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991.
A determination was made of the kinds and numbers of bacteria surviving when two soils were maintained in the laboratory under dry conditions for more than half a year. Certain non-spore-forming bacteria were found to survive in the dry condition for long periods. A higher percentage of drought-tolerant than drought-sensitive bacteria was able to grow at low water activities. When they were grown in media with high salt concentrations, bacteria generally became more tolerant of prolonged drought and they persisted longer. The percent of cells in a bacterial population that remained viable when exposed to drought stress varied with the stage of growth.  相似文献   
992.
Sandy loam soil was amended with different concentrations of glucose and was incubated at different pO2 levels. Under many conditions of incubation time and treatment, N2 ase activity as determined by 1-h aerobic C2H2 reduction assay (flushed with Ar:O2, 4:1 before assay) was significantly less than that determined by means of ambient assay (carried out at the pO2 of incubation without flushing with Ar:O2, 4:1 before assay). The difference between the N2ase activity in aerobic assay and that in ambient assay increased with decreasing glucose and O2 concentrations imposed during incubation. The inhition in aerobic assays was analogous to O2-induced shut-off of N2ase and amounted to 75 per cent inhibition after incubation at 0.06 atm pO2 of samples amended with 0.75% glucose (w/w). Similar O2 inhibition was observed after amendment with mannitol and with lactate. Times of incubation were chosen such that development of anaerobic N2ase activity was either absent or too low to account for the observed effects of O2 during assay. It was shown that 0.05 atm pC2H2 was adequate for routine 1-h assays of the soil system employed. Individual soil samples could be subjected to repeated 1-h assays (with removal of C2H2 and C2H4 by evacuation after each assay) thus avoiding side-effects of long exposure to C2H2.  相似文献   
993.
During a period of immobilization of nitrate-15N and mineralization of organic N in a sandy-loam, changes were recorded in: (a) the concentration of an added carbon source, glucose-14C: (b) evolution of 14CO2: (c) bacterial populations; (d) distribution and concentration of newly-synthesized, acid-soluble, amino acid-15N; and (e) distribution and activities of several oxidative and hydrolytic enzyme systems.Added glucose-14C was rapidly metabolized by the soil microflora. After 1.5 day's incubation, when only 3.6 per cent of the added glucose was present, 68 per cent of the 14C remained in the soil-microbial system. During this period there was a marked increase in viable bacterial numbers and an almost complete immobilization of nitrate-15N. On continued incubation, microbial metabolites were oxidized at decreasing rates, the more rapid phase corresponding to a period of net decline in the viable bacterial population.Soil was fractionated by a relatively mild procedure into components containing: (a) extractable proteins; (b) extractable amino acids and peptides; (c) particulate material containing microbial cells, cell debris and material bound to larger soil particles; and (d) microbial metabolites mainly bound to soil colloids. Although the total, acid-soluble, amino acid-15N remained relatively constant for about 50 days, there were marked changes in their concentration in different fractions, especially in the extracts and in the fraction containing fine colloidal material. However, the relatively large decline in labelled, acid-soluble, amino acid-15N occurred during the active phase of oxidation of microbial metabolites when little net mineralization of labelled compounds occurred.Increases in enzymic activities generally coincided with increased viable bacterial populations although there were some exceptions, notably casein and benzoyl arginine amide-hydrolysing enzymes. The stabilities of the newly-formed enzymes varied markedly. The greatest relative changes in activity occurred with the casein-hydrolysing enzymes. Their activity reached a maximal value after the main flush of bacterial growth, was short-lived and was to a large extent extractable. The formation and disappearance of this extracellular proteolytic activity coincided approximately with that of a secondary peak of extractable, newly-synthesized, protein-15N. In general however, changes in enzymic activity could not be identified with changes of protein-15N concentrations of the different fractions.  相似文献   
994.
995.
The Phuket, Thung Wa and Huai Pong soils of this study form the Phuket catena and are extensive in Narathiwat province in the southern part of peninsular Thailand where they were studied in the field and sampled. The Phuket soils on the higher-lying positions and the Huai Pong soils on the nearly-level, lower positions, have developed argillic horizons and are Ultisols. The Thung Wa soils, which occur on intermediate positions and receive sediments from upslope, have cambic horizons and are classified as Inceptisols.All soils formed from Late Cretaceous or Early Tertiary granite or from sediments derived frome these granites under a tropical rain forest climate. They contain kaolinite as the predominant clay mineral and are highly leached, with base saturation of less than 35% in their B horizons. Cation exchange capacities are less than 6 mequiv. per 100 g soil and exchange acidity and exchangeable aluminium are high. Field and thin-section studies as well as particle-size analysis indicate considerable clay translocation from A to B horizons in the Phuket and Huai Pong soils and little clay movement in the Thung Wa soils.  相似文献   
996.
A. Warren 《Geoderma》1973,9(1):75-78
The distinction between vegetation arcs and vegetation patterns is discussed. Some patterns are distinguished which are the result of ancient dune-building processes. These occur on an ancient series of aeolian sands.  相似文献   
997.
Measurements of downslope rock-creep, of the slow rate of plant establishment on alpine and subalpine road-cuts, and of the reduction in plant cover and plant production in two alpine plant communities, illustrate the degree of disturbance that can occur. The development of large trails or roads should not be permitted in areas of such active downslope rock movement, because of the natural rate of mass-wasting. If more people are to visit and enjoy the alpine communities, asphalt paths to confine their walking are desirable, for trampling experiments showed that plant cover and plant production were rapidly reduced in even one season of light use. Recovery during the following year was, however, quite rapid—especially in the snow-bank community. Different alpine and subalpine habitats have different sensitivities to disturbance, and any plans for increased human use must incorporate this kind of information.  相似文献   
998.
999.
While various experimental methods may show the number of butterflies in a given area, this, of itself, is no pointer either to general scarcity or to the reverse. Even less can it be taken as being the result of a particular set of outside influences. However, low levels in certain (otherwise plentiful) species may be taken as indicating unsuitable management trends.  相似文献   
1000.
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