Nitrite effect on nitrous oxide emission from denitrifying activated sludge

Laboratory-scale experiments were conducted to examine the N₂O emission during the denitrification process. For each of the 6 runs carried out, synthetic effluent was fed in a 10 l batch mixed liquor to investigate the effect of nitrite on N₂O emission and Helium was continuously bubbled through the reactor at constant rate (0.12 l/min) to favour N₂O transfer and detection. An increasing COD/NO₃⁻-N influent ratio from 3 to 7 was firstly applied (runs 1–3). Secondly, NO₂⁻ pulse additions were performed during run 4 and 5 (10 and 20 mg N/l, respectively). Finally, the reactor was fed with influent containing both NO₂⁻ and NO₃⁻. We showed that N₂O emission was detected shortly after NO₂⁻ accumulation, few minutes after the substrate feeding. The highest emission occurred at the lower COD/NO₃⁻-N ratio (=3) and at the higher NO₂⁻ addition (20 mg N/l). In addition, the higher nitrogen conversion to N₂O gas (14.4%) was obtained with an influent containing initially both NO₂⁻ and NO₃⁻. Our results suggest a direct effect of the NO₂⁻ concentration on the N₂O emission. We have also confirmed the inhibitory effect of NO₂⁻ concentration on N₂O reduction.     

Nitrogen mineralisation and plant nitrogen acquisition in a nitrogen-limited calcareous grassland

A field study measured the rate of soil mineral N supply and its effects on plant biomass and N accumulation in a 13-year-old, naturally regenerating, calcareous grassland. Gross rates of N mineralisation (2 μg g⁻¹ day⁻¹, i.e. 0.69 kg ha⁻¹ day⁻¹), assessed using ¹⁵N pool dilution, were at the lower end of the range previously reported for grasslands. Weekly additions of liquid N fertiliser ([NH₄]₂SO₄, NH₄NO₃ or KNO₃) and, to a lesser extent the addition of water, increased plant growth substantially, demonstrating that the primary constraint to plant growth was low N availability. In plants that had received NO₃⁻, the activity of the inducible enzyme nitrate reductase in shoots initially increased in proportion to the amount of NO₃⁻ supplied. However, as above-ground herbage accumulated, nitrate reductase activity declined to similar low levels in all treatments, despite the continuance of the constant NO₃⁻ additions. The decline in NR specific activity reflected declining tissue NO₃⁻ concentrations, although total plant NRA may have remained constant during the period of study. The study has shown that plant growth is limited by low N mineralisation rates and indeed the soil is a sink for much added N. Low water availability provides an additional constraint on N mineralisation in this calcareous grassland soil. Any disturbances in the N cycle which increase the availability of mineral N will result in a substantial increase in plant growth within this ecosystem.     

Nutrient removal by thermophilic Fischerella (Mastigocladus laminosus) in a simulated algaculture process

A novel nutrient removal/waste heat utilization process was simulated using semicontinuous cultures of the thermophilic cyanobacterium Fischerella. Dissolved inorganic carbon (DIC)-enriched cultures, maintained with 10 mg l⁻¹ daily productivity, diurnally varying temperature (from 55°C to 26–28°C), a 12:12 light cycle (200 μE sec⁻¹ m⁻²) and 50% biomass recycling into heated effluent at the beginning of each light period, removed > 95% of NO₃⁻ + NO₂⁻−N, 71% of NH₃-N, 82% of PO₄³⁻ −P, and 70% of total P from effluent water samples containing approximately 400 μg l⁻¹ combined N and 60 μg l⁻¹ P. Nutrient removal was not severely impaired by an altered temperature gradient, doubled light intensity, or DIC limitation. Recycling 75% of the biomass at the end of each light period resulted in unimpaired NO₃⁻ + NO₂⁻ removal, 38–45% P removal and no net NH₃ removal. Diurnally varying P removal, averaging 50–60%, and nearly constant > 80% N removal, are therefore projected for a full-scale process with continuous biomass recycling.     

Perchlorate and nitrate reductase activity in the perchlorate-respiring bacterium perclace

The perchlorate (ClO₄⁻)-respiring organism, strain perc1ace, can grow using nitrate (NO₃⁻) as a terminal electron acceptor. In resting cell suspensions, NO₃⁻ grown cells reduced ClO₄⁻, and ClO₄⁻ grown cells reduced NO₃⁻. Activity assays showed that nitrate reductase (NR) activity was 1.31 μmol min⁻¹ (mg protein)⁻¹ in ClO₄⁻ grown cells, and perchlorate reductase (PR) activity was 4.24 μmol min⁻¹ (mg protein)⁻¹ in NO₃⁻ grown cells. PR activity was detected within the periplasmic space, with activities as high as 14 μmol min⁻¹ (mg protein)⁻¹. The NR had a pH optimum of 9.0 while the PR had an optimum of 8.0. This study suggests that separate terminal reductases are present in strain perclace to reduce NO₃⁻ and ClO₄⁻.     

Kinetics of the reactions of nitrogen dioxide with glutathione,cysteine, and uric acid at physiological pH

Nitrogen dioxide (NO₂^•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO₂^• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO₂^• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO₂^• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO₂^• with glutathione, cysteine, and urate were estimated as 2 × 10⁷, 5 × 10⁷, and 2 × 10⁷ M⁻¹ s⁻¹, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO₂^• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 10⁷ M⁻¹ s⁻¹. The implications of these values are: (i) the lifetime of NO₂^• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO₂^• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO₂^• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO₂^•; and (v) reactions between NO₂^• and thiols/urate severely limit the likelihood of reaction of NO₂^• with NO• to form N₂O₃ in the cytoplasm.     

Differential distribution of ammonia- and nitrite-oxidising bacteria in flocs and granules from a nitrifying/denitrifying sequencing batch reactor

A lab-scale sequencing batch reactor was operated with alternating anoxic/aerobic conditions for nitrogen removal. Flocs and granules co-existed in the same reactor, with distinct aggregate structure and size, for over 180 days of reactor operation. Process data showed complete nitrogen removal, with temporary nitrite accumulation before full depletion of ammonia in the aerobic phase. Microbial quantification of the biomass by fluorescence in situ hybridisation showed that granules contained most of the nitrite-oxidising bacteria (NOB) whereas the ammonium-oxidising bacteria (AOB) seemed to be more abundant in the flocs. This was supported by microsensor measurements, which showed a higher potential of NO₂⁻ uptake than NH₄ uptake in the granules. The segregation is possibly linked to the different growth rates of the two types of nitrifiers and the reactor operational conditions, which produced different sludge retention time for flocs and granules. The apparent physical separation of AOB and NOB in two growth forms could potentially affect mass transfer of NO₂⁻ from AOB to NOB, but the data presented here shows that it did not impact negatively on the overall nitrogen removal.     

High cell density culture of the diatom Nitzschia laevis for eicosapentaenoic acid production: fed-batch development

A fed-batch process was developed for high cell density culture of the diatom Nitzschia laevis for enhanced production of eicosapentaenoic acid (EPA). Firstly, among the various medium components, glucose (Glu) was identified as the limiting substrate while nitrate (NO₃⁻), tryptone (Tr) and yeast extract (Ye) were found to promote cell growth by enhancing specific growth rate. Therefore, these components were considered essential and were included in the feed medium for subsequent fed-batch cultivation. With the optimized ratio of NO₃⁻:Tr:Ye being 1:2.6:1.3 (by weight), the relative proportions of glucose to the nitrogen sources in the feed were investigated. The optimal ratios of Glu:NO₃⁻ for specific growth rate and EPA productivity were both determined to be 32:1 (by weight). Finally, based on the residual glucose concentration in the culture, a continuous medium feeding strategy for fed-batch fermenter cultivation was developed, with which, the maximal cell dry weight and EPA yield obtained were 22.1 g l⁻¹ and 695 mg l⁻¹, respectively, which were great improvements over those of batch cultures.     

Oxidative phosphorylation coupled to oxygen uptake and nitrate reduction in Micrococcus denitrificans

A procedure is described for preparing particles from cells of Micrococcus denitrificans which were broken osmotically after treatment with lysozyme.

1. 1. The preparations catalysed ATP synthesis coupled to O₂ uptake or NO₃⁻ reduction. With NADH or succinate as the electron donors the P:O ratios were about 1.5 and 0.5, respectively; and the P:NO₃⁻ ratios were about 0.9 and 0.06, respectively.

2. 2. Addition of ADP or P_i to the reaction mixture increased the rates of NADH-dependent O₂ uptake and NO₃⁻ reduction. Addition of 1 mM 2,4-dinitrophenol, which inhibited phosphorylation by 50–60%, increased the basal rates of electron transport.

3. 3. Evidence derived from spectrophotometry and from the differential inhibition by antimycin A of O₂ and NO₃⁻ reduction leads to the conclusion that the nitrate reductase interacted with the respiratory chain in the region of the b-type cytochrome, and that the c-type cytochrome present was not involved in the reduction of NO₃⁻ to NO₂⁻.

Suspended sediments and nutrients in water ebbing from seagrass-covered and denuded tidal mudflats in a southern Australian embayment

The concentrations of suspended solids, nitrogen (NH₄⁺, NO₃⁻, NO₂⁻), phosphorus (total phosphorus, PO₄⁻³) and silicate were measured in water ebbing from seagrass-covered tidal mudflats and from adjacent ungrassed mudflats in Western Port, Victoria, Australia. Ebb water was collected on six occasions during January and February (summer) and once during July (winter). From both types of mudflats the concentrations of suspended solids, phosphorus and silicate increased during the last hour of ebb tide with maxima in concentrations reached 0.5 to 1 h before slack water ebb. During the last 1.5–2.5 h of ebb tide the suspended solids, phosphorus and silicate concentrations were higher in water ebbing from denuded mudflats than from seagrass-covered mudflats. The concentration of nitrogen remained low in water ebbing from both types of mudflats during the day, but increased significantly during a night-time ebb; light-dependent biological uptake of nitrogen is implicated. It is suggested that denudation of seagrass-covered tidal mudflats would lead to increased efflux of suspended solids, phosphorus and silicate from sediments to overlying water.     

Nitrite reduces the effects of HOCl on unsaturated phosphatidylcholines--a MALDI-TOF MS study

The concentration of nitrite (NO₂⁻) increases under inflammatory conditions. However, the physiological role of nitrite is so far controversial discussed: it was reported that effects of HOCl (an important inflammation mediator) on phospholipids (PL) may be enhanced but also reduced in the presence of nitrite.

In this paper a simple model system was used: unsaturated phosphatidylcholine (PC) vesicles were treated with HOCl in the presence of varying NaNO₂ concentrations and the yield of reaction products was determined by MALDI-TOF MS: the extent of chlorohydrin generation was significantly reduced in the presence of NaNO₂ because HOCl is consumed by the oxidation of NO₂⁻ to NO₃⁻.

Similar results were obtained when HOCl was generated by the myeloperoxidase (MPO)/H₂O₂/Cl⁻ system or the experiments were carried out in the presence of a simple peptide. It is concluded that the transient products of the reaction between HOCl and NO₂⁻ do not have a sufficient reactivity to modify PL.     

Inhibition of human surfactant protein A function by oxidation intermediates of nitrite

Nitration of protein tyrosine residues by peroxynitrite (ONOO⁻) has been implicated in a variety of inflammatory diseases such as acute respiratory distress syndrome (ARDS). Pulmonary surfactant protein A (SP-A) has multiple functions including host defense. We report here that a mixture of hypochlorous acid (HOCl) and nitrite (NO₂⁻) induces nitration, oxidation, and chlorination of tyrosine residues in human SP-A and inhibits SP-A’s ability to aggregate lipids and bind mannose. Nitration and oxidation of SP-A was not altered by the presence of lipids, suggesting that proteins are preferred targets in lipid-rich mixtures such as pulmonary surfactant. Moreover, both horseradish peroxidase and myeloperoxidase (MPO) can utilize NO₂⁻ and hydrogen peroxide (H₂O₂) as substrates to catalyze tyrosine nitration in SP-A and inhibit its lipid aggregation function. SP-A nitration and oxidation by MPO is markedly enhanced in the presence of physiological concentrations of Cl⁻ and the lipid aggregation function of SP-A is completely abolished. Collectively, our results suggest that MPO released by activated neutrophils during inflammation utilizes physiological or pathological levels of NO₂⁻ to nitrate proteins, and may provide an additional mechanism in addition to ONOO⁻ formation, for tissue injury in ARDS and other inflammatory diseases associated with upregulated ^•NO and oxidant production.     

Statistical media optimization for cell growth and azadirachtin production in Azadirachta indica (A. Juss) suspension cultures

Azadirachtin is one of the most widely used biopesticide originating from a plant source. Its production from plant cell cultivation was viewed to overcome constraints associated with its regular supply from the seed kernels. In order to select the effective carbon and nitrogen source, different concentrations of carbon (sucrose/glucose) and nitrogen (NO₃⁻/NH₄⁺ ratio) were studied in A. indica suspension culture. Glucose turned out to be a better carbon source over sucrose yielding high biomass (6.32 g/L) and azadirachtin (11.12 mg/L) content. Nitrate alone as nitrogen source was favorable for both biomass and azadirachtin accumulation. Plackett–Burman design was adopted to select the most important nutrients influencing the growth and azadirachtin accumulation in suspension culture. After identifying effective nutrients, Central Composite Design (CCD) was used to develop mathematical model equations, study responses and establish the optimum concentrations of the key nutrients for higher growth and azadirachtin production. A maximum of 15.02 g/L biomass and 2.98 mg/g azadirachtin was produced using optimum nutrient concentrations representing 99 and 96% validity of the model prediction with respect to biomass and azadirachtin, respectively. This optimized media can be used for cultivation of A. indica cells in bioreactor for mass production of azadirachtin.     

Particulate formate oxidase from Nitrobacter agilis

1. The nitrite oxidase particles obtained by sonic oscillation of Nitrobacter agilis cells also possessed appreciable formate oxidase activity, ranging from about 25 to 50% of the nitrite oxidase activity depending upon the N. agilis strain. Both activities distributed themselves in the same pattern and proportions during differential centrifugation, and resided solely in the pellet resulting from high-speed centrifugation.

2. Difference spectra of formate-reduced particles or intact cells demonstrated the presence of cytochromes of the c- and a-types like those of the NO₂⁻-reduced material. Under anaerobic conditions NO₃⁻ or fumarate acted as an alternate electron acceptor in place of O₂ in formate oxidation. Under aerobic conditions increasing NO₃⁻ concentrations resulted in (a) an increased role of NO₃⁻ as a terminal electron acceptor compared to O₂, (b) a greater total enzymatic transfer of electrons from formate than if O₂ were the sole electron acceptor, and (c) a partial inhibition of O₂ uptake suggestive of a competition for electrons by the two acceptors. The formate oxidase system failed to catalyze consistently the transfer of electrons to either added mammalian cytochrome c or Fe(CN)₆³⁻. The marked sensitivity of the system to certain inhibitors implicated cytochrome oxidase as an integral part of the formate oxidase. The system was also inhibited significantly by a variety of chelating agents, indicating a metal component in the formate dehydrogenase or early portion of the electron transfer sequence.

3. The stoichiometry of the formate oxidase system was shown to approach the theoretical value of 2 moles of CO₂ evolved per mole of O₂ or per 2 moles of formate consumed.

4. To a limited extent, phosphorylation occurred concomittantly with the oxidation of formate in the presence of the cell-free particulate system.     

Characterization of the particulate nitrite oxidase and its component activities from the chemoautotroph Nitrobacter agilis

1. Difference spectra, at room and liquid N₂ temperatures, of S₂O₄²⁻-, and NO₂⁻-reduced intact cells and cell-free preparations of Nitrobacter agilis demonstrated the presence of cytochromes of the c- and a-types. Reduction of cytochromes by succinate, and to a limited extent, by NADPH also occurred, provided KCN (0.1 mM) was also present.

2. A particulate, heat-labile nitrite oxidase having an absolute requirement for O₂ was prepared from N. agilis cells using sonic oscillation and differential centrifugation. The particles also possessed NADH oxidase, succinoxidase, formate oxidase and traces of NADPH oxidase activity. The stoichiometry of the nitrite oxidase reaction approached the theoretical value of 2 moles of NO₂⁻ consumed per mole of O₂ consumed. The pH optimum of the nitrite oxidase system shifted to progressively more alkaline values as the NO₂⁻ concentration was increased, changing from a pH value of 6.8 at 0.6 mM KNO₂ to pH 8.0 at 0.01 M KNO₂ with apparent K_m's of 0.2 and 1.2 mM NO₂⁻, respectively. Computations of the HNO₂ concentrations present under the above conditions showed an approx. 500-fold greater affinity for HNO₂ which was independent of pH, suggesting the involvement of HNO₂ as both a substrate and an inhibitor (at higher concentrations) of the nitrite oxidase system. The marked inhibition by NaN₃, NaCN and Na₂S, as well the light-reversible inhibition by CO, indicated the presence of cytochrome oxidase which was subsequently characterized. NO₂⁻ proved to be a competitive inhibitor of the nitrite oxidase system.

3. The particulate preparation also possessed a heat-labile nitrite-cytochrome c reductase activity which was energy independent and routinely measured under anaerobic conditions. As in the case of nitrite oxidase, the affinity of the enzyme for NO₃⁻ increased as the pH was lowered, but the pH optimum remained unaffected. In terms of calculated HNO₂ concentration an approximately constant K_m of about 0.2 μM was estimated at the several pH's examined. The inhibition by NO₃⁻ was shown to be competitive. The marked sensitivity of the reductase to several metal-binding agents implicated a metal component in the electron transport chain at the site prior to cytochrome c.

4. The membrane-like composition of the nitrite oxidase system is indicated.     

Effects of 15N application frequency on nitrogen uptake efficiency in citrus trees

Two irrigation systems were used to compare nitrogen uptake efficiency in citrus trees and to evaluate the NO₃⁻ runoff in «Navelina» orange trees [Citrus sinensis (L.) Osbeck] on Carrizo citrange rootstock (Citrus sinensis × Poncirus trifoliata Raf.). These were fertilized with 125 g N as labelled K¹⁵NO₃ and grown outdoors in containers filled with a sand-loamy soil. Two groups of 3 trees received this N dose either in five equally split applications by a flooding irrigation system or in 66 applications by drip. Trees were harvested at the end of the vegetative cycle (December) and the isotopic ratios of ¹⁵N/¹⁴N were measured in the soil-plant system. The N uptake efficiency of the whole tree was higher with drip irrigation (75 percnt;) than with flooding system (64 percnt;). In the 0-90 cm soil profile, the N immobilized in the organic fraction was similar for both irrigation methods (around 13 percnt;), whereas the N retained as NO₃⁻ was 1 percnt; of the N applied under drip and 10 percnt; under flooding. In the last case, most of NO₃⁻ remained under root system and it could be lost to leaching either by heavy rainfalls or excessive water applications. These results showed that a drip irrigation system was more efficient for improving water use and N uptake from fertilizer, in addition to potentially reduced leaching losses.     

Effects of external carbon source and empty bed contact time on simultaneous heterotrophic and sulfur-utilizing autotrophic denitrification

The effects of external carbon source (both type and concentration) and empty bed contact time (EBCT) on denitrification efficiency during simultaneous heterotrophic and sulfur-utilizing autotrophic denitrification were evaluated. Continuous experiments were conducted with up-flow mode sulfur packed bed reactors (SPBRs) fed with nitrified leachate containing 700–900 mg/l NO₃⁻–N. The fraction of NO₃⁻–N removed by heterotrophic denitrification (HDNR_fraction) for alkalinity production to balance the alkalinity consumption by autotrophic denitrification varied with the type of external carbon source. When methanol or sodium acetate was added at HDNR_fraction values of 60 and 44%, respectively, 100% denitrification was achieved without alkalinity addition. However, glucose required a HDNR_fraction value of 70% for complete denitrification and denitrification was not complete when molasses was used at a HDNR_fraction value of 70%. The EBCT and volumetric loading rate at which 100% denitrification efficiency could be achieved were 6.76 h and 2.84 kg NO₃⁻–N/m³ day, respectively. The maximum nitrogen removal rate was 5.05 kg NO₃⁻–N/m³ day observed with 89% removal efficiency. At short HRT, a clogging problem was observed near the bottom of the SPBR with excess growth of heterotrophic denitrifiers and gas accumulation within the pores of the SPBR. This problem may be eliminated by back-washing or by separating heterotrophic denitrification from sulfur-utilizing denitrification.     

Relative balance of the cost and benefit associated with carnivory in the tropical Utricularia foliosa

Investment by bladderwort (Utricularia foliosa) in carnivory, in terms of total C and N of bladders per leaf, was estimated in places with different nutrient concentrations from the Yahuarcaca Creek in the Colombian Amazon. The aims were to determine whether nutrient limiting conditions stimulate the investment in carnivory, and the relative balance between C and N invested in carnivory versus C and N obtained from prey. There were no significant differences either for phosphate (PO₄³⁻) concentration or for ammonia (NH₄⁺) concentration among five sampling areas, along approximately 5 km long stretch of the creek, with a pooled mean ± S.D. of 0.19 ± 0.06 and 8.6 ± 3.0 μM, respectively. However, there were significant differences in the nitrate (NO₃⁻) concentration ranging from 0.6 to 2.5 μM. Total C and N of bladders per leaf increased with decreasing NO₃⁻. This corroborates the hypotheses that the carnivorous plant U. foliosa optimises its investment in carnivory according to nutrient availability in the water, and that N is a limiting factor that stimulates the investment in carnivory. The numbers of prey per bladder were also higher under NO₃⁻ limitation, thus enhancing the input of nutrients toward the plant through the bladders. The ratio of total C of prey captured/total C invested in bladders was always lower than 1. However, the efficiency of N was higher since when NO₃⁻ concentration was lower than 1 μM, the ratio of total N of prey captured/total N invested in bladders ranged between 0.97 and 1.67.     

Simultaneous production of two different gel-forming exopolysaccharides by an Alteromonas strain originating from deep sea hydrothermal vents

Exopolysaccharide production by the marine bacterium Alteromonas sp. strain 1644 was shown to be stimulated by restricted growth conditions and was optimized in nitrogen limited fed-batch cultures. Exopolysaccharides were either partly secreted in the medium or stayed firmly cell-associated. The cell-polysaccharide associations could be destroyed by dialysis against distilled water, allowing polysaccharide purification. The chemical and rheological characterization of this last polysaccharide showed that it was different from the secreted polysaccharide that has been previously described (polysaccharide 1644). At low ionic concentration (below 0.03 M whatever the nature of the ions), solutions of this new polysaccharide had very low viscosities. However, at higher ionic concentration, it formed a gel or exhibited in solution at low polymer concentration an unusually high temperature dependent viscosity. This behaviour was also dependent on the nature of the ions and the following sequences for cations and anions were NH4 ⁺ > Mg²⁺ > Na ⁺ > Li⁺ > K⁺ > TMA⁺ and Br⁻ > NO₃⁻ > SO₄²⁻ > Cl⁻ > I⁻ respectively.     

环青海湖地区天然草地和退耕恢复草地植物群落生物量对氮、磷添加的响应

植物群落生物量反映了植被的初级生产能力, 是陆地生态系统碳(C)输入的最主要来源, 往往受到自然界中氮(N)、磷(P)元素供应的限制。该试验以青藏高原环青海湖地区的高寒草原为研究对象, 探讨了天然草地和退耕恢复草地植被群落生物量对N (10 g·m^-2)、P (5 g·m^-2)养分添加的响应。N、P添加显著增加了天然草地禾草的生物量, 进而促使地上总生物量显著提高。退耕恢复草地禾草和杂类草的生物量对N添加均有一致的正响应, 从而促使地上总生物量显著增加174%, 群落地上和地下总生物量显著增加34%; 而P添加对恢复草地生物量各项参数均无显著影响。回归分析显示: 天然草地植物群落地上生物量随土壤中NO₃^--N含量的增加而增加(p < 0.05), 退耕恢复草地植被地上、地下和总生物量均与土壤NO₃^--N含量显著正相关(p < 0.01), 说明环湖地区高寒草原植物生长主要受N供应的限制, P的限制作用随土地利用方式的转变和群落演替阶段的不同而变化; 相比天然草地, 恢复草地在现阶段植被初级生产力受N的限制作用更强烈, 土壤中可利用N含量是限制其植被自然恢复和重建的关键因子。     

Responses of plant community biomass to nitrogen and phosphorus additions in natural and restored grasslands around Qinghai Lake Basin

Aims Plant biomass reflects the primary productivity of community vegetation, and is the main resource of carbon input in the terrestrial ecosystem. It is usually limited by nitrogen (N) and phosphorus (P) availability in the soil. Alpine grassland around Qinghai Lake Basin has experienced extensive land-use changes due to the cultivation of native grassland and vegetation recovery on cropped land. In this experiment, two grassland types were chosen, natural alpine grassland (NG) and its adjacent restored grassland (RG), to determine the responses of plant community biomass to N and P additions with different land-use. Methods NH₄NO₃ and Ca(H₂PO₄)₂·H₂O were added in a completely randomized block design, with medium levels of 10 g N·m^-2 and 5 g P·m^-2. Soil NO₃^--N and available P contents, and the plant community biomass were measured in the two grasslands. Two-way ANOVA was used to determine the effects of nutrient additions on all measured indicators, and regression analysis was used to analyze the correlations between plant biomass and soil NO₃^--N and available P contents.Important findings Results showed: (1) N and P additions both increased grass biomass in the NG, and significantly elevated the total aboveground biomass, with the promoting effect of N addition higher than that of P addition; N addition significantly increased both grass and forb biomass in the RG, and markedly promoted the total aboveground biomass, while P addition had no effects on the functional groups and total aboveground biomass (p > 0.05). (2) N and P additions both had no effects on the belowground and total biomass in the NG, whereas N addition significantly increased the total biomass by 34% in the RG, which suggested that the effect of N limitation on the vegetation primary productivity was stronger in the RG at present stage. (3) The aboveground biomass in the NG increased with soil NO₃^--N content (p < 0.05), and the above- and below-ground as well as the total biomass were all positively correlated with soil NO₃^--N content in the RG (p < 0.01). These results indicated that the plant growth in alpine grassland around Qinghai Lake Basin was prone to N limitation, and the effect of P limitation changed with land-use. Soil available N might be the key limiting factor for vegetation restoration and reconstruction in the RG. The “Grain for Green” project (the land-use policy) and atmospheric N deposition are benefiting both plant growth and C accumulation in the alpine grassland ecosystem around Qinghai Lake Basin.