Development of a Soil N Test for Fertilizer Requirements for Corn Production in Quebec

Corn requires high nitrogen (N) fertilizer use, but no soil N test for fertilizer N requirement is yet available in Quebec. Objectives of this research were (1) to determine the effects of soil nitrate (NO₃ ^?)-N, soil ammonium (NH₄ ⁺)-N, and N fertilizer rates on corn yields and (2) to determine soil sampling times and depths most highly correlated with yields and fertilizer N response under Quebec conditions. Soil samples were taken from 0- to 30-cm and 30- to 60-cm depths at seeding and postseeding (when corn height reached 20 cm) to determine soil NH₄ ⁺ and NO₃ ^? in 44 continuous corn sites fertilized with four rates of N in two replications using a quick test (N-Trak) and a laboratory method. The N-Trak method overestimated soil NO₃ ^?-N in comparison with the laboratory method. Greater coefficients of determination were observed for soil NO₃ ^?-N analyses at postseeding compared with seeding.     

Soil inorganic nitrogen composition and plant functional type determine forage crops nitrogen uptake preference in the temperate cultivated grassland,Inner Mongolia

ABSTRACT

Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ ¹⁵N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO₃^?-N than NH₄⁺-N regardless of growth stages, and the NH₄⁺/NO₃^? uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH₄⁺-N rather than NO₃^?-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH₄⁺ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH₄⁺/NO₃^? uptake ratio was positively correlated with soil NH₄⁺/NO₃^? ratio. Our results suggest that the major forage crops prefer to absorb soil NO₃^?-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO₃^?-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.     

Fate of Fertilizer-15N to a Maize Crop Grown in Northern Zambia

Abstract

A field study with maize (Zea mays L.) was conducted in the 1988/89 cropping season to investigate the fate of ¹⁵NO₃-N-labelled NH₄ ¹⁵NO₃ applied at 40, 80 and 120 kg N ha^?1 (unlabelled N applied at 0, 80, 160 and 240 N ha^?1) with and without lime. The investigations were conducted in northern Zambia at Misamfu Regional Research Centre, Kasama on a Misamfu red sandy loam soil. The experimental design was a split plot arrangement with four replications with main plots receiving 0 and 2 Mg ha^?1 dolomitic limestone, while subplots received fertilizer N at various rates. Significant (p < 0.001) grain and DM yield responses to applied N up to 160 kg ha^?1 were observed. At higher rates little or no crop responses were observed and fertilizer use efficiency declined. Partitioning of amounts of total N and ¹⁵N in plants was in the order of seed = tassel > leaf> cob = earleaf> stem. Fertilizer N rates showed a highly significant (p < 0.001) effect on plant uptake of labelled N. Lime and its interaction with N rates had no effect on all measured parameters. Leaching of NO₃-N fertilizer to lower soil depths was in proportion to the rate of N applied, with highly significant (p < 0.001) differences among soil depths. Although higher concentrations of fertilizer-¹⁵N were recovered in the 0–20 cm depth the recovered portion at lower soil depths was still significant. Total recovery of labelled N by plant and by soil after crop harvest averaged 75, 55 and 54% of originally applied fertilizer-¹⁵N at 40, 80 and 120 kg N ha^?1, respectively. Corresponding unaccounted for ¹⁵N was 25, 45 and 46%. The most probable loss mechanism could have been by leaching to depths greater than 60 cm, gaseous losses to the atmosphere and root assimilation.     

Residual soil nitrogen credits for corn production along the upper Texas Gulf Coast region

Studies were conducted to evaluate response of dryland corn (Zea mays L.) along the upper Texas Gulf Coast to residual soil nitrate-nitrogen (NO₃-N) measured to depths of 15, 30, and 61 cm. Residual soil NO₃-N levels ranged from 3.4 to 31.6, 7.8 to 49.3, and 9.0 to 71.7 kg ha^?1, respectively, in 0 to 15, 15 to 30, and 30 to 61 cm depth increments, with cumulative NO₃-N ranging from 23.5 to 114.5 kg ha^?1 across sites-years. Where N fertilizer was reduced due to N crediting, yields and bushel weights at all 13 site-years showed no difference from those receiving full recommended N rates. A yield response to any level of added fertilizer N above the control was observed for only 6 of 13 site-years. These results indicate a high potential for success in crediting carryover soil NO₃-N to 61 cm as a means of reducing applied nitrogen fertilizer rates.     

Nitrification Inhibitor,Fertilizer Rate,and Temperature Effects on Nitrous Oxide Emission and Nitrogen Transformation in Loamy Sand Soil

An incubation study investigated the effects of nitrification inhibitors (NIs), dicyandiamide (DCD), and neem oil on the nitrification process in loamy sand soil under different temperatures and fertilizer rates. Results showed that NIs decreased soil nitrification by slowing the conversion of soil ammonium (NH₄⁺)-nitrogen (N) and maintaining soil NH₄⁺-N and nitrate (NO₃^?)-N throughout the incubation time. DCD and neem oil decreased soil nitrous oxide (N₂O) emission by up to 30.9 and 18.8%, respectively. The effectiveness of DCD on reducing cumulative soil N₂O emission and retaining soil NH₄⁺-N was inconsistently greater than that of neem oil, but the NI rate was less obvious than temperature. Fertilizer rate had a stronger positive effect on soil nitrification than temperature, indicating that adding N into low-fertility soil had a greater influence on soil nitrification. DCD and neem oil would be a potential tool for slowing N fertilizer loss in a low-fertility soil under warm to hot climatic conditions.     

Cassava (Manihot Esculenta Crantz) Yields,Soil Nitrous Oxide Emission,and Soil Nitrogen Transformation Affected by Nitrification Inhibitors in Loamy Sand Soil in Thailand

The effects of nitrification inhibitors (NIs) on soil nitrous oxide (N₂O) emission, soil ammonium (NH₄⁺) and nitrate (NO₃^?), and cassava (Manihot esculenta Crantz) yields were investigated in a loamy sand soil in eastern Thailand. Treatments were chemical fertilizer (CF) and CF plus dicyandiamide (DCD) or neem (Azadirachta indica) oil at two rates of 5% and 10%. DCD had a greater reduction of soil N₂O flux than the neem oil (P<0.10). DCD and neem oil retained NH₄⁺-N in the soil by 79% and 63% (P ≤ 0.10), respectively. The NI effect on soil NO₃^?-N was small due to a low N fertilizer rate. The cassava root yield and N uptake were increased 4–11% and 2–18%, respectively, by use of NIs, but they were only significant for DCD (P ≤ 0.10). These findings suggest that NIs application may be a promising method for minimizing nitrogen loss and enhancing crop yields in a tropical cassava field.     

Nitrification Inhibitor Effects on Nitrous Oxide Emission,Nitrogen Transformation,and Maize (Zea mays L.) Yield in Loamy Sand Soil in Thailand

The nitrification inhibitors (NIs) effects on soil nitrogen (N) fates and maize yields were investigated in a loamy-sand soil in Thailand. The treatments were chemical fertilizer (CF) and CF with dicyandiamide (DCD) or neem oil at two rates of 5% and 10%. Compared to the CF plot, DCD and neem oil reduced the cumulative nitrous oxide (N₂O) emission by the equivalent of 26% and 10%, respectively (P < 0.05). DCD and neem oil had a positive effect in slowing ammonium (NH₄⁺)-conversion and prolonging NH₄⁺-N in the soil with a maximum efficiency of 45% and 30%, respectively. NO₃^–N was higher in the NI plots (P < 0.05), but the effect was less pronounced later in the growing season. Adding the NIs increased maize yields and N uptake, but was only significant (P < 0.10) for neem oil. Results indicate that applying NIs is an effective method to mitigate soil N losses and enhancing N use efficiency in a tropical, agricultural field.     

Corn (Zea mays L.) Grain and Stover Yield as Affected by Soil Residual Mineral Nitrogen

Evaluation of nitrogen (N) dynamic in soil using regression equations is important for proper determination of N fertilization. A 3-year field experiment was conducted to (1) develop the best-fitted regression model relating corn grain and stover yield to soil residual ammonium (NH₄)-N and nitrate (NO₃)-N for corn yield prediction and (2) evaluate how such a model can be beneficial to the health of ecosystem by predicting the appropriate rates of N fertilization for corn production. Soil NH₄-N and NO₃-N were determined at corn harvest at the depths of 0–30 and 30–60 cm. Nitrogen fertilizer rates and soil mineral N accounted for a maximum of 93% variation in corn grain yield. Soil mineral N enhanced corn yield more than N fertilizer. Totals of 63.1 and 14.1 kg/ha of soil residual NO₃-N and NH₄-N were found in the 0- to 60-cm depth, indicating the importance of performing soil N tests.     

Leaching Methods Can Underestimate Mineralization Potential of Soils

Aerobic incubations to estimate net nitrogen (N) mineralization typically involve periodic leaching of soil with 0.01 M calcium chloride (CaCl₂), so as to remove mineral N that would otherwise be subject to immobilization. A study was conducted to evaluate the accuracy of leaching for analysis of exchangeable ammonium (NH₄⁺)-N and nitrate + nitrite (NO₃^?+ NO₂^–)-N, relative to conventional extractions using 2 M potassium chloride (KCl). Ten air-dried soils were used, five each from Illinois and Brazil, that had been amended with NH₄⁺-N (1 g kg^?1) and NO₃^–-N (0.6 g kg^?1). Both methods were in good agreement for inorganic N analysis of the Brazilian Oxisols, whereas leaching was significantly lower by 12–48% in recovering exchangeable NH₄⁺-N from Illinois Alfisols, Mollisols, and Histosols. The potential for underestimating net N mineralization was confirmed by a 12-wk incubation experiment showing 9–86% of mineral N recoveries from three temperate soils as exchangeable NH₄⁺.     

Nitrate Transformation and N2O Emission in a Typical Intensively Managed Calcareous Fluvaquent Soil: A 15-Nitrogen Tracer Incubation Study

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO₃-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N₂O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO₃-N decreased. When ¹⁵NO₃-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg^?1 only 1.1 ± 0.4 mg kg^?1 NO₃-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P < 0.05) with increasing amount of straw. Total N₂O-N emissions were at levels of only micrograms kg^?1 soil. The results indicate that application of straw can promote the immobilization of excessive nitrate with little emission of N₂O.     

Development of Soil N Testing for Wheat Production using Soil Residual Mineral N

The effects of soil residual nitrate (NO₃-N) and ammonium (NH₄-4) on wheat (Triticum aestivum L.) grain yield and N uptake were evaluated in a three-year experiment from 1991 to 1993, in the province of Quebec, Canada. Soil samples were collected at harvest from the depth of 0-30 and 30–60 cm from 20 wheat sites and analyzed in the laboratory for NO₃-N and NH₄-N. Wheat yield and N-uptake was also determined. Using statistical analysis system (SAS) and Matlab, data were analyzed and the best-fitted regression equations were plotted. Significant amounts of soil NO₃-N was found in the soil ranging from 31.6 to 61.6 kg/ha. The amount of soil NH₄-N varied from 9.2 to 23.3 kg/ha. The statistical effects of model parameters indicated that soil NO₃-N and soil NH₄-N significantly influenced wheat yield and N uptake. It may be possible to investigate wheat behavior at harvest using soil residual NO₃-N and NH₄-N.     

减氮配施有机物质对土壤氮素淋失的调控作用

采用室内土柱模拟试验方法,研究不同氮肥施用下1m土体中氮素的分布和移动特征,揭示土壤氮素动态变化规律。结果表明:FN(农民习惯施无机氮用量)、RN(根据土壤养分供应和作物需求确定的推荐无机氮用量)显著增加了土壤上层NH_4^+-N和NO_3^--N向下层淋失。RN+HA(与推荐无机氮纯养分相等的锌腐酸尿素)和RN40%+OMB(推荐无机氮肥减60%基础上配施自制有机调理物质)可延长上层土壤NH_4^+-N峰值出现时间,降低下层NH_4^+-N。淋溶结束后,等氮量下增施HA较RN降低60cm以下NH_4^+-N残留29.7%～54.2%;降低60—80cm NO_3^--N累积17.4%。RN40%+OMB处理无机氮肥用量最小,0—20cm的NH_4^+-N最高,40—100cm稳定在2.0mg/kg左右;0—20,20—40cm土层NO_3^--N较RN+HA增加12.3%和2.0%,显著降低40cm以下NO_3^--N残留。RN+HA和RN40%+OMB较RN的土壤总无机氮残留分别减少7.4%和20.2%,降低表观淋失率。因此,RN40%+OMB可较好地抑制氮素下移,降低氮素淋失风险,为减少氮素淋失、明确合理氮肥施用方式提供科学依据。     

宁夏引黄灌区稻田氮素浓度变化与迁移特征

过量施氮与不合理灌水是农田面源污染加剧的主要原因。为了寻求较优的水氮管理模式以促进农业生产和减少农田退水对黄河水体的污染, 在宁夏引黄灌区典型稻田中开展了不同水氮条件下稻田氮素迁移转化规律研究。结果表明: 不同水氮条件下稻田田面水NH₄⁺-N 与NO₃^--N 浓度伴随施肥出现明显峰值, NO₃^--N 峰值出现时间较NH₄⁺-N 晚, 且变化较平缓。3 次追肥时期和整个生育期田面水NH₄⁺-N 平均浓度与施氮量和灌水量都呈显著相关, 田面水NO₃^--N 平均浓度与施氮量呈显著正相关, 与灌水量相关性不显著。稻田30 cm与60 cm 深度的直渗水NH₄⁺-N 浓度受施肥影响较大, 与田面水NH₄⁺-N 浓度变化规律相似, 90 cm 处直渗水NH₄⁺-N 浓度峰值出现较为滞后, 且浓度较上层土体低, 120 cm 处直渗水NH₄⁺-N 浓度大体呈现持续上升趋势,整个生育期直渗水NH₄⁺-N 平均浓度与施氮量呈显著相关, 仅30 cm 处NH₄⁺-N 平均浓度与灌水量呈负相关, 其他土层深度不显著。30 cm 与60 cm 直渗水NO₃^--N 浓度在首次灌水后急剧下降, 在施肥后有较小幅度上升, 90 cm 与120 cm 直渗水NO₃^--N 浓度下降缓慢, 仅30 cm 处NO₃^--N 平均浓度与施肥量显著正相关。总的结果表明减少施肥或灌水均可达到减少农田氮素淋失的目的。     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.     

氮肥基追比对小麦产量、土壤水氮分布及利用的影响

为解决区域土壤质地类型针对性氮肥施用问题,在轻壤土和黏壤土上分别设置不施氮肥,氮肥基追比3∶7,4∶6,5∶5,6∶4和7∶3处理,研究小麦产量、水氮利用效率以及土壤含水量、贮水量、NH_4~+-N、NO_3~--N动态变化规律。结果表明:轻壤质土壤氮肥基追比4∶6的处理小麦产量、水分利用效率、氮肥生产效率最高分别为8 265.3 kg/hm~2,27.6 kg/(hm~2·mm),34.4 kg/kg。黏壤质土壤氮肥基追比5∶5的处理小麦产量、水分利用效率、氮肥生产效率最高分别为8 363.2 kg/hm~2,28.3 kg/(hm~2·mm),34.8 kg/kg。小麦不同生育期各土层含水量垂直分布变化较大,轻壤质土壤含水量在9.3%～26.2%,而黏壤质为9.7%～27.6%;小麦全生育期内土壤贮水量呈先升高后降低趋势,黏壤质土壤贮水量高于轻壤质。氮素追施量越多土壤表层NH_4~+-N与NO_3~--N含量越高,且随土层加深土壤NH_4~+-N与NO_3~--N含量降低,受降水影响轻壤质土壤NH_4~+-N与NO_3~--N更易于向土层深处淋溶,成熟期黏壤质各土层的NH_4~+-N和NO_3~--N含量均多于轻壤质。说明黏壤质土壤保水保氮肥能力强于轻壤质,氮肥基追比可以适当增加。     

Agro-Industry Sludge as a Potential Organic Fertilizer for Prompt Nitrogen Release

The aim of this study was to evaluate the sustainability of an agro-industry sludge as a nitrogen (N) fertilizer in perennial crops by assessing its dynamics of release of: 1) N as ammonium (NH₄⁺)- and nitrate (NO₃^?)-N, and 2) carbon as soil respiration. In incubation assay, application of agro-industry sludge promoted the maximum NH₄⁺-N concentration (50 mg kg^?1) 2 h after application, then it decreased with time until day 26. NO₃^?-N increased, reaching the maximum between day 60 and day 100, according to a second degree function. Agro-industry sludge showed a slower release of NO₃^?-N compared with urea. Soil basal respiration and cumulative amount of carbon dioxide were higher in sludge from day 71 on. In field, soil NO₃^?-N increased after fertilization treatments and was higher in Calcari Stagnic Cambisols soil. Sludge showed a potential lower environment impact in terms of mineral N release, than urea.     

Effect of crop residue C:N ratio on N2O emissions from Gray Lowland soil in Mikasa,Hokkaido, Japan

Abstract

We studied the effect of crop residues with various C:N ratios on N₂O emissions from soil. We set up five experimental plots with four types of crop residues, onion leaf (OL), soybean stem and leaf (SSL), rice straw (RS) and wheat straw (WS), and no residue (NR) on Gray Lowland soil in Mikasa, Hokkaido, Japan. The C:N ratios of these crop residues were 11.6, 14.5, 62.3, and 110, respectively. Based on the results of a questionnaire survey of farmer practices, we determined appropriate application rates: 108, 168, 110, 141 and 0 g C m^?2 and 9.3, 11.6, 1.76, 1.28 and 0 g N m^?2, respectively. We measured N₂O, CO₂ and NO fluxes using a closed chamber method. At the same time, we measured soil temperature at a depth of 5 cm, water-filled pore space (WFPS), and the concentrations of soil NH⁺ ₄-N, NO^? ₃-N and water-soluble organic carbon (WSOC). Significant peaks of N₂O and CO₂ emissions came from OL and SSL just after application, but there were no emissions from RS, WS or NR. There was a significant relationship between N₂O and CO₂ emissions in each treatment except WS, and correlations between CO₂ flux and temperature in RS, soil NH⁺ ₄-N and N₂O flux in SSL and NR, soil NH⁺ ₄-N and CO₂ flux in SSL, and WSOC and CO₂ flux in WS. The ratio of N₂O-N/NO-N increased to approximately 100 in OL and SSL as N₂O emissions increased. Cumulative N₂O and CO₂ emissions increased as the C:N ratio decreased, but not significantly. The ratio of N₂O emission to applied N ranged from ?0.43% to 0.86%, and was significantly correlated with C:N ratio (y = ?0.59 ln [x] + 2.30, r ² = 0.99, P < 0.01). The ratio of CO₂ emissions to applied C ranged from ?5.8% to 45% and was also correlated with C:N ratio, but not significantly (r ² = 0.78, P = 0.11).     

Does the Ammonium Fixation Capacity of Soils Have a Significant Effect on the Nitrogen Nutrition of Wheat?

Pot experiments were conducted on three soils differing in their ammonium (NH₄ ⁺) fixation capacity [high = 161 mg NH₄-nitrogen (N) kg^?1 soil; medium = 31.5 mg NH₄-N kg^?1 soil; and no = no NH₄-N was additionally fixed], and the effect of N fertilizer forms and doses on wheat (Triticum aestivum L.) was investigated. Grain yields responded to almost all forms of N fertilizer with 80, 160, and 240 kg N ha^?1 in the high, medium, and no NH₄ ⁺ fixing soil process, respectively. Agronomic efficiency of applied N fertilizers was significantly greater in the no NH₄ ⁺ fixing soil. Thousand grain weights (TGW) of wheat grown on the high and medium NH₄ ⁺ fixing soil decreased with increasing N. Grain protein increased with increasing NH₄ ⁺ fixation capacity. Nitrogen doses and the forms of N fertilizers affected grain protein at a significance level. The combination of urea + ammonium nitrate (NH₄NO₃) was most effective in increasing grain protein content.     

Comparison of Factors Affecting Soil Nitrate Nitrogen and Ammonium Nitrogen Extraction

Extraction of soil nitrate nitrogen (NO₃ ^?-N) and ammonium nitrogen (NH₄ ⁺-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO₃ ^?-N and NH₄ ⁺-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L^?1 calcium chloride (CaCl₂), 1 mol L^?1 potassium chloride (KCl), and 0.5 mol L^?1 potassium sulfate (K₂SO₄)] and concentration (0.5, 1, and 2 mol L^?1 KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (?18 ^oC, 4 ^oC, and 25 ^oC) of extracted solution. The recovery of soil NO₃ ^?-N and NH₄ ⁺-N was also measured to compare the differences of three extracting reagents (CaCl₂, KCl, and K₂SO₄) for NO₃ ^?-N and NH₄ ⁺-N extraction. Air drying decreased NO₃ ^?-N but increased NH₄ ⁺-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO₃ ^?-N and NH₄ ⁺-N concentrations compared to other treatments. The concentrations of extracted NO₃ ^?-N and NH₄ ⁺-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO₃ ^?-N and NH₄ ⁺-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO₃ ^?-N and 94.9% for NH₄ ⁺-N. K₂SO₄ was not found suitable for NO₃ ^?-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl₂ was only 57.3% for NH₄ ⁺-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L^?1 KCl could fully extract NO₃ ^?-N. A ratio of 10:1 of solution to soil was adequate for NO₃ ^?-N extraction, whereas the NH₄ ⁺-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at ?18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO₃ ^?-N concentration, whereas the NH₄ ⁺-N concentration varied greatly with storage temperature. Storing the extracted solution at ?18 ^oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO₃ ^?-N and NH₄ ⁺-N concentrations. Compared with the immediate extraction, the averaged NO₃ ^?-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH₄ ⁺-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L^?1 KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO₃ ^?-N, whereas the concentration of extracted NH₄ ⁺-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at ?18 ^oC could improve the reliability of NO₃ ^?-N and NH₄ ⁺-N results.     

Nitrification and denitrification in an acidic soil of tea (Camellia sinensis L.) field estimated by δ15N values of leached nitrogen from the soil columns treated with ammonium nitrate in the presence or absence of a nitrification inhibitor and with slow-release fertilizers

Forty-two-day-old wheat (Triticum aestivum L. var. Asakazekomugi) plants were treated with complete, K-free (—K), Ca-limited (—Ca), and Mg-free (—Mg) nutrient solutions for 10 days using 2 mM NH₄NO₃ as the nitrogen source, which was replaced with 4 mM ¹⁵ NH₄C1 or Na¹⁵NO₃ for the subsequent 2 days to investigate the absorption, translocation, and assimilation of inorganic nitrogen in relation to the mineral supply. In another experiment plants were grown on NO₃ ^?, NH₄ ⁺, NH₄N0₃, and K-free and Ca-limited NH₄N0₃ nutrient solutions for 10 days, and then in the latter three treatments the nitrogen source was replaced with NO₃ ^? and half of the —K plants received K for 6 days to examine the changes in the nitrate reductase activity (NRA).

Wheat plants absorbed NH₄ ^?N and NO₃-N at a similar rate. Influence of K on the absorption of N0₃-N was stronger than that on the absorption of NH₄-N in wheat plants. The supply of K to the —K plants increased the absorption of NO₃-N, while the absorption of NH₄-N still remained at a lower rate in spite of the addition of K. A limited supply of Ca and lack of Mg in nutrient media slightly affected the absorption of NH₄-N. The influence of K was stronger on the translocation of nitrogen from roots to shoots, while Ca and Mg had little effect. When K was supplied again to the —K plants the translocation of NO₃,-N was more accelerated than that of NH₄-N. Incorporation of NH₄-N into protein was higher than that of NO₃-N in all the tissues; root, stem, and leaf. Assimilation of NH₄-N and NO₃-N decreased by the —K and —Mg treatments.

Leaf NRA of wheat plants decreased in the —K and —Ca plants. Higher leaf NRA was found when K was given again to the —K plants than when the plants were continuously grown in K-free media. Replacement of NO₃ ^? with NH₄ ⁺ as the nitrogen source caused a decline of leaf NRA, while the supply of both NH₄ ^?N and NO₃-N slightly affected the leaf NRA.