不同水氮用量对日光温室黄瓜季硝态氮淋失的影响

于2010年3～7月,在河北省辛集市马庄农场研究了不同水氮用量对黄瓜季硝态氮淋失的影响,结果表明,通过调节不同生育阶段灌水量使黄瓜全生育期土壤含水量保持在18.7%～22.1%,不仅可以满足黄瓜生长发育对土壤水分的要求,而且可以减少用水量30%。不同处理中以节水灌溉、习惯施氮处理（W2N1）土壤硝态氮含量最高,习惯灌水、减量施氮处理（W1N2）最低。全生育期内,土体95cm深度硝态氮淋失量与土壤含水量、土壤硝态氮含量均呈正相关,其中以初瓜期和盛瓜期相关性系数最高。与农民习惯水氮处理（W1N1）相比,节水减氮处理（W2N2）在节水30%减施氮25%的情况下,可以显著降低黄瓜季土壤硝态氮淋失量,整个生育期降低淋失量35.0%。3年连续试验结果表明,节水减氮处理（W2N2）与习惯水氮处理（W1N1）间黄瓜产量结果差异不显著,说明河北省温室大棚蔬菜生产,目前农民习惯施氮和灌水量有很大的节水节肥空间,根据蔬菜不同生育期需肥量和土壤含水量来合理分配水、氮可取得明显的节水节氮效果。     

Nitrate leaching and adsorption in a Kenyan Nitisol

Abstract. ¹⁵N labelled NH₄NO₃ (fertilizer N) was applied at a rate of 50 kg N ha^–1 to an Ando-Humic Nitisol and two maize crops grown on it. About 20 months later, soil cores were taken to a depth of 2.5 m. Leached fertilizer N was found between 1.4 m and 1.8 m deep and was delayed relative to net drainage by between 4.2 and 4.9 pore volumes. Anion exchange capacity (AEC) increased ten-fold down the profile, up to 2.9 cmol_ckg^–1. The delay to fertilizer N leaching was predicted to be between 4.1 and 5.3 pore volumes when calculated from the AEC and from an equation relating delay due to AEC in laboratory columns of repacked soil obtained by Wong et al. (1990b). It was concluded that the nitrate leaching delay equation was also valid in undisturbed field profiles. Two concentration maxima for mineral N were found, which did not usually coincide with the fertilizer N and were thought to result from mineralization of soil organic matter and plant residues at the end of each season. The delay equation overestimated their leaching delay but the results were considered close enough to support the hypothesis for their formation.     

华北平原大孔隙优先流对农田氮素淋溶的影响

优先流是土壤水分入渗的一个重要途径,大孔隙是产生优先流的关键因素。研究优先流对于土壤水分和溶质运移研究及生态环境保护、制定合理的田间管理措施等具有重要意义。本研究将田间亮蓝染色示踪试验和WHCNS(soilwaterheatcarbonnitrogensimulator)模型模拟相结合,研究了华北平原冬小麦-夏玉米轮作体系存在大孔隙下,强降雨和不同施肥、灌溉情景下土壤水氮运移的情况,以此探讨大孔隙优先流对于土体中水分和硝态氮运移的影响。结果表明:明显含有虫洞的免耕土壤入渗深度和染色面积均高于旋耕土壤;免耕土壤的染色面积和稳定入渗速率的Pearson相关性不显著,染色示踪不能定量化土壤稳定入渗速率。同时WHCNS模拟的0～100cm土层硝态氮淋洗量结果显示:一方面,相较于无大孔隙情景,大孔隙存在会显著增加硝态氮的淋洗量;另一方面,大孔隙存在下优化施肥模式的硝态氮淋洗量比传统施肥模式减少46.0%。常规灌溉量下喷灌比漫灌处理的硝态氮淋洗量减少15.6%;强降雨导致硝态氮淋洗量增加119.4%。本研究为华北平原地区大孔隙存在条件下的农田水肥优化管理措施提供了理论指导。     

夏玉米生长期黄淮海平原土壤水氮利用效率模拟分析

以黄淮海平原为研究区域,建立并验证了基于地理信息系统(GIS)的土壤水、热、氮和作物生长联合模型。在1999年黄淮海平原的农村社会经济条件和土壤、气候等条件背景下,运用基于GIS的联合模型对夏玉米生长期土壤水氮利用效率和氮素损失量的区域分布规律进行评价。结果表明,由于自然条件和农田管理措施的差异,水分利用效率(WUE)、氮素利用效率(NUE)及土壤氮素淋失情况的空间分布在各地貌区划区之间有明显差异。多元线性逐步回归分析表明：1 m土体的氮素淋失量与施氮量、饱和导水率(Ks)、积温、降水和灌水量均呈极显著的正相关;WUE与施氮量呈极显著的正相关,而与积温呈极显著的负相关;NUE与降水和灌水量呈极显著的正相关,而与施氮量和土壤有机质含量呈极显著的负相关。     

Leaching of dissolved organic carbon and nitrogen under cotton farming systems in a Vertisol

Leaching with deep drainage is one of the loss pathways of carbon (C) and nitrogen (N) in cropping fields. However, field studies in irrigated row cropping systems are sparse. A 3‐year investigation on C and N leaching associated with deep drainage was overlaid on a long‐term experiment on tillage practices and crop rotations in Australia. The treatments included cotton (Gossypium hirsutum L.) monoculture and cotton–wheat (Triticum aestivum L.) or maize (Zea maize L.) rotations with maximum or minimum tillage. The deep drainage C and N concentrations at 0.6 and 1.2 m depth were measured after furrow irrigation with ceramic cup samplers during the 2014–15, 2015–16 and 2016–17 cotton seasons. Pre‐planting dissolved organic carbon (DOC) concentration in soil at 0.6–1.2 m depth during 2016–17 was 64 mg kg^?1 for maximum tilled cotton monoculture, 36 mg kg^?1 for minimum tilled cotton monoculture and 39 mg kg^?1 for cotton–wheat, and in maize and cotton subplots 51 and 41 mg kg^?1, respectively. Post‐harvest DOC values in soil were similar in all treatments (average of 32 mg DOC kg^?1). Total organic carbon (TOC) losses in deep drainage were equal to 2%–30% of TOC gained in irrigation water. Oxidized N losses in deep drainage ranged from 0.7% to 12% of applied N (260 kg ha^?1). NOx‐N concentrations in leachate under maize systems (20 mg L^?1) were up to 73% lower than those in cotton systems (75 mg L^?1). Maize sown in rotation with cotton can improve cotton yield, reduce N leaching and improve N use efficiency of subsequent cotton.     

Surface ozone concentrations and exposures during growing season at the Lithuanian rural site

The surface ozone (O₃) data show an increase by 2.6 % per year during the period 1982–1994 at the rural site of Lithuania. WHO (World Health Organization), UN-ECE (United Nations Economic Commission for Europe), CES (Commission of the European Communities) guideline values for the protection of vegetation from adverse effects are exceeded during the growing season at the Preila coastal station. Ozone exposures for different concentration threshold are estimated during daylight hours in April-September. These values above 60 g/m³ varied between 10 000 and 43 000 (g/m³) ·h, above 80 g/m³ — between 1700 and 15 000 (g/m³) ·h, above 100 g/m³ — between 130 and 3700 (g/m³) ·h during separate years. Maximum hourly ozone values were observed from 116 to 228 g/m³ during this period.     

Changes in the structure of differently tilled soil in a growing season

Changes in the structure of tilled soil over a growing season were investigated. Structural data from ten differently tilled plots were collected at the 40–50 mm depth on sectioned tilth block samples impregnated with paraffin wax. At the end of the growing season, significant increases in clod size and decreases in void size were observed. In some plots also a significant reduction in percentage of small (1–5 mm) aggregates was observed, but a crop cover (barley) increased the percentage of small aggregates.     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

The effects of crop husbandry and nitrogen fertilizer on nitrate leaching from a shallow limestone soil growing a five course combinable crop rotation

Abstract. Nitrogen (N) leaching losses from a shallow limestone soil growing a five course combinable croprotation (oilseed rape, wheat, peas, wheat, barley) were measured from 1990 until 1995 using porous ceramic cups, at 60 cm depth, and drainage estimates. The crops were grown with three husbandry systems and two levels of N fertilizer. The husbandry systems were designed to reflect local practice (Standard), the best possible techniques to reduce N loss (Protective) and an Intermediate system which was a compromise between the two. Nitrogen was applied at full and half recommended rates. Drainage started during September in four years and November in one year, with above average drainage in three years. Losses of N were largest after peas (58 kg/ha) and oilseed rape (42 kg/ha), and least (17 kg/ha) before peas sown in spring after a cover crop. Over five years, the Protective management system, which used early sowing and shallow cultivation wherever possible, lost least N (31 kg/ha/y) and the Standard system, with conventional drilling dates and ploughing as the primary cultivation, lost most (49 kg/ha/y). Halving the N fertilizer decreased N loss by 11 kg/ha/y, averaged over the rotation. None of the treatments gave mean drainage water nitrate concentrations of less than 50 mg/l, averaged over the five years. Changes to arable cropping alone will not eliminate the need for other measures to control nitrate concen-trations in public drinking water supplies.     

设施菜地土壤氮素运移及淋溶损失模拟评价

设施菜地因大水大肥管理方式导致的氮素淋失已成为当前关注焦点。探寻氮素淋失阻控技术需要首先探明土壤中NO_3~--N的运移和淋失过程,找到淋失阻控的关键点,从而实现蔬菜栽培高产量低环境成本。本研究以京郊设施菜地黄瓜-番茄轮作系统为研究对象,通过田间试验获取土壤温度、湿度、NO_3~--N含量等数据,对反硝化-分解(DNDC)模型进行参数校验,并以农民常规种植模式为基线情景,设置改变土壤基础性质、灌溉量、施氮量等不同情景,运用DNDC模型对设施菜地系统土壤氮素运移及淋溶损失进行定量评价。结果表明:经验证后的DNDC模型能够较好地模拟蔬菜产量、5 cm土壤温度和0～20 cm土壤孔隙含水率变化以及NO_3~--N的迁移过程,是模拟和评价氮素运移和损失的有效工具。模拟不同情景表明,设施菜地0～60 cm土壤NO_3~--N累积主要受灌溉水量和氮肥施入量的影响,此外土壤pH和土壤有机碳的变化也是影响NO_3~--N运移的重要因子。节水节肥是设施菜地氮素淋失减量的最有效方法,相比常规措施,同时减少20%灌溉量和20%施氮量可明显降低59.04%的NO_3~--N淋失量。同时,在节水节肥的基础上改变灌溉方式并提高20%土壤有机碳含量,在保证蔬菜产量的前提下,能够进一步降低69.04%的NO_3~--N淋失量。可见, DNDC模型为设施菜地NO_3~--N淋失评价和阻控提供了一个较好的解决方案。在当前重点关注减氮节水等管理措施的同时,提高土壤本身的质量,不失为一种更有效的减少设施菜地氮素淋失的途径。     

Overland flow systems for treatment of landfill leachates—Potential nitrification and structure of the ammonia-oxidising bacterial community during a growing season

Overland flow systems are useful for treating landfill leachates, because they provide favourable conditions for nitrification and they are easy to maintain. However, little is known about the microbial communities in such systems or the nitrification capacity of those microorganisms. In this study, seasonal variations in potential nitrification and in community composition of nitrifying bacteria were investigated in two overland flow areas receiving leachate from landfills at Korslöt and Hagby, Sweden. Samples were collected in the settling ponds sediment and at two depths in the overland flow areas (the macrophyte litter layer and the rhizosphere) in May, August and November 2003. A short-term incubation method was used to measure potential oxidation of ammonia and nitrite (designated PAO and PNO). The ammonia-oxidising bacterial (AOB) community was investigated using a 16S rRNA gene approach that included PCR amplification and analysis of PCR products by denaturing gradient gel electrophoresis (DGGE), followed by nucleotide sequencing and phylogenetic analysis.PAO was determined in the range 5-2700 (NO₂⁻+NO₃⁻)-N g⁻¹ dw d⁻¹ and PNO in the range 60-2000 μg NO₂⁻-N g⁻¹ dw d⁻¹. At Korslöt, PAO and PNO showed similar temporal variation in the different ecosystems, whereas no such relationship was noticed at Hagby. Considering both sites, there was no obvious change in the composition of the AOB community over the growing season. However, the composition did differ between the ecosystems: Nitrosomonas-like sequences were more common in the ponds, and in the litter layers they were found as often as Nitrosospira-like sequences, whereas Nitrosospira-like sequences were more common in the rhizospheres. Altogether, we found nine different AOB sequences, five Nitrosomonas-like and four Nitrosospira-like, which belonged to clusters 0, 2, 3b, 6a, 6b and 7. There was no apparent relationship between the number of AOB populations and the PAO in different soil layers and sediments.     

黄土高原北部生长季土壤氮素矿化对植被和地形的响应

氮素矿化是陆地生态系统氮循环的重要过程,对氮素有效性有着重要影响。本文在黄土高原北部六道沟小流域选取退耕年限相近的油松和柠条坡地,用原位培养法测定生长季节(4—10月)不同坡位冠层下和冠层外0~10 cm和10~20 cm土层土壤氮素矿化速率,以确定该区氮素矿化的季节动态特征和主要影响因素。结果表明,研究区生长季土壤矿质氮以铵态氮为主,其含量在0~10 cm和10~20 cm土层分别占矿质氮总量的61%和70%,并随生长季的推移而升高。油松林上坡位和中坡位土壤铵态氮显著高于下坡位土壤,柠条林不同坡位铵态氮差异不显著。土壤硝态氮和矿质氮不受坡位的影响,但与林型和采样位置有关,冠层下硝态氮在油松林与冠层外相近,在柠条林则高于冠层外。生长季土壤氮素矿化在0~10 cm土层由硝化作用引起,在10~20 cm土层则由硝化和铵化作用共同引起。铵化速率在生长季初期较高,中期较低,并受坡位、林型和采样位置的影响。土壤硝化和矿化速率在油松林不受采样位置影响,但是在柠条林则以冠层下较高。硝化和矿化速率在冠层下以下坡位土壤最高,在冠层外则以下坡位土壤最低。柠条林促进了冠层下土壤氮素的硝化和矿化过程,有利于矿质氮的积累;油松林对矿质氮和氮素矿化的影响不受采样位置影响。     

Changes of the displacement height and roughness length of maize during a growing season

Variations in the displacement height (d) and roughness length (z₀) of a maize crop were investigated through a growing season. A programme of measurement from which the wind profile, the Richardson gradient number and the turbulent fluxes of heat and momentum were estimated, was carried out.Two techniques were combined to obtain unique values for d and z₀: the log-profile fitting technique and the eddy correlation technique. Throughout the growing season, the displacement height appeared to correlate very well (r = 0.98) with the height (H) of the canopy. A mean value for d/H was 0.75. The roughness length was strongly correlated with the difference between the canopy height and the displacement height. A mean value of 0.26 for z₀/(H − d) was found (r = 0.86). If the ratio d/H was fixed, the roughness length did not show any clear dependence on wind speed or thermal stability.     

Soil phosphorus dynamics in a Vertisol as affected by cattle manure and nitrogen fertilization in soybean‐wheat system

Repeated application of phosphorus (P) as superphosphate either alone or in conjunction with cattle manure and fertilizer N may affect the P balance and the forms and distribution of P in soil. During 7 years, we monitored 0.5 M NaHCO₃ extractable P (Olsen‐P) and determined the changes in soil inorganic P (P_i) and organic P (P_o) caused by a yearly dose of 52 kg P ha^—1 as superphosphate and different levels of cattle manure and fertilizer N application in a soybean‐wheat system on Vertisol. In general, the contents of Olsen‐P increased with conjunctive use of cattle manure. However, increasing rate of fertilizer nitrogen (N) reduced the Olsen‐P due to larger P exploitation by crops. The average amount of fertilizer P required to increase Olsen‐P by 1 mg kg^—1 was 10.5 kg ha^—1 without manure and application of 8 t manure reduced it to 8.3 kg ha^—1. Fertilizer P in excess of crop removal accumulated in labile (NaHCO₃‐P_i and P_o) and moderately labile (NaOH‐P_i and P_o) fractions linearly and manure application enhanced accumulation of P_o. The P recovered as sum of different fractions varied from 91.5 to 98.7% of total P (acid digested, P_t). Excess fertilizer P application in presence of manure led to increased levels of Olsen‐P in both topsoil and subsoil. In accordance, the recovery of P_t from the 0—15 cm layer was slightly less than the theoretical P (P added + change in soil P — P removed by crops) confirming that some of the topsoil P may have migrated to the subsoil. The P fractions were significantly correlated with apparent P balance and acted as sink for fertilizer P.     

冬小麦生长期土壤固定态铵与微生物氮的动态研究

田间试验研究冬小麦生长期土壤固定态铵和微生物N动态变化结果表明 ,施入基肥后土壤固定态铵显著上升 ,春季后固定态铵显著下降 ,至扬花期降至最低点。作物生长后期随吸N量的降低 ,各施肥处理固定态铵含量约升至播前水平。冬小麦全生育期土壤微生物N呈明显季节变化 ,施基肥后短期内有所升高 ,且春季施肥后出现第 2次升高 ,至扬花期土壤微生物N降至最低点 ,至生长后期重新回升。     

甜玉米填闲减缓菜田土壤硝酸盐淋溶的研究

为提高菜田氮肥的利用率,降低氮肥对环境的污染风险。在中德合作项目东北旺试验田（土壤类型为潮土）,三年（1999～2001）9季蔬菜长期传统施氮灌水与推荐施氮灌水处理的地力基础上,于高温多雨的夏季选择甜玉米作为填闲作物,以休闲处理作对照,研究甜玉米在土壤硝态氮淋溶关键期对土壤残留硝态氮素分布及对后茬蔬菜产量的影响。研究结果表明甜玉米生长迅速、生物量大、吸收氮素能力强,吸氮量可达205.6～246.1 kg/hm²;与休闲处理相比,种植甜玉米能有效的吸收0～60 cm土壤中残留氮素,实现了土壤残留氮素的再利用。0～180 cm剖面中土壤硝态氮的残留量都有不同程度的降低,有效阻抑了氮素向土壤深层的淋洗。甜玉米也获得了较高的经济产量,穗净鲜重达9.2～10.2 t/hm²。后茬作物菠菜收获后未被利用的氮素大部分残留在土壤浅层;甜玉米处理与休闲处理相比未显著影响后季菠菜的生长,产量达18.4～20.7 t/hm²。该研究表明：甜玉米是较为理想的填闲作物。     

Calculation of nitrogen mineralization and leaching in fallow soil using a simple dynamic model

Simple models describing nitrogen processes are required both to estimate nitrogen mineralization in field conditions and to predict nitrate leaching at large scales. We have evaluated such a model called LIXIM, which allows calculation of nitrogen mineralization and leaching from bare soils, assuming that these are the dominant processes affecting N in bare soil. LIXIM is a layered, functional model, with a 1-day time step. Input data consist of frequent measurements of water and mineral N contents in soil cores, standard meteorological data and simple soil characteristics. The nitrate transport is simulated using the ‘mixing-cells’ approach. The variations in N mineralization with temperature and moisture are accounted for, providing calculation of the ‘normalized time’. An optimization routine is used to estimate the actual evaporation and the N mineralization rates that provide the best fit between observed and simulated values of water and nitrate contents in all measured soil layers. The model was evaluated in two field experiments (on loamy and chalky soils) including treatments, lasting 9–20 months. The water and nitrate contents in soil were satisfactorily simulated in both sites, and all treatments, including a ¹⁵N tracer experiment performed in the loamy soil. In the chalky soil, the calculated water balance agreed well with drainage results obtained in lysimeters and independent estimates of evaporation. At both sites, N mineralization was reduced by the incorporation of crop residues (wheat or oilseed rape straw); the amounts of nitrogen immobilized varied between 20 and 35 kg N ha^?1. In the treatments without crop residues, the mineralization rate followed first-order kinetics (against normalized time) in the loamy soil, and zero-order kinetics in the chalky soil. In the latter soil, the mineralization kinetics calculated in situ were close to the kinetics measured in laboratory conditions when both were expressed against normalized time.     

施氮量对潮土区冬小麦-夏玉米轮作农田氮磷淋溶的影响

潮土是我国华北地区主要土壤类型之一,潮土区是我国冬小麦-夏玉米作物的主要产区,研究不同施氮量潮土氮磷淋溶特征对于指导区域农田面源污染防控具有重要意义。本研究设置3个施肥处理,即传统施氮(CON)、优化施氮(OPT)和优化再减氮(OPTJ),利用田间渗漏池法,研究潮土冬小麦-夏玉米轮作农田硝态氮及总磷淋溶特征。结果表明:2016—2018年,冬小麦-夏玉米轮作周年不同施肥处理90cm土层年淋溶水量79.0～102.5 mm,不同淋溶事件间土壤淋溶液硝态氮浓度波动较大, CON、OPT和OPTJ处理单次淋溶事件硝态氮浓度分别为18.9～208.7(平均为72.7) mg·L~(-1)、9.0～99.2 (平均为33.8) mg·L~(-1)、4.7～55.5 (平均为15.4) mg·L~(-1)。本研究区域冬小麦-夏玉米轮作模式的氮素淋溶风险较高,磷素淋溶风险较低。传统施氮处理(CON)下农田硝态氮的平均淋溶量和表观淋失系数分别为66.4 kg·hm~(-2)和10.3%,而总磷(TP)为0.06 kg·hm~(-2)和0.04%。氮肥减施会显著降低氮素淋失,OPT和OPTJ处理的氮素淋溶减排率可达56.3%和78.9%。两个年度CON、OPT和OPTJ处理硝态氮平均表观淋失系数分别为10.3%、6.2%和4.9%,随着施氮量的增加,硝态氮淋失系数动态增加。氮淋溶具有较大的年际变化,降雨量高的2018年比降雨少的2017年硝态氮淋溶量多57.0%。两个年度CON、OPT和OPTJ处理总磷平均淋溶量分别为0.06 kg·hm~(-2)、0.06 kg·hm~(-2)和0.08 kg·hm~(-2)。适量减施氮肥会增加作物产量, OPT处理的作物产量是CON处理的1.08倍。然而,过量减施则会带来减产风险, OPTJ处理氮肥减施56%,作物产量比CON处理降低2.0%～8.1%。总之,潮土区农田硝态氮淋溶风险较大,适量减施氮肥能够在保证作物产量的基础上显著降低氮素淋失损失。     

Potential urea-derived nitrogen losses caused by ammonia volatilization and nitrogen leaching in a rainfed semiarid region,China

Abstract

In the rainfed semiarid region of the China Loess Plateau, rainfall is concentrated in the growing season and usually occurs in large storms. This makes for a high risk for fertilizer-derived nitrogen losses. However, relatively little attention has been given to fertilizer efficiency in this region. In this study, ammonia volatilization at natural field conditions as affected by urea-application rate was measured to evaluate the potential ammonia volatilization losses. Nitrogen-leaching potential in the 0–100 cm soil profile as affected by urea-application rate and -application depth for different rainfall-simulated irrigation regimes (two irrigation volumes: 70 and 105 mm; two irrigation methods: single irrigation and three-split irrigation) was also investigated. Results showed that the low initial soil moisture content at the crop-planting stage and a strong following rainfall made ammonia volatilization of low importance from an agricultural management perspective. The stock percentage of urea-derived nitrogen in the soil profile was significantly affected by irrigation method and irrigation volume but not by urea-application rate. Generally, the 105 mm irrigation volume and the single-irrigation method had a lower percentage of applied nitrogen remaining in the soil profile. The results of this rainfall-simulated irrigation investigation indicated that heavy rainfall, especially occurring in large storms, in this area increased the risk of nitrogen leaching. We recommend that split applications of urea should be adopted instead of a conventional lumped (single) application to reduce nitrogen losses in this rainfed semiarid region.