土壤硝化作用的抑制剂调控及其机理

既要保证氮肥在优质高效农业生产中的促进作用,又要保证农业和环境的可持续发展,如何提高氮肥利用率、减轻氮肥污染成了解决问题的关键.从氮素在土壤中的生物化学转化过程入手,通过抑制剂的施用来调控氮素的转化,减缓硝化过程的进行,是解决该问题的一种有效措施.本文从硝化抑制剂的定义、筛选硝化抑制剂的条件、国内外较受关注的硝化抑制剂品种以及硝化抑制机理等几个方面对目前的研究进展进行综述,并提出目前研究中存在的问题及今后的研究方向.     

Heterotrophic nitrification among denitrifiers.

Twelve denitrifying bacteria representing six genera were tested for an ability to nitrify pyruvic oxime heterotrophically. Six of these bacteria exhibited appreciable nitrification activity, yielding as much as 5.8 mM nitrite and little or no nitrate when grown in a mineral salts medium containing 7 mM pyruvic oxime and 0.05% yeast extract. Of the six active bacteria, four (Pseudomonas denitrificans, Pseudomonas aeruginosa, and two strains of Pseudomonas fluorescens) could grow on yeast extract but not pyruvic oxime, one (Pseudomonas aureofaciens) could grow slowly on pyruvic oxime, and one (Alcaligenes faecalis) could apparently grow on pyruvic oxime in the presence of yeast extract but not in its absence. Eight of the twelve bacteria in the resting state could oxidize hydroxylamine to nitrite, and P. aureofaciens was remarkably active in this regard. In general, those denitrifiers active in the nitrification of pyruvic oxime or hydroxylamine or both are abundant in soils. A possible advantage of having nitrification and denitrification capabilities in the same organism is discussed.     

Gliclazide mainly affects insulin secretion in second phase of type 2 diabetes mellitus.

We studied the effect of the acute administration of gliclazide at 160 mg on insulin release during hyperglycaemic clamps in 12 type 2 diabetes patients, age 50 +/- 9.0 years, diabetes duration 5.5 +/- 4.8 years, fasting blood glucose 9.6 +/- 2.1 mmol/L (means +/- SD). After a 210 min of hyperinsulinaemic euglycaemic clamp (blood glucose 4.6 +/- 0.14mmol/L), gliclazide or placebo (randomised, double-blind, cross-over) was administered; 60 minutes later, a hyperglycaemic clamp (4hr) at 8mmol/L was started. Plasma C-peptide levels increased significantly after the administration of gliclazide (increment 0.17 +/- 0.15 vs. 0.04 +/- 0.07 nmol/L, p = 0.024) before the clamp. After the start of the hyperglycaemic clamp, the areas under the curve (AUC) for insulin and C-peptide did not differ from 0-10 min (first phase) with gliclazide. However, second-phase insulin release (30-240 min) was markedly enhanced by gliclazide. AUC plasma insulin (30 to 240 min) was statistically significantly higher after gliclazide (12.3 +/- 13.9 vs. -0.56 +/- 9.4 nmol/L x 210 min, p = 0.022); similarly, AUC plasma C-peptide (30 to 240 min) was also higher: 128 +/- 62 vs. 63 +/- 50 nmol/L x 210 min, p = 0.002). In conclusion, in long-standing type 2 diabetes the acute administration of gliclazide significantly enhances second phase insulin release at a moderately elevated blood glucose level. In contrast to previous findings in mildly diabetic subjects, these 12 type 2 diabetes patients who had an inconsiderable first phase insulin release on the placebo day, only showed an insignificant increase in first phase with gliclazide.     

Bacterial nitrification in chloraminated water supplies.

Nitrifying bacteria were detected in 64% of samples collected from five chloraminated water supplies in South Australia and in 20.7% of samples that contained more than 5.0 mg of monochloramine per liter. Laboratory experiments confirmed that nitrifying bacteria are relatively resistant to the disinfectant. Increased numbers of the bacteria were associated with accelerated decays of monochloramine within distribution systems. The combination of increased concentrations of oxidized nitrogen with decreased total chlorine residuals can be used as a rapid indicator of bacterial nitrification.     

Bacterial nitrification in chloraminated water supplies.

Nitrifying bacteria were detected in 64% of samples collected from five chloraminated water supplies in South Australia and in 20.7% of samples that contained more than 5.0 mg of monochloramine per liter. Laboratory experiments confirmed that nitrifying bacteria are relatively resistant to the disinfectant. Increased numbers of the bacteria were associated with accelerated decays of monochloramine within distribution systems. The combination of increased concentrations of oxidized nitrogen with decreased total chlorine residuals can be used as a rapid indicator of bacterial nitrification.     

Modeling the temperature response of nitrification

To model nitrification rates in soils, it is necessary to have equations that accurately describe the effect of environmental variables on nitrification rates. A variety of equations have been used previously to describe the effect of temperature on rates of microbial processes. It is not clear which of these best describes the influence of temperature on nitrification rates in soil. I compared five equations for describing the effects of temperature on nitrification in two soils with very different temperature optima from a California oak woodland-annual grassland. The most appropriate equation depended on the range of temperatures being evaluated. A generalized Poisson density function best described temperature effects on nitrification rates in both soils over the range of 5 to 50 °C; however, the Arrhenius equation best described temperature effects over the narrower range of soil temperatures that normally occurs in the ecosystem (5 to 28 °C). Temperature optima for nitrification in most of the soils were greater than even the highest soil temperatures recorded at the sites. A model accounting for increased maintenance energy requirements at higher temperatures demonstrates how net energy production, rather than the gross energy production from nitrification, is maximized during adaptation by nitrifier populations to soil temperatures.     

Natural and artificial methods for inducing the luteal phase in the koala (Phascolarctos cinereus)

An experiment was conducted in which female koalas were mated for different durations of intromission and ejaculation to confirm that the luteal phase of the oestrous cycle in koalas is induced by the physical act of mating. Results showed that induction of a luteal phase in the koala usually required a complete duration of penile thrusting behaviour from the male. It is proposed that induction of a luteal phase in koalas may involve a copuloceptive reflex, triggered by the thrusting of the male's penis into the female's urogenital sinus. Although interrupted mating in koalas may be used to induce a luteal phase in preparation for an artificial insemination programme, this study showed that there is a 12.5% probability that pregnancy will result from semen prematurely emitted by the teaser male. A dose of 250 iu hCG was administered intramuscularly to eight oestrous females to determine whether it was possible to induce a luteal phase artificially. In contrast to control females, which received sterile saline injections, all females injected with hCG showed a significant increase in progestogen concentration above that of basal values, indicating that a luteal phase had been induced successfully.     

Aggregation of isolated myofibrils stimulated by their contraction. I. Origin of the second phase of optical changes during myofibril contraction

Aggregation of contracted myofibrils was shown to be responsible for spontaneous: slow reduction of optical density of myofibril suspension on the final stage of their contraction. This effect faded with an increase in light wavelength, with increased angle of view of the photocell and diminished sizes of myofibrils.     

Glucose-stimulated Cdc42 signaling is essential for the second phase of insulin secretion

The small Rho family GTPases Cdc42 and Rac1 have each been shown to function in insulin exocytosis and are presumed to function in actin remodeling and insulin granule mobilization. However, whether either GTPase is required for the mobilization phase of insulin release (second phase) and are linked in a common signaling pathway has remained unknown. Here we demonstrate that small interfering RNA-mediated depletion of Cdc42 from isolated islets results in the selective loss of second phase insulin release. Consistent with a role in this nutrient-dependent phase, Cdc42 activation was detected exclusively in response to D-glucose and was unresponsive to KCl or non-metabolizable glucose analogs in MIN6 beta-cells. Cdc42 activation occurred early in secretion (3 min), whereas Rac1 activation required approximately 15-20 min, suggesting Cdc42 as proximal and Rac1 as distal regulators of second-phase secretion. Importantly, Rac1 activation and function was linked in a common pathway downstream of Cdc42; Cdc42 depletion ablated glucose-induced Rac1 activation, and expression of constitutively active Rac1 in Cdc42-depleted cells functionally restored glucose-stimulated insulin secretion. Occurring at a time midway between Cdc42 and Rac1 activations was the phosphorylation of p21-activated-kinase 1 (Pak1), and this phosphorylation event required Cdc42. Moreover, small interfering RNA-mediated Pak1 depletion abolished Rac1 activation and glucose-stimulated insulin release, suggesting that Pak1 may mediate the link between Cdc42 and Rac1 in this pathway. Taken together, these data substantiate the existence of a novel signaling pathway in the islet beta-cell whereby Cdc42 functions as a key proximal transmitter of the glucose signal early in stimulus-secretion coupling to support the later stage of insulin release.     

Effect of chemicals used as nitrification inhibitors on the denitrification process.

Several chemicals used as nitrification inhibitors were tested to determine their effect on dentrification by a Pseudomonas sp. and in soil. Denitrification by the bacterium was suppressed by 2-chloro-6(-trichloromethyl)-pyridine (N-Serve) at a concentration of 50 ppm, while 2,5-dichloroaniline caused the accumulation of nitrite in the culture medium. The nitrification inhibitors had little effect on the denitrifying activity in soil under anaerobic conditions. 2-Sulfanilamidothiazole inhibited denitrification to some extent and samples supplied with potassium azide produced N2O rather than N2 as the predominant gas.     

Optimisation of the second phase of a two phase growth system for anthocyanin accumulation in callus cultures of Rudbeckia hirta

A two-phase growth system composed of modified Schenk-Hildebrandt medium and enriched Miller's medium was used to grow callus of R. hirta L. The second phase of the system has been optimised and the callus synthesised a 12-compound set of anthocyanins, comprising 4.97% of tissue mass (tubular flowers of the plant in nature only contain one compound comprising 0.28%). The beneficial effect of high content of cysteine on anthocyanin accumulation was noted. The predominant element of the anthocyanin set obtained in vitro is cyanidin-3-O-(6-O-malonyl-β-D-glucopyranoside), a relatively stable compound. This revised version was published online in June 2006 with corrections to the Cover Date.