Stem‐infused nitrogen‐15 enrichment for evaluation of nitrogen use in maize

Abstract

Nitrogen use efficiency (NUE) of N fertilizer can be accurately estimated by tracing the fate of soil applied ¹⁵N‐labeling. However, the quantity of N remobilization from non‐kernel components into kernels in maize (Zea mays L.) plants is difficult to determine. A field experiment involving stem infusion with four levels of enriched ¹⁵N solution plus non‐infusion or infusion with water was conducted at Ottawa (45°22'N, 75°43'W), Canada to determine the effect of ¹⁵N atom % enrichment (a.e.) on physiological processes associated with NUE. At anthesis, 30 mL of 35.7 mmol N solution as ¹⁵NH₄ ¹⁵NO₃ at 5.0 (N05), 33.0 (N33), 66.0 (N66) and 99.2 (N99) ¹⁵N% a.e. was infused into the internode below the primary cob. The control plants were infused with distilled water. Photosynthesis was measured at 2, 4, 6, 24 h and 1 wk after infusion. Plants were sampled and separated into components at 4 d after anthesis (D4A) and at physiological maturity (PM). Dry weight, total N concentration, NH₄‐ and NO₃‐N, and ¹⁵N% a.e. of each component were determined. At D4A, the N33 and N66 treatments resulted in component ¹⁵N enrichment similar to that of N99 treatment. At PM, however, only N66 treatment produced results similar to that of N99. None of the infused ¹⁵N treatments interfered with ear‐leaf photosynthesis or component NH₄‐ and NO₃‐N concentrations. Infused ¹⁵N was easily moved out of the internode where it was infused, into most components, with the majority in the dominant sinks (cob, husk, and kernels). Nitrogen remobilization in both N66 and N99 treatments accounted for 62% of kernel N. These findings indicate that stem infusion is an appropriate approach to study N remobilization. Based on the concentrations investigated in this study, 66% or higher ¹⁵N% a.e. is required for accurate labeling when soil available N is high.     

Slow-release 15N fertilizer formulations to measure N2-fixation by isotope dilution

Pot experiments were carried out to examine the effects of slow-release fertilizer formulations on estimates of N₂-fixation determined by the isotope dilution method. Soybeans were used as the N₂-fixing plants, with non-nodulated soybeans and maize as the non-fixing controls. The ¹⁵N-fertilizer formulations used were (¹⁵NH₄)SO₄, K¹⁵NO₃, gypsum-pelleted K¹⁵NO₃, (¹⁵NH₄)₂SO₄ + glucose, ground plant material enriched with ¹⁵N or ¹⁵N-oxamide. The estimate of the amount of N₂ fixed by the nodulated soybean plants depended upon both the control plant and the fertilizer formulation used. Maize took up N later than non-nodulated soybean and estimates of soil N-pool (soil “A” value + fertilizer N added) calculated from the enrichment of this control were about twice as large as those calculated from the enrichment of non-nodulated soybean receiving the same fertilizer treatment. As a consequence, estimates of N₂-fixation relative to this control were lower than those relative to non-nodulating soybean (mean 140 mg N per pot compared with 292 mg N per pot). With unstablilized ¹⁵N salts errors were sufficient to produce negative estimates of fixation relative to maize. Even with a “well-matched” control (non-nodulated soybean) estimates of fixation varied with fertilizer formulation.     

一种直接测定硝化—反硝化气体的15N示踪—质谱法

本文对¹⁵N示踪—质谱法的可靠性进行了检验。结果表明,在不同的¹⁵N丰度气体样品的测定中,用两种方法(反硝化作用源的¹⁵N丰度法和气样的¹⁵N丰度法)计得的反硝化损失量基本一致,故建立起来的¹⁵N示踪—质谱法是可靠的。该方法的测定偏差随气样¹⁵N丰度的降低而增大。此外,回收率结果表明,(N₂+N₂O+NO_x)-¹⁵N累积释放量占加入NO₃-¹⁵N量的94.1％。因此,这一方法可用于直接测定氮肥的硝化—反硝化损失的研究中。     

Fate of 15N-Labeled Potassium Nitrate in Different Citrus-Cultivated Soils: Influence of Spring and Summer Application

The fate of ¹⁵N-labeled potassium nitrate (8.5% ¹⁵N excess) was determined in 3-year-old Valencia orange trees grown in 1-m³ containers filled with different textured soils (sandy and loamy). The trees were fertilized either in spring (24 March) or summer (24 July). Spring fertilized trees gave higher fruit yields in sandy than in loamy soils, which exceeded summer fertilized trees in both cases. Summer fertilized trees had greater leaf biomass than spring fertilized trees. Fibrous root weight was 1.9-fold higher in sandy than in loamy soil. At the end of the cycle, tree N recovery from spring application was 45.7% for sandy and 37.7% for loamy soil; from summer fertilization, N recovery was 58.9% and 51.5% for sandy and loamy soils, respectively. The ¹⁵N recovered in the inorganic soil fraction (0?C90?cm) was higher for loamy (1.3%) than for sandy soil (0.4%). Fertilizer N immobilized in the organic matter was lower in sandy (2.5%) than in loamy soil (6.0%). Potential nitrate leaching from fertilizer (¹⁵NO ₃ ^? ?CN in the 90?C110-cm soil layer plus ¹⁵NO ₃ ^? ?CN in drainage water) was 34.8% higher in sandy than in loamy soil. The low N levels in sandy soil resulted from both higher NO ₃ ^? ?CN leaching losses and higher N uptake of plants grown in the former. The great root mass and higher soil temperatures could account for raised plant N uptake in sandy soil and in summer, respectively.     

北京地区潮土表层中NO3--N的转化积累及其淋洗损失

本试验利用渗滤池设施，采用化学分析和同位素技术相结合的方法研究了北京地区潮土表层中施用氮肥后ＮＯ３＾－－Ｎ转化积累及其１３０ｃｍ土体的淋洗状况。常规分析结果表明，在春小麦和夏玉米的生育前期可以观察到氮素明显地向ＮＯ３＾－－Ｎ的转化积累，其强度随尿素施用量的增加而明显增加，而尿素、硝铵、硫铵等不同氮肥品种处理之间有差异但大多不显著。同时夏玉米期间转化积累作用比春小麦期间强烈。＾１５Ｎ标记试验结果表明     

Potato uptake and recovery of nitrogen 15N-enriched ammonium nitrate

Tuber yield and nitrogen uptake in potatoes were recorded during 1996 and 1997 in Southern Bavaria. Recovery of applied fertilizer nitrogen was measured by using ¹⁵N (¹⁵NH₄¹⁵NO₃). Nitrogen fertilizer was brought out either broadcast or in the ridge; 150 kg N ha⁻¹ were applied either at planting or in split doses of 50 kg N ha⁻¹ (at planting, emergence and at 20-cm plant height). Due to unfavorable conditions, tuber yield and fertilizer N recovery were lower in 1996 as compared with 1997. Fertilizer N recovery in plant biomass (tuber and foliage) ranged from 35.9% to 68.5% at growth stage EC 79; the main fraction was allocated to tubers. Placement of fertilizer N in the ridge had a positive effect on N recovery, when the total N amount was applied at planting. In broadcast application, fertilizer N recovery was higher when the fertilizer doses were split, as compared with a single broadcast application at planting. When fertilizer N was applied in split doses, the effect of N placement became negligibly small. Fertilizer N recovery in soil ranged from 19.5% to 24.6%, and total recovery ranged from 60.1% to 88.0%. Rainfall between planting and plant emergence, and conditions restricting plant development in early developmental stages were related with unaccounted fertilizer N losses. Therefore, the positive effects of split N applications or fertilizer placement are most likely to occur under unfavorable growing conditions.     

Petiole and leaf nitrate studies in sunflower

Abstract

This study was conducted at two sites in Mississippi to determine whether petiole and leaf NO^‐ ₃ monitoring could be used as a management tool in making fertilizer N recommendations for sunflower (Hellanthus annuus L.). Petiole and leaf samples were taken at the four leaf stage at both sites, and later at two week intervals at Brooksville. Petiole and leaf NO^‐ ₃ at the four leaf stage was significantly influenced by rate of N application at both sites. The level of petiole and leaf NO^‐ ₃ was highly correlated with rate of N application as well as with seed yield. The concentration of NO^‐ ₃ in petioles and leaves was greatest at the four leaf stage and showed quadratic declines as the season progressed. Petiole and leaf NO^‐ ₃ showed the highest correlations with rate of N application and seed yield at the four leaf stage than at any other sampling time at Brooksville, indicating that this was the “best” period for taking petiole and leaf samples. However, analysis of petioles and leaves at the four leaf growth stage for NO^‐ ₃ may have limited potential of becoming a useful tool in making N fertilizer recommendations for sunflower. This is due to the sensitivity of both petiole and leaf NO^‐ ₃ to time of sampling and locational differences, as well as lack of information on response of sunflower to N applied after this stage of growth.     

Denitrification losses of nitrogen fertilizer applied to winter wheat following ley and arable rotations as estimated by acetylene inhibition and 15N balance

We studied the fate of 222 kg N ha^?1 applied in spring as K¹⁵NO₃ to winter wheat test crops which followed either continuous arable cropping (Arable) or a rotation in which a 3-year grass/clover ley preceded the wheat (Ley). Denitrification losses (measured by an acetylene-inhibition method) of over 1 kg N ha^?1 d^?1 were measured for short periods following heavy rain in mid-May. However the generally dry and cool weather resulted in accumulated losses by denitrification between fertilizer application and anthesis equivalent to only 5.3% and 3.6% (±2%) of the applied N for the arable and ley treatments respectively. The smaller loss from the ley was despite this treatment containing more inorganic N and available carbon. ¹⁵N balance indicated that, at anthesis, 1.5% and 11.5% (± 7%) of the labelled N was lost from the arable and ley treatments respectively. Given the precision of the 15N and the acetylene-inhibition methods, the results are not significantly different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because ¹⁵N balance does not measure losses of unlabelled N. These were probably very small on the arable treatment but could have increased total N loss by 25% to c. 32 kg ha^?1 on the ley treatment compared with the 8 kg ha-1 measured as denitrified. Such a large difference is unlikely to be an error but was probably due to ammonia volatilization from this crop which was severely infected by mildew. The results were thus a poor test of the acetylene-inhibition method, but revealed another loss process which could be significant in some situations.     

Elevated CO2 concentrations increase leaf nitrate reduction by strengthening sink activity in soybean plants

An experiment was conducted to examine the effect of CO₂ enrichment on the nitrate uptake, nitrate reduction activity, and translocation of assimilated-N from leaves at varying levels of nitrogen nutrition in soybean using ¹⁵N tracer technique. CO₂ enrichment significantly increased the plant biomass, apparent leaf photosynthesis, sugar and starch contents of leaves, and reduced-N contents of the plant organs only when the plants were grown at high levels of nitrogen. A high supply of nitrogen enhanced plant growth and increased the reduced-N content of the plant organs, but its effect on the carbohydrate contents and photosynthetic rate were not significant. However, the combination of high CO₂ and high nitrogen levels led to an additive effect on all these parameters. The nitrate reductase activity increased temporarily for a short period of time by CO₂ enrichment and high nitrogen levels. ¹⁵N tracer studies indicated that the increase in the amount of reduced-N by CO₂ enrichment was derived from nitrate-N and not from fixed-N of the plant. To examine the translocation of reduced-N from the leaf in more detail, another experiment was conducted by feeding the plants with ¹⁵NO₃-N through a terminal leaflet of an upper trifoliated leaf under depodding and/or CO₂ enrichment conditions. The export rate of ¹⁵N from the terminal leaflet to other plant parts decreased by depodding, but it increased by CO₂ enrichment. CO₂ enrichment increased the percentage of plant ¹⁵N in the stem and / or pods. Depodding increased the percentage of plant ¹⁵N in the leaf and stem. The results suggested that the increase in the leaf nitrate reduction activity by CO₂ enrichment was due to the increase of the translocation of reduced-N from leaves through the strengthening of the sink activity of pods and / or stem for reduced-N.     

Immobilization of 15N in forest litter studied by 15N CPMAS NMR spectroscopy

Solutions labelled with ¹⁵N were applied as (¹⁵NH₄)₂SO₄ or K¹⁵NO₃ to isolated microplots in the floor of mountain beech forest (Nothofagus solandri var. cliffortioides) and incubated for 135 days under field conditions of moisture and temperature. Solid state ¹⁵N CPMAS NMR spectra of the forest litter layer showed that more than 80% of the total signal intensity was attributable to the secondary amide-peptide peak. The degree of ¹⁵N enrichment or form of N did not alter the relative intensity of signals attributable to ¹⁵N in peptides, nucleic acids and aliphatic amine groups (amino sugars and free NH₂ on amino acids). Combinations of ¹³C and ¹⁵N-NMR spectra, edited by a process that exploited differences in proton spin properties between distinct categories of organic matter, indicated incorporation of ¹⁵N in humified organic matter rather than partly degraded plant material. This application demonstrated that solid state ¹⁵N CPMAS NMR has potential for use in studies of N immobilization under field conditions and with materials containing little N and small ¹⁵N enrichment.     

A 15N tracing technique-based analysis of the fate of fertilizer N: a 4-year case study in eastern China

The fate of fertilizer N applied with different irrigation amounts in tobacco fields was quantitatively studied by applying ¹⁵N double-labelled NH₄NO₃ in lysimeters. The ¹⁵N (fertilizer N originating from the fertilizer applied in 2011) in tobacco plants, ¹⁵N in soils and ¹⁵N loss were observed continuously from 2011 to 2014. The results showed that 21.6% of ¹⁵N was utilized by tobacco plants, 72.1% remained in the 0–60 cm soil layer and 6.3% was lost from the soil–plant system after the first season’s harvest (2011) of flue-cured tobacco. During the four seasons from 2011 to 2014, cumulative utilization of ¹⁵N by tobacco plants was 34.3%, while 54.2% remained in the 0–60 cm soil layer, and 11.5% was lost via mechanisms such as leaching and volatilization. The fate of ¹⁵N in terms of accumulation in plants and soils or losses from the soil–plant system from 2012 to 2014 was greatly affected by the fertilizer and irrigation management strategies in 2011. The results of this investigation suggest that the major amount of fertilizer N applied during the first season remains available in the soil for utilization by tobacco plants after 4 years.     

Fate of fertilizer nitrogen applied to grassland. II. Nitrogen-15 leaching results

Results are presented from a 3-year investigation into nitrate leaching from grassed monolith lysimeters treated with double (¹⁵NH₄¹⁵NO₃) or single (¹⁵NH₄NO₃) labelled ammonium nitrate at three rates, 250, 500 and 900 kg N ha^?1 a^?1. Over the 3 years of the experiment, 0.14%, 3.1% and 18.1% of the applied fertilizer was recovered in the leachate at 250, 500 and 900kg N ha^?1 respectively. This represented 9%, 39% and 75% of the overall nitrate leaching at the three application rates. A significant proportion of the fertilizer leached as nitrate at the three application rates was derived, via nitrification, from the fertilizer ammonium. Increasing fertilizer applications caused a rise in the leaching of both soil and fertilizer derived nitrogen, although whether the increase reflected a true priming effect was not clear.     

Aufnahme und Verlagerung von 15N-Stickstoff bei Sommergerste in Abhängigkeit von K-Ernährung und Mehltaubefall

¹⁵N uptake and distribution by spring barley in relation to K nutrition and mildew attack Barley cv. Aramir was grown in complete nutrient solution containing 0,5 (K₁), 1,5 (K₂) and 4,5 mM K (K₃) respectively till maturity. Part of the plants was infected by mildew (Erysiphe graminis), whereas control treatments were kept mildew-free by fungicide sprays. Symptoms of mildew attack were less at K₂ and K₃ and grain yields of infected plants were raised from 98 (K₁) to 160 (K₂) and 165 g/pot (K₃). In control treatments grain yields were 130 (K₁), 164 (K₂) and 163 (K₃) g/pot. During anthesis nitrogen in the nutrient solution was labelled with ¹⁵N for two days and ¹⁵N metablism was studied within 7 consecutive days. The aim of this pulse-chase experiment was to find out, how mildew attack and K nutrition influence N metabolism. The following results were obtained: a) ¹⁵N uptake increased from K₁ to K₂ in infected and healthy plants, though at slightly lower levels in the infected treatments. b) During 7 days after the ¹⁵N pulse up to 17,1% of the ¹⁵N absorbed were translocated into the grains. Better K status as well as spray treatments stimulated nitrogen translocation. c) Mildew attack of leaves does not seem to influence the conversion of grain-¹⁵N into grain proteins. Better K supply stimulates the rate of protein synthesis in the grain. d) In mildew infected leaves relatively more of the ¹⁵N absorbed is incorporated into leaf proteins. But this may be a consequence of slower grain growth, because already 7 days after anthesis infected plants in all K treatments had significantly lower grain yield. In spite of the short duration of the experiment the ¹⁵N data do not indicate that the visible depression of mildew attack by better K nutrition of the crop might inter alia be due to K stimulated defense reactions within N metabolism. This is in agreement with a previous experiment.     

Effect of rate of nitrapyrin and source of N on soil pH and response of burley tobacco

Abstract

A field experiment was conducted on Maury silt loam soil (Typic Paleudalf) during 2 years to determine the effects of rate of nitrapyrin and source of N fertilizer on soil pH and response of burley tobacco (Nicotiana tabacum L.cv.^xKy‐14'). All sources of N were applied at the rate of 280 kg N ha^‐1. The information was needed to increase the efficiency of N fertilizer use and improve the growth and safety of tobacco.

Results indicated that application of a NO₃ ^‐source of N fertilizer or low rates of nitrapyrin (0.56 to 2.24 kg ha^‐1) decreased surface soil acidification and the concentration of plant Mn, while plant dry weight early in the growing season was increased. The early growth benefits noted for .nitrapyrin did not lead to increased cured leaf yields or value. Cured leaf yield and value were highest in plots receiving Ca(NO₃)₂, followed by KH₄NO₃, then urea.

Concentration of protein N, total alkaloids, and total volatile nitrogenous bases of cured leaves increased and NO₃ ^‐‐N decreased as rate of nitrapyrin increased. Total N concentration of cured leaf, however, was not significantly affected by nitrapyrin application, indicating that the proportion of absorbed N as NH₄ ⁺increased as nitrapyrin rate increased.     

Late nitrogen fertilization affects nitrogen remobilization in wheat

A pot experiment with wheat plants was carried out to study how late application of nitrogen (N) fertilizer affects the use of pre‐anthesis N reserves during the grain‐filling period. Increasing doses of N fertilizer were applied (0, 40, and 52 mg N plant^–1), either in two amendments (growth stages GS20 and GS30, according to Zadoks scale) or in three amendments (GS20, GS30, and GS37). The experiment was arranged in a complete randomized three‐block design with 129 plants per treatment. The plants were watered daily, harvested every 2 d between anthesis and maturity, and were separated into roots, leaf sheaths, leaf blades, and ears for further N determination. Grain N concentration improved due to a late N application in GS37 by 14% (higher N dose) and by 7% (further splitting the same N‐fertilizer dose, respectively). The higher the N‐fertilizer dose applied, the greater was the amount of pre‐anthesis reserves in vegetative organs, these reserves became later available for remobilization. Although splitting the same N dose in three amendments did not increase the N reserves, these reserves were more efficiently remobilized allowing an improvement in grain N concentration. The fertilizer management did not change the temporary pattern of N accumulation in the ear, but did induce a change in the amount of N remobilized and in the contribution of each organ (root, leaf sheath, leaf blade) to this remobilization. Late N amendment allowed a greater N availability of leaf blades and ear N reserves (from 20% up to 26% and from 19% up to 22%, respectively) for remobilization towards the grain, decreasing the root contribution from 28% down to 15%, while the contribution of leaf sheaths was maintained around 35% irrespective of the N applied.     

Uptake and translocation of nitrogen in patumma (Curcuma alismatifolia) by leaves or root

An adequate supply of nitrogen (N) is important for patumma growth and flower quality. This study aimed to compare the uptake and translocation of N by foliar and root application. Fertilization with ¹⁵ nitrate (NO₃)-N via roots or leaves was carried out at four stages, at the 1st to 4th fully expanded leaf (FEL) stages, and the plants were sampled at each successive stage. The uptake and translocation of ¹⁵N from foliar or root applications showed relatively similar patterns at all stages. Although the N fertilizer utilization rate by roots was higher than that via leaves, the foliar application stimulated reproductive growth by earlier flowering. The N supplied at the 1st FEL and the 2nd FEL was utilized mainly in leaves, whereas supplying N at the 3rd and 4th FEL promoted flower quality. Fertilizer application method and stage of application influence the utilization rate and translocation of N to the sink organs.     

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.     

不同管理方式对夏玉米氮素吸收、分配及去向的影响

【目的】本文利用15N同位素示踪技术探讨传统(CT)和优化(YH)两种管理方式对夏玉米氮素吸收、分配及去向的影响。分析目标产量下化肥氮的变化,解析夏玉米花前、花后氮素利用及转移规律,探讨肥料氮、土壤氮与作物氮之间的关系,为该地区夏玉米的科学合理施氮提供合理依据。【方法】在传统和优化两种管理方式定位试验中设置15N微区,采用将15N标记的尿素表施的方法,分析植株和土壤样品。新鲜土壤用1 mol/L KCl浸提,滤液用TRACCS 2000型流动分析仪测定土壤的NH+4-N和NO-3-N含量。15N标记的土壤和植物全氮的测定用烘干样(过0.15 mm筛),然后用美国THERMO finnigan公司生产的稳定同位素质谱仪DeltaplusXP进行测定。【结果】在该试验条件下,优化方式下夏玉米籽粒产量和总吸氮量显著高于传统方式,分别增加12%和10%。作物收获后,优化方式的15N吸收量及利用率显著高于传统方式,利用率分别为20.81%、32.54%。夏玉米各器官中氮素的积累量和向籽粒中的转移量土壤氮显著高于肥料氮,传统方式籽粒中氮素的57.73%、优化方式籽粒中氮素的45.15%来自各器官的转移,近一半的氮素是在花后积累的,基施高氮对作物生长作用不大。开花期土壤表层硝态氮含量传统方式显著高于优化方式,收获后有所降低,而土壤深层含量明显增加,有向下淋洗的趋势。夏玉米收获后,传统方式各土层的原子百分超均高于优化方式,而且在20—40 cm处出现了明显的15N累积峰,与开花期相比,40 cm以下土层的原子百分超明显增大,氮肥随水向下淋洗强烈。夏玉米收获后传统方式土壤氮素残留率高达56.18%,表现为土壤残留损失作物吸收;优化方式则表现为土壤残留作物吸收损失。【结论】在优化方式中夏玉米施氮量为N 185 kg/hm2时,玉米达到高产水平且氮肥的利用率高。适当减少施氮量及增加后期追肥次数可实现夏玉米的高产和肥料的高效利用。     

MONITORING NITROGEN POLLUTION FROM SUGARCANE RUNOFF USING 15N ANALYSIS

In many forested wetlands of Louisiana, surface water quality is being deteriorated by nutrient input from adjacent agricultural production areas. This field study assesses the input of fertilizer N, applied to sugarcane fields, to forested wetlands. The potential use of natural abundance variations in ¹⁵N¹⁴N ratios for identification and tracing surface water N sources (NH ₄ ⁺ - and NO₃ ^--N) was evaluated. Runoff and surface water samples were collected from sugarcane fields and bordering forested wetlands (6 stations) over a 16 month period and analyzed for NH ₄ ⁺ -N, NO ₃ ^- -N, and associated NH ₄ ⁺ -δ¹⁵N and NO ₃ ^- -δ¹⁵N ratios. Fertilizer N draining into adjacent forested wetland was estimated to be only a small fraction of the amount applied. Concentrations of NH ₄ ⁺ - and NO ₃ ^- -N in the collected water samples were low and ranged from 0.02 to 1.79 mg L^-1. Isotopic analysis revealed NH ₄ ⁺ -δ¹⁵N and NO ₃ ^- -δ¹⁵N means were distinctive and may have the potential to be used as tracers of N contamination. The mean NH ₄ ⁺ -δ¹⁵N value was +18.6 ± 7.1‰ and the NO ₃ ^- -δ¹⁵N mean was +8.3 ± 3.1‰. Anomalously high NO ₃ ^- -δ¹⁵N values (>30‰) were attributed to denitrification.     

硝酸盐反射仪和SPAD法对玉米氮素营养诊断的比较

精准的营养诊断是了解作物氮素营养及推荐施肥的基础。本文在田间滴灌条件下利用SPAD叶绿素仪(SPAD-502 Plus)和硝酸盐反射仪(RQ flex10)两种诊断方法对玉米关键生育时期的氮素营养诊断进行研究,旨在筛选出适宜的诊断方法,并依据诊断值建立滴灌玉米不同生育时期的施肥模型。试验设置0 kg(N)·hm~(-2)(N0)、225 kg(N)·hm~(-2)(N225)、330 kg(N)·hm~(-2)(N330)、435 kg(N)·hm~(-2)(N435)和540 kg(N)·hm~(-2)(N540)5个施氮水平,在不同生育时期测定了玉米叶片SPAD值和叶鞘NO_3~-含量,并分别与施氮量、植株全氮含量、产量进行方程拟合,比较两种诊断方法对玉米氮素营养的响应。研究结果表明:1)玉米叶片SPAD值和叶鞘NO_3~-含量均随施氮量的增加而显著升高,且在拔节期对施氮量的响应最敏感。叶鞘NO_3~-含量对施氮量变化的响应较SPAD值大,其与施氮量及玉米产量的拟合度均高于SPAD值,说明硝酸盐反射仪法对滴灌玉米氮素丰缺的反应更灵敏。2)玉米全氮含量与叶片SPAD值呈显著线性关系,而与叶鞘NO_3~-含量则以线性加平台表示。当叶鞘NO_3~-含量小于186 mg·L~(-1)时,植株全氮与NO_3~-间呈显著线性相关;当叶鞘NO_3~-含量大于186 mg·L~(-1)时,植株全氮随NO_3~-含量增加趋于不变。3)本农作区滴灌玉米最佳经济施氮量为402.5 kg·hm~(-2),对应的玉米产量为17 049 kg·hm~(-2)。玉米拔节期、抽雄吐丝期和灌浆期的临界叶鞘NO_3~-含量分别为729.3 mg·L~(-1)、536 mg·L~(-1)和81.2 mg·L~(-1)。SPAD叶绿素仪和硝酸盐反射仪均可对滴灌玉米进行氮素营养诊断,但硝酸盐速测值能更敏感地反映氮素丰缺状况,基于硝酸盐反射法进行作物氮素营养诊断及推荐施肥具有较好的准确性与适用性。