甲醇为碳源时C/N对反硝化过程中亚硝酸盐积累的影响

如何获得稳定的NO2--N作为厌氧氨氧化细菌的电子受体是城镇污水通过厌氧氨氧化途径脱氮的瓶颈问题。为此考虑利用反硝化途径获取稳定的NO2--N。以甲醇为碳源,采用小试装置的SBR反应器,通过控制进水C/N(COD与NO3--N质量浓度比)的策略,研究了反硝化过程中的NO2--N积累的状况。试验结果表明以甲醇为碳源且投加量不足时(C/N3.2),反硝化过程中和反硝化结束后会产生稳定的NO2--N积累;在C/N不足的前提下,NO2--N积累量随甲醇投加量的增加而增加;进水C/N为2.4～3.2时,可获得约25%的NO2--N积累率;进水C/N为0.8时,NO2--N积累率仅为5.6%;C/N1时,NO2--N与NO3--N的还原速率随着COD浓度的增加而增加;C/N≥1时,COD浓度不再影响NO2--N与NO3--N的还原速率。     

Stabilization and dewatering of wastewater treatment plant sludge using combined bio/Fenton-like oxidation process

Wastewater sludge usually contains large amounts of water and organic materials; therefore, its stabilization and dewatering are of particular importance. The present study aimed to investigate the possibility of sludge stabilization and dewatering from wastewater sludge by bioleaching (Thiobacillus ferrooxidans), Fenton/bioleaching, and bioleaching/Fenton-like processes. To evaluate sludge stabilization and dewatering, specific resistance to filtration (SRF), volatile suspended solids (VSS), total suspended solids (TSS), and soluble chemical oxygen demand (SCOD) were measured. In biological treatment with T. ferrooxidans with Fe²⁺ (2?g?L^?1), 99.75, 33, 37, and 72% reduction were observed in SRF, VSS, TSS, SCOD, respectively, after 2 days. In the combined treatment of Fenton before bioleaching (including Fe²⁺ 2?g?L^?1 and H₂O₂ 1?g?L^?1 with Fenton oxidation for 30?min followed by biological treatment with T. ferrooxidans for 2 days), the reduction rates in TSS, VSS, SCOD, and SRF were 40.18, 40.88, 60.95, and 75.43%, respectively. In treatment with the combined method of bioleaching before Fenton-like oxidation, the removal rates of the aforementioned parameters were 52.5, 54.4, 88, and 99.82%, respectively. In comparison to Fenton oxidation and bioleaching alone, combined biological method of bioleaching/Fenton-like oxidation using a lower dose of H₂O₂ and Fe²⁺ significantly improved sludge dewatering and stabilization.     

厌氧同时反硝化产甲烷研究进展

厌氧体系中同时反硝化产甲烷因具有同时去碳脱氮的特点,成为目前研究的热点。本文阐述了厌氧同时反硝化产甲烷工艺的原理及物质和电子流向,总结了反硝化产甲烷工艺中微生物种群和主要影响因素的研究现状,探讨了目前研究中存在的问题并提出未来发展方向。分析认为,还需要优化反硝化和产甲烷各自的电子流分量、探索更加合适的COD/NO^-_x -N比,合理分配电子流使产生的气体具有能源价值;污泥形态也影响同时反硝化产甲烷过程的进行,厌氧颗粒污泥或生物膜能缓解氮氧化物对产甲烷的抑制,更适合应用于厌氧同时反硝化产甲烷,但是还需考虑传质阻力以及进水高SS对系统的影响。     

Performance assessment of a UASB–anoxic–oxic system for the treatment of tomato‐processing wastes

An upflow anaerobic sludge blanket (UASB)–anoxic–oxic system was used to achieve biochemical oxygen demand, NH₄ and total suspended solids (TSS) criteria of 15, 1 and 15 mg dm^?3 at 1.17 days of system hydraulic retention time during treatment of tomato‐processing waste. The incorporation of an anoxic tank was found to affect the improvement in sludge‐settling characteristics, as reflected by about 25–33% reduction in the sludge volume index, along with final effluent TSS and soluble biochemical oxygen demand concentrations of 13 and 9 mg dm^?3, respectively, which met the discharge criteria. Despite incomplete denitrification, sludge settleability was very good (sludge volume index < 60 cm³ g^?1) owing to reduction in volatile suspended solids/TSS ratio from 0.75 to 0.6 as a result of higher alkalinity in the UASB effluent. Also in this study, phosphorus release was observed in the anoxic tank, predominantly due to abundance of acetic acid in the UASB effluent. A phosphate release of 5.4 mg P dm^?3 was observed in the anoxic tank with subsequent P uptake in the following aerobic stage. Copyright © 2006 Society of Chemical Industry     

厌氧-限氧SBR处理低C/N生活污水SNDPR启动及N2O释放

以厌氧-限氧方式运行序批式生物反应器（SBR）,采用逐步降低进水碳氮比（C/N）方式驯化聚磷菌（PAOs）和聚糖菌（GAOs）,启动了低C/N生活污水同步脱氮除磷过程（SNDPR）,并考察了SNDPR内PAOs、GAOs间竞争关系及系统脱氮除磷性能过程N₂O释放特性。结果表明,C/N=7.0,SBR限氧段脱氮和除磷效率分别为83.5%和90%以上,N₂O产量为0.54 mg/L;C/N=3.0~3.5,脱氮和除磷效率分别降至60.1%和80.5%,N₂O产量达1.09 mg/L。SBR内不同反应阶段内源物质变化均表现出PAOs-GAOs共存特性。高C/N有利于微生物合成聚-β-羟基烷酸酯（PHA）并促进N₂O还原。C/N降低,SBR内污泥内源物质转化倾向于富集GAOs的降解特性。氨氧化菌（AOB）好氧反硝化过程及GAOs以PHA作为电子供体的内源反硝化过程促进了N₂O的释放。随C/N降低,SBR内污泥平均胞外聚合物（EPS）由43.4 mg/g VSS增至50.5 mg/g VSS,污泥容积指数（SVI）由99 ml/g增至127 ml/g。疏松型EPS（LB-EPS）内,蛋白质（PN）与多糖（PS）之比（PN/PS）随C/N增加而降低,污泥亲水性增加,不利于污泥脱水。     

The effect of increasing application rate of granular calcium ammonium nitrate on net nitrification in a laboratory study of grassland soils

Soil incubation studies were undertaken in controlled environment cabinets at 15°C to investigate the effect of increasing application rates of calcium ammonium nitrate (CAN) on net nitrification in two grassland soils. Granular CAN was applied to the surface of freshly collected, moist soil, at a rate equivalent to 0, 100, 200, 400, 800 and 1600µg NH ₄ ⁺ -N and NO ₃ ^- -N per gram of oven dry soil. In half the treatments finely ground CaCO₃ was incorporated into the moist soil to raise the starting pH. Changes in soil mineral N and pH were measured at weekly intervals up to six-weeks. The most probable number (MPN) technique was used to enumerate the NH ₄ ⁺ -N and NO ₂ ^- -N oxidizers at the beginning and end of the incubation.At low rates of CAN application there was considerable NH ₄ ⁺ -N oxidation to NO ₃ ^- -N during the incubation of both soils. Lime stimulated this N transformation. At high application rates (i.e. 800 and 1600 ppm) there was little change in NH ₄ ⁺ -N or NO ₃ ^- -N on either soil during the 6 week incubation, in the presence or absence of lime. The rate of NO ₃ ^- -N produced peaked at 5.6 and 3.8 mg NO ₃ ^- -N kg^–1 d^–1 on soil 1 and 2 respectively, in the presence of lime. Above a level of 400 ppm CAN (equivalent to 38 kg N ha^–1) the rate of NO ₃ ^- -N produced decreased. The higher rate of net nitrification in soil 1 compared with soil 2 was probably due to a higher number of nitrifying bacteria. Although high rates of CAN decreased the nitrifying activity of both soils there was little difference between treatments in the actual numbers of NH ₄ ⁺ -N and NO ₂ ^- -N oxidizers determined by the MPN technique.The results showed that the rate of granular CAN applied to the soil surface can influence the local activity of nitrifying bacteria and subsequent N transformations. At application rates of CAN generally used agriculturally for grass production, it is likely that net nitrification of the NH ₄ ⁺ -N in the fertilizer granule will be inhibited.     

厌氧复合床处理模拟焦化废水的反硝化动力学

采用厌氧复合床(UBF)处理模拟焦化废水,进水中投加NaNO₃模拟硝化液回流,实现了同时反硝化/产甲烷,COD去除率达到94%,NO₃^--N去除率达到99%。取复合床下部污泥做反硝化动力学研究,在存在亚硝酸盐积累的情况下,以NO₃^--N+0.6NO₂^--N作为反硝化电子受体,采用双基质的Monod方程对实验数据进行拟合,拟合曲线与实验测定值相关性良好。其次,采用双基质的Monod微分方程组对NO₃^--N和NO₂^--N的浓度变化进行拟合,得到相关参数:硝态氮的最大比降解速率和半饱和常数分别为1.13 d^-1和2.0 mg·L^-1;亚硝态氮的最大比降解速率和半饱和常数分别为0.66 d^-1和2.5 mg·L^-1,基于硝态氮和亚硝态氮的有机物半饱和常数分别为90.8 mg·L^-1和96.8 mg·L^-1。     

Improvement of the settling properties of Anammox sludge in an SBR

Biological systems for the treatment of wastewater have to provide optimum sludge retention to achieve high removal efficiencies. In the case of slow‐growing micro‐organisms, such as anaerobic ammonia‐oxidizing (Anammox) bacteria, episodes of flotation involving biomass wash‐out are especially critical. In this study a strategy based on the introduction of a mix period in the operational cycle of the Anammox Sequencing Batch Reactor (SBR) was tested for its effects on biomass retention and nitrite removal. Using this new cycle distribution the biomass retention inside the reactor improved as the solids concentration in the effluent of the SBR decreased from 20–45 to 5–10 mg VSS dm^?3 and the biomass concentration inside the reactor increased from 1.30 to 2.53 g VSS dm^?3 in a period of 25 days. A decrease of the sludge volume index (SVI) from 108 to 60 cm³ g VSS^?1 was also observed. Complete depletion of nitrite was achieved in the reactor only with the new cycle distribution treating nitrogen loading rates (g N‐NO₂^? + g N‐NH₄⁺ dm^?3 d^?1) up to 0.60 g N dm^?3 d^?1. Copyright © 2004 Society of Chemical Industry     

Effect of COD/NO3-N ratio on the performance of a hybrid UASB reactor treating phenolic wastewater

The effect of COD/O₃⁻-N ratio on the biodegradation of complex phenolic mixture was studied in bench scale hybrid upflow anaerobic sludge blanket (HUASB) reactors. HUASB reactor is a combination of a UASB unit at the lower part and an anaerobic fixed film at the upper end. The aim of this study was to evaluate the biodegradability of phenolic mixture (from synthetic coal wastewater) as the only carbon and energy source in continuous experiments using nitrate as the final electron acceptor. Synthetic coal wastewater contained phenol (490 mg/L); m-,o-,p-cresols (123.0 mg/L, 58.6 mg/L, 42 mg/L); 2,4-, 2,5-, 3,4- and 3,5 dimethyl phenols (6.3 mg/L, 6.3 mg/L, 4.4 mg/L and 21.3 mg/L) as major phenolic compounds representing the complex phenolic mixture. Nitrate nitrogen loading was increased from 0.11 g/m³/d to 0.5 g/m³/d in order to keep COD/NO₃⁻-N ratio as 20.1, 14.85, 9.9, 6.36 and 4.45. An input phenolics concentration of 752 mg/L and hydraulic retention time (HRT) of 24 h was maintained through out the study. Removal of phenolic mixture was found to increase with the lowering of COD/NO₃⁻-N ratio. Maximum phenolics removal of 98% was achieved at a COD/NO₃⁻-N ratio of 6.36. However, phenolics removal got adversely affected when COD/NO₃⁻-N ratio was reduced below 6.36. A nitrogen production efficiency of 78% was obtained according to nitrate consumption. Simultaneous denitrification and methanogenesis was observed in all the reactors throughout the study, demonstrating that denitrification is a feasible alternative for the treatment of coal wastewater. Granules degrading complex phenolic mixture were of diameter 1.6–2.25 mm.     

Crop nitrogen utilization and soil nitrate loss in a lettuce field

Low N use efficiency and high nitrate (NO ₃ ^- ) pollution potentials are problems in intensive vegetable production systems. The purpose of this study was to quantify N utilization by lettuce (Lactuca sativa L. cv Salinas), and identify periods of NO ₃ ^- loss in an on-farm study in the Salinas Valley in coastal California. During autumn and winter, surface moisture remained low, and NO ₃ ^- concentrations increased, reflecting high net mineralizable N, as determined by anaerobic incubation, and nitrification potential, as determined by the chlorate inhibition method. At the onset of a large winter storm, tracer levels of¹⁵NO ₃ ^- were injected in the top 5 mm of soil in 30 cm-deep cylinders. After two weeks, most of the¹⁵N was present as¹⁵NO ₃ ^- at 10–30 cm depth. By difference, losses to denitrification accounted for ~ 25% of the surface-applied¹⁵N. Leaching below 30 cm did not occur, since no¹⁵N enrichment of NO ₃ ^- -N was measured in anion-exchange resin membranes placed at the base of each cylinder. During the crop period, NO ₃ ^- losses were most pronounced after irrigation events. Uptake of N by two crops of lettuce (above- and belowground material) was approximately equal to fertilizer inputs, yet simulation of N fates by the Erosion/Productivity Impact Calculator (EPIC) model indicated losses of 14.6 g-N m^–2 by leaching and 2.5 g-N m^–2 by denitrification during the 6-month crop period. The large NO ₃ ^- losses can be attributed to accumulation of soil NO ₃ ^- during winter that was leached or denitrified during the irrigated crop period.     

Simultaneous organic and nutrient removal from municipal wastewater by BSACNR process

Biological nutrient removal was investigated under a biological synthetic activated ceramic nutrient removal (BSACNR) process. Tests were made to establish whether organic compounds and nutrients (N, P) from municipal wastewater were eliminated effectively in a lab-scale BSACNR process by increasing the hydraulic retention time (HRT) from 4 hr to 10 hr. In the system, synthetic activated ceramic (SAC) media were packed in each reactor for attached growth of both nitrifying bacteria and denitrifying bacteria; the sludge of the clarifier was returned to the anaerobic reactor to release phosphate. In this configuration, nitrification, denitrification and phosphorus removal could be performed at their respective conditions. The influent was synthetic wastewater, and the mean concentration of COD, NH⁺ ₄ -N and T-P in the influent was about 200 mg/L, 20 mg/L and 8 mg/L, respectively. At a total HRT of the system of 4-10 hr, the system worked successfully obtaining the removal of COD, NH⁺ ₄-N, T-N and T-P: 90.5-97.5%, 72.9-94.4%, 56.5-73.7% and 36.0-61.1%, respectively. The results of this research show that a biological synthetic activated ceramic nutrient removal (BSACNR) process packed with SAC media could be applicable for treatment of organic and nutrient from municipal wastewater.     

Comparison of a sMBR and a CASP system for wastewater reclamation and re-use

The performance of a submerged membrane bioreactor (sMBR) for municipal wastewater reclamation and re-use was compared with that of a current classic activated sludge process (CASP). The average chemical oxygen demand (COD) of CASP effluent was 75 mg/l, while the average COD of sMBR filtrate was 15 mg/l. In general, COD removal was above 98%. However, the best results were obtained at a sludge rentention time (SRT) of 50 days. The total phosphorus (TP) content of the filtrate decreased to a level <1 mg/l under aerobic conditions in which aeration occurred continuously. It was shown that the filtrate TP and orthoposphate (Orto-P) concentrations increased dramatically during the periods of intermittent aeration because phosphorus is released under anoxic conditions. In the CASP effluent, the average TP and Orto-P were 7.9 mg/l and 7.1 mg/l, respectively. The filtrate was free of suspended solids (SS) and total coliform bacteria. The effluent SS in the CASP was 40 mg/l. The turbidity removal in the sMBR reached almost 99%, i.e. generally less than 1 NTU, while the average turbidity of the CASP effluent was almost 15 NTU. The removal of ammonium nitrogen (NH₄⁺-N) in the sMBR was almost 99.8%. In addition, the nitrate concentration in the filtrate decreased to 2.4 mg/l under both aerobic & anoxic conditions. It was shown that both nitrification and denitrification could be successfully reduced through intermittent aeration. Average total Kjeldehl nitrogen (TKN) and NH₄⁺-N in the CASP effluent were 30.2 mg/l and 20 mg/l, respectively, i.e. the nitrification efficiency was 42.9%, and the denitrification value was not available. When these results are compared with those in the CASP, it indicated that the sMBR could be successfully used for reclamation and re-use of municipal wastewater.     

Improving Dewaterability and Other Properties of Citric Acid Wastewater Treatment Sludge by Ozone and Peroxone

ABSTRACT

The efficacies of ozonation and peroxone (O₃/H₂O₂) pretreatments were compared for citric acid wastewater sludge conditioning with the objective of improving dewatering characteristics of the sludge. Treatment with 84 mg O₃/g dry solid (DS) and 12.5 mg/g DS H₂O₂ greatly enhanced the effectiveness of ozonation, providing sludge dewaterability similar to that obtained by ozonation at 250 mg O₃/g DS. Most importantly, treatment of citric acid wastewater sludge with 84 mg O₃/g DS and 12.5 mg/g DS H₂O₂ led to the preservation of the nutrient elements nitrogen, phosphorus, and potassium in the sludge with a minimal volatile suspended solids/total suspended solids reduction of 5.5%, which is much lower than that with ozonation at 250 mg O₃/g DS.     

Deterioration of denitrification by oxygen and cost evaluation of electron donor in an uncovered pre-denitrification process

Specific nitrate uptake rates (SNURs) under two test conditions were measured to evaluate effects of oxygen inhibition on denitrification. A test condition was that activated sludge was completely prevented from contacting of oxygen (SNUR _closed ), the other was that activated sludge was contacted to free air (SNUR _open ). Municipal wastewater and acetate were used as electron donors. SNUR _closed was 2.42 mg NO₃-N/g VSS-hr and SNUR _open was 1.09 mg NO₃-N/g VSS-hr when municipal wastewater was used as electron donor. Meanwhile, when acetate was used as electron donor, SNUR _closed was 24.65 mg NO₃-N/g VSS-hr and SNUR _open was 18.00 mg NO₃-N/g VSS-hr. The operating costs for electron donors were calculated based on the unit price of acetate to remove nitrate. When municipal wastewater was used as electron donor the ratio of cost _open to cost _closed was 0.45. Cost evaluation showed the adverse impacts on denitrification and explained why an anoxic reactor should be sequestered from oxygen.     

Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N₂O production in a soil profile. The N₂O produced by all processes may mix to form one pool before being reduced to N₂ by denitrification. Reliable methods are needed for measuring the fluxes of N₂O and N₂ simultaneously from agricultural soils. The C₂H₂ inhibition and ¹⁵N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C₂H₂ is that as well as blocking N₂O reductase, it also blocks nitrification and dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ . Potentially the ¹⁵ N gas-flux method can give reliable measurements of the fluxes of N₂O and N₂ when all N transformation processes proceed naturally. The analysis of ¹⁵N in N₂ and N₂O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha^-1day^-1 for N₂ and 4 to 15 g N ha^-1day^-1 for N₂O. By measuring the ¹⁵N content and distribution of ¹⁵N atoms in the N₂O molecules, information can also be obtained to help diagnose the sources of N₂O and the processes producing it. Only a limited number of field studies have been done using the ¹⁵N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N₂O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO ₃ ^- and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N₂O than a single application because of the time lag in the synthesis of N₂O reductase. Increasing the NO ₃ ^- concentration above the K _m value for NO ₃ ^- reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N₂O. The effect of NO ₃ ^- concentration on the mole fraction of N₂O is enhanced at low pH. Manipulating the interaction between NO ₃ ^- supply and soil pH offers the best hope for minimising N₂O and N₂ fluxes.     

Biological nutrient removal with volatile fatty acids from food wastes in sequencing batch reactor

Volatile fatty acids (VFAs) derived from food waste were used as an alternative carbon source in biological nutrient removal. The pH Profiles were monitored during the nutrient removal in an Na-acetate fed sequencing batch reactor (SBR) (C source). Effluent N, P and SCOD concentrations of 0.5 and 0.1 mg/L were achieved with 5.5 hour of HRT (hydraulic retention time) when influent concentrations of NH₄⁺-N, PO₄³⁻ and SCOD were 42.5, 5.92 and 180 mg/L. Then the SBR was fed with four solutions of VFAs produced under different acidogenesis conditions of food wastes. VFAs-added SBR showed similar specific nitrification rates (3.0 to 3.9 mg-N/g MLSS · h) to that of acetate, but specific denitrification rates (3.2 to 4.2 mg NO₃⁻N/g MLSS·h) were slightly lower than with acetate of 4.67 mg NO₃⁻-N/g MLSS·h. VFAs-introduced SBR efficiently removed phosphorus except when the SBR was fed with a VFA-solution containing high amounts of valerate and caproate.     

缺氧环境对颗粒污泥强化除磷系统释磷的影响

以除磷颗粒污泥为研究对象,采用批次试验,以混合丙酸和乙酸（2∶1）为外加碳源,在厌氧起始阶段投加不同浓度的NO^-₃-N（0, 20, 30, 50和75 mg·L^-1, pH=8.0）和NO^-₂-N（10,20,40和60 mg·L^-1, pH=7.0, 7.5和8.0）,研究缺氧环境对颗粒污泥强化除磷系统释磷的影响。结果表明,硝态氮的投加对聚磷菌释磷无明显抑制,系统中VFA的吸收、磷的释放和硝态氮的反硝化同时发生,挥发性脂肪酸（相似文献   

Parametric study of the factors influencing simultaneous denitrification and enhanced biological phosphorus removal

In this study, the effect of various factors such as C:N ratio, carbon source, percentage P content in the sludge influencing the simultaneous denitrification and enhanced biological phosphorus removal was investigated in batch tests on bean and tomato waste sludge from an upflow anaerobic sludge blanket reactor–anoxic/aerobic system and municipal sludge from a circulating fluidized bed bioreactor. A correlation between the change in redox potential and rate of P release was developed. Interestingly, maximum P release was observed at positive redox potential in some of the batch tests. Simultaneous denitrification and P release under anoxic conditions was observed during all the batch tests. Sludge acclimatization improved the efficiency of the sludge and proved independency of maximum specific denitrification rate and P content of sludges. The contribution of denitrifying PAOs to anoxic P uptake was determined through the denitrification control test at an initial level of PO₄‐P of 100–120 mg dm^?3. Copyright © 2006 Society of Chemical Industry     

AAO-BAF反硝化除磷系统的二次启动特性

利用厌氧/缺氧/好氧工艺与曝气生物滤池组成的双污泥强化反硝化除磷系统（AAO-BAF）处理生活污水,通过对比污染物的去除性能,分析了该系统除污染物性能的恢复情况。试验用反硝化除磷污泥在室温下放置了1个月。试验期间比较了两种启动方式,AAO反应器中厌氧、缺氧和好氧区的容积比（V_r）分别为2：5：2和2：6：1,记为1^#和2^#。试验结果表明,系统去除有机物和氨氮的性能基本没有恶化。第17 d,两种启动方式得到的TN和PO₄^3--P的去除率分别达到了约75%和92%;去除单位NO^-₃-N吸收的PO₄^3--P量也由试验初的0.5 mg PO₄^3--P·（mg NO^-₃^-N）^-1增加到了1.5 mg PO₄^3--P·（mg NO^-₃-N）^-1 左右。试验结果显示,AAO-BAF系统更适宜采用有利于除磷性能和污泥沉降性能快速恢复的方式1（1^#）进行二次启动。     

Downward movement of nitrate and ammonium nitrogen in a flatland ultisol

Three field trials were conducted in the early and late rainy season on a Piarco Series soil (Aquoxic Tropudults) in Central Trinidad to monitor downward movement of NH ₄ ⁺ -N and NO ₃ ^- -N under flat-tilled and ridge-tilled conditions as affected by mulch application. The first experiment was carried out in the early rainy season under bare-fallowed conditions, while okra (Abelmoschus esculentus) was the test crop used in the two remaining trials, which were conducted during the early and late rainy season periods. The plots were fertilized with urea seven days after crop emergence and 14 days after land preparation for the cropped and bare-fallowed experiments respectively. Soil samples were collected weekly after fertilizer application and analyzed for NH ₄ ⁺ -N, NO ₃ ^- -N and soil water.Urea application increased NH ₄ ⁺ -N levels in the soil and NH ₄ ⁺ -N was the dominant inorganic N form observed for the first four weeks after fertilization. Mulch application decreased NH ₄ ⁺ -N and NO ₃ ^- -N soil levels. Ridging the soil increased downward movement of NH ₄ ⁺ -N and NO ₃ ^- -N. Under bare-fallowed conditions, downward movement of NH ₄ ⁺ -N was noted in the 30 to 45 cm soil layer at seven days after fertilization, while under cropped conditions its movement was restricted to the 15 to 30 cm layer within the same period. In bare-fallowed soil, increased NO ₃ ^- -N and its downward movement was noted after four weeks of fertilization. In the cropped soil, downward movement of NO ₃ ^- -N was observed one week after fertilization in the early rainy season and three weeks after fertilization in the late rainy season.