摘要:

There is evidence that N concentration in hardwood leaf litter is reduced when plants are raised in an elevated CO2atmosphere. Reductions in the N concentration of leaf litter have been found for tree species raised under elevated CO2, with reduction in N concentration ranging from ca. 50% for sweet chestnut (Castanea sativa) to 19% for sycamore (Acer platanoides). However, the effects of elevated CO2on the chemical composition of litter has been investigated only for a limited number of species. There is also little information on the effects of increased CO2on the quality of root tissues. If we consider, for example, two important European forest ecosystem types, the dominant species investigated for chemical changes are just a few. Thus, there are whole terrestrial ecosystems in which not a single species has been investigated, meaning that the observed effects of a raised CO2level on plant litter actually has a large error source. Few reports present data on the effects of elevated CO2on litter nutrients other than N, which limits our ability to predict the effects of elevated CO2on litter quality and thus on its decomposability. In litter decomposition three separate steps are seen: (i) the initial stages, (ii) the later stages, and (iii) the final stages. The concept of "substrate quality," translated into chemical composition, will thus change between early stages of decomposition and later ones, with a balanced proportion of nutrients (e.g., N, P, S) being required in the early decomposition phase. In the later stages decomposition rates are ruled by lignin degradation and that process is regulated by the availability of certain nutrients (e.g., N, Mn), which act as signals to the lignin-degrading soil microflora. In the final stages the decomposition comes to a stop or may reach an extremely low decomposition rate, so low that asymptotic decomposition values may be estimated and negatively related to N concentrations. Studies on the effects of changes in chemical composition on the decomposability of litter have mainly been made during the early decomposition stages and they generally report decreased litter quality (e.g., increased C/N ratio), resulting in lower decomposition rates for litter raised under elevated CO2as compared with control litter. No reports are found relating chemical changes induced by elevated CO2to litter mass-loss rates in late stages. By most definitions, at these stages litter has turned into humus, and many studies demonstrated that a raising of the N level may suppress humus decomposition rate. It is thus reasonable to speculate that a decrease in N levels in humus would accelerate decomposition and allow it to proceed further. There are no experimental data on the long-term effect of elevated CO2levels, and a decrease in the storage of humus and nutrients could be predicted, at least in temperate and boreal forest systems. Future works on the effects of elevated CO2on litter quality need to include studies of a larger number of nutrients and chemical components, and to cover different stages of decomposition. Additionally, the response of plant litter quality to elevated CO2needs to be investigated under field conditions and at the community level, where possible shifts in community composition (i.e., C3 versus C4 ; N2fixers versus nonfixers) predicted under elevated CO2are taken into account.Key words: climate change, substrate quality, carbon dioxide, plant litter, chemical composition, decomposition.

ISSN:1208-6053    

DOI:10.1139/a97-013

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Boreal peatlands occupy about 1.14 x 106km2in North America. Fires can spread into peatlands, burning the biomass, and if moisture conditions permit, burning into the surface peat. Charred layers in peat sections reveal that historically bogs in the subhumid continental regions and permafrost peatlands of the subarctic regions have been the most susceptible to fires. Fire return periods were estimated from the numbers and ages of the charred peat layers. Based on average moisture conditions of the surface, about 0.5% of the peatlands (6420 km2) can be expected to burn annually, but the surface peat layer is expected to burn only in a small portion of this area (1160 km2). Carbon losses from aboveground combustion, in the form of CO2, CO, CH4, and nonmethane hydrocarbons, are the highest in forested swamps at 2.03 Tg C ·year-1. Carbon losses due to combustion of surface peat is the highest in the driest peatlands (e.g., raised bogs underlain by permafrost) at 5.82 Tg C ·year-1. The total estimated carbon release due to aboveground combustion is 2.92 Tg C ·year-1and due to belowground peat combustion is 6.72 Tg C ·year-1. These estimates of direct carbon emissions to the atmosphere due to wildfires suggest a globally significant, but relatively small source in contrast with emissions from wildfires in uplands. The effects of a possible climate change are expected to be most prominent in the continental and northern parts of North America. A lower water table would result in increased CO2but decreased CH4emissions from the peatlands. A drier climate may mean increased fire frequency and intensity, resulting in more fires in peatlands and an increased probability of the fires consuming part of the peat.Key words: fire, peatlands, carbon, boreal, permafrost, gas flux.

ISSN:1208-6053    

DOI:10.1139/a98-002

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A review and synthesis of experimental decomposition data was performed with the objective of finding parameter values for a decomposition model. Experimental data were retrieved from the literature and included data on mass loss rates, nitrogen mineralization rates, carbon dioxide evolution rates, and growth rates of bacteria and fungi. Environmental variables included in the synthesis were air temperature, soil moisture, and soil acidity (concentration of H+and Al3+in soil solution). The variables were assumed to act as separate, multiplicative rate regulating factors on soil microbial processes. The model outline includes four organic matter pools: (i) easily decomposable compounds, (ii) holocellulose, (iii) lignin, and (iv) resistant compounds. It was assumed that the decomposition of the easily decomposable substances can be modeled as the growth rate of bacteria, while the decomposition of lignin and resistant compounds can be modeled as the growth rate of fungi. The decomposition of the substances included in the holocellulose pool was assumed to follow an intermediate function. Results show that enough data are available for the parameterization of a model of the suggested type. The effect of temperature on decomposition rate seems to increase with decreasing nutrient concentration of the substrate. Decomposition rates increase with relative soil moisture saturation. Bacterial growth rates are generally more sensitive to low pH than fungal growth rates. Decomposition of mixed organic material is inhibited in an intermediate fashion. The combined impact of H+and Al3+on the growth rates of bacteria and fungi can be modeled with an ion-exchange expression, preferably the Vanselow expression. It was concluded that some additional experiments would be needed for further model development purposes. Such experiments should be set up as mass loss experiments and last for a minimum of 2 weeks.Key words: decomposition, model, acidification, aluminum, pH.

ISSN:1208-6053    

DOI:10.1139/a98-001

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

In this paper the effects of anthropogenic acidification of soils and waters on bird populations are reviewed. Acidification causes (i) declines in the reproductive success and the density of piscivorous birds through declines in the fish populations, (ii) shifts in the forest bird community from forest birds to birds of open woodland through large-scale forest dieback, and (iii) leads to a lower reproductive success of birds in calcium-poor areas through a decline in the availability of calcium-rich material (needed for eggshell formation and skeletal growth). Acidification may also affect the availability of food and nest sites for insectivorous and hole-nesting birds, but there are no consistent effects on the population sizes of these birds. Effects of declines in populations of invertebrates in aquatic habitats may be mitigated by reduced competition from fish, and acidification in forests in less-advanced stages of dieback can both lead to an increase and a decrease in insect and seed abundance in forests, the outcome depending on species, extent of leaf and needle loss, and other factors. There is some evidence that acidification may strongly affect avian reproduction through an increased exposure to toxic metals such as aluminium. Anthropogenic acidification on a worldwide scale is expected to continue during the next decades. Future research on the effects of acidification on bird populations should focus on remedial action and effects on population sizes in moderately acidified areas, and should more often apply an experimental approach than in the past.Key words: acid rain, birds, reproduction, calcium, toxic metals, forest dieback.

ISSN:1208-6053    

DOI:10.1139/a98-003

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The current state of knowledge on the recycling of pulp sludge in the forest and agricultural lands as an alternative to disposal is reviewed. Effects of land application of pulp sludge on chemical and physical properties of soils, on leaching of chemical constituents to groundwater, and on yields of crops are discussed. Regions in Europe where land application of pulp sludge are potentially most beneficial are identified. Information on pulp production, pulping and bleaching methods, and treatments of the effluents, as well as its environmental implications, are also briefly reviewed.Key words: Europe, Mediterranean region, pulp sludge, land application, forest and agricultural sites, environmental impacts.

ISSN:1208-6053    

DOI:10.1139/a98-004

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The use of plants to reduce contaminant levels in soil is a cost-effective method of reducing the risk to human and ecosystem health posed by contaminated soil sites. This review concentrates on plant-bacteria interactions that increase the degradation of hazardous organic compounds in soil. Plants and bacteria can form specific associations in which the plant provides the bacteria with a specific carbon source that induces the bacteria to reduce the phytotoxicity of the contaminated soil. Alternatively, plants and bacteria can form nonspecific associations in which normal plant processes stimulate the microbial community, which in the course of normal metabolic activity degrades contaminants in soil. Plants can provide carbon substrates and nutrients, as well as increase contaminant solubility. These biochemical mechanisms increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants or reduce the phytotoxicity of the contaminated soil. However, the specificity of the plant-bacteria interaction is dependent upon soil conditions, which can alter contaminant bioavailability, root exudate composition, and nutrient levels. In addition, the metabolic requirements for contaminant degradation may also dictate the form of the plant-bacteria interaction i.e., specific or nonspecific. No systematic framework that can predict plant-bacteria interactions in a contaminated soil has emerged, but it appears that the development of plant-bacteria associations that degrade contaminants in soil may be related to the presence of allelopathic chemicals in the rhizosphere. Therefore, investigations into plants that are resistant to or produce allelopathic chemicals is suggested as one possible method of identifying plant-bacteria associations that can degrade contaminants in soil.Key words: phytoremediation, mechanisms, rhizosphere, bacterial inoculants.

ISSN:1208-6053    

DOI:10.1139/a98-005

出版商:NRC Research Press    

年代:1998

数据来源: NRC