Fertilizer ammonium : nitrate ratios determine phosphorus uptake by young maize plants

We investigated the interacting effects of inorganic nitrogen and the main inorganic phosphorus form in dairy manure (dicalcium phosphate, CaHPO₄) on growth, nutrient uptake, and rhizosphere pH of young maize plants. In a pot experiment, three levels of CaHPO₄ (0, 167, and 500 mg P pot^?1) were combined with nitrogen (637 mg N pot^?1) applied at five NH₄‐N : NO₃‐N ratios (0 : 100, 25 : 75, 50 : 50, 75 : 25, and 100 : 0) and a nitrification inhibitor in a concentrated layer of a typical acid sandy soil from Denmark. ¹⁵N‐labeled NH₄‐N was applied to differentiate the role of nitrification and to partition nitrogen uptake derived from NH₄‐N. Among treatments including nitrogen, shoot biomass, rooting and phosphorus uptake were significantly higher at the five‐leaf stage when CaHPO₄ was applied with NH₄‐N : NO₃‐N ratios of 50 : 50 and 75 : 25. In these treatments, rhizosphere pH dropped significantly in direct proportion with NH₄‐N uptake. The fertilizers in the concentrated layer had a root‐inhibiting effect in treatments without phosphorus supply and in treatments with pure NO₃‐N or NH₄‐N supply. Increased nitrogen uptake as NH₄‐N instead of NO₃‐N reduced rhizosphere pH and enhanced acquisition of applied CaHPO₄ by young maize plants, which may have positive implications for the enhanced utilization of manure phosphorus.     

不同铵硝配比对低温胁迫棉花幼苗生长及抗氧化酶活性的影响

【目的】新疆春季的“倒春寒”是严重阻碍棉花前期的生长发育,影响产量及品质的主要非生物限制因子之一,而氮素供应及氮素形态对棉花 (Gossypium hirsutum) 的生长发育具有显著影响。研究低温胁迫下不同铵硝配比对棉花幼苗抗氧化酶活性及质膜脂质过氧化伤害的影响,为通过调节氮素供应提高棉花抗寒性提供理论依据。 【方法】以‘新陆早 13 号’为供试棉花品种,在人工气候室内采用营养液水培法调节铵硝营养配比,研究了常温和低温条件下,不同铵硝配比对低温胁迫下棉花幼苗电解质渗出率、丙二醛 (MDA)、游离脯氨酸 (Pro)、可溶性蛋白质 (SP) 含量及抗氧化酶活性的影响。 【结果】常温条件 (25℃) 下,铵硝混合营养较单纯铵态氮或硝态氮营养对棉苗各器官生物量均有显著的提高 (P < 0.05),地上部分和根系干物质重量在 NH₄+/NO₃– 比为 50/50 处理时最大,纯铵营养处理时最小;对棉苗生物量的影响效果表现出铵硝混合营养处理优于纯铵或纯硝营养处理。低温胁迫 (15℃) 后棉苗各器官生物量明显减小 (P < 0.05)。低温胁迫下棉花幼苗电解质渗出率、MDA、Pro 含量显著增加 (P < 0.01),SP 含量差异不大 (P > 0.05),超氧化物歧化酶 (SOD)、过氧化物酶 (POD)、过氧化氢酶 (CAT) 活性显著降低 (P < 0.01),抑制了棉花幼苗的生长发育。相同温度条件下,棉花幼苗电解质渗出率和 MDA 含量均随营养液中铵营养比例的增加而呈现出先减小后增大的变化趋势,在营养液中 NH₄+-N/ NO₃–-N 比例为 50/50 处理时达到最小,纯硝营养处理次之,纯铵营养处理最大;而同温下 SP、Pro 含量先增加后减少,NH₄+-N/NO₃–-N 为 25/75 最大。增铵营养明显增强了 SOD、CAT 活性 (P < 0.01),POD 活性则随着 NH₄+-N/NO₃–-N 比例增加表现出先降低后升高的趋势。 【结论】低温胁迫下,铵硝混合营养降低了棉花幼苗电解质渗出率和 MDA 含量,增加了渗透调节物质积累,提高了抗氧化酶活性,降低了脂质过氧化伤害,增强棉花幼苗对低温的抗性,尤其 NH₄+-N/NO₃–-N 比例为 50/50 的营养液效果更明显。     

Mitigation of ammonium toxicity by silicon in tomato depends on the ammonium concentration

Ammonium toxicity in hydroponically grown crops can affect tomato development. However, it has been shown that the silicon (Si) attenuates ammonium toxicity in plants depending on the plant species, the stage of development and the ammonium concentration in the nutrient solution. Thus, in order to investigate how Si attenuates stress caused by ammonium in tomato, a study was carried out involving plants cultivated up to 40 days after seed germination using nutrient solutions containing ammonium concentrations (1, 2, 4, 6 and 8?mmol?L^?1), in the absence or presence of Si (1?mmol?L^?1). The accumulation and efficiency of nitrogen and Si use, as well as the concentrations of chlorophyll, carotenoids, malondialdehyde, hydrogen peroxide and growth parameters was assessed. At a concentration of 1?mmol?L^?1 ammonium, Si increases the accumulation of nitrogen and Si, the nitrogen use efficiency, the root area and dry biomass of the shoot. At concentrations of 1 and 2?mmol?L^?1 ammonium, Si increases the leaf area and root dry biomass, and in higher concentrations, there was no effect of Si after the supply of ammonium. It was observed that the addition of Si mitigates ammonium toxicity by 1?mmol?L^?1 ammonium, and we can recommend its use in the nutrient solution (Si?=?1?mmol?L^?1) to grow tomato cropsthat employs ammonium concentration of 1?mmol?L^?1.     

不同氮素形态比对雾培马铃薯生长和原原种产量的影响

通过雾培试验研究4种不同比例硝态氮(NO_3~--N)和铵态氮(NH_4~+-N)对马铃薯品种米拉和川芋802植株生长及原原种生产的影响,为种薯生产的氮肥高效利用提供理论依据。结果表明,定植60 d,全硝态氮处理(0∶4)的植株干物质积累、匍匐茎数量及叶面积指数最高;而增铵营养更利于植株定植60 d后的生长发育,其中铵硝比(2∶2)处理下植株干物质积累、匍匐茎数量、叶面积指数增幅最大,表明块茎形成期全硝态氮更利于植株生长。铵硝比(2∶2)处理下,两个品种的氮积累最高,表明增加一定的铵态氮能增强马铃薯氮素利用率。产量上,全硝态氮处理(0∶4)单株结薯数最高(米拉115.5粒/株,川芋802为42.5粒/株),随铵硝比增大,单株结薯数降低,但大薯比例随之提高,最高为铵硝比(3∶1)处理(米拉64%,川芋802 76%)。因此,全硝态氮能增加结薯数量,增铵能促进马铃薯块茎膨大。     

Glucose metabolism in detached leaves of tomato plants grown with ammonium and nitrate as nitrogen sources

The metabolism of exogenous glucose-¹⁴C in the light and the dark was studied in the detached leaves of tomato plants grown with ammonium nitrogen and nitrate nitrogen. In the light, ¹⁴CO₂, release and incorporation of glucose into insoluble materials were hardly affected at all by the nitrogen sources. Among the soluble labelled amino acids, serine had the greatest amount of label in the ammonium-plants while aspartate had the greatest amount in the nitrate-plants. This aspartate was synthesized from C₃-compounds by carboxylation. During dark-light transition, the change in the composition of soluble amino acids was more rapid in the ammonium-plants than in the nitrate-plants. In the dark, ¹⁴CO₂-release, which was ten times as much as in the light, was larger in the ammonium-plants than in the nitrate-plants; but the synthesis of high molecular compounds from glucose in the ammonium-plants was about half that in the nitrate-plants. So, it is considered that respiration operates sufficiently in the ammonium-plants. The effects of DCMU and a 100% O₂ atmosphere on glucose metabolism in both groups of plants were studied and the respiration of leaves in the light was discussed.     

Responses of Polygonatum odoratum seedlings in aeroponic culture to treatments of different ammonium: nitrate ratios

The objective was to determine the most favorable nitrogen (N) source ratio of ammonium (NH₄⁺) to nitrate (NO₃^?) for aeroponic culture of Chinese fragrant solomonseal Polygonatum odoratum (Mill.) Druce seedlings. Seedlings were cultured with solutions based on 50% Hoagland formula containing 0:100, 10:90, 20:80, and 30:70 NH₄⁺:NO₃^? ratios for 21 days. Activities of anti-oxidant enzymes and glutathione contents of leaves with treatments of 10:90 and 20:80 NH₄⁺:NO₃^? ratios were higher than that of all-nitrate treatment, and malondialdehyde (MDA) concentrations were lower than that of all-nitrate treatment. The maximum quantum yield ?P_o value was normal among all of the treatments, whereas the absorption energy ABS/CSm value with 20:80 NH₄⁺:NO₃^? treatment was highest and the heat dissipation DIo/CSm value with 20:80 NH₄⁺:NO₃^? treatment was lowest among all of the treatments. These results supported that moderate proportion of 20% NH₄⁺ in the solution provided optimal growth condition for (P) P. odoratum aeroponic culture.     

Absorption,translocation, and assimilation of foliar-applied urea compared with nitrate and ammonium in tomato plants

To evaluate the use of foliar application of N fertilizer and the occurrence of leaf injury in tomato plants (Lycopersicon esculentum Mill., cv. Momotaro), the effects of the form and concentration of N and solution pH on the leaf injury were studied in the first experiment (Expt. 1). The effects of solution pH and leaf surface on the absorption, translocation, and assimilation of urea were compared with those of nitrate and ammonium in the second experiment (Expt. 2). In Expt. 1, no leaf injury was observed regardless of N sources applied at the N concentration of 1.0 g L^-1. Compared with nitrate or ammonium, the index of leaf injury was the lowest in the leaf to which urea had been applied (hereafter referred to as “urea-applied leaf”), when the N level increased from 2.0 to 10.0 g L^-1. Leaf injury was not affected by the solution pH in the case of urea, but it increased in the case of ammonium and decreased when nitrate was applied with increasing solution pH. In Expt. 2, the absorption of nitrate and ammonium by a leaf within 4 d was 34% and 74% of that of urea, respectively. N absorption at the lower leaf surface was much greater than that at the upper leaf surface for each N source. No apparent effect of solution pH on the absorption of urea was detected. With increasing solution pH, however, the absorption of nitrate decreased. The absorption of ammonium was the greatest at solution pH 7.5. Total-¹⁵N translocation from applied leaf to other plant parts within 4 d was the largest in the urea-applied plants. Effects of solution pH and leaf surface on ¹⁵N distribution were not appreciable. ¹⁵N assimilation was the quickest in the urea-applied plants. Two days after application, ¹⁵N assimilation in the whole plant was up to 76.9% in the urea-applied plants, but only 33.7% and 43.0% in the nitrate- and ammonium-applied plants, respectively. Urea was an appropriate foliar N source due to the low ability to injure foliage because of the rapid absorption and translocation, fast assimilation, and the wide and suitable range of solution pH. Foliar application of N to the lower leaf surface was recommended.     

Effect of ammonium or nitrate nutrition on photosynthesis,growth, and nitrogen assimilation in tomato plants

Nitrogen (N) is taken up by most plant species in the form of nitrate (NO ) or ammonium (NH ). Plant response to continuous NH nutrition is species‐dependent. In this study, we compare the responses of tomato (Solanum lycopersicum L. cv. Rio Grande) plants to N source (NO or NH ). To this end, early plant growth, photosynthesis, chlorophyll, carbohydrate, and N‐compound concentrations as well as the activities of main enzymes involved in N metabolism (nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate dehydrogenase) were analyzed. Early plant growth was remarkably ameliorated under NH ‐ in comparison to NO ‐based nutrition. Concomitantly, photosynthetic activity, total chlorophyll, and carbohydrate concentrations were significantly increased. With increasing external NH concentration, NH accumulated mainly in roots. In addition, root protein concentration was significantly increased, reflecting high NH incorporation into organic nitrogen. Root glutamine synthetase (GS) activity was enhanced by NH for concentrations below 5 mM, whereas root glutamate dehydrogenase (GDH) activity increased in parallel to NH availability. Together with the positive effect of NH on tomato plant cv. Rio Grande growth, these results reveal that GDH could have, in addition to GS, a possible role in NH detoxification and tolerance of NH ‐based nutrition.     

施用铵态氮对森林土壤硝态氮和铵态氮的影响

对取自武夷山的红壤、黄壤、黄壤性草甸土分别在对照(CK,N 0 mg/kg)、低氮(LN,N 50 mg/kg)、高氮(HN,N 100 mg/kg)3种氮(N)水平处理下开展培养实验,研究施加NH4+-N对森林土壤N转化的短期影响.结果表明,添加NH4+-N可显著(p＜0.05)降低土壤NO3--N含量4.5％～25.7％,但LN与HN处理差异不显著,NO3--N降低可能与NO3--N反硝化和异氧还原有关;然而,黄壤性草甸土NO3--N没有降低.与培养前比较,在第56天红壤NO3--N含量显著增加5倍左右;桐木关黄壤增加40％左右,而黄冈山25 km黄壤仅在CK处理下增加16％,但是黄壤性草甸土显著降低;结果显示LN与HN处理土壤NO3--N含量变化幅度小于CK.与CK相比,LN和HN处理红壤NH4+-N分别显著(p＜0.05)升高24.1％ ～ 96.5％和68.7％～114.1％,且随培养进行没有累积,可能与微生物固N有关;桐木关NH4+-N分别升高17.6％ ～ 39.6％和37.6％～95.8％ (p＜0.05),LN处理黄冈山25 km黄壤NH4+-N只有第7天升高17.8％ (p＜0.05),HN处理第7、14、28、42天显著升高17.5％～48.6％(p＜0.05).LN处理黄壤性草甸土的NH4+-N在前3周显著降低11.6％～28.5％ (p＜0.01); HN处理在第7天和14天分别降低10.8％(p＜0.01)和7.5％,但是在第28～56天显著增加17.6％～20.4％(p=0.002).随着培养进行,CK处理红壤NH4+-N逐渐降低,桐木关黄壤、黄冈山25 km黄壤和黄壤性草甸土升高;LN和HN处理黄壤和黄壤性草甸土NH4+-N逐渐升高.可见,不同海拔土壤类型对NH4+-N添加响应存在差异.     

Testing foliar nutritional changes in space and over time in greenhouse tomato

The aim of this trial was to study the spatial and temporal variability of the leaf nutrient concentration in tomato intensive crop under greenhouse, to know the number of sub-samples needed to be taken to obtain a representative sample and the influence by the position in the greenhouses. The experiment consisted on the selection of 20 sample points over the greenhouse with one tomato plant per sample point. One fully expanded leaf by sample point with an interval of a week were taken. Nitrogen (N), phosphorus (P), potassium (K), sulfur (S), sodium (Na), and chloride (Cl) were determined in each sample. According to these results, N, P, K, S, Cl, and Na concentration showed no variation over the time. The spatial variability of N, P, and K could be related with the light intensity and yield. The suggested number of leaf sub-sample ranging from 25 to 126 depending on nutrient.     

Tuber formation as influenced by the C : N ratio in potato plants

Increasing the tuber number per plant and promoting tuber formation are important goals for potato production. Thus, it is of significance to understand how environmental factors affect tuber formation, which relates to the development of agronomical practices. The objective of this research was to test whether changing the CO₂ or N supply level could alter the C : N ratio in potatoes, and whether a change of the C : N ratio in potatoes closely correlates with tuber formation in potato plants. Potato plantlets were grown in greenhouse with varied CO₂ or N levels. The C and N concentrations of plants were measured, and the tuber number and tuber weight were recorded after different growth times. The results show that the C : N ratios in the potato plants increased with increasing CO₂ concentration and that potato plants treated with higher CO₂ concentrations form tubers earlier. A decrease in the N supply also resulted in a higher C : N ratio and earlier tuber formation. Therefore the hypothesis proposed previously is supported by the present work. In addition, the results that C concentration in plants remained stable under different CO₂ or N levels imply that the variation of C : N ratio in the potato plants is mainly due to a change of N concentrations which decreased as the environmental CO₂ concentration increased, while increased as the N supply level increased.     

磷酸二氢钙和氯化钾对氯化铵处理黄泥土水溶性铵态氮和硝态氮的影响

氮磷钾是农业生产中大量施用并且经常共同施用的肥料,三者在土壤中的相互作用对养分的迁移转化、吸收和代谢有着深远影响.本文模拟生产中氮磷钾肥料同施,研究了田间持水量条件下磷酸二氢钙、氯化钾对氯化铵处理土壤水溶性铵态氮和硝态氮的影响.结果表明,铵态氮施入土壤后,随着培养时间的延长,土壤中水溶性铵态氮含量下降,硝态氮含量升高,两者之间存在着显著相关性.磷酸二氢钙延缓了铵态氮向其他形态氮的转变,使培养中期土壤水溶性铵态氮显著高于氯化铵处理土壤,并对培养中后期硝态氮的增加有抑制作用.氯化钾增加了培养前中期氯化铵处理土壤铵态含量,但显著抑制了氯化铵处理土壤培养后期硝态氮的含量.因此,农业生产中氯化铵和氯化钾共施,氯化铵和磷酸二氢钙共施,氯化铵、氯化钾和磷酸二氢钙共施,对提高氮肥利用率,降低硝态氮淋失损失均有重要作用.     

Arabidopsis under ammonium over-supply: Characteristics of ammonium toxicity in relation to the activity of ammonium transporters

Because of highly intensive farming practices, crop plants could suffer relatively long terms of ammonium (NH₄⁺) excess stress introduced by overdose application of nitrogen fertilizers. However, the lack of sufficient understanding of plant responses to NH₄⁺ excess stress impairs the detection of effective solutions to this problem. The present work examined the biological influences of over-supplied NH₄⁺ in Arabidopsis thaliana using two mutant lines each with an ammonium transporter (AMT) gene (AtAMT1;1 or AtAMT1;3) knocked out. Our results indicated that lacking one of the major components of root NH₄⁺ -absorbing systems significantly alleviated the toxicity effects on Arabidopsis plants by reducing the accumulation of free NH₄⁺, suggesting that persistent absorption of NH₄⁺ through AMT was the main cause of excessive accumulation of free NH₄⁺ in the plants. Shading treatment led to a reduced transpirational driving force and thereby constrained the accumulation of toxic NH₄⁺ in the plants, finally resulting in higher NH₄⁺ -promoted growth in the wild type (WT). Under the shading treatment, the amt1;1 and amt1;3 mutant plants acquired insufficient NH₄⁺ and showed reduced growth when compared with the WT. Furthermore, the foliar application of sucrose notably alleviated the inhibitory effects on plant growth in the WT but had no effect on either the amt1;1 or amt1;3 mutant plants, indicating that carbon scarcity associated with NH₄⁺ excess is probably a major cause of NH₄⁺ toxicity in plants. Accordingly, increasing carbon source could be a potentially effective approach that alleviates the inhibition caused by NH₄⁺ excess and increases nitrogen use efficiency under NH₄⁺ over-supply.     

Different responses of ash and beech on nitrate versus ammonium leaf labeling

The effects of tree species on the N cycle in forest systems are still under debate. However, contradicting results of different ¹⁵N labeling techniques of trees and N tracers in the individual studies hamper a generalized mechanistic view. Therefore, we compared Ca(¹⁵NO₃)₂ and ¹⁵NH₄Cl leaf‐labeling method to investigate: (1) N allocation patterns from aboveground to belowground, (2) the cycles of N in soil‐plant systems, and (3) to allow the production of highly ¹⁵N enriched litter for subsequent decomposition studies. 20 beeches (Fagus sylvatica ) and 20 ashes (Fraxinus excelsior ) were ¹⁵N pulse labeled from aboveground with Ca(¹⁵NO₃)₂ and 40 beeches and 40 ashes were ¹⁵N pulse labeled from aboveground with ¹⁵NH₄Cl. ¹⁵N was quantified in tree compartments (leaves, stem, roots) and in soil after 8 d. Beech and ash incorporated generally more ¹⁵N from the applied ¹⁵NH₄Cl compared to Ca(¹⁵NO₃)₂ in all measured compartments, except for ash leaves. Ash had highest ¹⁵N incorporation [45% of the applied with Ca(¹⁵NO₃)₂] in its leaves. Both tree species kept over 90% of all fixed ¹⁵N from Ca(¹⁵NO₃) in their leaves, whereas only 50% of the ¹⁵N from the ¹⁵NH₄Cl tracer remained in the leaves and 50% were allocated to stem, roots, and soil. There was no damage of the leaves by both salts, and thus both ¹⁵N tracers enable long‐term labeling in situ field studies on N rhizodeposition and allocation in soils. Nonetheless, the ¹⁵N incorporation by both salts was species specific: the leaf labeling with ¹⁵NH₄Cl results in a more homogenous distribution between the tree compartments in both tree species and, therefore, ¹⁵NH₄Cl is more appropriate for allocation studies. The leaf labeling with Ca(¹⁵NO₃)₂ is a suitable tool to produce highly enriched ¹⁵N leaf litter for further long term in situ decomposition and turnover studies.     

The effect of elemental sulfur,gypsum and ammonium thiosulfate as sulfur sources on yield of rapeseed

Abstract

A growth chamber experiment was conducted to compare ammonium thiosulfate, gypsum and elemental sulfur in the form of Agrisul as sources of sulfur for rapeseed (Brassica napus var. Regent). Rapeseed supplied with ammonium thiosulfate or gypsum produced significantly higher yields than treatments supplied with elemental sulfur. Powdering and mixing of elemental sulfur, as opposed to banding granules, significantly increased dry matter yield of rapeseed. While not always significant, there was a trend towards higher dry matter yields where gypsum granules were mixed as opposed to banded and where ammonium thiosulfate was placed in a band as opposed to being mixed throughout the soil.     

水氮耦合供应对温室番茄果实硝酸盐累积的影响

采用2水平灌水量(W1:4541.0、W2:2270.6 m3/hm2)×3水平氮肥追施量(N1:747.4、N2:373.7及N3:0 kg/hm2),研究了不同灌溉、施氮量对日光温室番茄果实硝酸盐累积的影响。结果表明,番茄果实硝酸盐含量随果实成熟度的提高而降低、随结果部位的提高而提高。水氮耦合供应可以显著影响番茄果实中硝酸盐含量。在施氮量相同的情况下,果实硝酸盐含量随着灌水量的增加而降低;而在灌水量相同的情况下,果实硝酸盐含量随施氮量的增加而增加。     

Nitrogen concentrations and proportions of ammonium and nitrate in the nutrition and growth of yellow passion fruit seedlings

Abstract

Despite the importance of nitrogen (N) supply to plants, there are still doubts concerning the optimal relations of ammonium and nitrate in the nutrition of yellow passion fruit seedlings. This study aims to evaluate the interaction between nitrogen concentrations and ammonium and nitrate proportions in the nutrition, growth, and dry matter production of passion fruit seedlings grown in a substrate with a nutrient solution. The experiment was conducted in a greenhouse in randomized complete block design with three replications in a 4 × 5 factorial design, consisting of four N concentrations (2.5, 5.0, 10.0 and 20.0?mmol L^?1) and five ammonium proportions (0, 25, 50, 75 and 100% in relation to the total N supply). At 60?days after transplanting, green color index; accumulation of N, potassium, calcium, and magnesium in roots and shoots; stem diameter; leaf area; root length; nitrogen use efficiency (NUE); and dry matter of roots and shoots were evaluated. For the formation of seedlings of yellow passion fruit, the nutrient solution should have 13?mmol L^?1 of N, with 40% of this nutrient in the form of ammonium. The passion fruit is a plant tolerant to ammonium. However, a critical concentration above 5.7?mmol L^?1 of NH₄⁺ in the nutrient solution decreases absorption of cations, NUE, and production of dry matter.     

Indices of dissolved organic nitrogen, ammonium and nitrate across productivity gradients of boreal forests

Organic nitrogen (N) uptake, rather than solely inorganic N (DIN), is considered a significant pathway for plant nutrition, especially in arctic, alpine and boreal ecosystems. Assays of plant-available N in these ecosystems might therefore be improved with measures of dissolved organic N (DON). We examined DON and DIN abundance from an in situ 5-week incubation across plant associations that represent the widest range in site potential in southern boreal forests of British Columbia, Canada. The supply of N from forest floors and mineral soils (20 cm depth) was measured separately and then combined (kg ha⁻¹) to facilitate comparisons of sites. DON was the predominant form of extractable N, and was increasingly supplemented, rather than replaced, by NH₄⁺ and NO₃⁻ on productive sites. The amount of DIN produced in the soils was very low, perhaps too small to support forest needs, and the correlation of DIN to asymptotic stand height (a measure of site potential) was significant but nonlinear. The combined amount of DON+DIN was considered a more effective index of plant-available N because it was strongly significant as a linear correlation to stand height and more typical of annual forest N uptake. The relative shift in N forms, from a predominance of DON to progressively greater ratios of DIN:DON, was consistent with the current paradigm of N forms across gradients of N availability, although the actual amounts of DON increased, rather than decreased, with site potential. Based on this, we suggest organic N uptake has the potential to contribute to plant nutrition across the entire productivity gradient of soils in southern boreal forests. Although other N indices were effective in characterizing forest productivity, a combined assay of DON+DIN production could provide new insights into functional differences in plant-available N.