Investigation of the permeability of humic molecules using zeta potential measurements

Zeta potential measurements have been performed on colloidal humate in the presence of differing concentrations of sodium ions at pH = 6.0. A series of calculations has been performed in which the radius of the humic colloid was varied until the predicted surface potential was equal to the experimentally determined zeta potential. The results of the calculations showed that as the ionic strength increases, the apparent radius of the colloid decreases. Similar calculations in which a model colloid was treated as a rigid sphere, i.e., the radius of the colloid was kept constant, failed to simulate the observed zeta potential measurements.     

Seasonal leaching and biodegradation of dicamba in turfgrass

The leaching of surface-applied herbicides, such as dicamba (2methoxy-3,6-dichlorobenzoic acid), to ground water is an environmental concern. Seasonal changes in soil temperature and water content, affecting infiltration and biodegradation, may control leaching. The objectives of this study were to (i) investigate the leaching of dicamba applied to turfgrass, (ii) measure the degradation rate of dicamba in soil and thatch in the laboratory under simulated field conditions, and (iii) test the ability of the model EXPRES (containing LEACHM) to simulate the field transport and degradation processes. Four field lysimeters, packed with sandy loam soil and topped with Kentucky bluegrass (Poa pratensis L.) sod, were monitored after receiving three applications (May, September, November) of dicamba. Concentrations of dicamba greater than 1 mg L(-1) were detected in soil water. Although drying of the soil during the summer prevented deep transport, greater leaching occurred in late autumn due to increased infiltration. From the batch experiment, the degradation rate for dicamba in thatch was 5.9 to 8.4 times greater than for soil, with a calculated half-life as low as 5.5 d. Computer modeling indicated that the soil and climatic conditions would influence the effectiveness of greater degradation in thatch for reducing dicamba leaching. In general, EXPRES predictions were similar to observed concentration profiles, though peak dicamba concentrations at the 10-cm depth tended to be higher than predicted in May and November. Differences between predictions and observations are probably a result of minor inaccuracies in the water-flow simulation and the model's inability to modify degradation rates with changing climatic conditions.     

An idiosyncratic assessment of the role of mathematical models in environmental sciences

This paper summarizes a number of studies in two disciplines of particular interest to environmental scientists and managers: academic ecology and academic economics. These studies suggest that too often in the past we have been extremely cavalier in our acceptance and use of some theoretical mathematical models purporting great importance and generality. Of particular concern is that on a number of occasions, data have been presented as substantiation of certain theoretical models while in fact, on closer examination, they are not. In many cases, the use of these formulations have led to unnecessary environmental destruction and illogical economic decisions. I conclude that, although there are many good reasons to continue to formalize our knowledge about systems through explicit model development, we must not continue to confuse mathematical rigor with scientific rigor, and we must subject our theoretical formulations to more rigid tests vis-a-vis nature than we have in the past.     

Vertical and horizontal structure in the picoplankton communities of a coastal upwelling system

The distribution of prokaryotic and eukaryotic picoplankton in the west coast upwelling-region off the South Island of New Zealand was investigated during midwinter (1988) the time of year when several commercially important fish species migrate into the region to breed. Picoplanktonic cells were major contributors to the autotrophic biomass, with > 80% of the particulate nitrogen and 39 to 55% of the total chlorophylla contained in the <2µm size-fraction. The prokaryotic picoplankton concentrations ranged from 6.3 × 10⁵ to 2.1 × 10⁷ cell l^–1, and the eukaryotic picoplankton between 3.9 × 10⁵ to 1.2 × 10⁷ cells l^–1. Picoplankton numbers increased with distance offshore to a maximum of ~ 3.0 × 10⁷ cells l^–1 at ~ 35 km from the coast, and then diminished towards the outer shelf and open ocean. The ratio of prokaryotic to eukaryotic cells varied between 1.01 and 4.71 in the mixed layer. Both groups declined substantially beneath the pycnocline, with no evidence of deep maxima. Prokaryotic cells dominated the planktonic cell concentrations at all but two stations, but eukaryotic cells dominated picoplankton biovolume as a result of their larger average cell size. The prokaryotic to eukaryotic picoplankton cell-number ratios in this system were considerably lower than often recorded elsewhere, and were inversely correlated with nitrate concentration. These observations show that a eukaryoticdominated picoplankton community makes a substantial contribution to autotrophic biomass in this nutrient-rich upwelling system, and may thereby play a major role in the food-web dynamics of this coastal fishery.     

Significance of dredging on sediment denitrification in Meiliang Bay, China： A year long simulation study

An experiment for studying the effects of sediment dredging on denitrification in sediments was carried out through a one-year incubation of undredged (control) and dredged cores in laboratory. Dredging the upper 30 cm of sediment can significantly affect physico-chemical characteristics of sediments. Less degradation of organic matter in the dredged sediments was found during the experiment. Denitrification rates in the sediments were estimated by the acetylene blockage technique, and ranged from 21.6 to 102.7 nmol N2/(g dry weight (dw)·hr) for the undredged sediment and from 6.9 to 26.9 nmol N2/(g dw·hr) for dredged sediments. The denitrification rates in the undredged sediments were markedly higher (p0.05) than those in the dredged sediments throughout the incubation, with the exception of February 2006. The importance of various environmental factors on denitrification was assessed, which indicated that denitrification was regulated by temperature. Nitrate was probably the key factor limiting denitrification in both undredged and dredged sediments. Organic carbon played some role in determining the denitrification rates in the dredged sediments, but not in the undredged sediments. Sediment dredging influenced the mineralization of organic matter and denitrification in the sediment; and therefore changed the pattern of inherent cycling of nitrogen.     

氮沉降的全球化:对于陆地生态系统的意义

人类活动产生的氮(N),包括农田氮肥施肥和矿物燃料燃烧过程中所固定的氮,其来源和分布正在迅速地扩展到全球范围.这部分氮进入到陆地生态系统中,将会促进植物的生长,并产生过量的氮,最终通过淋溶和痕量气体排放的形式,增加了氮的损失.在某种情况下,还会导致物种组成的变化和生态系统功能的衰减.但并非所有的生态系统都对氯沉降具有类似的响应.它们的响应情况,取决于氮沉降的连续性、生态系统的类型,生态系统对氮的需求或保持能力、土地利用的历史、土壤状况、地形条件、气候背景以及氮沉降的速度、时间和类型等因素.我们认为,起重要作用的是:一些对人类影响具有显著意义的条件、一些控制生态系统响应变化的因素以及一些由于资料缺乏而使不确定性增大的地方.