Assessing Nitrification and Denitrification in the Seine River and Estuary Using Chemical and Isotopic Techniques

Downstream from metropolitan Paris (France), a large amount of ammonium is discharged into the Seine River by the effluents of the wastewater treatment plant at Achères. To assess the extent of nitrification and denitrification in the water column, concentrations and isotopic compositions of ammonium (δ¹⁵N–NH₄⁺) and nitrate (δ¹⁵N–NO₃⁻, δ¹⁸O–NO₃⁻) were measured during summer low-flow conditions along the lower Seine and its estuary. The results indicated that most of the ammonium released from the wastewater treatment plant is nitrified in the lower Seine River and its upper estuary, but there was no evidence for water-column denitrification. In the lower part of the estuary, however, concentration and isotopic data for nitrate were not consistent with simple mixing between riverine and marine nitrate. A significant departure of the nitrate isotopic composition from what would be expected from simple mixing of freshwater and marine nitrates suggested coupled nitrification and denitrification in the water, in spite of the apparent conservative behavior of nitrate. Denitrification rates of approximately 0.02 mg N/L/h were estimated for this part of the estuary.     

Denitrification measurements in aquatic sediments: A comparison of three methods

Measurements of denitrification using the acetylene inhibition,¹⁵N isotope tracer, and N₂ flux methods were carried out concurrently using sediment cores from Vilhelmsborg sø, Denmark, in an attempt to clarify some of the limitations of each technique. Three experimental treatments of overlying water were used: control, nitrate enriched, and ammonia enriched water. The N₂ flux and¹⁵N tracer experiments showed high rates of coupled nitrification/denitrification in the sediments. The acetylene inhibition method did not capture any coupled nitrification/denitrification. This could be explained by acetylene inhibition of nitrification. A combined¹⁵N tracer/acetylene inhibition experiment demonstrated that acetylene inhibition of N₂O reduction was incomplete and the method, therefore, only measured approximately 50% of the denitrification due to nitrate from the overlying water. Similar rates of denitrification due to nitrate in the overlying water were measured by the N₂ flux method and the acetylene inhibition method, after correcting for the 50% efficiency of acetylene inhibition. Rates of denitrification due to nitrate from the overlying water measured by the¹⁵N tracer method, however, were only approximately 35% or less of those measured by the acetylene inhibition or N₂ flux methods.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol,lignin, and nitrogen contents

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and ¹⁵N-recovery methods. Mature tops of alfalfa (Medicago sativa L.), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), leucaena (Leucaena leucocephala Lam., deWit), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde) were incorporated in soil at the rate of 100 mg legume N kg^-1 soil. The medic and vigna were labeled with ¹⁵N. Sorghum-sudan hybrid (Sorghum bicolor, L. Moench) was used as the test crop. A non-amended treatment was used as a control. Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa. With the two legumes that contained less than 20 g kg^-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment. The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 0.05 level and at the 0.01 level at 6 weeks (r²=0.866). Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time. The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r²=0.692). The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized. Estimates of legume-N mineralization by the difference and ¹⁵N recovery methods were significantly different at all sampling dates for both ¹⁵N-labeled legumes. After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the ¹⁵N recovery method. This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from ¹⁵N recovery experiments.     

Rapid abiotic transformation of nitrate in an acid forest soil

Nitrate immobilization into organic matter isthought to require catalysis by the enzymes ofsoil microorganisms. However, recent studiessuggest that nitrate added to soil isimmobilized rapidly and this process mayinclude abiotic pathways. We amended living andsterilized soil with ¹⁵N-labeled nitrateand nitrite to investigate biotic and abioticimmobilization. We report rapid transformationof nitrate in incubations of the O layer offorest soils that have been sterilized toprevent microbial activity and to denaturemicrobial enzymes. Approximately 30, 40, and60% of the ¹⁵N-labeled nitrate added tolive, irradiated, or autoclaved organic horizonsoil disappeared from the extractableinorganic-N pool in less than 15 minutes. About5% or less of the nitrate was recovered asinsoluble organic N in live and sterilizedsoil, and the remainder was determined to besoluble organic N. Added ¹⁵N-nitrite,however, was either lost to gaseous N orincorporated into an insoluble organic N formin both live and sterile organic soils. Hence,the fate and pathway of apparent abioticnitrate immobilization differs from thebetter-known mechanisms of nitrite reactionswith soil organic matter. Nitrate and nitriteadded to live A-horizon soil was largelyrecovered in the form added, suggesting thatrapid conversion of nitrate to solubleorganic-N may be limited to C-rich organichorizons. The processes by which this temperateforest soil transforms added nitrate to solubleorganic-N cannot be explained by establishedmechanisms, but appears to be due to abioticprocesses in the organic horizon.     

Measurements of abundances of 15N and 13C as tools in retrospective studies of N balances and water stress in forests: A discussion of preliminary results

Preliminary attempts to make retrospective studies of N balances and water stress in forest fertilization experiments by analyzing changes in the abundances of ¹⁵N and ¹³C, respectively, are discussed. Most evidence is from the Swedish Forest Optimum Nutrition Experiments, which have been running for two decades. Annual additions of N have been given either alone or in combination with other elements, notably P and K, every third year. Processes leading to loss of N, e.g. volatilization of ammonia, nitrification followed by leaching or denitrification, and denitrification alone, discriminate against the heavy isotope ¹⁵N. A correlation was found between fractional losses of added N and the change in ¹⁵N () during 19 years in current needles in a Scots pine forest, irrespective of source of N. Isotope effects were larger on urea than on ammonium nitrate plots (2 as compared to 9 ¹⁵N ()) because of ammonia volatilization and higher rates of nitrification. They developed gradually over time, which opens possibilities to analyse the development of N saturation. However, the analysis may be confounded by shifts in ¹⁵N abundance of fertilizer N. In another trial, N isotope effects could be seen in both plants and soils 10 years after the last fertilization; they were smaller in soils because of a large pretreatment memory effect, but we expect them to persist there for decades.The enzyme RuBisCo discriminates strongly against the heavy isotope ¹³C during photosynthesis, but this effect becomes less expressed as stomata close because of water stress. The supply of N may also affect the ¹³C () via effects on rates of photosynthesis, and the source of N may have an influence directly via non-RubisCo carboxylations, and indirectly via effects on water use efficiency. In a trial with Norway spruce, the effect of N fertilization on the ¹³C () of current needles was strongly correlated with production and weakly so with foliar biomass a dry year, but not a wet year. This suggested that these variations are primarily related to induced differences in the balance between supply and demand for water. Hence, studies of {au13}C abundance can disentangle the role of water as such from its effects on mineralization of N and flow of N.     

Natural15N abundance as a method of estimating the contribution of biologically fixed nitrogen to N2-fixing systems: Potential for non-legumes

The¹⁵N abundance of plants usually closely reflects the¹⁵N abundance of their major immediate N source(s); plant-available soil N in the case of non-N₂-fixing plants and atmospheric N₂ in the case of N₂ fixing plants. The¹⁵N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the¹⁵N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural¹⁵N abundance method of estimating the relative contribution of atmospheric N₂ to N₂-fixing plants growing in natural and agricultural settings. The natural¹⁵N abundance method has certain advantages over more conventional methods, particularly in natural ecosystems, since disturbance of the system is not required and the measurements may be made on samples dried in the field. This method has been tested mainly with legumes in agricultural settings. The tests have demonstrated the validity of this method of arriving at semi-quantitative estimates of biological N₂-fixation in these settings. More limited tests and applications have been made for legumes in natural ecosystems. An understanding of the limits and utility of this method in these systems is beginning to emerge. Examples of systematic measurements of differences in¹⁵N abundance between non-legume N₂-fixing systems and neighbouring non-fixing systems are more unusual. In principle, application of the method to estimate N₂-fixation by nodulated non-legumes, using the natural¹⁵N abundance method, is as feasible as estimating N₂-fixation by legumes. Most of the studies involving N₂-fixing non-legumes are with this type of system (e.g., Ceanothus, Chamabatia, Eleagnus, Alnus, Myrica, and so forth). Resuls of these studies are described. Applicability for associative N₂-fixation is an empirical question, the answer to which probably depends upon the degree to which fixed N goes predominantly to the plant rather than to the soil N pool. The natural¹⁵N abundance method is probably not well suited to assessing the contribution of N₂-fixation by free-living microorganisms in their natural habitat, particularly soil microorganisms.This work was supported in part by subcontracts under grants from the US National Science Foundation (DEB79-21971 and BSR821618)     

Seasonal variation of nitrogen transformations in the pelagial of selected nearshore waters of the Baltic Sea with emphasis on the particulate pool

Physical and chemical conditions, particulate matter and N-uptake were characterized at two sampling sites at the eastern German coast of the Baltic Sea (Pomeranian Bay) over the annual period of 1997 (February–November). The inshore sampling sites (5 m water depth) differed with respect to the potential influences of river run-off and salt water exchange (mean values of salinity: 7.05 and 8.72 PSU), respectively. The mean org-C_diss/org-C_part-ratios (4.9 and 12.6) fell in the same order of magnitude (1.0–12.6) as values of neighboring inshore waters, and increasing values reflect an enhancement of the trophic level. Beside differences of nitrogen concentrations (dissolved inorganic nitrogen: 1.8–23.8 and 0.9–9.9 mol l^–1), particulate nitrogen (4.30–41.01 and 2.69–9.08 mol l^–1) and absolute uptake of N-nutrients (mean sum of NH₄ ⁺, urea, NO₃ ^– uptake rates: 0.141 and 0.087 mol l^–1 h^–1), specific uptake of ¹⁵N-labelled nutrients (NH₄ ⁺, urea, NO₃ ^–) as well as the relationships between the measured variables characterize distinguishable inshore systems. The high variability at the shallow sampling sites prevents, however a simple resolution of the seasonal courses. Light dose could be identified as a potential key in order to describe long-term variations of N-uptake at the station with higher organic matter concentration (station KW), but phytoplankton development is better reflected in the seasonal course of N-uptake at the other station. Specific nitrogen uptake rates (NH₄ ⁺: 0.0009–0.0353 h^–1, urea: 0.0001–0.0137 h^–1, NO₃ ^–: 0.000004–0.0009 h^–1) and relative nitrogen preferences indicate extraordinary importance of reduced nitrogenous nutrients (NH₄ ⁺, urea) at both stations throughout the year.     

15N natural abundance in plants of the Amazon River floodplain and potential atmospheric N2 fixation

Summary The¹⁵N natural abundance values of various Amazon floodplain (várzea) plants was investigated. Samples of young leaf tissues were collected during three different periods of the river hydrography (low water, mid rising water and high water) and during one period in the Madeira River (high water). A large variation of¹⁵N abundance was observed, both among the different plant types and between the different flood stages. This variation probably, reflected, in part, the highly variable nature of the floodplain, sometimes dry and oxygenated and at other times inundated and anaerobic and, in part, changes in plant nitrogen metabolism. Comparison of the nitrogen isotopic composition of leguminous plants with that of non-leguminous plants showed that, on average, the¹⁵N abundance was lower in the legumes than non-legumes, suggesting active N-fixation. Also, the¹⁵N natural abundance in aquatic grasses of the generaPaspalum, was in general, lower than the¹⁵N abundance of aquatic grasses of the generaEchinochloa. As both of these grasses grow in the same general habitat, it appears thatPaspalum grasses may also be nitrogen fixers.     

Forest floor-mineral soil interactions in the internal nitrogen cycle of an old-growth forest

Seasonal patterns and annual rates of N inputs, outputs, and internal cycling were determined for an old-growth mixed-conifer forest floor in the Sierra Nevada Mountains of California. Rates of net N mineralization within the forest floor, and plant N-uptake and leaching of inorganic N from the forest floor were 13, 10, and 9 kg-N ha^-1 yr^-1, respectively. The Mediterranean-type climate appeared to have a significant effect on N cycling within this forest, such that all N-process and flow rates showed distrinct seasonal patterns. We estimated the forest floor supplies less than one-third of the total aboveground plant N-uptake in this forest. The rate of net nitrification within the forest floor was always low (1 kg-NO₃ ^--N ha^-1 30d^-1). Mean residence times for organic matter and N in the forest floor were 13 and 34 years, respectively, suggesting that this forest floor layer is a site of net N immobilization within this ecosystem. We examined the influence of the forest floor on mineral soil N dynamics by injecting small amounts of¹⁵N-enriched (NH₄)₂SO₄ solutions into the surface mineral soil with the forest floor present (+FF) or removed (-FF). K₂SO₄-extractable NO₃ ^--N, total inorganic-N, and total-N pool sizes in the mineral soil were initially increased after forest floor removal (after 4 months), but NO₃ ^--N and total inorganic-N were not significantly different thereafter. Microbial biomass-N and K₂SO₄-extractable total-N pool sizes were also found to be larger in mineral soils without a forest floor after 1 and 1.3 years, respectively. Total¹⁵N-recovery was greater in the +FF treatment compared to the -FF treatment after 1-year (about 50% and 35%, respectively) but did not differ after 1.3 years (both about 35%), suggesting that the forest floor delays but does not prevent the N-loss from the surface mineral soil of this forest. We estimated using our¹⁵N data that fungal translocation from the mineral soil to the forest floor may be as large as 9 kg-N ha^-1 yr^-1 (similar in magnitude to other N flows in this forest), and may account for all of the observed absolute increase of N in litter during the early stages of decomposition at this site. Our results suggest that the forest floor acts both as a source and sink for N in the mineral soil.     

15N abundance of soils and plants along an experimentally induced forest nitrogen supply gradient

¹⁵N abundances of soils and a grass species (Deschampsia flexuosa (L.) Trin.) were analysed in a forest fertilization experiment 10 years after the last fertilization. Nitrogen had been given as urea, at seven doses, ranging from 0 to 2400 kg N ha^-1. Previously, we have shown that plants in systems experiencing large losses of N become enriched with ¹⁵N. This was explained by the fact that processes leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrification and denitrification itself, tend to fractionate against ¹⁵N. In this experiment, ¹⁵N abundance increased with dose of N applied in both grass and soil total-N, but more so in the grass. This was interpreted to be due to the grass sampling small but active pools of N subject to losses. In contrast, soil total-N largely consists of inactive N that does not immediately exchange with pools of N from which fractionating losses occur. Hence, soil total-N shows a large pretreatment ¹⁵N memory effect, and is, therefore, and integrator of the long-term N balance. When short-term changes (years, decades) in N balances are monitored using variations in ¹⁵N abundance, plants are more suitable indicators of such change than is soil total-N.     

Long-term behaviour of 15N in an alpine grassland ecosystem

Nitrogen dynamics in semi-natural environments is crucial for the development and ecological stability of these systems. The present paper shows the results of the reinvestigation of a ¹⁵N-tracer experiment, which was established in the Grossglockner massif in Austria at 2300ma.s.l. in 1974/1975. We show that large quantities of nitrogen introduced by a single pulse labelling (amounting to approximately 1.7% of the nitrogen in the system) into an alpine grassland remain in the soil–plant system, with only 55% being lost during 27–28 years. In the first 10cm of the four investigated soil profiles 40% of ¹⁵N was recovered, being mainly bound in organic forms. A simple site specific model was established on the basis of the results considering a biological, residual and labile N-pool, the latter being the source for N-losses. By the model a long mean residence time close to 100 years was derived for the remaining ¹⁵N.     

Effects of variations in soil water potential,depth of N placement,and cultivar on postanthesis N uptake by wheat

This study was conducted to investigate the influence of soil water potential, depth of N placement, timing, and cultivar on uptake of a small dose of labeled N applied after anthesis by wheat (Triticum aestivum L.) Understanding postanthesis N accumulation should allow better control of grain protein concentration through proper manipulation of inputs. Two hard, red spring-wheat cultivars were planted in early and late fall each yr of a 2-yr field experiment. Less than 1 kg N ha^–1 as K ¹⁵NO₃ was injected into the soil at two depths: shallow (0.05 to 0.08 m) and deep (0.15 to 0.18 m). In both years an irrigation was applied at anthesis, and injections of labeled N were timed 4, 12, and 20 days after anthesis (DAA). Soil water potential was estimated at the time of injection. Mean recovery of ¹⁵N in grain and straw was 57% of the ¹⁵N applied. Recovery did not differ between the high-protein (Yecora Rojo) and the low-protein (Anza or Yolo) cultivars. Mean recovery from deep placement was 60% versus only 54% from shallow placement (p < 0.01). Delaying the time of injection decreased mean recovery significantly from 58% at 4 DAA to 54% at 20 DAA. This decrease was most pronounced in the shallow placement, where soil drying was most severe. Regressions of recovery on soil water potential of individual cultivar x yr x planting x depth treatments were significant only under the driest conditions. Stepwise regression of ¹⁵N recovery on soil water potential and yield parameters using data from all treatments of both years resulted in an equation including soil water potential and N yield, with a multiple correlation coefficient of 0.64. The translocation of ¹⁵N to grain was higher (0.89) than the nitrogen harvest index (0.69), and showed a highly significant increase with increase in DAA. This experiment indicates that the N uptake capacity of wheat remains reasonably constant between 4 and 20 DAA unless soil drying is severe.     

Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO₃ and NH₄, floodwater NO₃, and N loss from added ¹⁵N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha^–1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH₄ in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha^–1) soil NO₃ accumulations. Soil NO₃, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered ¹⁵N in ¹⁵N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH₃ volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH₄ and NO₃.     

天山林区土壤总氮矿化过程对季节性冻融的响应

森林土壤总氮矿化对冻融过程的响应机制尚不明确,氮矿化速率和转化情况尚缺乏定量刻画。通过土壤原位法与室内培养分析相结合,利用¹⁵N同位素稀释技术,研究冻融期间天山林区乔木林地、灌丛、草地3种群落类型土壤总氮矿化及转化累积量的动态,分析土壤总氮矿化速率与土壤温度、含水率及微生物量氮(MBN)的相互关系。结果表明:(1)冻融过程及群落类型对总氮矿化速率和MBN含量有极显著的影响(P<0.01),秋、春季冻融期的总氮矿化速率相比冻结期更高;(2)季节性冻融期间,乔木林地土壤总氨化累积量在3种群落类型中最高(163.9 kg N hm^-2 a^-1),秋、春冻融期占整个时期的比值约为66%;而总硝化累积量在3种群落类型中相差较小,秋、春冻融期占比均约为77.4%;(3)土壤温度和含水率显著影响总氮矿化速率、净氮矿化速率和MBN速率,随土壤温度增加,总氨化速率(林地和灌丛)显著升高(P<0.05);随土壤含水率增加,净氨化速率(灌丛)和净硝化速率(灌丛)显著降低(P<0.05)。通过揭示天山林区土壤总氮矿化速率(总氨...     

Nitrogen-flow in the rotifer Brachionus rotundiformis and its significance in mass cultures

The nitrogen budget in the rotifer Brachionus rotundiformis wasmeasured by the stable-isotope technique. The budget was estimatedusing the difference in the turnover time between egestion andexcretion. The rotifer was fed on the algae Nannochloropsiswhich was labeled with ¹⁵N as a tracer. The turnover time ofegestion and excretion were 20 min and 2.5 hours, respectively. Where77% of the ingested nitrogen was egested, and of the assimilated23%, 18% were devoted to growth and 5% to excretion.As for the unassimilated nitrogen egested as faeces, it recycled tothe rotifer through bacteriovory. When the algae provided as foodwere almost fully consumed, bacteriovory became dominant. Thethreshold occurred when the concentration of algae in the culture wasbetween 1.5 and 0.5 million cells of Nannochloropsis per ml. Ina chemostat operated with un-limited food condition, bacterialnitrogen corresponding to 20% of algal feeding, was consumed by therotifer.In a semi-continuous mass culture where food condition was limited,bacteriovory was more effective in supporting the rotiferreproduction. It contributed to the extremely high nitrogen recoveryfrom the provided foods (algae and oil-yeast) to the harvestedrotifers. The rapid and large nitrogen outflow from rotifersaccelerated the propagation of edible bacteria and can explain thestrange paradox observed in the culture; daily supply of foods didnot cover the sum of growth and excretion.It is not too exaggerated to state that the rotifer mass culture issupported by bacteria. The future strategy for maintenance of masscultures should consider this aspect.     

水氮添加条件下高寒草甸主要植物种氮素吸收分配的同位素示踪研究

资源利用方式的分化可以减小物种间对相同资源的竞争,是群落物种多样性维持的主要机制。在全球变化背景下,土壤温度和水分条件的变化可能影响高寒草甸生态系统植物的氮素(N)营养。该实验在经N、水处理3年的高寒草甸开展,通过15NH415NO3的15N稳定性同位素注射,比较高寒草甸主要植物种对N、水处理的响应方式,以及N吸收能力、分配和根冠比特点,研究其营养吸收和资源分配方式的分化。结果发现不同植物种对N、水处理响应差异显著,N吸收能力、根N含量和根冠比等功能性状种间差异显著;回归分析发现植物种N吸收能力和根N含量之间的关系不显著,和根冠比之间呈显著线性负相关。说明高寒草甸生态系统不同植物种间N吸收具有生态位分化,并且存在N营养吸收能力和资源分配策略的权衡。     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Variations in 15N abundance in a forest fertilization trial: Critical loads of N,N saturation,contamination and effects of revitalization fertilization

¹⁵N abundances of current needles of Norway spruce collected during 23 yrs of a forest fertilization experiment were studied in order to follow ecosystem gains and losses of N. Unlabelled ammonium nitrate at four rates (N0–N3), phosphorus at three rates (P0–P2), and potassium plus other elements including micronutrients at two rates (K0–K1), had been applied to plots in a complete factorial design. Nitrogen had been applied annually at average rates of 0, 34, 68 and 102 kg N ha^-1 yr^-1. Tree growth had responded positively to additions of N, but the response was remarkably more positive to the N2P2K1 treatment. In N1 treatments, δ¹⁵N (‰) declined over time. This was consistent with an earlier study, and should reflect a change in ¹⁵N abundance towards that of fertilizer N (minus discrimination during uptake), which in turn means accretion of most of the N added. As in the earlier study, in which N3 plots lost most of the N added, the present N3 plots showed an increasing δ¹⁵N (‰). This pattern was not significantly affected by additions of P and K plus other elements, although a weak negative effect of P on N accretion was indicated, i.e. there was a tendency δ¹⁵N (‰) to be higher when P was added. This, and another recent result based on an N budget, shows that so-called revitalization fertilization may well increase growth of trees, but also promotes losses of N from the ecosystem. As in the previous study, a decline in δ¹⁵N (‰) on control plots provided evidence of contamination. Given a removal of 100 kg N ha^-1 at stem harvest and a leaching of 2 kg N ha^-1 yr^-1, our data on ¹⁵N suggest that a load of 9 kg N ha^-1 yr^-1 would saturate the ecosystem after 100 years. This load is only about twice the annual deposition at the site.     

Significance of organic nitrogen acquisition for dominant plant species in an alpine meadow on the Tibet plateau, China

Though the potential of plants to take up organic N (e.g., amino acids) is well established, the true significance of organic N acquisition to plant N nutrition has not yet been quantified under field conditions. Here we demonstrate that organic N contributes significantly to the annual N uptake of three dominant plant species (Kobresia humilis, Saussurea superba and Stipa aliena) of alpine meadows on the Tibet Plateau, China. This was achieved by using double-labelled (¹⁴C and ¹⁵N) algae as a source for slow and continuous release of amino acids, and tracing both labels in the above- and below-ground plant biomass. Four months after addition of algae, between 0.5% and 2.6% of ¹⁴C and 5% and 14% of ¹⁵N from added algae were recovered in the plants, which translate into an uptake of organic N between 0.3 mg N m⁻² and 1.5 mg N m⁻². The calculated contribution of organic N to total N uptake was estimated to range between 21% and 35% for K. humilis, and between 13% and 21% for S. aliena and S. superba, respectively, implying that organic N uptake by grassland plants is quantitatively significant under field conditions in the studied alpine meadows. This finding has important ecological implications with regard to competition for organic N between microorganisms and plant roots.