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.     

Leaching Methods Can Underestimate Mineralization Potential of Soils

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

Nitrogen transformation from three soil organic amendments in a sandy soil

Abstract

A sandy soil was amended with various rates (20 – 320 g air-dry weight basis of the amendments per kg of air-dry soil) of chicken manure (CM), sewage sludge (SS), and incinerated sewage sludge (ISS) and incubated for 100 days in a greenhouse at 15% (wt/wt) soil water content. At the beginning of incubation, NH₄-N concentrations varied from 50 – 280 mg kg^?1 in the CM amended soil with negligible amounts of NO₃-N. Subsequently, the concentration of NH₄-N decreased while that of NO₃-N increased rapidly. In soil amended with SS at 20 – 80 g kg^?1 rates, the NO₃-N concentration increased sharply during the first 20 days, followed by a slow rate of increase over the rest of the incubation period. However, at a 160 g kg^?1 SS rate, there were three distinct phases of NO₃-N release which lasted for160 days. In the ISS amended soil, the nitrification process was completed during the initial 30 days, and the concentrations of NH₄-N and NO₃-N were lower than those for the other treatments. The mineralized N across different rates accounted for 20 – 36%, 16 – 40%, and 26 – 50% of the total N applied as CM, SS, and ISS, respectively.     

干污泥和堆肥污泥施用于红壤中N素的释放特性

在红壤自然状况下,模拟了施肥沟,对红壤不同污泥施肥处理的N素释放特性进行了研究。试验结果表明,干污泥配比在10%~20%时,碱解氮、铵态氮和硝态氮累计释放量分别为:25.71%~33.48%,9.57%~14.85%和4.08%~7.65%。堆肥污泥配比在20%~33%时,其累计释放量分别为13.55%~15.65%,2.03%~4.23%和3.11%~5.37%。干污泥处理的释放量大于堆肥污泥处理的释放量,释放过程变化较堆肥污泥剧烈,铵态氮和硝态氮均有明显峰值,铵态氮最大含量532.98±10 mg/kg,释放量最大达10.95%;硝态氮含量最大为149.2±14 mg/kg,释放量最大时为3.32%。无论是从氮的肥效角度,还是氮释放的环境风险角度考虑,污泥堆肥处理后施肥方式均优于干污泥处理施肥方式。     

Does the potential ammonium fixation of soils have an impact on the optimum nitrogen fertilizer rate?

Field experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH₄) fixation capacity [high = 161 mg fixed NH₄-N kg^?1 soil, medium = 31.5 mg fixed NH₄-N kg^?1 soil and no = nearly no fixed NH₄-N kg^?1 soil]. On high NH₄⁺ fixing soil, 80 kg N ha^?1 Urea+ ammonium nitrate [NH₄NO₃] or 240 kg N ha^?1 ammonium sulfate [(NH₄)₂SO₄]+(NH₄)₂SO₄, was required to obtain the maximum yield. Urea + NH₄NO₃ generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH₄⁺ fixing soil, 80 kg N ha^?1 urea+NH₄NO₃ was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH₄⁺ fixing soil than in the others. Grain protein was highly affected by NH₄⁺ fixation capacities and N doses. Harvest index was affected by the NH₄⁺ fixation capacity at the 1% significance level.     

Ammonia volatilization following urea application at maize fields in the East African highlands with different soil properties

Use of nitrogen (N) fertilizer is underway to increase in Sub-Saharan Africa (SSA). The effect of increasing N rates on ammonia (NH₃) volatilization—a main pathway of applied-N loss in cropping systems—has not been evaluated in this region. In two soils (Alfisols, ALF; and Andisols, AND) with maize crop in the East African highlands, we measured NH₃ volatilization following urea broadcast at six rates (0–150 kg N ha^?1) for 17 days, using a semi-open static chamber method. Immediate irrigation and urea deep placement were tested as mitigation treatments. The underlying mechanism was assessed by monitoring soil pH and mineral N (NH₄⁺ and NO₃^?) concentrations. More cumulative NH₃-N was volatilized in ALF than in AND at the same urea-N rate. Generally, higher urea-N rates increased proportional NH₃-N loss (percent of applied N loss as NH₃-N). Based on well-fitted sigmoid models, simple surface urea application is not recommended for ALF, while up to 60 kg N ha^?1 could be adopted for AND soils. The susceptibility of ALF to NH₃ loss mainly resulted from its low pH buffering capacity, low cation exchange capacity, and high urease activity. Both mitigation treatments were effective. The inhibited rise of soil pH but not NH₄⁺ concentration was the main reason for the mitigated NH₃-N losses, although nitrification in the irrigation treatment might also have contributed. Our results showed that in acidic soils common to SSA croplands, proportional NH₃-N loss can be substantial even at a low urea-N rate; and that the design of mitigation treatments should consider the soil’s inherent capacity to buffer NH₃ loss.     

坡缕石包膜对尿素氮行为的影响

采用静态吸收和土柱淋溶试验方法,分析对比了3种不同用量坡缕石包膜尿素与普通尿素施入土壤后对尿素氮行为的影响,结果表明:在土壤中施用坡缕石包膜尿素较普通尿素减少10.38%～26.24%的氨挥发损失,减少5.88%～27.74%的氮素(NO3--N+NH4+-N)淋溶损失,20%的坡缕石包膜尿素能显著提高土柱土壤NH4+-N含量,3种坡缕石包膜尿素都能极显著提高土柱土壤NO3--N含量.坡缕石包膜后能减少尿素氨的挥发,降低NH4+-N和NO3--N的淋失,提高土壤NH4+-N和NO3--N含量,以20%的坡缕石包膜尿素的综合生态效应最好.     

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

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

Influence of the nitrification inhibitor DMPP on the community composition of ammonia-oxidizing bacteria at microsites with increasing distance from the fertilizer zone

The effect of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N transformations and composition of ammonia-oxidizing bacteria (AOB) communities was investigated at the centimeter scale in a microcosm experiment under laboratory conditions. After 28 days, samples were collected from soil treated with urea or urea and DMPP at increasing distance from the fertilizer zone; this distance ranged from 0 to 5 cm in both horizontal and vertical directions. The results showed that DMPP application significantly increased soil pH and NH ₄ ⁺ -N and mineral N (NH ₄ ⁺ -N, NO ₃ ^? -N, and NO ₂ ^? -N) concentrations but decreased (NO ₃ ^? + NO ₂ ^? )-N concentration, and such effect was decreased by increasing the distance from the fertilizer zone. Fingerprint profiles of denaturing gradient gel electrophoresis showed that the number of bands decreased by increasing the distance from the fertilizer zone due to decreasing NH ₄ ⁺ -N concentrations in the urea treatment. Compared to urea applied alone, DMPP application increased NH ₄ ⁺ -N concentrations and decreased AOB diversity from 0 to 3 cm but promoted diversity from 3 to 5 cm distance from the fertilizer zone. A phylogenetic analysis showed that AOB communities were dominated by Nitrosospira cluster 3. Therefore, the nitrification inhibitor DMPP modified the composition of AOB communities by increasing the distance from the fertilizer zone and this probably was related to the changes in soil pH and inorganic N concentration.     

Crop Utilization of Nitrogen in Swine Lagoon Sludge

Swine lagoon sludge is commonly applied to soil as a source of nitrogen (N) for crop production but the fate of applied N not recovered from the soil by the receiver crop has received little attention. The objectives of this study were to (1) assess the yield and N accumulation responses of corn (Zea mays L.) and wheat (Triticum aestivum) to different levels of N applied as swine lagoon sludge, (2) quantify recovery of residual N accumulation by the second and third crops after sludge application, and (3) evaluate the effect of different sludge N rates on nitrate (NO₃-N) concentrations in the soil. Sludge N trials were conducted with wheat on two swine farms and with corn on one swine farm in the coastal plain of North Carolina. Agronomic optimum N rates for wheat grown at two locations was 360 kg total sludge N ha^?1 and the optimum N rate for corn at one location was 327 kg total sludge N ha^?1. Residual N recovered by subsequent wheat and corn crops following the corn crop that received lagoon sludge was 3 and 12 kg N ha^?1, respectively, on a whole-plant basis and 2 and 10 kg N ha^?1, respectively, on a grain basis at the agronomic optimum N rate for corn (327 kg sludge N ha^?1). From the 327 kg ha^?1 of sludge N applied to corn, 249 kg N ha^?1 were not recovered after harvest of three crops for grain. Accumulation in recalcitrant soil organic N pools, ammonia (NH₃) volatilization during sludge application, return of N in stover/straw to the soil, and leaching of NO₃ from the root zone probably account for much of the nonutilized N. At the agronomic sludge N rate for corn (327 kg N ha^?1), downward movement of NO₃-N through the soil was similar to that for the 168 kg N ha^?1 urea ammonium nitrate (UAN) treatment. Thus, potential N pollution of groundwater by land application of lagoon sludge would not exceed that caused by UAN application.     

Nitrogen Availability from Compost in High Tunnel Tomato Production

High yield agricultural systems, such as high tunnel (HT) vegetable production, require a large supply of soil nutrients, especially nitrogen (N). Compost is a common amendment used by HT growers both to supply nutrients and to improve physical and biological soil properties. We examined commercially-available composts and their effects on soil N, plant N uptake, and tomato yield in HT cultivation. In addition, a laboratory study examined N and carbon (C) mineralization from the composts, and the usefulness of compost properties as predictors of compost N mineralization was assessed under field and laboratory conditions. The field study used a randomized complete block design with four replications to compare four compost treatments (all added at the rate of 300 kg total N ha^?1) with unamended soil and an inorganic N treatment (110 kg N ha^?1). Tomatoes were grown in Monmouth, Maine during the summers of 2013 and 2014. Compost NO₃^?-N and NH₄⁺-N application rates were significantly correlated with soil NO₃^?-N and NH₄⁺-N concentrations throughout the growing season. Marketable yield was positively correlated with compost total inorganic N and NO₃^?-N in both years, and with NH₄⁺-N in 2014. There were no significant differences among composts in percentage of organic N mineralized and no correlations were observed with any measured compost property. In the laboratory study, all compost-amended soils had relatively high rates of CO₂ release for the initial few days and then the rates declined. The compost-amended soils mineralized 4%–6% of the compost organic N. This study suggested compost inorganic N content controls N availability to plants in the first year after compost application.     

控释氮肥养分控释效果及合理施用研究

试验采用{3,3}单形重心设计方法,研究了普通尿素和2种包膜尿素D90、D60配比对土壤NH4+-N、NO3--N及矿质态氮(Nmin)含量的影响。结果表明,供试的7种包膜肥料初期溶出率均12.0%,微分溶出率在0.26%～2.49%之间。各处理土壤NH4+-N含量均随时间逐渐降低,而NO3--N和Nmin含量随时间逐渐增加。整个培养期内单独施用尿素处理,土壤NH4+-N、NO3--N及Nmin含量最高;2种控释肥单施或其配比施用土壤NH4+-N、NO3--N及Nmin含量最低;尿素与控释肥配合施用,土壤NH4+-N、NO3--N及Nmin含量居中。不同时期内土壤NH4+-N的来源不同,0～20d内,尿素对土壤NH4-N含量贡献最大;30～50d内,土壤NH4+-N主要来自D60;整个培养期内尿素对土壤NO3--N和Nmin的贡献均最大。肥料配比中随着尿素比例的减少,土壤NH4+-N、NO3--N及Nmin均逐渐减少。研究结果初步验证了混料设计在肥料配比研究中的可行性。     

Nitrogen fertilization inhibits soil microbial respiration regardless of the form of nitrogen applied

We tested how amendments of different forms of nitrogen (N) affect microbial respiration rates by adding six different forms of N (NH₄NO₃, (NH₂)₂CO (urea), KNO₃, NH₄Cl, (NH₄)₂SO₄, Ca(NO₃)₂) to three distinct soils. All inorganic N forms led to a net reduction in microbial respiration, and the magnitude of the observed response (up to 60 % reduction) was consistent across all soils and negatively correlated with N concentration. Urea also reduced respiration rates in nearly all cases, but the effect was attenuated by the associated input of labile organic carbon. We observed decreases in respiration regardless of soil type, the specific N counter ion, N added as NH₄⁺ or NO₃⁻, or the effects of N form on soil pH, suggesting that decreases in respiration rates were mainly a direct result of the increase in soil N availability, rather than indirect effects caused by the form of N added.     

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.     

灌溉施用氮肥后氮素在土壤中的转化及淋失状况: 土柱模拟研究

A soil column method was used to compare the effect of drip fertigation (the application of fertilizer through drip irrigation systems, DFI) on the leaching loss and transformation of urea-N in soil with that of surface fertilization combined with flood irrigation (SFI), and to study the leaching loss and transformation of three kinds of nitrogen fertilizers (nitrate fertilizer, ammonium fertilizer, and urea fertilizer) in two contrasting soils after the fertigation. In comparison to SFI, DFI decreased leaching loss of urea-N from the soil and increased the mineral N (NH₄⁺-N + NO₃^--N) in the soil. The N leached from a clay loam soil ranged from 5.7% to 9.6% of the total N added as fertilizer, whereas for a sandy loam soil they ranged between 16.2% and 30.4%. Leaching losses of mineral N were higher when nitrate fertilizer was used compared to urea or ammonium fertilizer. Compared to the control (without urea addition), on the first day when soils were fertigated with urea, there were increases in NH₄⁺-N in the soils. This confirmed the rapid hydrolysis of urea in soil during fertigation. NH₄⁺-N in soils reached a peak about 5 days after fertigation, and due to nitrification it began to decrease at day 10. After applying NH₄⁺-N fertilizer and urea and during the incubation period, the mineral nitrogen in the soil decreased. This may be related to the occurrence of NH₄⁺-N fixation or volatilization in the soil during the fertigation process.     

Impact of addition of different rates of rice-residue biochar on C and N dynamics in texturally diverse soils

Impacts of crop residue biochar on soil C and N dynamics have been found to be subtly inconsistent in diverse soils. In the present study, three soils differing in texture (loamy sand, sandy clay loam and clay) were amended with different rates (0%, 0.5%, 1%, 2% and 4%) of rice-residue biochar and incubated at 25°C for 60 days. Soil respiration was measured throughout the incubation period whereas, microbial biomass C (MBC), dissolved organic C (DOC), NH₄⁺-N and NO₃^–N were analysed after 2, 7, 14, 28 and 60 days of incubation. Carbon mineralization differed significantly between the soils with loamy sand evolving the greatest CO₂ followed by sandy clay loam and clay. Likewise, irrespective of the sampling period, MBC, DOC, NH₄⁺-N and NO₃^–N increased significantly with increasing rate of biochar addition, with consistently higher values in loamy sand than the other two soils. Furthermore, regardless of the biochar rates, NO₃^–-N concentration increased significantly with increasing period of incubation, but in contrast, NH₄⁺-N temporarily increased and thereafter, decreased until day 60 in all soils. It is concluded that C and N mineralization in the biochar amended soils varied with the texture and native organic C status of the soils.     

Relationships between microbial biomass nitrogen,nitrate leaching and nitrogen uptake by corn in a compost and chemical fertilizer-amended regosol

Abstract

To determine the relationships between microbial biomass nitrogen (N), nitrate–nitrogen leaching (NO₃-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L^?1 K₂SO₄ extractable N (extractable N), NO₃-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha^?1 containing 158 kg N ha^?1 of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO₃-N leaching were monitored in soil treated with SDC (20 Mg ha^?1) + rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha^?1 containing 0, 15, 29, 44 and 59 kg N ha^?1) and in soil treated with CF in 2001. Nitrogen was applied as (NH₄)₂SO₄ at rates of 220 kg N ha^?1 for SDC and SDC + RS treatments and at a rate of 300 kg N ha^?1 for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha^?1 at 7 days after treatment (DAT) and was maintained at 60–70 kg N ha^?1 after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO₃-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO₃-N leaching in the CF treatment was 146 kg N ha^?1, which was equal to half of the applied N, but only 53 kg N ha^?1 in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO₃-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO₃-N leaching without impacting negatively on N uptake by plants.     

Effect of biuret content on transformation of urea nitrogen in soil

The effect of the biuret content (0.0, 1.0, 2.5, 5.0 and 10.0% of urea) on transformations of urea-N was studied in a sandy loam (pH 7.7). While biuret did not affect urea hydrolysis, it inhibited the conversion of NH⁺₄ to NO^?₂ and the subsequent oxidation of NO^?₂ to NO^?₃. This resulted in the accumulation of larger amounts of both NH⁺₄-N and NO^?₂-N in soil as compared to soil receiving urea alone. The results suggest that biuret impurity in urea fertilizer is likely to enhance nitrite toxicity.     

Ammonium and nitrate in the rhizosphere of spring barley, hordeum vulgare L., and take-all disease

The incidence and severity of take-all disease, due to Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker, was observed on spring barley plants growing in soil in two glasshouse experiments. Soil amendments of NH⁺₄-N significantly increased the number of diseased plants and roots during the first month after germination in comparison with controls unamended with N (P < 0.05). No significant difference in the incidence of take-all disease was detected between more mature barley plants growing in soil amended with either NH⁺₄ or NO^?₃-N and unamended controls. The least take-all disease in 3 month-old barley plants was observed when N was supplied as foliar sprays of urea at 0.5 mg N kg^?1 soil (P < 0.01). There was no significant correlation between the degree of infection and the NH⁺₄-N to NO^?₃-N ratio in the rhizosphere soil     

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.