Assimilation and transport of nitrogen in rice II. 15N-labelled nitrate nitrogen

An investigation was made to study the assimilation and transportof ¹⁵N-labelled nitrate nitrogen in rice plant (Oryza sativaL.). Nitrogen from labelled nitrate at the end of plant feedingwas found mainly in nitrate form, and was more prevalent inroots, stem and leaf sheaths. The nitrite fraction had the nextlargest ¹⁵N enrichment. The ¹⁵NO₃ assimilation in the newlyemerged panicle was mainly in amide and amino acid. The ¹⁵N-incorporation at day 0 was greatest in amino acid andnitrate of roots and decreased towards the stem and leaves.Incorporation in these fractions considerably decreased fromday 0 to day 10. Probably most of the nitrogen from the nitratesource was transported from the roots to the shoot in nitrateand amino acid forms. A decrease of ¹⁵N-incorporation in the soluble N fraction andincrease in the insoluble N fraction from day 0 to day 10 inplant parts, particularly the blades, suggested that proteinsynthesis occurred mostly in young parts of the shoot duringthis period. The marked variation in ¹⁵N distribution in differentparts of the plant during the 10 days indicated that the nitrogenin roots and tillers was probably remobilized and transportedto other parts, particularly the upper leaf blades. Ammonium and nitrate nitrogen transport in rice plant are compared. (Received May 11, 1974; )     

Uptake of fertilizer nitrogen and soil nitrogen by rice using15N-labelled nitrogen fertilizer

Data from five field experiments using labelled nitrogen fertilizer were used to determine the relative effects of soil nitrogen and fertilizer nitrogen on rice yield. Yield of grain was closely correlated with total aboveground nitrogen uptake (soil+fertilizer), less closely correlated with soil nitrogen uptake and not significantly correlated with fertilizer nitrogen uptake. When yield increase rather than yield was correlated with fertilizer nitrogen uptake, the correlation coefficient was statistically significant.Contribution from the Laboratory for Flooded Soils and Sediments, Agronomy Dept., Louisiana Agri. Exper. Sta., Louisiana State Univ., Baton Rouge, LA 70803, and Univ. of Florida, Agricultural Research and Education Center, Sanford, FL 32771.     

Contribution of N2 fixing cyanobacteria to rice production: availability of nitrogen from 15N-labelled cyanobacteria and ammonium sulphate to rice

This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N₂-fixing cyanobacteria abundance and N₂ fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha^-1. No further increase was observed with 140 kg N ha^-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from ¹⁵N fertilizer. Recovery of applied ¹⁵N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of ¹⁵N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application. This revised version was published online in June 2006 with corrections to the Cover Date.     

A kinetic study of the assimilation of 15N-labelled ammonium in rice seedling roots

The pathway of ammonium nitrogen assimilation, its incorporationinto amino acids and synthesis of protein was studied with theaid of nitrogen-15. The analysis of ¹⁵N involves the use ofoptical emission spectrometry. Kinetic analysis of nitrogen assimilation by the roots indicatesthat glutamine and glutamic acid were the primary products ofammonium assimilation. Possibly some of the amino acids, suchas aspartic acid and alanine received their amino nitrogen directlyfrom free ammonia in the roots. Amino groups were transformedinto other amino acids from these primary products, especiallyfrom glutamic acid through transamination. (Received April 1, 1974; )     

Availability of nitrogen from 15N-labelled Azolla pinnata and urea to flooded rice

In view of the recently generated interest in Azolla and the high cost of N fertilizers, this field study was aimed at measuring the availability of Azolla-N applied in two split application in comparison to urea-N. Azolla was cultivated and labelled with ¹⁵N isotope in the field. A total of about 60 kg N ha^-1 was applied as Azolla, urea or Azolla and urea in combination, in two equal splits at transplanting and at maximum tillering, i.e. 30 days after transplanting (30 DAT).The recovery by the crop of Azolla-N applied at 30 DAT was significantly higher than that applied at transplanting, viz. 30.2% and 20.2%, respectively. The recoveries of urea-N applied at the same stages were similarly low, viz. 22.5% at transplanting and 38.6% at 30 DAT. Total recoveries of fertilizer N at the time of harvest were 26.8% from Azolla, 30.7% from urea applied in the same two splits and 49.1% from urea applied in locally recommended three splits. Recoveries of labelled Azolla-N in succeeding rice crop were twice higher than those of labelled urea-N. The recoveries ranged from 1.9 to 2.1% from urea-N and 4.0 to 4.9% from Azolla-N. There were no differences in residual ¹⁵N recovery in the succeeding crop between Azolla and urea either applied at transplanting or at 30 DAT.     

Assimilation of ammonium ions and reutilization of nitrogen in rice (Oryza sativa L.)

A major source of inorganic nitrogen for rice plants grown in paddy soil is ammonium ions. The ammonium ions are actively taken up by the roots via ammonium transporters and subsequently assimilated into the amide residue of glutamine (Gln) by the reaction of glutamine synthetase (GS) in the roots. The Gln is converted into glutamate (Glu), which is a central amino acid for the synthesis of a number of amino acids, by the reaction of glutamate synthase (GOGAT). Although a small gene family for both GS and GOGAT is present in rice, ammonium-dependent and cell type-specific expression suggest that cytosolic GS1;2 and plastidic NADH-GOGAT1 are responsible for the primary assimilation of ammonium ions in the roots. In the plant top, approximately 80% of the total nitrogen in the panicle is remobilized through the phloem from senescing organs. Since the major form of nitrogen in the phloem sap is Gln, GS in the senescing organs and GOGAT in developing organs are important for nitrogen remobilization and reutilization, respectively. Recent work with a knock-out mutant of rice clearly showed that GS1;1 is responsible for this process. Overexpression studies together with age- and cell type-specific expression strongly suggest that NADH-GOGAT1 is important for the reutilization of transported Gln in developing organs. The overall process of nitrogen utilization within the plant is discussed.     

Uptake of 15N-labelled alanine,ammonium and nitrate in Pinus sylvestris L. ectomycorrhiza growing in forest soil treated with nitrogen,sulphur or lime

The uptake of ¹⁵N-labelled alanine, ammonium and nitrate was studied in ectomycorrhizal morphotypes of intact Pinus sylvestris seedlings. PCR-RFLP analysis of the ITS-region of fungal rDNA was used to identify the morphotypes. Seedlings were grown in forest soil collected at an experimental site in southern Sweden. The treatments compared were a control, N fertilisation (600 kg N ha^-1 as urea), sulfur application (1200 kg S ha^-1) and lime application (6000 kg CaCO₃ ha^-1). The forest, which had been dominated by Picea abies, was clear-cut two years before the forest soil was sampled. Soil was also collected from an adjacent standing forest. The aim of the present study was to detect changes in the ectomycorrhizal communities in forest soils and relate these changes to the functional parameter of uptake of nitrogen from organic (alanine and protein) and inorganic (ammonium and nitrate) sources.Liming resulted in the detection of a morphotype not found in other samples, and one morphotype was only found in samples from the standing forest (the fungi in these two morphotypes could not be identified). All mycorrhizal root tips showed a higher ¹⁵N concentration after exposure to different nitrogen forms than non-mycorrhizal long roots. Uptake of¹⁵ N from a labelled solution of alanine or ammonium was higher (about tenfold) than uptake from a ¹⁵N-labelled solution of nitrate. Uptake of ammonium and alanine varied between 0.2 and 0.5 mg N g^-1 h^-1 and between 0.1 and 0.33 mg N g^-1 h^-1, respectively, among the different morphotypes.In seedlings grown in the control soil and in soil from standing forest, alanine and ammonium were taken up to a similar degree from a supply solution by all morphotypes, whereas ammonium uptake was higher than alanine uptake in seedlings grown in lime-treated soil (about twofold) and, to a lesser extent, in the nitrogen- and sulfur-treated soils. The higher ammonium uptake by morphotypes from the limed soil was confirmed in pure culture studies. In cases where ammonium was used as the N source, an isolate of the S. variegatus morphotype collected in the limed soil produced more biomass compared with isolates of S. variegatus collected in nitrogen- or sulphur-treated soil. One isolate of a silvery white morphotype produced about equal amounts of biomass on alanine and ammonium, whereas all S. variegatus isolated performed better with ammonium as their N source. Based on the results it is hypothesised that liming can induce a shift in the ectomycorrhizal community, favouring individuals that mainly utilise inorganic nitrogen over those that primarily utilise organic nitrogen.     

A kinetic study of the assimilation of 15N-labelled nitrate in rice seedlings

A ¹⁵N kinetic-analysis of the assimilation of nitrate nitrogenin the roots of rice seedlings indicated that (1) nitrate wasrapidly reduced to ammonia in the roots, where it was incorporatedinto glutamine and glutamic acids; (2) the pattern of nitrateassimilation into amino acids was very similar to that of ammoniumassimilation; and (3) the pattern of nitrogen incorporationinto protein was also similar to that of the incorporation withNH₄-feeding. In the shoots, alanine, serine, glutamic acid, -amino butyricacid and aspartic acid were relatively strongly labelled with¹⁵N as compared with the other amino acids. A different mechanismof nitrogen assimilation seems to operate in between the photosyntheticand non-photosynthetic organs of the plants. (Received August 19, 1974; )     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

^15N标记稻草中N,C在羊体内的转化和利用

应用~(13)N标记稻草饲喂3只山羊,以探明羊对稻草N、C化合物的消化、吸收、排泄和转化规律。结果表明,已宰杀的2只羊消化、吸收、转化为羊机体的~(15)N占试验日粮中~(15)N富集总量的38.54和23.78％,平均为31.16％。3只羊从粪尿中排泄的~(15)N各占饲料中~(15)N的34.78、33.88和33.18％,平均为33.95±0.80％,已屠宰的2只羊对饲料~(15)N总回收率为73.32和56.96％,损失率为26.68和43.04％。饲料~(15)N的回收利用率与饲料中氨基酸的消化率(％)相吻合。1、2、3号羊对饲料碳水化合物的消化率分别为76.40、68.66和65.19％。其中饲喂2、3号羊的饲料中都含稻草50％左右,羊对碳水化合物的平均消化率为66.93％。     

Biochar application to rice with 15N-labelled fertilizers,enhanced leaf nitrogen concentration and assimilation by improving morpho-physiological traits and soil quality

Leaf nitrogen (N) concentration plays an important role in biochemical and physiological functions, and N availability directly influences rice yield. However, excessive N fertilization is considered to be a root cause of environmental issues and low nitrogen use efficiency. Therefore, the selection of appropriate nutrient management practices and organic amendments is key to maximizing nitrogen uptake and maintaining high and sustainable rice production. Here, we evaluated the effects of different ¹⁵N-labelled nitrogen sources (urea, ammonium nitrate, and ammonium sulfate at 315 kg ha⁻¹) with or without biochar (30 t ha⁻¹) on paddy soil properties, root growth, leaf gas exchange, N metabolism enzymes, and N uptake in the early and late seasons of 2019. We found significant differences among N fertilizer sources applied with or without biochar (P < 0.05). Across the seasons, the combination of biochar with N fertilizers significantly increased soil organic carbon by 51.21% and nitrogen availability by 27.51% compared with N fertilizers alone. Correlation analysis showed that rice root morphological traits were strongly related to soil chemical properties, and higher root growth was measured in the biochar treatments. Similarly, net leaf photosynthetic rate averaged 9.34% higher, chlorophyll (Chl) a concentration 12.91% higher, and Chl b concentration 10.05% higher in the biochar treatments than in the biochar-free treatments across the seasons. Notably, leaf ¹⁵N concentration was 23.19% higher in the biochar treatments in both seasons. These results illustrated higher activities of N metabolism enzymes such as NR, GS, and GOGAT by an average 23.44%, 11.26% and 18.16% in the biochar treatments across the seasons, respectively. The addition of biochar with synthetic N fertilizers is an ecological nutrient management strategy that can increase N uptake and assimilation by ameliorating soil properties and improving the morpho-physiological factors of rice.     

Fate of 15N-labelled nitrogen fertilizer applied to kiwifruit (Actinidia deliciosa) vines

The fate of ¹⁵N-labelled ammonium fertilizer applied once to six-year-old field-grown kiwifruit (Actinidia deliciosa Hayward) vines was measured over three years. The three main treatments were nitrogen (N) applied singularly at 100 or 200 kg N ha^–1 in early spring (two weeks before bud burst) or split with 100 kg N ha^–1 (unlabelled) in early spring and 100 kg N ha^–1 (¹⁵N-labelled) ten weeks later. All N treatments were applied to vines with a history of either 50 or 200 kg N ha^–1 yr^–1. For three years after ¹⁵n application, components of the vines and soil (0–600 mm depth) were sampled at harvest in late autumn and the N and ¹⁵N contents determined.By the first harvest, all plant uptake of ¹⁵N had occurred and this represented 48–53% of the ¹⁵N applied. There was no significant effect of current N fertilizer treatment or of N history on ¹⁵N recovery by vines. Removal of ¹⁵N in harvested fruit was small at 5–6% in the first year and 8% over 3 years. After 2–3 years, most plant ¹⁵N occurred in the roots and this component declined only slowly over time. In contrast, there was a large temporal decline in ¹⁵N in above-ground plant components due to the annual removal in leaf fall and pruning. An associated experiment showed that when ¹⁵N-labelled prunings and leaves were mulched and returned to the soil, only about 9% was recovered by plants within 2 years. Almost all remaining mulched material had been immobilised into the soil organic N.In all treatments, about 20% of the added ¹⁵N remained in soil at the first harvest. This was almost entirely in organic fractions (<0.4% in inorganic N) and mostly in the surface 150-mm layer. The ¹⁵N content in soil changed little over time (from 20 to 17% between the first and third harvests respectively) and indicated that most of the N had been immobilised into stable humus forms.     

Effects of gypsum on the uptake,assimilation and cycling of 15N-labelled ammonium and nitrate-N by perennial ryegrass

Solution culture studies have shown that plant uptake of NH₄ ⁺ and NO₃ ^- can be improved by increasing the concentration of Ca²⁺ in the root environment: the same may be true for grass grown in soil culture. An experiment was set up to see whether gypsum (CaSO₄ 2H₂O) increased the rate at which perennial ryegrass absorbed ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^- from soil.The results demonstrated that gypsum increases the rates of uptake of both NH₄ ⁺ and NO₃ ^- by perennial ryegrass. However because there was little potential for mineral-N loss from the experimental system, either by gaseous emission or by N immobilization, long term improvements in fertilizer efficiency were not observed. Nitrogen cycling from shoots to roots commenced once net uptake of N into plants had ceased. Labelled N transferred thus to roots underwent isotopic exchange with unlabelled soil N. It was suggested that this exchange of N might constitute an energy drain from the plant, if plant organic N was exchanged for soil inorganic N. The fact that the exchange occurred at all cast doubt on the suitability of the ¹⁵N-isotope dilution technique for assessing fertilizer efficiency in medium to long term experiments. There was evidence that the extra NO₃ ^--N taken up by plants on the all-nitrate treatments as a result of gypsum application, was reduced in root tissue rather than in shoots, but to the detriment of subsequent root growth and N uptake.     

Metabolism of 15N-labelled ammonium by the ectomycorrhizal fungus Pisolithus tinctorim (Pers.) Coker & Couch

Summary Glutamine was the major product accumulated following transfer of nitrogen-limited cultures of the ectomycorrhizal fungus Pisolithus tinctorius to an ammonium medium. Experiments in which mycelium was transferred to [¹⁵N]H ₄ ⁺ showed glutamine amide was the most heavily labelled product. Assimilation of ammonium into glutamate was markedly inhibited by azaserine. The kinetics of ¹⁵N-labelling and the effects of azaserine and methionine sulphoximine on the distribution of ¹⁵N-labelled products are entirely consistent with the operation of the glutamate synthase cycle. No evidence was found for ammonium assimilation via glutamate dehydrogenase. The labelling pattern observed in mycelium treated with aminooxyacetate suggests that transamination reactions are an important source of glutamate for the synthesis of glutamine.     

Mineralization of15N-labelled legume residues in soils with different nitrogen contents and its uptake by Rhodes grass

Summary Soil was collected from pots that had grown 1,3 or 6 soybean (Glycine max) or Siratro (Macroptillium atropurpureum) crops that had received organic residue returns from each crop.¹⁵N-labelled residues were added to half the pots in the experiment and the other half left unamended. Half of each group was then sown to Rhodes grass (Chloris gayana) which was grown, under glasshouse conditions, for 12 weeks.Ten grams of organic matter residues were added to each pot (1.5 kg soil) and the pots subjected to two wetting and drying cycles. At the end of the second wet cycle, soil mineral N values ranged from 6 to 64 ppm in unamended soils and from 19 to 177 ppm in amended soils. These levels generally declined over a 12 week period both in the presence and absence of sown grass.Nitrogen uptake by the grass increased with the number of previous cycles and was higher in Siratro than soybean soils. Recovery of¹⁵N by plant growth from the incorporated soybean residues was little effected by previous crop history and averaged 15.4%. On the other hand, Siratro recoveries were 13.7, 42.4 and 55.5% from soils that had grown 1, 3 and 6 previous Siratro crops, respectively.The addition of organic residues stimulated the release of native organic N (positive priming effect) on all soils.These results show that the turnover rate of nitrogen from organic residues can be high and the net result of these additions depends on the nature of the organic residues and the soil system to which they are added. These data emphasise the need to consider the rate of nutrient turnover from organic sources rather than concentrate on the nature and size of the resident nutrient pools.     

Transformation of15N-labelled urea in rice-wheat cropping system

Summary In a udic chromusterts the transformation of an initial application of¹⁵N-urea @ 80 kg N ha^–1 to rice (Oryza sativa L.) in rice-wheat (R-W) and to wheat (Triticum aestivum L.) in wheat-rice (W-R) rotations was followed in 6 successive crops in each rotation. All rice crops were grown in irrigated wetland and wheat in irrigated upland conditions.The first wheat crop in W-R rotation utilized 22 kg fertilizer N ha^–1 as compared to 19 kg by the corresponding rice crop in R-W rotation. But the latter absorbed more soil N than the former. About 69% of the total N uptake in rice was derived from mineralization of soil organic N as compared to 61% in wheat.The succeeding wheat crop in R-W rotation utilized 6.7% of the residual fertilizer N in the soil but the corresponding rice crop in W-R rotation only 2.2%. The higher utilization appeared to be related to a greater incorporation of labelled fertilizer N in mineral and hexosamine fractions of the soil N. After the second crop in each rotation, the average residual fertilizer N utilization in the next 4 crops ranged between 3 and 4%.The total recovery of¹⁵N-urea in all crops amounted to 21.7 and 24.3 kg N ha^–1 in R-W and W-R rotation, respectively. At the end of the experiment, about 9 to 10 kg ha^–1 of the applied labelled N was found in soil upto 60 cm depth. Most of the labelled soil N (69–76%) was located in the upper 0–20 cm soil layer indicating little movement to lower depths despite intensive cropping for 4 years.