Nitrogen‐use efficiency of young citrus trees as influenced by the timing of fertilizer application

The aim of this study was to improve the fertilizer‐nitrogen‐use efficiency (NUE) for a sustainable citrus production where yield, fruit quality, and environment are reconciled. A soil pot experiment was conducted using 5‐year‐old orange trees (Citrus sinensis cv. Navel Lane Late) to determine the response of NUE to timing of N‐fertilizer supply. An equal N rate (50 g tree^–1) following two seasonal supply distributions was tested: (1) ENS (early supply from March to June of 75% of the N rate, with the remaining 25% administered between July and October) and (2) LNS (late supply of the bulk of the N rate with 25% between March and June and 75% between July and October). Labeled fertilizer (5 atom% ¹⁵N excess) was applied in order to accurately quantify N uptake, its partitioning among plant–soil compartments and NUE at the end of fruit drop and at fruit maturity. LNS resulted in a significant increase in NUE in both stages (up to 19% at the end of fruit drop and 9% at fruit maturity), while also increasing summer/autumn flush development. ENS showed lower fruit abscission, an enhanced final fruit load (up to 45%), and a more profuse development of spring flush. It is worth mentioning that LNS led to higher ¹⁵N content in the majority of the tree storage organs available for next spring growth resumption. The amount of ¹⁵N remaining in the soil of ENS trees at the end of the trial, which represented 16% of the supplied ¹⁵N, was 1.5‐fold higher than that of LNS trees as a result of the lower NUE of the former. Irrespective of the seasonal distribution of the fertilizer, mean ¹⁵N recovery in the soil–plant system at the end of the trial was about 71%. The results clearly confirm that the N seasonal distribution curve affects NUE in young citrus trees and NUE increases when the bulk of supply of the N rate is delayed to the summer months.     

Species‐specific differences in nitrogen uptake and utilization by six European tree species

Species‐specific uptake and allocation mechanisms for N are scarce, in particular when trees are cultivated in potted soil under more natural conditions than in hydroponic culture. The objective of this study was to compare specific N‐uptake rates for economically and ecologically important tree species in Central European forests: pine (Pinus sylvestris), spruce (Picea abies), oak (Quercus petraea), beech (Fagus sylvatica), lime (Tilia cordata), and ash (Fraxinus excelsior) when they grow in mineral soil from an old fallow site with a pH of 6. We used an ¹⁵N‐labeling method to measure tree seedling ¹⁵N uptake in potted soils (Humic Cambisol) when both N forms NH$ _4^+ $ and NO$ _3^- $ were simultaneously present in the soil solution for interspecies comparison and assessment of relationships between specific ¹⁵N‐uptake rates and amino acid–accumulation rates or relative growth rates (RGR). The results demonstrate that tree species varied significantly in their capacity to take up NH$ _4^+ $ or NO$ _3^- $ into roots, stems, or leaves, but indicate only marginal differences in their preference for NH$ _4^+ $ or NO$ _3^- $ when they grow in mineral soil. The ranking of specific ¹⁵N‐uptake rates for NH$ _4^+ $ and NO$ _3^- $ was oak < beech < spruce < pine < lime < ash. Fine roots of all species had the highest specific ¹⁵N‐uptake rates for both N forms, followed by total roots, leaves/needles, and stems. As regards tree seedling species, we found negative relationships between glutamine (Gln)‐accumulation rates in leaves/needles and total ¹⁵N‐uptake rates in fine roots. Noteworthy was the fact that, at high Gln‐accumulation rates, the N‐uptake system in fine roots of ash was probably lower under feedback inhibition by the amino acid.     

Nitrogen‐use efficiency traits of mini‐watermelon in response to grafting and nitrogen‐fertilization doses

Two experiments were conducted to study the effect of grafting on nitrogen‐use efficiency (NUE) in mini‐watermelon plants. In the first study, mini‐watermelon plants (Citrullus lanatus [Thumb.] Matsum. and Nakai cv. Minirossa) either ungrafted or grafted onto Macis, Vita (Lagenaria siceraria [Mol.] Standl.), PS1313, and RP15 (Cucurbita maxima Duchesne × Cucurbita moschata Duchesne) rootstocks grown in hydroponics were compared in terms of shoot dry biomass, leaf area, root‐to‐shoot ratio, SPAD index, shoot N uptake, and nitrate reductase (NR) activity 40 d after transplantation in response to nitrate concentration in the nutrient solution (0.5, 2.5, 5, 10, 15, or 20 mM of NO$ _3^- $ ). In the second experiment, the suitability of a selected rootstock with high NUE (Vita) to improve crop performance and NUE of grafted mini‐watermelon plants was evaluated under field conditions. In the hydroponic experiment mini‐watermelon grafted onto Vita rootstock needed the lowest nitrate concentration (1.31 mM of NO₃) in the nutrient solution to reach half maximum shoot dry weight. Total leaf area, SPAD index, and shoot N uptake increased in response to an increase of N concentration in the nutrient solution. At 2.5 mM NO$ _3^- $ , mini‐watermelon grafted on either Vita or RP15 had the highest NR activity whereas no significant difference was observed at 10 mM NO$ _3^- $ . The open‐field study indicated that increasing N‐fertilization rates from 0 to 100 kg ha^–1 improved total and marketable yields of mini‐watermelon plants while decreasing NUE. When averaged over N levels, the marketable yield, NUE, N‐uptake efficiency, and N‐utilization efficiency were significantly higher by 39%, 38%, 21%, and 17%, respectively, in Minirossa grafted onto Vita compared to ungrafted Minirossa plants. Therefore, grafting mini‐watermelon plants onto selected rootstocks can be used as a quick and effective method for improving productivity and NUE.     

The role of biochar in retaining nutrients in amended tropical soils

This study investigated the effect of biochar amendments on the retention and availability of plant nutrients and Al in seven acidic tropical soils from Zambia and Indonesia. The experiments carried out investigated whether the adsorption capacity of NH$ _4^+ $ in the soils increased upon the addition of biochar and which effect biochar had on available concentrations of NO$ _3^- $ , K⁺, Mn²⁺, Mg²⁺ , PO$ _4^{3‐} $ , and Al³⁺. These nutrients were selected as they represent those important to plant growth and soil quality. No significant increases or decreases in aqueous NH$ _4^+ $ ‐N concentration with additions of biochar were detected. The Gaines–Thomas model was used in order to calculate selectivity coefficients for NH$ _4^+ $ exchange (K_gt values). Following the addition of biochar to soil, K_gt values decreased showing a reduction in the selective binding of NH$ _4^+ $ in the biochar amended soil compared to the control. The concentration of NO$ _3^- $ increased following the addition of biochar to the soils. The addition of 5 and 10% biochar to the Indonesian soil did not significantly alter (t‐test confidence level 0.05) the sorption of PO$ _4^{3‐} $ to the soil–biochar mixtures as compared to the soil alone. However, the addition of biochar to the soil from Zambia increased the sorption of PO$ _4^{3‐} $ compared to the soil alone. The concentrations of K⁺ and Mg²⁺ were significantly increased for almost all soils (t‐test at the 0.05 confidence level) following the addition of biochar. Addition of biochar to all but two soils significantly decreased (t‐test confidence level 0.05) Mn²⁺ concentrations. The concentration of Al³⁺ in the soils decreased exponentially significantly (t‐test confidence level 0.05) following the amendment of biochar in accordance with the increase in pH observed when biochar was added to the soil. These results show that biochar has the ability to release essential plant growth nutrients as well as alleviate Al toxicity in these soils.     

The combined effects of salinity and nitrogen forms on Catharanthus roseus: The role of internal ammonium and free amino acids during salt stress

Catharanthus roseus is widely cultivated at the seaside of Hainan island of China for production of vinblastine with anticancer activity. Improving growth of this plant and vinblastine content under saline conditions is one of the agronomic challenges. In this study, we compare the responses of C. rosesus upon nitrate (N1) or ammonium nitrate (N2) nutrition to salinity. Biomass accumulation, reproductive growth, photosynthetic gas exchange, and the concentrations of soluble sugars (SS), free amino acids (FAA), ammonium (NH$ _4^+ $ ), sodium (Na⁺), and vinblastine were determined during 3 weeks of treatment in N1‐control, N1‐salt, N2‐control, and N2‐salt plants. The retarded growth caused by salt stress was largely ameliorated by the N2 nitrogen form. One of the reasons for this improved salt resistance in N2‐salt plants was related to the competitive uptake of NH$ _4^+ $ and Na⁺. The N2‐salt plants displayed a more rapid response to salinity, leading to decreased transpiration rate, well‐kept water relations, and reduced uptake of Na⁺. The N2‐control plants rapidly increased the concentrations of soluble sugars and free amino acids, such as sucrose, glucose, proline, and threonine in the first week. These compounds may act as compatible solutes to adjust osmotic stress and retain ion homeostasis in salt‐stressed plants. The increased FAA was found to be at the expense of glutamate accumulation, and the activities of nitrogen assimilation enzymes were tightly linked with this process. The medical alkaloid vinblastine in C. roseus was significantly increased in N2‐salt plants in the first week and in N1‐salt ones in the third week compared to their controls. It is concluded that the supplied nitrogen regime modified salinity‐induced vinblastine production in a time‐dependent way, and this process was tightly related to NH$ _4^+ $ status.     

Nitrogen transformation and plant growth in response to different urea‐application methods and the addition of DMPP

The differences in soil inorganic‐nitrogen (N) concentration and distribution, plant biomass, and root growth in the presence or absence of the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) under different urea‐application methods (placement versus homogeneously applied) were explored in a short‐term microcosm experiment. Spring wheat (Triticum aestivum L.) was grown in a microcosm with six different treatments: no amendment (CK), DMPP homogeneously applied (DMPP‐hom), urea homogeneously applied (Urea‐hom), urea with DMPP homogeneously applied (Urea + DMPP‐hom), urea placement (Urea‐place), and urea with DMPP placement (Urea + DMPP‐place). After 28 d, plant biomass, soil inorganic nitrogen content, distribution of soil inorganic nitrogen and plant roots in the soil were analyzed. The soil inorganic N and plant roots tended to be distributed asymmetrically in the placement treatment but were distributed symmetrically in the homogeneous treatment. DMPP addition significantly increased the soil NH$ _4^+ $ ‐N content and decreased the NO$ _3^- $ ‐N content, especially near the fertilized zones in the placement treatment. Compared to the urea‐only treatments, DMPP application significantly increased the shoot biomass and root lengths of the wheat in the homogeneous treatment but decreased them in the placement treatment. Therefore, homogeneously applied urea and DMPP may produce a more uniform nutrient distribution, leading to greater nitrogen retention in the soil and thus accelerating wheat growth.     

Effects of earthworms and plant growth–promoting rhizobacteria (PGPR) on availability of nitrogen,phosphorus, and potassium in soil

Both earthworms and plant growth–promoting rhizobacteria (PGPR) are ubiquitous and important for promoting circulation of plant macronutrients. Two series of laboratory experiments were conducted to investigate the effects of earthworm casts and activities on the growth of PGPR, and the inoculation of earthworms and PGPR on the availability of N, P, and K in soils, respectively. During a short incubation period (0–34 h), the extracts of earthworm (Pheretima guillelmi)‐worked soil significantly (p < 0.05) increased the abundance of the three species of PGPR, including N‐fixing bacteria (NFB) (Azotobacter chroococcum HKN‐5), phosphate‐solubilizing bacteria (PSB) (Bacillus megaterium HKP‐1), and K‐solubilizing bacteria (KSB) (B. mucilaginous HKK‐1), in Luria‐Bertani (LB) broth. There were synergistic effects of dual inoculation of earthworms and PGPR on increasing the concentrations of NH$ _4^+ $ ‐N, (NO$ _3^- $ + NO$ _2^- $ )‐N, NaHCO₃‐extractable P, and NH₄OAc‐extractable K in the corresponding soils. Bioavailable N (the sum of NH$ _4^+ $ ‐N and [NO$ _3^- $ + NO$ _2^- $ ]‐N) in the dual inoculation was 4 to 24 times those inoculated with earthworms or NFB alone, respectively. The significantly higher concentrations of bioavailable N and P in the dual inoculation of earthworms and NFB or PSB may be due to the higher abundance of PGPR and/or higher activities of urease and acid phosphatase than those of single inoculation of NFB or PSB, respectively. Dual inoculation of earthworms and PGPR would be most effective in reducing the need for chemical fertilizers in agriculture.     

Nitrification is the key process determining N use efficiency in paddy soils

Nitrogen (N) fertilizer use efficiency (NUE) in flooded paddy fields is relatively low. Many N fertilizer management options have been proposed to enhance NUE and minimize environmental damage. However, few investigations are focusing on the role of the characteristics of soil N transformations in regulating NUE and N losses in paddy fields. In this study, we test the role of soil N transformations on NUE and N losses under rice growth conditions in two paddy soils collected from Jiangxi (JX) and Sichuan (SC) in China. The N recoveries of applied ¹⁵N either as nitrate or ammonium in plant and soil, and N losses estimated by ¹⁵N balance were investigated in rice pot experiments using a ¹⁵N tracing technique. The results showed that gross nitrification rates in soil collected from JX were much lower than those in soils collected from SC either at 60% water holding capacity (WHC) or rice growth (flooding) conditions, which could be due to the difference in soil pH. The ‐N concentration in soil solution was maintained at a relatively high level for a long time period after N fertilizer application in the JX soil (41 d) compared to the SC soil (26 d), caused by different nitrification rates owing to different soil pH. The ¹⁵N uptake by rice in the JX soil (29–78%) was always significantly higher than that in the SC soil (22–54%), while N losses from the plant–soil system in the JX soil (17–21%) were always significantly lower than those from the SC soil (20–34%) at the same rice growth stage in the labeled ¹⁵N ammonium treatment. However, there were no significant differences in ¹⁵N uptake by rice and N losses in applied treatment between the two studied soils. These results indicate that nitrification, not denitrification, was the key process determining NUE and N losses in paddy soils. The results of the N application gradient experiment also indicated that higher amounts of N fertilizer should be applied for the same amount of N uptake, however, this caused higher N losses, in soils characterized by high nitrification rate (e.g ., the alkaline soil). Results highlighted that soil N transformations in particular nitrification rate provided a very good guideline for an optimized N management.     

Charcoal in Amazonian paddy soil—Nutrient availability,rice growth and methane emissions

We investigated the effects of charcoal under flooded (anoxic) rice cultivation at low and high fertilizer levels during 2 y in the Maranhão lowlands, eastern periphery of Amazonia. Two applications (at onset of first and second year) of 15 Mg ha^–1 of fine (< 2 mm) charcoal derived from the endocarp of the babassu (Attalea speciosa Mart.) palm nut had little influence on soil fertility, rice growth, yield, and nutritional status. Exception to this were negative impacts of charcoal on first‐year N availability, with lower sub‐superficial soil NH$ _4^+ $ availability paired with lower rice tissue N and a responsiveness of grain yields to (mainly N‐) fertilization following charcoal application. This N‐limitation effect was, however, limited to the first year and—though statistically significant—without agronomic relevance. The most consistent charcoal effect on flooded‐soil fertility was the strong increase in K availability in the second year, at low and to a lesser extent at intermediate, but not at high fertilizer level. Low K concentrations of our charcoal exclude the possibility of direct K inputs via charcoal, suggesting other indirect mechanisms for K availability increases. Methane fluxes in the second year were significantly reduced (–43.8%) by charcoal application, charcoal‐induced reductions were stronger under high‐ (–47.3%) than under low‐fertilizer regime (–26.0%). Thus, charcoal could be a valuable tool for reducing methane emissions associated with intensely fertilized flooded rice, without significantly affecting grain yields.     

Plant availability and leaching of (heavy) metals from ammonium‐, calcium‐, carbohydrate‐, and citric acid‐treated uranium‐mine‐dump soil

Remediation of an uranium‐mine soil from Settendorf (East Germany) includes phytoextraction under conditions which make its heavy metals more plant‐available but less leachable. A second way is active inhibition of heavy metal uptake by the plant. In a pot trial with Chinese cabbage (Brassica chinensis L.), planted and unplanted soil samples were daily irrigated with deionized water or aqueous solutions with a total of (g (kg soil)^–1) CaCl₂ (0.26 Ca), NH₄Cl (1.39), casein, sucrose, citric acid (13), and an extract of rape (B. napus L.) shoots (13 DW) in a phytotron for 26 d. Water‐irrigated plants were also treated with a 50 mM citric acid solution (10.5 g (kg soil)^–1) 6 and 7 d prior to harvesting. Total elements in plant tissue and soluble elements in aqueous extracts from control and postharvest soils were determined by ICP‐AES. Supplements of NH , and the NH ‐generating casein and rape extract reduced soil pH during nitrification, and increased plant uptake of Cd, Cu, Ni, and Zn. Citric acid at 50 mM adjusted soil to pH 4.5–6.0 and enhanced uptake of all elements. Long‐term application of sucrose and citric acid increased pH and inhibited uptake of Cd, Cr, Cu, Ni, and Zn. Contemporarily, leaching of heavy metals and humic substances was lowest with Ca and NH and highest with sucrose and citric acid amendments. It is concluded that Chinese cabbage grown for chelate‐assisted phytoextraction should be supplied with Ca and NH to obtain a high plant biomass on soil with a low hazard of leaching. Metal uptake should be stimulated by application of chelator 7 d prior to harvesting. Undesired uptake of heavy metals by Chinese cabbage determined as food should be inhibited with carbohydrate amendments. Long‐term application of NH or chelator, which reduces the solubility of certain elements but increases their uptake moderately, is recommended as a tool for continuous phytoextraction technologies.     

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.     

MODULATION IN YIELD AND JUICE QUALITY CHARACTERISTICS OF CITRUS FRUIT FROM TREES SUPPLIED WITH ZINC AND POTASSIUM FOLIARLY

Citrus, especially K innow (Citrus deliciosa × Citrus nobilis), fruit yield and quality in Pakistan is not competitive with that of other countries which could be mainly attributed to the lack of good nutrient management for citrus orchards. The yield losses in this fruit crop occur mainly due to heavy fruit dropping. Experiments to overcome these problems were conducted at four different sites one each in Faisalabad, Toba Tek Singh, Jhang and Sargodha districts of Punjab, Pakistan. The soil and leaf chemical analysis showed severe deficiency of Zn and our pervious results have shown that soil amendment with potassium (K) at 75 K₂O kg ha^?1 improved the citrus fruit yield and quality at all selected sites. In the present experiments, effect of foliar application of Zn and K alone or in combination was studied on nutrient uptake, fruit yield, fruit dropping and juice quality. The fruit trees were pretreated with a selected K level of sulfate of potash (SOP) or muriate of potash (MOP), i.e., 75 kg K₂O ha^?1 along with recommended nitrogen (N) and phosphorus (P) doses. Zinc [Zn, 1% zinc sulfate (ZnSO₄) solution], K [1% potassium sulfate (K₂SO₄) solution] and Zn + K (solution containing 0.5% each of ZnSO₄ and K₂SO₄) were sprayed at the onset of spring and flush of leaves or flowers, fruit formation and at color initiation on fruit. Overall, application of Zn, K or Zn + K was effective in improving the nutrient uptake, yield and quality parameters of citrus fruit at all sites. Fruit dropping was also reduced by the foliar spray of Zn, K or Zn + K but the most promising results were recorded with foliar spray containing both Zn and K.     

不同深施方式对太湖地区稻田氨挥发和氮肥利用率的影响

本研究以太湖地区稻田为研究对象开展连续两年的田间试验，通过设置不施氮肥（CK）、常规施氮（CN）、减氮表施（RN）、减氮侧深施（RNS）和减氮穴施（RNP）5种施氮处理，探究不同深施方式对稻田氨挥发与氮肥利用率的影响。结果表明，与表施处理（CN和RN）相比，RNS和RNP通过降低田面水NH₄⁺-N浓度和pH分别减少30.95%~41.54%和66.71%~72.23%的氨挥发排放（P<0.05）。相较于RN处理，RNP促进水稻根系生长并增加根区土壤有效氮含量，进而增加水稻产量（6.23%），提高氮肥利用率（50.15%），降低土壤氮盈余（63.92%）（P<0.05）。与CN处理相比，RNS显著降低土壤氮盈余（29.20%）（P<0.05），但水稻吸氮量和氮肥利用率均未显著增加。相较于RNS，RNP进一步降低氨挥发损失（50.84%）和土壤氮盈余（51.07%），提高氮肥利用率（40.40%）（P<0.05）。综上所述，RNP的农学和环境效益最高，但因穴施机械及肥料造粒技术等因素的限制，尚难应用于实际生产；而侧深施肥在我国水稻大规模集约化生产中效益较高且切实可行。     

Bio‐inoculant improves nitrogen‐use efficiency and cotton yield in saline soils

Improved nutrient‐use efficiency is important to sustain agricultural production. The goal of our study was to investigate the effects of Azovit® (Azotobacter chroococcum) inoculation of seed with N fertilization on crop yield, nutrient uptake, and N‐use efficiency (NUE) of irrigated cotton (Gossypium hirsutum L. cv. C‐6524) in secondary saline soil under continental climatic conditions of Uzbekistan. A randomized complete block design in a 4 × 2 split‐plot experiment was established in the fall of 2013. The main plot was N fertilization (0, 140, 210, and 280 kg ha^?1) and the subplot was Azovit inoculation. Azovit inoculation consistently increased the seed and lint yields of cotton by 25 and 27.9%, respectively, at 210 kg N ha^?1 compared to the respective control. Azovit with 210 kg N ha^?1 significantly increased the cotton harvest index by 21%, when compared to the control. Likewise, nutrient uptake and NUE of cotton were higher when N (210 kg ha^?1) was applied with Azovit, as compared to other treatment combinations. An extrapolation of the relationship of relative yield vs. N fertilization showed that Azovit at 210 kg N ha^?1 was sufficient to obtain near‐maximum cotton production (90%) with highest NUE, as compared to the respective control. The results suggest that Azovit with 210 kg N ha^?1 produces cotton yield higher and/or comparable with the currently used rates of 280 kg N ha^?1 or higher, suggesting savings of 70 kg N ha^?1 for cotton production in saline soils under continental climatic conditions.     

Comparison of urease inhibitor N‐(n‐butyl) thiophosphoric triamide and oxidized charcoal for conserving urea‐N in soil

Charcoal‐based amendments have a potential use in controlling NH₃ volatilization from urea fertilization, owing to a high cation‐exchange capacity (CEC) that enhances the retention of NH . An incubation study was conducted to evaluate the potential of oxidized charcoal (OCh) for controlling soil transformations of urea‐N, in comparison to urease inhibition by N‐(n‐butyl) thiophosphoric triamide (NBPT). Four soils, ranging widely in texture and CEC, were incubated aerobically for 0, 1, 3, 7, and 14 d after application of ¹⁵N‐labeled urea with or without OCh (150 g kg^?1 fertilizer) or NBPT (0.5 g kg^?1 fertilizer), and analyses were performed to determine residual urea and ¹⁵N recovery as volatilized NH₃, mineral N (as exchangeable NH , NO , and NO ), and immobilized organic N. The OCh amendment reduced NH₃ volatilization by up to 12% but had no effect on urea hydrolysis, NH and NO concentrations, NO accumulation, or immobilization. In contrast, the use of NBPT to inhibit urea hydrolysis was markedly effective for moderating the accumulation of NH , which reduced immobilization and also controlled NH₃ toxicity to nitrifying microorganisms that otherwise caused the accumulation of NO instead of NO . Oxidized charcoal is not a viable alternative to NBPT for increasing the efficiency of urea fertilization.     

Plant‐available nitrogen in the soil: Relationships between pre‐plant and pre‐sidedress nitrate tests for corn production

Corn (Zea mays L.) producers in the rainfed regions sometimes sidedress fertilizer N according to pre‐plant–nitrate test (PPNT) results based on the assumption that there is a linear relationship between pre‐sidedress nitrate test (PSNT) and the PPNT. There has been no report on such relationship in Ontario (Canada) and elsewhere in the nonirrigated corn‐growing regions. A field study was conducted near Ottawa, Canada for 7 y to (1) determine changes in soil available N from pre‐planting to shortly after the sidedress stage (late June) for corn and (2) establish a quantitative relationship between PPNT and PSNT. In each year, soil samples from fields of three to four plot experiments with different cropping histories, soil textures, and management levels, taken at 7 to 10 d intervals, and from on‐farm trials taken at pre‐planting and pre‐sidedress, were extracted with 2 M KCl. The concentrations of NO ‐N were determined colorimetrically. It was found that soil NO ‐N concentration of PSNT was a linear function of PPNT with an average slope of 1.7. However, the slope of the regression equations differed dramatically among cropping sequences, and to a lesser extent, soil textures. The NO ‐N concentration after planting to pre‐sidedress was influenced by air temperature and precipitation during this period of time. Both PPNT and PSNT positively correlated with corn‐grain yield. Our data suggest that cautions must be taken when deciding the rate of fertilizer N for sidedress application to corn based on PPNT test, especially under more humid northern climate conditions.     

Depletion of non‐exchangeable NH$ _4^+ $ at the root–soil and hyphae–soil interface of VA mycorrhizal maize (Zea mays) and parsley (Petroselinum sativum)

Mobilization of non‐exchangeable ammonium (NH ) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, ¹⁵NH was added to the soil to differentiate between the native non‐exchangeable NH and the non‐exchangeable NH derived from N fertilization. Non‐exchangeable NH was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH in the soil solution and of exchangeable NH . However, the NH concentration was still too high to permit the release of non‐exchangeable NH . The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH might be found.     

Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

Nitrogen (N) deposition can affect grassland ecosystems by altering biomass production, plant species composition and abundance. Therefore, a better understanding of the response of dominant plant species to N input is a prerequisite for accurate prediction of future changes and interactions within plant communities. We evaluated the response of seven dominant plant species on the Tibetan Plateau to N input at two levels: individual species and plant functional group. This was achieved by assessing leaf N : P stoichiometry, leaf δ¹⁵N and biomass production for the plant functional groups. Seven dominant plant species—three legumes, two forbs, one grass, one sedge—were analyzed for N, P, and δ¹⁵N 2 years after fertilization with one of the three N forms: NO$ _3^- $ , NH$ _4^+ $ , or NH₄NO₃ at four application rates (0, 7.5, 30, and 150 kg N ha^–1 y^–1). On the basis of biomass production and leaf N : P ratios, we concluded that grasses were limited by available N or co‐limited by available P. Unlike for grasses, leaf N : P and biomass production were not suitable indicators of N limitation for legumes and forbs in alpine meadows. The poor performance of legumes under high N fertilization was mainly due to strong competition with grasses. The total above‐ground biomass was not increased by N fertilization. However, species composition shifted to more productive grasses. A significant negative correlation between leaf N : P and leaf δ¹⁵N indicated that the two forbs Gentiana straminea and Saussurea superba switched from N deficiency to P limitation (e.g., N excess) due to N fertilization. These findings imply that alpine meadows will be more dominated by grasses under increased atmospheric N deposition.     

Small amounts of ammonium (NH$ _4^+ $) can increase growth of maize (Zea mays)

Nitrate (NO$ _3^ - $ ) and ammonium (NH$ _4^+ $ ) are the predominant forms of nitrogen (N) available to plants in agricultural soils. Nitrate concentrations are generally ten times higher than those of NH$ _4^+ $ and this ratio is consistent across a wide range of soil types. The possible contribution of these small concentrations of NH$ _4^+ $ to the overall N budget of crop plants is often overlooked. In this study the importance of this for the growth and nitrogen budget of maize (Zea mays L.) was investigated, using agriculturally relevant concentrations of NH$ _4^+ $ . Maize inbred line B73 was grown hydroponically for 30 d at low (0.5 mM) and sufficient (2.5 mM) levels of NO$ _3^ - $ . Ammonium was added at 0.05 mM and 0.25 mM to both levels of NO$ _3^ - $ . At low NO$ _3^ - $ levels, addition of NH$ _4^+ $ was found to improve the growth of maize plants. This increased plant growth was accompanied by an increase in total N uptake, as well as total phosphorus, sulphur and other micronutrients in the shoot. Ammonium influx was higher than NO$ _3^ - $ influx for all the plants and decreased as the total N in the nutrient medium increased. This study shows that agriculturally relevant proportions of NH$ _4^+ $ supplied in addition to NO$ _3^ - $ can increase growth of maize.