are used to estimate regional losses of nitrate from agricultural land to the environment. The estimated leaching losses are associated with uncertainty arising from uncertainty in the input data used. In the present case study we have assessed this uncertainty by use of Monte Carlo analysis, using the Latin hypercube sampling technique. Input data have preferably been adopted from publicly available data. Data which could not be retrieved from the databases was assessed by guided estimates or based on local data. The estimated annual leaching loss from the study region was around 106 kg N ha⁻¹, which is in agreement with previous findings. The uncertainty in the leaching expressed in terms of coefficients of variation (CV) depended on the agricultural practices. CV's for arable farm rotations, cattle farm rotations, and pig farm rotations were around 20, 30 and 40%, respectively. Breakdown of the total uncertainty into contributions of different error sources did not isolate one single all important source.     

Polymerization kinetics of wheat gluten upon thermosetting. A mechanistic model

Size exclusion high-performance liquid chromatography analysis was carried out on wheat gluten-glycerol blends subjected to different heat treatments. The elution profiles were analyzed in order to follow the solubility loss of protein fractions with specific molecular size. Owing to the known biochemical changes involved during the heat denaturation of gluten, a mechanistic mathematical model was developed, which divided the protein denaturation into two distinct reaction steps: (i) reversible change in protein conformation and (ii) protein precipitation through disulfide bonding between initially SDS-soluble and SDS-insoluble reaction partners. Activation energies of gluten unfolding, refolding, and precipitation were calculated with the Arrhenius law to 53.9 kJ x mol(-1), 29.5 kJ x mol(-1), and 172 kJ x mol(-1), respectively. The rate of protein solubility loss decreased as the cross-linking reaction proceeded, which may be attributed to the formation of a three-dimensional network progressively hindering the reaction. The enhanced susceptibility to aggregation of large molecules was assigned to a risen reaction probability due to their higher number of cysteine residues and to the increased percentage of unfolded and thereby activated proteins as complete protein refolding seemed to be an anticooperative process.     

桓台县高产农田土壤硝态氮淋失动态研究

试验研究高产农田生态系统条件下N肥施用量和秸秆还田对土体硝态氮（NO3^--N）的时空分布动态结果表明，NO3^- -N含量在空间上随土壤深度而降低，这一相关关系可用Y=aX^b函数表达。小麦-玉米2季秸秆还田同单季小麦秸秆还田对NO3^- -N的动态影响较小，但相同施N量下未进行玉米秸秆还田0-40cm土层土壤中NO3^- -N含量偏高，土体NO3^- -N有淋失较强的趋势。土体NO3^- -N含量年度内波动大小与施N量密切相关，0-40cm土层土壤内NO3^- -N含量起伏最大，60cm土层以下相对稳定。各土层内NO3^- -N含量与施N量相关密切，这一相关关系影响到2m土层深度。土体中NO3^- -N含量周年内出现2次峰值和1次低谷，峰值出现在玉米和小麦收获后，低谷发生在小麦苗期-开花期土体养分大量吸收时期。9月下旬2m土层土壤NO3^- -N含量可高达10mg/kg，而且有淋失出2m土体的趋势。     

基于DNDC模型的环渤海典型小流域农田氮素淋失潜力估算

为了定量评价流域尺度氮素污染的可能性并探明氮素污染的主要来源,以期指导农业生产实际保护农田生态环境,该文主要运用农业生态系统生物地球化学模型(DNDC)模拟的方法,以环渤海典型小流域——小清河流域为例,在GIS流域数据库支持下对该流域氮素淋失潜力进行了估算。研究结果表明,2006年小清河流域年均氮淋失负荷范围为10.44×103～36.86×103t,平均为23.65×103t。以当年氮肥投入总量222.2×103t计算,该流域平均氮素流失量占氮肥投入的10.6%。不同地区氮素淋失空间分布差别较大,与氮肥施用量的空间分布规律大体一致。其中,44%和27%的地区氮素潜在淋失量分别集中在20～40和>40～80kg/hm2,这些地区主要分布在小清河两侧沿岸及寿光市大部分地区,给流域水环境造成了较大影响。研究结果显示流域氮淋失存在较大的空间区域差异,根据不同地区的实际情况进行水氮管理,减少氮素的无效丢失十分必要。     

施氮量对白萝卜硝酸盐含量和土壤硝态氮淋溶的影响

在保护地栽培条件下,通过6个施氮水平的田间小区试验,结合土层原位渗滤装置,研究了施用氮肥对白萝卜（Raphanus sativus L.）产量和硝酸盐含量及土壤硝态氮淋溶的影响。结果表明,施氮处理白萝卜产量比不施氮处理仅增加6.04%～10.92%,当尿素氮施用量大于N 100 kg/hm2时,增产幅度开始下降。不同施氮处理白萝卜产量没有显著差异,说明在土壤基础肥力较高的情况下,增施氮肥不能明显提高白萝卜的产量;单施有机肥白萝卜体内硝酸盐含量为 196.86 mg/kg,比不施氮处理降低 5.08%。在此基础上加施尿素后,硝酸盐含量随氮肥施用量的增加显著升高（p0.05）;0—100cm土壤剖面硝态氮累积量随氮肥施用量的增加而增加,且与氮肥施用量显著正相关（r=0.993, r0.01=0.917）;白萝卜生长期间收集到的土壤淋溶液中硝态氮浓度较高,平均为32.88 mg/L,硝态氮的淋失量为 4.42～6.14 kg/hm2,不同施氮量处理之间没有显著差异。     

Progress in studies of nitrate leaching from grassland soils

Abstract. The large input of research effort on aspects of nitrate leaching over the last two decades has produced many innovative scientific and practical results. The MAFF Nitrate Programme has enabled considerable progress to be made in unravelling much of the complexity of the grassland nitrogen (N) cycle, and identifying gaps as essential first stages in providing improved managements for N in grassland systems. From a practical standpoint, there have been key outputs which have allowed the identification of options for policy, and which should allow grassland farmers to increase the efficiency of N use throughout their farming system and thereby improve the sustainability of their enterprises. As well as quantifying N transformations, transfers and losses, other important outcomes have been the development of user-friendly models of N cycling (NCYCLE and variants) and an easy to use field kit to determine mineral N in pasture soils. The use of modelling to produce fertilizer recommendations with a Decision Support System and of new approaches developed within the Programme, in particular system synthesis desk studies, and 'farmlet' investigations to determine the consequences of modifying N flows and losses, have allowed us to produce solutions to satisfy the dual aims of meeting environmental and economic production targets.     

Testing a mechanistic model. I. The effects of time and temperature on the reaction of fluoride and molybdate with a soil

A mechanistic model that had been developed to describe the reaction of phosphate with soil was applied to the reactions of fluoride and molybdate with soil. The model assumes that: there is an initial adsorption reaction between ions and charged surfaces; that the surfaces are not uniform in their properties; and that the initial adsorption is followed by diffusive penetration. The model was developed beyond that previously published to permit the rate of the initial adsorption reaction to be included. The model reproduced the effects of solution concentration, and of increasing period and temperature of incubation, on the retention of both fluoride and molybdate. Increasing the temperature of incubation increased the retention, or decreased the solution concentration, and this was reproduced by the effect of temperature on the rate of diffusive penetration. Desorption of fluoride was also reproduced. This supports the argument that slow desorption is mostly due to the need to reverse the diffusive penetration. After incubation at constant temperature, increasing the temperature at which solution concentration of molybdenum was measured, increased the concentration. The direction of this effect of temperature was also reproduced. It arises because of the effect of temperature on the position of the equilibrium between adsorbed molybdate (in the strict sense of the term) and molybdate in solution.     

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。     

N mineralization and nitrate leaching from vegetable crop residues under field conditions: a model evaluation

Six different vegetable crop residues were incorporated in the field and N mineralization from the residues and from an unamended plot was followed over 4 months by periodically monitoring mineral N contents of the soil. The crop residues were also fractionated according to a modified Stevenson chemical fractionation. Nitrogen mineralization parameters of the first order kinetic model N(t)=N_A(1−e^−kt) were derived from the chemical fractionation data. The first order model was used in combination with a model describing the temperature dependence of N mineralization and a simple leaching model to predict N mineralization rates and nitrate redistribution after crop residue incorporation under field conditions. Comparison of predicted and measured mineral N contents in the upper soil layer (0–30 cm) before the start of leaching showed that the model was able to predict N mineralization from both soil organic matter and crop residues under field conditions. From the onset of leaching, mineral N contents were slightly overestimated in the upper layer and underestimated in the lower soil layers. Although the Burns leaching model underestimated the leaching rate, the general pattern of nitrate movement was simulated satisfactorily. Statistical analysis using the variance ratio test yielded small but significant F values, indicating that the model can still be improved. The modelling efficiency was rather high and the coefficient of residual mass very close to zero. Linear regression between measured and simulated nitrate contents over the whole profile (0–120 cm) for all samplings yielded Y=9.6+0.876X (r=0.94***) with all deviations smaller than 25 kg N ha⁻¹. Total N mineralization ranged from 48 kg N ha⁻¹ for the control plot to 136 kg N ha⁻¹ for the plots with cauliflower residues and cumulative leaching losses from 26–66 kg N ha⁻¹, with most of the mineral N left in the 60–120 cm layer. These results show that N losses by leaching in winter can be high when vegetable crop residues are incorporated, even when there is little mineral N in the soil at the time of incorporation.     

Development and testing of a model for predicting tillage effects on nitrate leaching from cracked clay soils

Both water movement and nitrate leaching in structured soils are strongly influenced by the nature of the macro-porosity. That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential impacts of tillage on rates of nitrate leaching from structured soils, a model specific to these soils, CRACK-NP was developed. The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal infiltration and root uptake (evaporation). The model reflects the influence of diffusion limitation in the release of solutes to by-passing water. The model was then used to investigate the influence of variable ped spacings which were created by variations in tillage practices. The results both from the model and from the field data demonstrated that finer soil structures, which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction between peds and the water moving around them, and so release more nitrates through the drainage waters.     

Effect of nitrate leaching in oxisol columns on 15N abundance and nitrate breakthrough curves

Abstract

Nitrate (20 mg N0₃‐ N l^?1) was leached through 180 cm columns of oxisol subsoil until the leachate attained the initial nitrate concentration. Leaching was continued with water until no nitrate was detectable in the leachate.

The Δ¹⁵N for the first aliquot of leachate containing nitrate was 2.2 units lower than that of the added solution indicating that ¹⁵Nwas preferentially adsorbed to ¹⁴N. The breakthrough curve for nitrate indicated that nitrate adsorption decreased after six pore volumes. The implications for modelling nitrate leaching are discussed.     

Impact of excreted nitrogen by grazing cattle on nitrate leaching

Abstract. At De Marke experimental farm, data on water and nitrogen flows in the unsaturated zone were gathered on two grazed pastures on sandy soils during the years 1991 to 1994. These provided a basis for calibration and validation of simulation models. The different levels of nitrate-N concentrations of the two plots could largely be explained by differences in crop uptake and simulated denitrification as influenced by different groundwater levels. The irregular distribution of excreta was taken into account by a simulation study quantifying the variability of nitrate-N concentrations under a grazed field. The resulting distribution of simulated nitrate-N concentrations explained the average and peak values of the measured concentrations. Temporal variability of weather was used to assess the nitrate leaching risk under urine patches deposited in either July or September. At site A the probability of exceeding the EC-directive by drinking water (11.3 mg/1 nitrate-N) under a urination deposited in either July or September was respectively 10 and 25%. The average field concentration at this site will hardly ever be a high risk for the environment under the current farm management. At site B the EC-directive will be exceeded under any urine patch in almost 100% of the years, affecting the field average concentration. In field B careful grazing management would result in less nitrate leaching, but the environmental goals would not be reached.     

The impact of nitrogen deposition on upland surface waters in Great Britain: A regional assessment of nitrate leaching

A national dataset of water chemistry collected for critical loads mapping is used to make a regional assessment of surface water nitrate concentrations in Great Britain. The primary data are dominated by high concentrations in lowland regions Where N inputs are dominated by non-atmospheric sources. Land cover data are used to screen out sites with potential catchment sources of N, allowing the evaluation of nitrate leaching due to atmospheric deposition alone. In the screened dataset several upland regions show elevated nitrate concentrations, notably Wales, the Pennines, Cumbria, Galloway and the Cairngorms, and there is a clear relationship between surface water nitrate and total N deposition.     

高产农田土壤硝态氮淋失与地下水污染动态研究

对桓台县区域农田监测研究表明,水肥管理不同的2个监测区域郭家区、李家区高产农田土体内NO_3~--N淋失迁移动态有差异,地下水污染亦不同。春天始土体内NO_3~--N含量趋于持续降低,浅层地下水NO_3~--N含量则持续升高,雨季后地下水中NO_3~--N含量尤剧烈升高,并达年内最高值,表现出农田N肥对地下水的直接污染,这可能与李家区灌溉次数多、土壤质地较轻和地下水位较浅有关。     

Effects of forage type and gibberellic acid on nitrate leaching losses

Nitrogen (N) leaching from soil into water is a significant concern for intensively grazed forage‐based systems because it can cause a decline in water quality and is a risk to human health. Urine patches from grazing animals are the main source of this N. The objective of this study was to quantify the effect that forage type and gibberellic acid (GA) application had on N leaching and herbage N uptake from urine patches on perennial ryegrass–white clover (RGWC), Italian ryegrass and lucerne. A lysimeter study was conducted over 17 months to measure herbage growth, N uptake and N loss to water beneath each of the three forage types with the following treatments: control, urine (700 kg N/ha) and urine with GA (8 g GA active ingredient/ha). Compared with RGWC (205 kg N/ha), N leaching losses were 35.3% lower from Italian ryegrass (133 kg N/ha) and 98.5% higher from lucerne (407 kg N/ha). These differences in leaching loss are likely to be due to winter plant growth and N uptake. During the winter months, Italian ryegrass had higher N uptake, whereas lucerne had lower N uptake, compared with RGWC. The application of GA had no effect on N leaching losses, DM yield or N uptake of forage treated with 700 kg N/ha urine.