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71.
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73.
We investigated the effect of increased N-supply on productivity and potential litter decay rates of Carex species, which are the dominant vascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-supply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) under conditions of P-limited plant growth, increased N-supply will not affect productivity but it will lead to lower C:N ratios in plant litter and thereby to a higher potential decay rate of that litter. These hypotheses were tested by fertilization experiments (addition of 10 g N m-2 year-1) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry and potential leaf litter decay. In a P-limited peatland, dominated by Carex lasiocarpa, there was no significant increase of net C-fixation by plant biomass upon enhanced N-supply, although N-uptake had increased significantly compared with the unfertilized control. Due to the N-fertilization the C:N ratio in the plant biomass decreased significantly. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The potential decay rate of that litter, measured as CO2-evolution from the litter under aerobic conditions, was significantly increase upon enhanced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply, whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in response to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. This implies that atmospheric N-deposition may affect the CO2-sink function of peatlands, but the effect is dependent on the nature of nutrient limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under conditions of P-limited plant growth, however, the net C-accumulation will decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is increased. As plant growth in most terrestrial ecosystems is N-limited, increased N-supply will in most peatlands lead to an increase of net C-accumulation. 相似文献
74.
Long-term field experiments in Norway spruce stands on fertile sites (site indices 27–35 m) in southwestern Sweden were analysed with respect to volume increment. Three treatments were included (0=No fertilization, N = Fertilization with N, NP = Fertilization with N and P).Volume growth was monitored for 18 years in 10 blocks. No significant differences in annual volume increment between the treatments were detected. Volume increments in the N treatment were 97%, 99% and 107% as high as those in the 0 treatment for the periods 1–5, 6–10 and 11–15 years after the first fertilization. Corresponding values for the NP treatment were 104%, 108% and 110%, indicating that P has a small positive effect.The amount of N-fertilization would correspond to an annual N deposition of 20 kg ha-1 during the next 30 years in southwestern Sweden. For this period, the results imply that this N deposition would not affect the growth of Norway spruce stands on fertile sites. 相似文献
75.
Acid atmospheric deposition can cause losses of metal nutrients from the organic layer of a soil. The size of these losses
depend on the sizes of the different pools in which the metals are present, as these pools differ in mobility. The metal pools
in an organic soil layer of a Douglas fir forest in the Netherlands subjected to acid deposition were determined by means
of extractions and percolations. Na was mainly dissolved and exchangeably adsorbed, K dissolved, exchangeably adsorbed and
present in the soil microbial biomass, Ca exchangeably adsorbed and present in organic precipitates, Mg exchangeably adsorbed
and present in the soil biomass, and Mn exchangeably adsorbed and present in inorganic precipitates. The main part of the
metals was exchangeably adsorbed. The adsorption affinity increased in the order Na < K < Mg < Mn ≈ Ca. The vertical distribution
of the metals in the organic layer showed that all metals were continuously lost from the organic layer. The differences between
the metals in retention and vertical distribution patterns were in agreement with their differences in deposition rate, pool
distribution, and exchange affinity. Since the metals were mainly exchangeably adsorbed, and the acidifying cations dominated
the atmospheric deposition, acid deposition and cation exchange must be processes that strongly affect the losses of metals
from this organic soil layer. R F Huettl Section editor 相似文献
76.
An automatic system was installed for continuous analyses of ozone, sulphur dioxide, nitrogen monoxide and nitrogen dioxide in an experimental orchard with a canopy ofPrunus cerasifera plants in summer 1993. Air samples from three elevations (0.8 m, 1.6 m and 3 m above ground) were sequentially analyzed. Ozone concentrations above the canopy were usually higher than within the canopy; their relationships with stomatal resistance have been investigated. Sulphur dioxide levels were negligible. Nitrogen oxides showed a complex profile, with no particular trend, likely due to a reciprocal exchange between the atmosphere and the ground surface. 相似文献
77.
Summary Synthetic protocols are presented both for D-PheSar and the corresponding cyclised diketopiperazine, prepared from N-t-butoxycarbonylprotected D-PheSar. Deprotection conditions could be manipulated to yield either D-Phenylalanylsarcosine or (R)-1-methyl-3-(phenylmethyl)-2,5-piperazinedione. Molecular modelling revealed several low energy conformers which contained a Z-peptide bond and which were readily amenable to cyclisation. Cyclisation was found by HPLC to be fastest in strongly acidic conditions.Abbreviation HBTU
o-Benzotriazolyl-tetramethyluronium hexafluorophosphate 相似文献
78.
Acid proteinase production by the fungus Humicola lutea 120-5 in continuous culture was studied. The maximum activity of the culture broth reached 2200 U/ml at a dilution rate (D) of 0.05/h. The continuous process was carried out for 1 month without any bacterial contamination, due to low pH (3.0–3.5) during the cultivation. 相似文献
79.
Regio- and stereo-specific nitrile hydrolysis by the nitrile hydratase from Rhodococcus AJ270 总被引:1,自引:0,他引:1
Alan J.Blakey JohnColby EdwinWilliams CatherineO'Reilly 《FEMS microbiology letters》1995,129(1):57-61
Abstract Acid phosphatase activity was measured in individual cells by determining their optical densities through a scanning confocal laser microscope. The naphthol AS-TR (3-hydroxy-2-naphtoic acid 4'-chloro-2'-methylanilide) phosphate-hexazotized para-rosanilin method was used to visualise the acid phosphatase content in the light microscope. Evidence was obtained that the amount of enzyme varied in exponential growth phase cells as the fission age increased. By comparing the acid phosphatase activity with the rate of food vacuole formation, it appeared that the amount of enzyme inside the cells decreased in early clonal life, whereas the rate of food uptake increased. It was assumed that the reduction of acid phosphatase content could lead to a more extended life of vacuoles and to a decreased membrane recycling rate. In turn, the reduced supply of membrane available for food vacuole formation could partly be responsible for the decrease of the food uptake rate observed after the initial increase. 相似文献
80.
Sulfur cycling in forests 总被引:6,自引:5,他引:1
D. W. Johnson 《Biogeochemistry》1984,1(1):29-43
Sulfur is essential for the production of certain amino acids in plants. As amino acid sulfur is the major form of sulfur in trees, there is a strong relationship between organic S and organic N in tree tissue. Sulfur deficiencies occur in parts of southeastern Australia and northwestern North America, remote from pollutant inputs. Since bilogical S requirements of forests are modest (< 5 kg · ha–1 yr–1 for net vegetative increment), however, atmospheric S inputs in polluted regions (10–80 kg · ha–1 yr–1 ) often exceed not only the forest ecosystem S requirement but also its ability to biologically accumulate S. There is some increase in the SO2–
4–S content of forest vegetation in response to elevated atmospheric S inputs, but this capacity is apparently easily saturated. Soil SO2–2
4adsorption is often the dominant feature of S cycling in polluted ecosystems and often accounts for net ecosytem S accumulations.Contribution from a symposium on the role of sulfur in ecosystem processes held August 10, 1983, at the annual meeting of the A.I.B.S., Grand Forks, ND; Myron Mitchell, convenor. 相似文献