Denitrification in the vadose zone and in surficial groundwater of a sandy entisol with citrus production

A portion of nitrate (NO ₃ ⁻ ), a final breakdown product of nitrogen (N) fertilizers, applied to soils and/or that produced upon decomposition of organic residues in soils may leach into groundwater. Nitrate levels in water excess of 10 mg L⁻¹ (NO₃–N) are undesirable as per drinking water quality standards. Nitrate concentrations in surficial groundwater can vary substantially within an area of citrus grove which receives uniform N rate and irrigation management practice. Therefore, differences in localized conditions which can contribute to variations in gaseous loss of NO ₃ ⁻ in the vadose zone and in the surficial aquifer can affect differential concentrations of NO₃–N in the groundwater at different points of sampling. The denitrification capacity and potential in a shallow vadose zone soil and in surficial groundwater were studied in two large blocks of a citrus grove of ‘Valencia’ orange trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jambhiri (L.)) under a uniform N rate and irrigation program. The NO₃–N concentration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collected from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundwater interface (SGWI). Groundwater samples from the monitoring wells (MW) were collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capacity ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N₂O–N kg⁻¹ soil at the surface soil and at the soil/groundwater interface, respectively. The denitrification potential increased by 100-fold with the addition of 200 mg kg⁻¹ each of N and C. The denitrification potential in the groundwater also followed a pattern similar to that for the soil samples. Denitrification potential in the soil or in the groundwater was greatest near the monitor well with shallow depth of vadose zone (MW3). Cumulative N₂O–N emission (denitrification capacity) from the SGWI soil samples and from stagnant water samples strongly correlated to microbial most probable number (MPN) counts (r² = 0.84 – 0.89), and dissolved organic C (DOC) (r² = 0.96 – 0.97). Denitrification capacity of the SGWI samples moderately correlated to water-filled pore space (WFPS) (r² = 0.52). However, extractable NO₃-N content of the SGWI soil samples poorly (negative) correlated to denitrification capacity (r² = 0.35). However, addition C, N or both to the soil or water samples resulted in significant increase in cumulative N₂O emission. This study demonstrated that variation in denitrification capacity, as a result of differences in denitrifier population, and the amount of readily available carbon source significantly (at 95% probability level) influenced the variation in NO₃–N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spatial distribution of nitrogen on grazed karst landscapes

The impact on water quality by agricultural activity in karst terrain is an important consideration for resource management within the Appalachian region. Karst areas comprise about 18% of the region"s land area. An estimated one-third of the region"s farms, cattle, and agricultural market value are located on karst terrain. Mean nitrate concentrations in several karst springs in southeastern West Virginia exhibit a strong linear relationship with the percentage of agriculture land cover. Development of best management practices for efficient nitrogen (N) use and reduction of outflow of N to water from karst areas requires knowledge about N dynamics on those landscapes. Water extractable NO3-N and NH4-N were measured along transects at four soil depths in two grazed sinkholes and one wooded sinkhole. Distribution of soil NO3-N and NH4-N were related to frequency of animal presence and to topographic and hydrologic redistribution of soil and fecal matter in the grazed sinkholes. Karst pastures are characterized by under drainage and funneling of water and contaminants to the shallow aquifer. Control of NO3-N leaching from karst pasture may depend on management strategies that change livestock grazing behavior in sinkholes and reduce the opportunity for water and contaminants to quickly reach sinkhole drains.     

Using the combined bioelectrochemical and sulfur autotrophic denitrification system for groundwater denitrification

A combined bioelectrochemical and sulfur autotrophic denitrification system (CBSAD) was evaluated to treat a groundwater with nitrate contamination (20.9-22.0mgNO(3)(-)-N/L). The reactor was operated continuously for several months with groundwater to maximize treatment efficiency under different hydraulic retention times (HRT) and electric currents. The denitrification rate of sulfur autotrophic part followed a half-order kinetics model. Moreover, the removal efficiency of bioelectrochemical part depended on the electric current. The reactor could be operated efficiently at the HRT ranged from 4.2 to 2.1h (corresponding nitrogen volume-loading rates varied from 0.12 to 0.24 kg N/m(3)d; and optimum current ranged from 30 to 1000 mA), and the NO(3)(-)-N removal rate ranged from 95% to 100% without NO(3)(-)-N accumulation. The pH of effluent was satisfactorily adjusted by bioelectrochemical part, and the sulfate concentration of effluent was lower than 250 mg/L, meeting the drinking water standard of China EPA.     

Nitrate loss from a restored floodplain in the Lower Cosumnes River, California

Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO ₃ ⁻ ) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO ₃ ⁻ -N loss from the overlying water in both sandy and clayey soils and at three levels of NO ₃ ⁻ -N (ambient, +1 mg N l⁻¹, +5 mg N l⁻¹). In addition, we examined NO ₃ ⁻ -N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N₂O–N cm⁻³ h⁻¹. In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 μg N l⁻¹ h⁻¹ over all treatments and all 3 months examined. Significant loss of NO ₃ ⁻ -N (60–93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO ₃ ⁻ -N uptake by phytoplankton. When scaled up, potential NO ₃ ⁻ -N loss from the restored floodplain represented 0.6–4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and ~24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO ₃ ⁻ sink.     

乡村城镇化水污染的生态风险及背景警戒值的研究

对江南地区乡村城镇化引起水污染的生态风险进行定量估测与相关分析表明 ,地表水BOD5、NO 3 N、苯酚、表面活性剂和降水 pH、Pb、NO 2 N、苯酚及地下水NO 3 N等水质参数与小城镇人口总数存在着一元线性显著正相关 ,地下水氯化物、细菌总数、洗涤剂和地表水CODCr等水质参数与小城镇人口总数存在着指数显著正相关 ,地表水BOD5、苯酚、表面活性剂和地下水NO 3 N、氯化物、细菌总数、洗涤剂及降水pH、苯酚、Pb等水质参数与小城镇人口密度存在着一元线性显著正相关 ,降水NO 2 N与小城镇人口密度存在着指数显著正相关 .并对该地区小城镇人口的背景极限值和最大允许背景密度进行了推算 .     

退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况,同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定,随草地植被状况和植物生长时期变化不大,说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低,到植物枯黄期含量又回复到较高的水平;土壤NH+4-N含量随植物生长有逐渐升高的趋势;IN则随着植物的生长出现低-高-低-高的特点,且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N,NO－3-N与植被地上绿色生物量有显著的负相关性,是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01）,Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01）,说明土壤微生物对土壤NH+4-N有偏好吸收。总体上,不同形态的氮素在各土壤层次间差异显著,随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后,各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著;而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致,即以前过度和中度放牧处理的Micro-N含量较高,无牧和轻牧含量较低;IN、NH+4-N和NO－3-N变化比较复杂,在不同放牧恢复处理上结果并不一致。总的来看,以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高,存在潜在损失的可能。经过两年的恢复,植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。     

Nitrate removal by a novel autotrophic denitrifier (Microbacterium sp.) using Fe(II) as electron donor

A novel denitrifying bacterium was isolated using bicarbonate as the sole carbon source in a defined medium. Strain W3 was isolated from deep sediments of East Lake (Wuhan, China). In this study, analysis of 16S rRNA genes showed that strain W3 was affiliated with Microbacterium sp. When using Fe2+ as the only electron donor, this strain could convert 88.6 % of NO3 −-N to N2, corresponding to an Fe2+ oxidation rate of 80 %. Meanwhile, neither NO2 −-N nor NH4 +-N was accumulated after the experiment. In similar experiments with Fe(II)-EDTA, cell encrustations did not occur and supplementary substrates were consumed. The accumulated NO2 −-N was below 2.5 mg L−1. In addition, PCR revealed five kinds of key denitrifying genes: narG, napA, nirS, norB and nosZ. These results indicated that strain W3 could be used as an alternative autotrophic denitrifier for the treatment of groundwater and low C/N ratio wastewater.     

太湖上游不同类型过境水氮素污染状况

利用GPS定位,在春、夏、秋、冬4个季节,对地处太湖上游宜兴地区受农田径流、农村生活污水、水产养殖和畜禽养殖污水污染的过境水体氮素污染状况进行了调查,对不同类型水体中不同形态氮浓度的季节性变化特征进行了分析,比较了不同类型水体的15N自然丰度.结果表明:不同类型过境水氮素污染严重,人为影响强烈,不同的农业生产和人类活动具有不同的污染特征;受农田径流影响的河流,水体受铵态氮污染的风险较小,受硝态氮的污染较重;农村居民区河流主要受农村生活污水影响,水体铵态氮负荷较高;畜禽养殖场附近的河流受养殖污水的影响,水体的氮素污染最为严重,尤其是铵态氮负荷最高;相比之下,养殖鱼塘水体总氮最低,主要以有机氮为主;水体氮素污染受河流季节性环境演变的影响,表现为夏季浓度最低,冬季浓度最高;农田施肥是太湖氮污染的主要来源,但在丰水期生活污水和畜禽养殖废水也会成为太湖重要的污染源.     

不同施肥模式调控沿湖农田无机氮流失的原位研究——以南四湖过水区粮田为例

为探索山东南四湖沿岸麦玉轮作区玉米季内减少土壤无机氮素淋溶和径流损失的施肥策略,降低其对湖区水质产生的潜在威胁,采用田间原位安装淋溶水采集器和地表水径流池收集水样结合室内分析不同形态氮含量的方法,研究了不同施肥模式下无机氮素淋溶和径流损失特征。结果表明：土壤淋溶水量及地表水径流量与降水呈显著正相关关系,其水量受秸秆类物质还田的影响;硝态氮(NO3--N)与铵态氮(NH4 -N)随地表水径流损失的浓度及总量均明显高于淋溶水,由径流方式损失的氮素占2/3以上,是氮素以水溶液形式流失的主要途径;淋溶和径流均以NO3--N损失为主(径流损失中NO3--N占总量的82.9%-90.8%,淋溶损失中NO3--N占63.5%-72.9%),地表径流水NO3--N浓度对水质有较大影响,但土壤淋溶水NO3--N浓度对地下水污染不构成威胁;农民习惯施肥处理在玉米整个生育期淋溶和径流氮损失最高。在保证玉米产量前提下,降低氮素流失造成湖区的污染,平衡施用氮磷钾肥、施用控释氮肥、有机替代无机和秸秆还田等措施均可在沿南四湖区农田使用。     

水网平原地区不同种植类型农田氮磷流失特征

采用田间径流小区定位研究方法,在浙江省绍兴县选择27块农田,研究了自然降雨条件下水网平原地区7种种植类型农田N、P的径流流失特征、负荷及影响因素.结果表明： 农田径流总P（TP）、水溶态P（DP）和颗粒态P（PP）的年流失量平均分别为4.75、0.74和4.01 kg·hm^-2;PP占TP的比例高于DP.径流总N（TN）、水溶态总N（DTN）、水溶态有机N（DON）、NH₄⁺-N和NO₃^--N的年流失量平均分别为21.87、17.19、0.61、3.63和12.95 kg·hm^-2;流失的DTN各组分以NO₃^--N为主,其次为NH₄⁺-N,DON的比例较低.不同种植类型农田径流TN、DTN、DON和NO₃^--N的流失量由低至高依次为：休闲地<苗木地<单季晚稻农田<双季稻农田<油菜(或小麦)-单季水稻农田<小麦-早稻-晚稻农田<蔬菜地,而径流TP和PP的流失量依次为：休闲地<苗木地<单季晚稻、双季稻农田<小麦-早稻-晚稻农田<油菜(或小麦)-单季水稻农田<蔬菜地,不同种植类型间的DP流失量差异较小.N、P流失主要发生在作物生产期间,TN和TP的流失比例随作物复种指数的提高而增加.TN、DTN和NO₃^--N流失量主要与N肥施用量有关,土壤中NO₃^--N含量对TN和DTN流失量也有明显影响;农田DON的流失除与N肥施用量有关外,还受土壤全N和有机质积累的影响;NH₄⁺-N的流失量主要与土壤NH₄⁺-N水平有关,受N肥施用量的影响不明显;径流TP和PP的流失量受P肥施用量、土壤P积累的共同影响,而DP的流失与施P量关系不大,但与土壤全P和有效P都存在显著相关关系.     

公园水体浮游植物与环境因子的关系

于2006年10月—2007年9月,对上海市10个公园景观水体水质环境因子及浮游植物群落结构进行逐月监测,应用主成分分析(PCA)和典范对应分析(CCA)探讨了浮游植物数量与水质环境因子之间的关系,评价了城市水环境状况,以期为公园水体的水质管理提供科学依据。结果表明:共鉴定公园水体浮游植物8门167种,浮游植物丰度范围为2.16×106～7.87×106cells·L-1,主要以蓝藻、硅藻、绿藻为主,优势种由皮状席藻(Phormidium corium)、窝形席藻(Ph.fovedarum)、微小平裂藻(Merismopedia tenuissima)、尖针杆藻(Synedraacus)、银灰平裂藻(M.glauca)、啮蚀隐藻(Cryptomonas erosa)等组成;公园景观水体水温变幅为7.9～29℃,水深0.79～1.05m,透明度0.5～0.70m,总氮0.896～3.9mg·L-1,铵氮0.224～1.979mg·L-1,硝酸盐0.126～0.346mg·L-1,亚硝酸盐0.015～0.140mg·L-1,总磷0.063～0.372mg·L-1,活性磷0.007～0.194mg·L-1,化学需氧量为5.418～10.685mg·L-1。PCA分析表明,水温、透明度、氮磷营养因子以及化学需氧量是影响浮游植物密度变化的主要因素。CCA分析表明,总氮、总磷、透明度和水温是影响浮游植物群落结构季节变化的主要驱动因子。     

华南赤红壤无机复合肥氮磷淋失特征

采用室内土柱淋溶模拟试验,研究了不同施肥水平下无机复合肥中氮和磷在华南赤红壤中的淋失特征.结果表明: 铵态氮、硝态氮和总氮淋失量均随施肥量增加而提高;肥料中氮的淋失率在36.8%～49.2%之间,总氮淋失量（y）与施肥量（x）之间的回归方程为y=0.3667x+66.483（R²=0.992）;铵态氮和总氮的淋失主要集中在前5次淋洗,硝态氮的淋失持续时间更长.施肥量对可溶性磷淋失量无显著影响,但颗粒磷和总磷淋失量均随施肥量增加呈上升趋势;肥料磷淋失率在0.002%～0.010%之间,总磷淋失量（y）与施肥量（x）之间的回归方程为y=7e^-5x+0.0538（R²=0.931）;磷的淋失动态与氮显著不同,为缓慢的持续累积过程.硝态氮与铵态氮淋失量比值和可溶性磷与颗粒磷淋失量比值均随施肥量增加呈下降趋势.     

太湖地区氮素湿沉降动态及生态学意义：以常熟生态站为例

在常熟生态站2001年6月至2003年5月连续两年定位收集湿沉降,对太湖地区氮素湿沉降动态进行研究．结果表明,湿沉降氮输入量季节变化显著,夏、春季高,秋、冬季低．在湿沉降输入氮中NH4^+-N、NO3^--N和DON的比例分别为47．6％、35．1％和17．4％．湿沉降中NH4^+-N主要来自当地农田的氨挥发,湿沉降NH4^+-N月输入量随月降雨量增加而增加(R0=0．3178^**)．该地区空气中NO3^--N浓度相对比较稳定,湿沉降中NO3^--N浓度与降雨量呈负相关(R^2=0．4205^***)．湿沉降NO33^--N月输入量与月降雨次数呈直线正相关(R^2=0．6757***),而与月降雨量相关性较差(R^2=0．1985^*)．湿沉降TN年输入量为27．0kg·hm^-2,并在所有降雨中,氮浓度均超过水体富营养化阈值(0．2mg·L^-1)．     

三江平原小叶樟和毛果苔草中N素营养动态分析

讨论了沼泽湿地优势种小叶樟（Ｃａｌａｍａｇｒｏｓｔｉｓ ａｎｇｕｓｔｉｆｏｌｉａ）和毛果苔草（Ｃａｒｅｘ ｌａｓｉｏｃａｒｐａ）生物量和生长率变化情况,不同生长期各器官中Ｎ素含量及储量动态变化,以及植物对Ｎ素利用和区域养分限制情况。结果表明,两种植物地上生物量生长符合模式ｐ＝γ＋ａｔ＋βｔ＾２,地下生物量符合曲线ｐ＋ａ０＋ｂ０ｔ;受土壤水分、养分、气温和植物本身特点及其对Ｎ素选择吸收作用等多种因素     

盐渍区农田氮肥施用量对土壤硝态氮动态变化的影响

土壤硝态氮动态变化和残留与农田硝态氮淋溶以及地下水硝态氮污染密切相关。为了促进海河低平原盐渍区农田氮肥合理利用以及农业可持续发展,试验在盐化潮土条件下,通过设计不同施氮量（0,70,140,210kg N hm^-2）处理,重点研究了该区农田氮肥施用量对土壤硝态氮动态、残留以及土壤氮损失的影响。结果表明：（1）0～100cm土壤剖面硝态氮总量随施氮量显著增加,施用氮肥没有改变剖面硝态氮总量随玉米生育进程波状变化趋势,但明显增强了其变化幅度;（2）施氮改变了硝态氮土壤剖面空间分布状态,表现出施氮后上部土层（0～40cm）硝态氮比例显著增加而后迅速降低的趋势;（3）硝态氮残留与氮素损失随施氮量增加而增加,且N210和N140处理下氮素损失量显著高于N70和N0。     

The effects of land use change and irrigation water resource on nitrate contamination in shallow groundwater at county scale

The objective of this study was to assess the impacts of land use changes and irrigation water resource on the nitrate contamination in shallow groundwater. 394 water samples were sampled from the same irrigation wells during a period of five years (from 2002 to 2007) in Huantai County in the North China Plain. NO₃-N concentration in irrigation wells was measured. Geostatistical method combined with GIS technique was used to analyze the spatio-temporal distribution of groundwater NO₃-N concentrations in Huantai County. Land use type and irrigation water resource were combined with the variation of NO₃-N concentrations by statistical approach to investigate the relationship between them. The distribution map showed that the percentages of area increased by 13.06%, 14.37%, 12.23% and 3.85% for that had nitrate concentrations of 10–15, 15–20, 20–30 mg L^?1 and greater than 30 mg L^?1 for shallow groundwater, respectively, while decreased by 28.87% and 14.63% for 0–5 and 5–10 mg L^?1. In the well-irrigated field, the NO₃-N concentrations in shallow groundwater had increased for vegetables, wheat–vegetables and wheat–maize rotations. In contrast, fast-growing tree system could act as a buffer to retain shallow groundwater nitrate content which resulted in reduced NO₃-N concentrations. Under the same land use condition, irrigation with sewage, or well and sewage by turns would both enormously add nitrate to groundwater.     

PS1沼泽红假单胞菌对集约化对虾养殖废水的净化作用

应用光合细菌PS1——沼泽红假单胞菌（Rhodopseudomonas palustris）净化集约化对虾养殖废水,探讨不同温度、不同菌浓度下PS1对养殖废水的净化效果。研究结果表明：PS1可有效降低对虾养殖废水中的COD、NH4^＋-N、NO3^-N、PO4^3--P,但对NO2^--N无降解效果,反而使之持续升高;PS1对养殖废水96h的降解率受温度和添加菌浓度的影响显著（P〈0．05）。当温度为26℃时,PS1对废水的降解活性较好;不同菌浓度组间的净化效果差异明显（P〈0．05）,综合净化效果,以12．5×10^5 CFU／ml的菌浓度为宜。     

莴笋对不同形态氮素的反应

探讨了不同形态氮素对莴笋生长发育的影响及其营养特性。结果表明,莴笋幼苗根系对NH4^+ -N的亲和力稍大于NO3^- -N的亲和力;分别供给NO3^- -N+NO3^- -N及NH4^+ -N,莴笋的生物学产量和吸N量均依次递减（分别为100：56.9：12.4,100：48.9：8.6）,因此在水培条件下,NO3^- -N是最适合莴笋生长发育的氮源,NH4^+ -N与NO3^- -N各占50％时对莴笋的生长发育已有一定的抑制作用,仅以NH4^+ -N作氮源则莴笋很难生长;NH4^+ -N与NO3^- -N各占50％时,莴笋倾向于吸收较多的NH4^+ -N,而且在培养不同阶段NH4^+/NO3^-吸收比例均大于1,莴笋表现出喜铵性,但NH4^+ -N并非莴笋很适合的氮源;营养液中NO3^- -N不足,主要影响莴笋茎叶的生长,而NH4^+ -N所占比例达50％时,莴笋根系生长受到抑制,且有明显的受害症状;以NO3^- -N作氮源预培养两周,以含微量NO3^- -N的自来水为水源,再单独以NH4^+ -N为氮源,对莴笋生长有极大的促进作用,同时还大幅度降低了体内硝酸盐的含量。尿素作氮源莴笋未出现受害症状,但莴笋的生长发育状况明显劣于其它氮源。     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: subsurface solute patterns

Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian zones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN; NH₄ ⁺- and NO₃ ⁻-N) were low in stream water (< 110 ug/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO₃ ⁻-N (550 ug/L) and oxygen concentrations were high (5.2 mg/L). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH₄ ⁺-N (200–700 ug/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 ug/L) than at Icacos (600 ug/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH₄ ⁺ and NO₃ ⁻ were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appear to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes through the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zone. However, geomorphology appears to strongly modify the importance of specific biotic components.     

不同土地利用类型对丹江口库区土壤氮矿化的影响

氮(N)素是陆地生态系统净初级生产力的重要限制因子, 土地利用类型的变化对生态系统氮循环过程有着重要的影响。采用PVC顶盖埋管原位培养的方法, 对丹江口库区清塘河流域相邻的侧柏(Platycladus orientalis)人工林、人工种植灌木林地和农田3种土地利用类型的氮素矿化和硝化作用进行了研究。结果表明, 侧柏人工林、灌木林地和农田的NH₄⁺-N浓度(mg·kg^-1)依次为1.33 ± 0.20、1.67 ± 0.17和1.62 ± 0.13, 不同土地利用类型间的NH₄⁺-N浓度无显著性差异; 而3种土地利用类型下土壤NO₃^--N浓度(mg·kg^-1)差异显著, 农田NO₃^--N浓度(9.00 ± 0.73)显著高于侧柏人工林(1.27 ± 0.18)和灌木林地(3.51 ± 0.11)。NO₃^--N在灌木林地和农田中分别占土壤无机氮库的67.8%和84.8%, 是土壤无机氮库的主要存在形式; 而侧柏人工林中NO₃^--N和NH₄⁺-N浓度则基本相等。土壤硝化速率(mg·kg^-1·30 d^-1)从农田(7.13 ± 2.19)、灌木林地(2.56 ± 1.07)到侧柏人工林(0.85 ± 0.10)显著性降低。侧柏人工林、灌木林地和农田的矿化速率(mg·kg^-1·30 d^-1)依次为0.98 ± 0.12、2.52 ± 1.25和6.58 ± 2.29。矿化速率和硝化速率显著正相关, 但是矿化速率在不同的土地利用类型间差异不显著。培养过程中灌木林地和农田NH₄⁺-N的消耗大于积累, 氨化速率为负值, 导致灌木林地和农田矿化速率小于硝化速率。氮素的矿化和硝化作用受土壤含水量和土壤温度的影响, 并对土壤含水量更为敏感。土壤C:N与土壤矿化和硝化速率显著负相关。研究结果表明: 土地利用类型的变化会改变土壤微环境和土壤C:N, 进而会影响到土壤氮循环过程。