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1.
Dry matter production, net photosynthetic rate, leaf nutrient status and trunk anatomical characteristics of Fagus crenata seedlings grown in brown forest soil acidified by adding H2SO4 solution were investigated. The soil acidification leaded to decreased (Ca+Mg+K)/Al molar ratio in the soil solution. Dry mass per plant of the seedlings grown in the soil treated with H+ at 120 mg·L?1 was significantly reduced compared with the control value at 0 mg·L?1. When net photosynthetic rate was reduced in the seedlings grown in the soil treated with H+ at 120 mg·L?1, the carboxylation efficiency and maximum net photosynthetic rate at saturated CO2-concentration were lower than the control values. The addition of H+ to the soil at 120 mg·L?1iinduced a reduction in the concentration of Ca in the leaf. By contrast, the concentration of Al in the leaf was increased with increasing the amount of H+ added to the soil. The annual ring formed in the seedlings grown in the soil treated with H+ at 120 mg·L?1 was significantly narrower than that at 0 (control), 10, 30, 60 or 90 mg·L?1. Based on the results obtained in the present study, we conclude that Fagus crenata is relatively sensitive to a reduction in the (Ca+Mg+K)/Al molar ratio of soil solution compared with Picea abies.  相似文献   

2.
Abstract

The diurnal net photosynthesis of Ficus benjamina L., cultivar Cleo, was studied at different daylengths (12, 18 and 24 h day?1), photosynthetic photon flux densities (40 and 120 μmol m?2 s?1 PPFD) and CO2 concentrations (350 and 700 μmol mol?1). Net photosynthesis increased to a maximum after 5–6 and 6–7h of light at 12 and 18h day?1photoperiods, respectively, followed by a decrease towards the end of the photoperiod. At a photoperiod of 18 h day?1 similar diurnal curves were found at 350 and 700 μmol mol?1 CO2, and at 40 and 120 μmol m?2 s?1 PPFD. Five days after the photoperiod was changed from 18 to a 24h day?1the diurnal rhythm disappeared. Transpiration followed the same diurnal rhythm as that for photosynthesis. The water-use efficiency was enhanced by raising the CO2 concentration. A decrease in the CO2 concentration from 700 to 350 μmol mol?1after six days at high CO2 first significantly decreased the photosynthesis, but three days later it reached the same level as that at high CO2.  相似文献   

3.
New experiments have been conducted to determine the speciation of dissolved mercury (Hg) over wide pH (1–12) and sulfide concentration ranges (0.5–30 mM) and in the presence of elemental sulfur (S0) or Hg0, conditions that encompass those of near-bottom and pore waters of sediments. Samples containing synthetic red mercuric sulfide (HgS, cinnabar), buffer solution, aliquots of bisulfide (HS?1) solution, and, in special cases, S0 or Hg0 were prepared anaerobically and allowed to equilibrate for several months. Filtered samples were analyzed for pH, total sulfide (ΣS2?), and total mercury [Hg]tot. Plots of [Hg]tot values vs. pH at varying ΣS2? verified the formation of three previously known mercury-sulfide complexes (HgS2Hn n?2) and revealed that a new Hg2SOH+ complex is important at low pH and low ΣS2?. Our constants for ionic strength (I) 0.7 and 250 C are as follows: K1=10?5.76(+0.71, ?1.02) for HgScinn+H2S ? HgS2H2 0; K2=10?4.82(+0.72, ?1.10) for HgScinn+HS? ? HgS2H?; K3=10?13.41(+0.76, ?0.93) for HgScinn+HS? ? HgS2 2?+H+; K4=10?8.36(+0.71, ?0.93) for 2HgScinn+H++H2O ? Hg2SOH++H2S. With decreasing pH, below 1, Hg solubility decreased sharply, indicating the formation of a new solid phase, inferred to be corderoite (Hg3S2Cl2). From our solubility data, we calculated the free energy of formation (ΔGf o) of Hg3S2Cl2 to be ?396 (+3, ?11) kJ/mol. In experiments where excess S0(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K5=10?1.99(+0.69, ?1.27) for 2HgScinn+2HS? + nS0 ? Hg3S4 IISn oH2 2?. Data from experiments where Hg0(aq) was added confirmed the reversibility of HgS dissolution. An application of our mercury-sulfide speciation model to a natural anoxic basin, Saanich Inlet, British Columbia, is discussed.  相似文献   

4.
Rain water at two forested sites in Guangzhou (south China) show high concentrations of SO4 2?, NO3 ? and Ca2+ and display a remarkable seasonal variation, with acid rain being more important during the spring and summer than during the autumn and winter. The amount of acid rain represents about 95% of total precipitation. The sources of pollutants from which acid rain developed includes both locally derived and long-middle distance transferred atmosphere pollutants. The seasonal variation in precipitation chemistry was largely related to the increasing neutralizing capacity of base cations in rainwater in winter. Soil acidification is highlighted by high H+ and Al3+ concentrations in soil solutions. The variation in elemental concentration in soil solution was related to nitrification (H+, NH4 + and NO3 ?) and cation exchange reaction (H+, Al3+) in soil. The negative effect of soil acidification is partly dampened by substantial deposition of base cations (Ca2+, Mg2+ and K+) in this area.  相似文献   

5.
Sandy loam soil, with added glucose, was incubated anaerobically under N2 and subjected to repeated 1-h C2H2 reduction assays. In the presence of 1% glucose the addition of 50 μg NH4+ ?N/g or of 20 μg NO?3 N/g (untreated soil contained 1.2 μg NH+4?N and 7.10 μg NO?3-N/g) caused at least some suppression of nitrogenase activity. Activity developed when the KCl-extractable soil inorganic nitrogen concentration dropped below 35 μg/g. In the presence of 0.1 or 0.05% glucose the addition of 5 μg NH+4?N/g caused some suppression of nitrogenase activity. However, activity developed when the soil NH4+-N concentration dropped below about 4 μg/g. With 0.1% glucose and 5 μg added NO?2 N/g, activity did not develop until the soil NO?2 -N concentration dropped to zero. Added NO?3 N was rapidly reduced and denitrified to NO?2- N, N2O-N and NH+4 N and furthermore caused some inhibition of CO2 evolution. The data from NH4?-addition experiments are consistent with a nitrogenase repression/ derepression threshold of 4 and 35μg NH+4-N/g at 0.05 and 1% glucose concentrations, respectively. The data from NO?2- and NO?3-addition experiments suggest a combination of repression and toxicity effects in the presence of added NO?3 N.  相似文献   

6.
Production of surface casts and the removal of plant litter from the soil surface by earth-worms had similar seasonal variations, with maximum values in May and minimum values occurring in July and August. Seasonal variations in the total nitrogen (TN) and oxidizable carbon contents of casts were closely related to variations in litter production. The C:N ratio of casts (10.7) was consistently smaller than that of underlying soil material (15.0 and 14.2 for the 0–5 and 18–22 cm depths, respectively), which is probably due to the mineralization of plant-derived organic material during passage through earthworms and utilization of low C:N ratio litter. Seasonal changes in the amounts of inorganic N forms in casts showed a build-up of NH4+N in the cooler winter months (July and August), attaining a maximum of 112 μg.g?1. with a decrease in autumn (April and May) and early spring (September and October), reaching a minimum of 54 μg.g?1. The opposite trend existed for seasonal variations in the NO3?-N content of the casts. Because only minor fluctuations in the amounts of N forms were obtained for underlying soil material during the casting period, the more dramatic changes observed in the casts could not be explained by soil variations. Seasonal variations in urease enzyme activity, associated with fluctuations in organic matter content, were more important than the effect of temperature on enzyme activity in accounting for seasonal variations in the NH4+-N content of casts. It was calculated that 73% of the TN content of litter removed from the surface by earthworms was accumulated in casts, indicating both the importance of earthworms in incorporating litter N with soil material and the inefficiency of N digestion by earthworms.  相似文献   

7.
Fogwater, fog drip and rainwater chemistry were examined at a tropical seasonal rain forest in Xishuangbanna, southwest China between November 2001 and October 2002. During the period of observation, 204 days with the occurrence of radiation fog were observed and the total duration of fog was 1949 h, of which 1618 h occurred in the dry season (November to April), accounting for 37.0% of the time during the season. The mean pH of fogwater, fog drip and rainwater were 6.78, 7.30, and 6.13, respectively. The ion with the highest concentration for fog- and rainwater was HCO3 ?, which amounted to 85.2 and 37.3 μeq l?1, followed by Ca2+, Mg2+ and NH4 +. Concentrations of NO3 ?, HCO3 ?, NH4 +, Ca2+, and K+ in fogwater samples collected in the dry season were significantly greater when compared to those collected in the rainy season. It was found that the ionic concentrations in fog drip were higher than those in fogwater, except for NH4 + and H+, which was attributed to the washout of the soil- and ash-oriented ions deposited on the leaves and the alkaline ionic emissions by the leaves, since biomass burns are very common in the region and nearby road was widening.  相似文献   

8.
Reduction of Cr(VI) by soil humic acids   总被引：1，自引：0，他引：1  
The rate of hexavalent chromium reduction by a soil humic acid (SHA) was investigated in aqueous solutions where concentrations of Cr(VI), H+, and SHA were independently varied. Rate experiments were done with a large excess of SHA over Cr(VI). Rates of reduction depend strongly on [H+], increasing with decreasing pH. Typical Cr(VI)-SHA reactions display a nonlinear reduction of Cr(VI) with time that cannot be modelled with simple first- or second-order rate equations. An empirical rate equation is developed for Cr(VI)-soil humic acid reactions over a range of experimental conditions. The model is in part based on a reactive continuum concept developed for soil fulvic acids. The rate equation describing Cr(VI) reduction by SHA is: R= -(k0+k[H+]1/2)[HCrO4?]1/2Xe?1, where k0 is (8·3 ± 1·2) × 10?12, s?1k is (2·04 ± 0·05) × 10?9 l1/2 mol?1/2 s?1, and Xe is the equivalent fraction of SHA oxidized. The rate equation adequately models Cr(VI) reduction in an experiment with [Cr(VI)]0 four times greater than the maximum concentration used in its derivation. Cr(VI) reduction at pH 3 by two other SHAs can also be modelled using the rate equation. The difference between the rate coefficients for the humic acid and the fulvic acid from the same soil was greater than the difference in the rate coefficients for humic acids from different soils.  相似文献   

9.
Odors, such as the malodorous and toxic hydrogen sulfide (H2S), are released during leachate collection, storage, and treatment. A full-scale biofilter was applied to treat H2S emitted from a leachate equalization basin in a sanitary landfill site. The inlet concentration of H2S was 26.3?C213.0?mg?m?3. In steady state, total removal efficiency was over 90?% in summer and over 80?% in winter. The maximum elimination capacity achieved 9.1?g?m?3?h?1 at a loading rate of 10.5?g?m?3?h?1. The biofilter was effective at reducing H2S. Factors on the level of H2S inlet concentration and performance of the biofilter were investigated. The H2S inlet load and removal efficiency relied on ambient and biofilter temperature, respectively. The water containing rate and relative humidity presented seasonal variation, according to which the interval period of irrigation could be optimized. The main product of H2S degradation was sulfate, and sulfur also could be observed from the biofilter. Spatial and temporal shifts in bacterial community composition in the biofilter were determined by polymerase chain reaction-denaturing gradient gel electrophoresis followed by DNA sequence analysis. The present study revealed a correlation between biofilter performance and bacterial community structure, especially in a real application case.  相似文献   

10.
The effect of nitrogen (30 and 120 mg N per cuvette) on photosynthetic rate of four cultivars of triticale (‘Bolero’, ‘Grado’, ‘Largo’, and ‘Lasko’) grown 14 days in phytotron was strongly modified by water content (75, 45 and 35% of full water capacity). For plants grown under 35% of full water capacity, it was higher when they were grown under 30 than under 120 mg N/cuvette (9.88 and 8.76 μmol CO2 m?2 s?1, respectively) but for plants grown under 45 and 75% of full water capacity there were not significant differences. Transpiration, stomatal conductance, photosynthetic water use efficiency, and internal water use efficiency were not influenced by nitrogen doses independently of water content. Photosynthetic rate, transpiration, stomatal conductance, photosynthetic water use efficiency, and dry matter of studied cultivars of triticale grown under 45 and 35% of full water capacity and both nitrogen doses were lower than for plants grown under 75% of full water capacity. With lowering of water content stomatal conductance was decreasing similarly as photosynthetic rate e.g. for plants grown under 35% of full water capacity as compared with those grown under 75% of full water capacity average stomatal conductance decreased from 0.209 to 0.138 mol H2O m?2 s?1 and photosynthetic rate from 13.69 to 9.32 μmol CO2 m?2 s?1 and as a result there were not significant differences in internal water use efficiency for all studied combinations (67.09 μmol CO2 mol?1 H2O) which shows that stomatal factors were mainly responsible for changes of photosynthetic rate. With lowering of water content from 75 to 35% of full water capacity the decrease of photosynthetic rate and stomatal conductance was much higher than the decreases of transpiration (from 3.57 to 3.02 mmol H2O m?2 s?1) what shows not direct dependence of transpiration on stomatal conductance (water use efficiency decreased from 3.87 to 3.10 μmol CO2 mmol?1 H2O). The effect of nitrogen on dry matter production was strongly modified by water availability e.g. for plants grown under 35% of full water capacity, dry matter was similarly independent of nitrogen dose but for plants grown under 45 and 75% of full water capacity dry matter was significantly higher than when they were grown under 120 (79.05 and 86.75 mg, respectively) or with 30 mg N/cuvette (74.03 and 80.30 mg, respectively).  相似文献   

11.
The diurnal and seasonal variations of soil respiration (SR) were studied at a subtropical mangrove wetland in the Jiulong River Estuary from May 2010 to April 2011.SR rates were measured continuously from 08:00 to 06:00 local time (24-h time system) on July8–9 and October 3–4,2010;and January 15–16 and April 11–12,2011.Similar patterns in the diurnal variation of SR were observed on October 2–3 and April 11–12,with the maximum values at 14:00 and the minimum at 00:00.However,the diurnal dynamics of SR on July 8–9,2010 and January 15–16,2011 showed diferent patterns,with the maximum values at 08:00–10:00 on above sampling dates and the minimum at 22:00 on July 8 and at 04:00 on January 16.The daily mean values of SR approximated to the values measured at 08:00.SR fluctuated with distinct seasonal patterns.The seasonal variation was characterized by a mono-peak pattern,with the highest rate (6.18μmol CO2m-2s-1) in July and the lowest rate (0.36μmol CO2m-2s-1) in December.The results showed that the variation of SR in mangrove wetland was mainly controlled by soil temperature,and there was no significant correlation between SR and soil water content.It also implied that the model of SR in mangrove wetland should not only consider the efect of soil temperature,but also incorporate other factors,such as water level,precipitation,microbial activity and photosynthesis,which also could affect SR.  相似文献   

12.
Total P concentrations, chlorophyll concentrations, and phytoplankton production were investigated bi-weekly in Tibbs Run Lake, Monongalia County, West Virginia, from March 1977 to March 1978. Mean H+ concentration in the lake was 25.1 μeq 1?1 (pH 4.6). The acidic condition of the lake is attributed to inputs of acid via precipitation (mean H+ concentration of the bulk precipitation was 79 μeq 1?1, pH 4.1), and the low buffering capacity of the watershed (bedrock composition of sandstone). Effect of the watershed is shown by the net retention of imput of P (ca. 26%) and H+ (ca. 68%). Total P loading to the lake was 0.495 g P m?2 yr?1. The single inflow accounted for 95% of the total loading while bulk precipitation accounted for the remainder. Mean summer chlorophyll concentration was 22.2 mg m?2. Phytoplankton production expressed volumetrically as aP-vol-x value was 9.78 mg C m?3 h?1. Regression analysis indicated that H+ do not affect chlorophyll concentrations or phytoplankton production but rather that P limits algal biomass. Trophic status of Tibbs Run Lake based on a P budget model, chlorophyll concentration, and volumetric production all indicate that the lake is meso-eutrophic.  相似文献   

13.
Sodium chloride, at rates up to 100 mg g?1, was added to a Sassafras sandy loam amended with finely-ground alfalfa to determine the effect of NaCl on CO2 evolution, ammonification, and nitrification in a 14-week study. A NaCl concentration of 0.25 mg g?1 significantly reduced CO2 evolution by 16% in unamended soil and 5% in alfalfa-amended soil. Increasing NaCl progressively reduced CO2 evolution, with no CO2 evolved from the soil receiving 100 mg NaCl g?1. A 0.50 mg NaCl g?1 rate was required before a significant reduction in decomposition of the alfalfa occurred. The NO?2-N + NO?3-N content of the soil was significantly reduced from 40 to 37 μg g?1 at 0 and 0.25 mg NaCl g?1, respectively in the unamended soil. In the alfalfa amended soil, nitrification was significantly reduced at 5 mg NaCl g?1. At 10 mg NaCl g?1, nitrification was completely inhibited, there being only 6 and 2 μg NO?2-N + NO?3-N g?1 in the alfalfa amended and unamended soil, respectively. In the alfalfa amended soil NH+4-N accumulated from 6 μg g?1 at the 0 NaCl rate to a maximum of 54 μg g?1 with 25 mg NaCl g?1. These higher NH+4-N values resulted in a 0.5 unit increase in the pHw over that of the 0 NaCl rate in the alfalfa amended soil. At NaCl concentrations above 25 mg g?1 there was a reduction in NH+4-N. The addition of alfalfa to the soil helped to alleviate the adverse affects of NaCl on CO2 evolution and nitrification.  相似文献   

14.
《Journal of plant nutrition》2013,36(10):1561-1573
The interactions between NaCl and different NO3 ?NH4 + ratios were investigated. Tomato plants (Lycopersicon esculentum Mill.) were grown in a greenhouse, in 120L capacity containers filled with continuously aerated Hoagland nutrient solution. Treatments were added to observe the combined effect of two NaCl levels (30 and 60 mM) and three millimolar ratios of nitrate: ammonium (14:0, 12:2, 10:4) on growth, nutrition, and contents of chlorophyll and sugars. Saline treatments decreased growth, which was partly restored by NH4 + treatment. The leaf mineral composition showed a marked effect of nitrogen (N) source, while salinity only affected NO3 ? concentration. Changing the NO3 ?:NH4 + ratio from 14:0 to 12:2 and 10:4 produced progressive increases in the concentrations of iron (Fe), chlorophyll, and reducing sugars in leaves. Therefore, the deleterious effect of salinity on biomass production can be minimized by the use of nutrient solutions containing higher NH4 + concentrations, since this seemed to be correlated with increases in nitrogen assimilation and the levels of Fe and chlorophyll.  相似文献   

15.
Varying amounts of CaCl2 or MgCl2 were first equilibrated with a dilute suspension of H+-kaolin and then with spiked (high activity radioactive 64Cu) water. After filtration, through 0.45 μm Millipore membrane the amount of radioactivity on the filter and the filtrate was measured separately using a Gamma ray well counter. Adsorption of Cu (expressed as distribution coefficient ?K d ) was plotted against Ca or Mg concentration. K d values decreased sharply at Ca or Mg concentration from 0 to 10 mg l?1 and changed only slightly from 10 to 200 mg l?1. Increase in the concentration of Ca or Mg reduces the Cu binding capacity of the clay.  相似文献   

16.
One-year field measurements were conducted in a Japanese cedar (Cryptomeria japonica) forest, located in Gunma Prefecture, Japan. On the basis of the meteorological and atmospheric concentration data, the dry deposition of SO2, HNO3, NO2 and HCl was estimated using the inferential method. The annual dry deposition of H+ was estimated at 721 eq ha?1yr?1, which was 40% larger than the measured annual wet deposition of H+ (514 eq ha?1yr?1). Therefore, dry deposition is an important pathway for the atmospheric input of H+ to the forest in the study site. The contribution of each gas to the dry deposition of H+ was as follows: SO2, 25%; HNO3, 32%; NO2, 10%; and HCl, 33%. The extremely high contribution of HCl appeared to be caused by the high emission intensity of HCl due to waste incineration in the site region. The differences between estimated deposition and throughfall and stemflow measurements indicated that about 80% of the total deposition of H+ was taken up by the canopy.  相似文献   

17.
Blanket‐bog peats, mapped as the Winter Hill and Crowdy associations by the Soil Survey of England and Wales, are an oceanic manifestation of the ombrotrophic ‘raised‐bog’ (Hochmoor) peats that cover large tracts in the boreal zone of the northern hemisphere. This paper examines monthly analyses from 1992 to 1997 of major ions and other variables from an upland blanket bog in southwest England in relation to seasonality, rainfall, and the chemical composition of rainwater. Average ionic composition of surface water (and peat) integrates variable atmospheric solute inputs over the years. The dominant ions in the surface water, Na+ and Cl–, showed only weak seasonality, but divalent cations a stronger seasonal pattern with a summer maximum. Mean pH ranged from c. 4.4 in February to c. 4.2 in August. Changes in concentration of different cations were closely interlinked by cation exchange. The anion deficit, accounted for by anionic groups on the dissolved organic matter, was strongly seasonal with a summer maximum, as was optical absorbance at 320 nm. Nitrate and NH4+ were both at much smaller concentrations than in rain. Nitrate exceeded 1 μmol l?1 only during cold periods in winter, mainly following drought in the summer of 1995; NH4+ reached a few μmol l?1 only in summer. There was evidence of net retention of S by the peat in wet sites and during wet periods, and of net release of SO42– (and acidity) under dry conditions. The 1995 summer drought and ensuing dry year in 1996 had marked and persistent effects on pH, apparent ion deficit (DEF), SO42–, the divalent cations and Fe.  相似文献   

18.
The effect of the supply of ammonium (NH4 +) and phosphorus (P) in gel on the amounts of hydrogen ion (H+) excreted from plant roots was studied with Brachiaria humidicola (a highly acid‐soil tolerant tropical grass) and B. brizantha (less acid‐soil tolerant) grown in soil in a glasshouse. The H+ production was measured over 24 h in agar gel containing full nutrient solution with a range of NH/‐N levels (0, 0.25, 0.5, and 5.0 mM NH4 +‐N). Highly soluble P, K2HPO4, or relatively insoluble P, rock P, was supplied at four concentrations (0, 11.5, 34.5, or 115 μM p) in the gel. Increasing NH4 + concentration in the gel increased H+ production for both grasses, but there was some inhibition of growth for B. brizantha at the highest N concentration. For B. humidicola, but not B. brizantha H+ production was greater with 34.5 μM K2HPO4 than 11.5 μM K2HPO4. At 34.5 μM P for both grasses there was no difference in H+ production when P was supplied as rock P or K2HPO4. With 11.5 μM P both grasses produced less acid in the gel with the rock P compared with K2HPO4. The reduced H+ production is probably due to a lower availability of P in the rock P compared with K2HPO4. This effect was greater with B. brizantha than B. humidicola, implying that 11.5 μM rock P was not able to supply sufficient P for the growth of B. brizantha. Brachiaria humidicola was able to dissolve more rock P than B. brizantha or alternatively, the growth of B. humidicola was less adversely affected by the low P supply from rock P than B. brizantha. Plant‐induced acidity does not seem to occur as a response to a lack of available P, but rather these grasses only produce acid if there are enough nutrients for growth, i.e., both NH4 + and P. If either N or P is limiting, growth is limited as is NH4 + uptake, so that H+ production is curtailed.  相似文献   

19.
The effects of five environmental factors on variation in the rate of C2H2 reduction in two cowpea (Vigna unguiculata L. Walp.) and two soybean (Glycine max L. Merrill) cultivars were examined at two stages of growth in the field. Diurnal changes in C2H2 reduction, estimated as μmoles C2H4 produced·g?1 nodules·h?1, over a 30 h period were compared with changes in soil, canopy and air temperatures, global radiation, and vapour pressure deficit. Although the environmental factors showed one maximum and one minimum during a diurnal cycle, the C2H2 reduction rate in nodulated cowpea roots showed two peaks: one between 0600–1200 and another between 1800–2400 and two minima: one between 1200–1600 and another between 2400–0600. Variation in C2H2 reduction in soybean nodules did not show any definite pattern. Vapour pressure deficit appeared the most likely factor influencing the decline in C2H2 reduction between 1200–1600.The rate of C2H2 reduction in cowpeas was greater at the pre-flower than at the early pod-fill period; the rate in soybeans was not greatly different at early flower or early pod-fill.  相似文献   

20.
The nitrogenase activity (potential nitrogen fixation) of a mixed white clover-grass sward growing on a range of soils containing up to 216 μg g?1 Cd, 30 000μg g?1 Pb and 20 000 μg g?1 Zn was determined using the acetylene-reduction assay. The plants were incubated in situ using an intact soil-core technique. Little change in the rate of C2H2 reduction was observed during the daylight hours although a marked seasonal fluctuation was found, the maximum activity during the spring and declining to 20% of this by the autumn. Some reductions of nodule and plant size, and in nitrogenase activity, was observed in the most heavily contaminated sites. These effects were small; the plants and nodules otherwise appeared healthy. The potential for nitrogen fixation was equally high in all the sites at up to 80 g N ha?1 h?1 in the spring.  相似文献   

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