DEVELOPMENT OF A SOIL N TEST FOR FERTILIZER REQUIREMENTS FOR WHEAT

Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO^? ₃)- N, soil ammonium (NH ⁺ ₄)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO^? ₃-N and NH ⁺ ₄-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH ⁺ ₄-N and NO^? ₃ -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO^? ₃-N.     

Interactive Effects of Nitrogen Source and Salinity on Growth Indices and Ion Content of Indian Mustard

ABSTRACT

The interactions between salinity and different nitrogen (N) sources nitrate (NO₃ ^?), ammonium (NH₄ ⁺), and NO₃ ^? + NH₄ ⁺ were investigated on Indian mustard (Brassica juncea cv. RH30). Treatments were added to observe the combined effect of two salinity levels (8 and 12 ds m^{? 1}) and three nitrogen sources (NO₃ ^?, NH₄ ⁺, and NO₃ ^? + NH₄ ⁺) on different growth parameters and mineral composition in different plant parts, i.e., leaves, stem, and root. Salinity has been known to affect the uptake and assimilation of various essential nutrients required for normal growth and development. Different growth parameters, i.e., leaf area, dry weight of different plant parts, absolute growth rate (AGR), relative growth rate (RGR), and net assimilation rate (NAR) declined markedly by salinity at pre-flowering and flowering stages. All growth indices were less sensitive to salinity (12 d s m^{? 1}) with the nitrate form of nitrogen. It is pertinent mention that a high dose (120 kg ha^{? 1}) of nitrogen in ammonium form NH₄ ⁺, acted synergistically with salinity in inhibiting growth. Plants fed with combined nitrogen (NO₃ ^? + NH₄ ⁺) had an edge over individual forms in ameliorating the adverse effects of salinity on growth and yield. Under salt stress, different nutrient elements such as N, phosphorus (P), potassium (K⁺), and magnesium (Mg^{2 +}) were decreased in different plant parts (leaves, stem, and root). The maximum and minimum reduction was observed with ammoniacal and combined form of nitrogen, respectively, while the reverse was true of calcium (Ca^{2 +}), sodium (Na⁺), chloride (Cl^?), and sulfate (SO₄ ^2?) at harvest. Nitrogen application (120 Kg ha^{? 1}) in combined form had been found to maintain highest concentrations of N, P, Mg^{2 +}, and Ca^{2 +} along with reduced concentrations of Na⁺, Cl^?, and SO₄ ^{2 ?}. However, reverse was true with ammoniacal form of nitrogen.     

Root Carbon Enrichment Alleviates Ammonium Toxicity in Cucumber Plants

ABSTRACT

The addition of carbonates to a nutrient solution to alleviate ammonium (NH₄ ⁺) toxicity in hydroponically-grown cucumber (Cucumis sativus L.) plants was investigated. Stable isotopes [nitrogen (¹⁵N) and carbon (¹³C)] were used to assess the uptake of nitrogen [NH₄ ⁺ or nitrate (NO₃ ^?)] as well as carbon [bicarbonate (HCO₃ ^?)/carbonate (CO₃ ^2?)] by the roots. Ammonium as the sole N source at 5 mM decreased plant fresh weights compared to NO₃ ^?. However, at lower concentrations of NH₄ ⁺ (25% of 5 mM total N), growth was increased compared to NO₃ ^? alone. Inorganic C enrichment [calcium carbonate (CaCO₃)] of the nutrient solution increased the fresh weight of NH₄ ⁺ grown plants with up to 150% relative to control plants receiving calcium hydroxide [Ca(OH)₂] for pH regulation. Root ¹⁵N enrichment was lower in ¹⁵NH₄ ⁺ supplied plants compared to ¹⁵NO₃ ^?, while the ¹³C enrichment in leaves was increased by NH₄ ⁺ nutrition compared to NO₃ ^? or NH₄NO_3. The enhanced C capture was associated with high PEPCase activity in the roots. It is concluded that inorganic carbon enrichment of the root medium may alleviate NH₄ ⁺ toxicity via increased synthesis of C skeletons and, accordingly, increased capacity for NH₄ ⁺ assimilation and N export to the shoots.     

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.     

Characteristics of ammonium and nitrate fluxes along the roots of Picea asperata

Nitrogen (N), ammonium (NH₄⁺) and nitrate (NO₃^?), is one of the key determinants for plant growth. The interaction of both ions displays a significant effect on their uptake in some species. In the current study, net fluxes of NH₄⁺ and NO₃^? along the roots of Picea asperata were determined using a Non-invasive Micro-test Technology (NMT). Besides, we examined the interaction of NH₄⁺ and NO₃^? on the fluxes of both ions, and the plasma membrane (PM) H⁺-ATPases and nitrate reductase (NR) were taken into account as well. The results demonstrated that the maximal net NH₄⁺ and NO₃^? influxes were detected at 13–15?mm and 8–10.5?mm from the root apex, respectively. Net NH₄⁺ influx was significantly stimulated with the presence of NO₃^?, whereas NH₄⁺ exhibited a markedly negative effect on NO₃^? uptake in the roots of P. asperata. Also, our results indicated that PM H⁺-ATPases and NR play a key role in the control of N uptake.     

不同铵硝配比对弱光下白菜氮素吸收及相关酶的影响

以黑色遮阳网覆盖模仿弱光环境, 使光照强度为自然光的20%左右, 以自然光照为对照, 采用精确控制水培溶液氮素营养, 研究NH₄⁺-N/NO₃^--N 比例分别为0/100、25/75、50/50、75/25、100/0 对弱光下白菜氮代谢及硝酸还原酶和谷氨酰胺合成酶活性的影响。结果表明, 弱光下, 白菜的鲜重及叶片总氮量以NH₄⁺-N/NO₃^--N 比为25/75 时最大, NH₄⁺-N/NO₃^--N 比为100/0 时最低。随弱光处理的进行, 白菜叶片中硝酸还原酶活性及谷氨酰胺合成酶活性均呈下降趋势, 但NH₄⁺-N/NO₃^--N 比为25/75 时, 可维持叶片内较高的硝酸还原酶活性及谷氨酰胺合成酶活性。试验表明, NH₄⁺-N/NO₃^--N 比25/75 是白菜在弱光下生长的较适宜氮素形态配比。     

Effects of nitrogen rate and nitrogen form on glycine uptake by pakchoi (Brassica chinensis L.) under sterile culture

It is well known that plants are capable of taking up intact amino acids. However, how the nitrogen (N) rates and N forms affect amino acid uptake and amino acid nutritional contribution for plant are still uncertain. Effects of the different proportions of nitrate (NO₃^?), ammonium (NH₄⁺) and ¹⁵N-labeled glycine on pakchoi seedlings glycine uptake were investigated for 21 days hydroponics under the aseptic media. Our results showed that plant biomass and glycine uptake was positively related to glycine rate. NO₃^? and NH₄⁺, the two antagonistic N forms, both significantly inhibited plant glycine uptake. Their interactions with glycine were also negatively related to glycine uptake and glycine nutritional contribution. Glycine nutritional contribution in the treatments with high glycine rate (13.4%–35.8%) was significantly higher than that with low glycine rate (2.2%–13.2%). The high nutritional contribution indicated amino acids can serve as an important N source for plant growth under the high organic and low inorganic N input ecosystem.     

Tomato growth and mineral composition as influenced by nitrogen form and light intensity

Abstract

Tomato plants were grown in sand culture with NH⁺ ₄, and NO^? ₃, forms of N and three levels of light. Plants supplied with NH⁺ ₄, nutrition under high light intensity had symptoms of stunting, leaf roll, wilting, interveinal chlorosis of the older leaves, and one third the dry weight of N0₃‐fed plants. In contrast, growth of plants receiving NH⁺ ₄, nutrition under shade appeared normal although dry weight was reduced. NH₄‐N nutrition suppressed K, Ca and Mg accumulation in tissues and increased P contents as compared to NO₃‐N nutrition.     

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.     

聚天门冬氨酸钙盐对水稻田面水中三氮动态变化的影响

利用桶栽试验探究不同浓度水平的聚天门冬氨酸钙盐(PASP-Ca)对水稻田面水中铵态氮(NH_4~+)、硝态氮(NO_3~-)和总氮(TN)浓度动态变化的影响。结果表明,施氮后,田面水中TN、NH_4~+和NO_3~-分别于第1,3,9天达到最大值,随后逐渐降低。NO_3~-/TN多在0.1以下,(NH_4~++NO_3~-)/TN多在0.5以上。因此,可以将NH_4~+和TN作为农田水污染防治的主要监测指标,NO_3~-作为辅助指标。添加一定浓度的PASP—Ca能对田面水中氮素浓度的变化起到缓释作用,其中0.3%浓度水平的PASP—Ca效果相对较好,田面水中NH_4~+和TN的下降速率分别为3.452,4.806mg/(L·d),与单施氮肥(CK)相比,分别降低了11.68%和16.25%;同时,NH_4~+的平均浓度为6.999mg/L,较CK低了3.88%;NO_3~-的平均浓度为0.396mg/L,较CK低了24.83%;TN的平均浓度为20.077mg/L,较CK提升了3.10%。施氮后田面水中TN浓度随时间呈对数递减,而NH_4~+浓度在施氮后3天内随时间呈对数增加,之后随时间呈对数递减趋势。施氮后的9天内是防止稻田田面水中氮素流失的关键时期。     

Influence of Pre-Plant Micronutrient Mixes and Ammonium to Nitrate Ratios in Fertilizer Solution on Growth and Micronutrient Contents of Marigold in Plug Culture

ABSTRACT

The purpose of this work was to determine the effects of pre-plant micronutrient mixes and various ammonium/nitrate (NH₄ ⁺/NO₃ ^?) ratios in a fertilizer solution on growth and the nutrient uptake of French marigold ‘Orange Boy’ in a plug culture. Two kinds of granular glass frits containing six micronutrients were produced and incorporated into peatmoss+vermiculite (1:1, v/v) substrate at a rate of 0.3 g · L^{? 1}. The five NH₄ ⁺/NO₃ ^? ratios in fertilizer solution were 0/100, 25/75, 50/50, 75/25, and 100/0, each giving a total nitrogen (N) supply of 80, 100, and 120 mg · L^{? 1}in the root substrate during stages 2, 3, and 4, respectively. The plants produced a significantly higher fresh and dry matter yield at 35 days after sowing when grown in 25/75 N nutrition in micronutrient fertilizer 1 (MF1) and 50/50 in micronutrient fertilizer 2 (MF2) than in the other NH₄ ⁺/NO₃ ^? ratios tested. Treatments of the highest accumulation of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), and boron (B) in plants were 25/75, 50/50, 50/50, 25/75 and 50/50 in MF1 and 50/50, 25/75, 50/50, 50/50, 50/50 in MF2, respectively. The trends in accumulations of Fe, Mn, Zn, Cu, and B in plants were quadratic (Q), linear (L) and Q, L and Q, L and Q, and Q, respectively, in MF1. Those in MF2 were Q, Q, Q, L and Q, and Q, respectively.     

Growth Patterns of Jojoba Male and Female Leaf Callus with Various Ammonium and Nitrate Sources

Abstract

Male and female leaf discs of Jojoba [Simmondsia chinensis (Link) Schneider] were cultured on Murashige and Skoog (MS) media supplemented with various nitrate:ammonium ratio and phytohormones concentrations. For the optimum callus growth, hormonal concentrations were remained equal for both male and female leaf tissues i.e., 0.4 mg L^?1 2,4‐dichlorophenoxyaceticacid, 1.25 mg L^?1 6‐benzyladenine and 0.5 mgL^?1 kinetin. However, a statistically significant difference was observed when Murashige and Skoog media was supplemented with an additional nitrogen source. In female leaf tissue, maximum fresh and dry weights were recorded in Murashige and Skoog media supplemented with an additional source of NO₃ ^?:NH₄ ⁺ (60 mM) whereas in male leaf tissue this addition was inhibitory. This study suggests that nitrogen requirement may be different for optimum callus growth in both male and female leaf tissues.     

Role of Ammonium to Limit Nitrate Accumulation and to Increase Water Economy in Wild Swiss Chard

ABSTRACT

The present study was performed to characterize the interaction between nitrogen (N) form and availability with respect to growth, water relations, and mineral nutrition of wild swiss chard (Beta macrocarpa Guss). Plants were cultured hydroponically with two levels of N concentrations, high-N (2.5 mM) or low-N (0.5 mM), added as nitrate (NO^? ₃) or ammonium (NH⁺ ₄). At high N, growth was affected significantly by N form. If the NO^? ₃ medium was considered as control, the use of NH⁺ ₄ decreased dry matter production and leaf area by ca. 35%. Use of NH⁺ ₄ led to water economy and did not affect the nutrient content of the plant tissues. Compared to growth with high N, plants growth fell in either low- NO^? ₃ or low- NH⁺ ₄ medium. In this case, the difference between the two N sources was not significant. Our results showed that the replacement of NO^? ₃ by NH⁺ ₄ as the N source decreased the NO^? ₃ concentration in consumable leaves and increased the water use efficiency.     

Salinity effects on nitrogen metabolism in plants – focusing on the activities of nitrogen metabolizing enzymes: A review

Nitrogen (N) metabolism is of great economic importance because it provides proteins and nucleic acids which in turn control many cellular activities in plants. Salinity affects different steps of N metabolism including N uptake, NO₃^? reduction, and NH₄⁺ assimilation, leading to a severe decline in crop yield. Major mechanisms of salinity effects on N metabolism are salinity-induced reductions in water availability and absorption, disruption of root membrane integrity, an inhibition of NO₃^? uptake by Cl^?, low NO₃^? loading into root xylem, alteration in the activities of N assimilating enzymes, decrease in transpiration, and reduction in relative growth rate which results in a lower N demand. However, the effects of salinity on N metabolism are multifaceted and may vary depending on many plant and soil factors. The present review deals with salinity effects on N metabolism in plants, emphasizing on the activities of N metabolizing enzymes in a saline environment.     

Growth,Nitrate Accumulation,and Macronutrient Concentration of Pakchoi as Affected By External Nitrate-N: Amino Acid-N Ratio

ABSTRACT

A study was carried out to determine the influence of nitrogen (N) sources on the growth, nitrate (NO₃ ^?) accumulation, and macronutrient concentrations of pakchoi (Brassica chinensis L.) in hydroponics. Plants were supplied with NO₃ ^? and two amino acids (AA), glutamic acid (Glu), and glutamine (Gln), at six NO₃ ^?-N/AA-N molar ratios: (1) 100:0, (2) 80:20, (3) 60:40, (4) 40:60, (5) 20:80, (6) 0:100. The total N concentration was 12.5 mmol/L for all treatments in nutrient solutions. Both AAs reduced plant growth with decreasing NO₃ ^?-N/AA-N ratios, but the reduction was for Gln than for Glu. At 80:20 NO₃ ^?-N: Gln-N ratio, the Gln had no significant effect on pakchoi fresh weights. Decreasing NO₃ ^?-N/AA-N ratios reduced NO₃ ^? concentrations in the plant, regardless of AA sources. Adding an appropriate portion of AA-N to nutrient solutions for hydroponic culture increased concentrations of N, phosphorus (P), and potassium (K) in pakchoi shoots. Substituting 20% or less of NO₃ ^?-N with Gln-N in hydroponic culture will increase the pakchoi quality by reducing NO₃ ^? concentration and increasing mineral nutrient concentrations in shoots without significant reduction of crop yields.     

Effect of Nitrogen Fertilizer Forms on Growth,Photosynthesis, and Yield of Rice Under Cadmium Stress

ABSTRACT

A pot experiment was conducted to study the influence of four nitrogen (N) fertilizer forms [Urea; calcium nitrate, Ca(NO₃)₂; ammonium sulfate, (NH₄)₂SO₄; and organic N] on growth, photosynthesis, and yield of rice under two cadmium (Cd) levels (0 and 100 mg Cd kg^?1 soil). Cadmium addition significantly reduced photosynthetic rate, and the reduction varied with N fertilizer form, with ammonium (NH₄ ⁺)-N and urea treated plants having more reduction. Nitrogen form had a distinct effect on SPAD value, and the effect was also dependent on Cd level and growth stage. Cadmium-stress significantly reduced flag leaf area, but for the second leaf, only the plants supplied with organic N showed the reduction. There was a significant difference in plant height among four N forms, with NH₄ ⁺- and nitrate (NO₃ ^?)-treated plants having the highest and lowest height, respectively. Cadmium stress caused significant reduction in grains per panicle and total plant weight, and the reduction varied with N form, with organic N treatment showing more reduction. There were significant differences among N forms in N and Cd concentrations of the plants subjected to Cd stress, with NH₄ ⁺-N treated plants having highest N and lowest Cd concentrations and NO₃ ^?-treated plants having lowest N and highest Cd uptake. The results showed that the inhibition of Cd stress on growth and yield formation of rice is closely related to N fertilizer form.     

Effects of Salinity and Nitrogen on Growth,Contents of Pigments,and Ion Accumulation of a Euhalophyte Suaeda Salsa in an Intertidal Zone and on Saline Inland

The different responses of two populations of Suaeda salsa (Linn.) Pall. (saline seepweed) from an intertidal zone and a saline inland zone to salinity [1 or 500 mM sodium chloride (NaCl)] and nitrogen [N; 0.05, 1, or 10 mM nitrate (NO₃ ^?)‐N] were investigated. Greater NO₃ ^?‐N supply (10 mM) increased shoot dry weight for the two populations of S. salsa, especially for S. salsa from the saline inland zone. Greater NO₃ ^?‐N supply (10 mM) increased the concentrations of chlorophyll and carotenoid in leaves and the NO₃ ^? and potassium (K⁺) concentrations in shoots for both populations. Greater NO₃ ^?‐N supply (10 mM) increased shoot Na⁺ in S. salsa from the intertidal zone. In conclusion, S. salsa from the saline inland zone is more responsive to NO₃ ^?‐N supply than the intertidal population. Greater NO₃ ^?‐N supply can help the species, especially the intertidal population, to grow and to mediate ion homeostasis under high salinity.     

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH₄⁺) and nitrate (NO₃⁻). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃⁻ assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH₄⁺ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO₃⁻ and NH₄⁺ was estimated based on the uptake of ¹⁵N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH₄⁺ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO₃⁻ assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO₃⁻ than NH₄⁺. Heterotrophic microbes assimilated less NH₄⁺ than NO₃⁻ probably because NH₄⁺ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH₄⁺ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO₃⁻ immobilization.     

Effect of Nitrate/Ammonium Ratios on Growth,Root Morphology and Nutrient Elements Uptake of Watermelon (Citrullus Lanatus) Seedlings

Nitrogen is taken up by most plant species in the form of nitrate and ammonium. The objective of this study was to investigate the effect of different nitrogen forms on the growth of watermelon seedlings. Plants were grown in hydroponic culture with five nitrate (NO₃^?)/ammonium (NH₄⁺) ratios (100/0, 75/25, 50/50, 25/75, 0/100). When the proportion of NH₄⁺ was increased, the leaf number, leaf area, shoot height, net photosynthesis, biomass, and root growth were significantly decreased. Higher concentrations of nitrogen (N) and phosphorus (P) were observed when plants were supplied with mixed NO₃^? and NH₄⁺ compared to NO₃^? or NH₄⁺ alone, whereas the concentrations of potassium (K), calcium (Ca), and magnesium (Mg) were decreased with increasing NH₄⁺. The microelements concentrations were generally increased with more NH₄⁺ added. In addition, plants fed with higher NO₃^?/NH₄⁺ ratios resulted in more minerals accumulation.     

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.