摘要:

The effects of nitrogen (N) deposition on plant litter and soil organic matter decomposition differ depending on the stage of decomposition (early, late, and final stages). The effects can be divided further into direct and indirect ones.Direct effects: additions of ammonium and nitrate to fresh, newly shed litter stimulate the initial decomposition of celluloses and solubles. By contrast, addition of the same compounds to humus (final stages) clearly suppresses activity. This was seen in all studies reviewed and for several types of humus.Indirect effects: long-term deposition leads to increases in litter concentrations of N and other nutrients. This N in litter forms "natural" organic compounds and the resulting effects are similar to those resulting from natural variation among litter types. Thus, initial decomposition is generally higher for N (nutrient) rich plant litters than for litters with a lower N (nutrient) content. In later stages, at which lignin-degradation rates regulate litter decomposition, N has a retarding effect on decomposition. Significant negative correlations have also been found between N concentrations in humus and respiration rate. There probably is a sink for deposited N in the humus. We may conclude that N storage in humus is regulated by a positive feedback mechanism. Raised levels of N resulting from N deposition cause more humus to be left in the system, and the resulting lower levels of Mn further retards humus decomposition, thus leading to an increased storage of N in humus. Thus, when calculating critical loads it would be incorrect to assume that N pools in the humus remain at a steady state.

ISSN:1208-6053    

DOI:10.1139/a96-017

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The substantial size of some hydroelectric projects and the extensive total surface area covered by reservoirs globally require that research determining the impacts of these developments be done at ever-increasing spatial and temporal scales. As a consequence of this research, new views are emerging about the spatial extent and longevity of the environmental and social impacts of such developments. New findings challenge the notion of hydroelectric development as a benign alternative to other forms of power generation. This review examines the intertwined environmental and social effects of methylmercury bioaccumulation in the food web, emission of greenhouse gases from reservoirs, downstream effects of altered flows, and impacts on biodiversity, each of which operates at its own unique spatial and temporal scales. Methylmercury bioaccumulation occurs at the smallest spatial and temporal scales of the four impacts reviewed, whereas downstream effects usually occur at the largest scales. Greenhouse gas emissions, the newest surprise connected with large-scale hydroelectric development, are relatively short term but eventually may have important global-scale consequences. Limitation of biodiversity by hydroelectric development usually occurs at intermediate spatial and temporal scales. Knowledge developed from working at expanded spatial and temporal scales should be an important part of future decision making for large-scale hydroelectric development.

ISSN:1208-6053    

DOI:10.1139/a97-001

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Exposures to the all pervasive ambient ozone (O3) can and has resulted in visible foliar injury and (or) reduction of crop growth and yield. However, most of our knowledge regarding the latter effect is derived from above ambient, artificial O3fumigations in field exposure chambers. In the most recent years, such methodologies have been the subject of much criticism. Further, in such studies, efforts to establish cause-effect relationships have mainly relied on the use of single-point, season-end statistical descriptors of the O3exposure and crop yield measurements. Such models do not consider the time lag in feedback and the extent of plant stress, repair, or compensation that regulate the final crop biomass in response to highly variable ambient O3exposures in time and space. Therefore, no single such model has performed consistently. In this paper we provide explanations for the problem. Further, we discuss the need to couple atmospheric properties that are conducive for the transfer of O3onto plant canopies and crop properties that promote O3uptake (absorbed dose), a prerequisite to the observed effect. Additional discussion is directed to considerations for performing experiments in chamberless, ambient environments and for the use of multipoint exposure-response models. Issues are highlighted as to the difficulties and uncertainties associated with the available data in establishing ambient air quality regulations to protect crops.

ISSN:1208-6053    

DOI:10.1139/a97-002

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Understorey vegetation in boreal and subboreal ecosystems is critical to ecosystem dynamics by contributing to nutrient cycling, soil aggregation, and soil fertility. As well, it provides habitat for animals and microorganisms and creates suitable microsites for the regeneration of tree species. Within a context of holistic or ecological management, it is important to understand the dynamics of understorey vegetation after disturbance. However, there is a lack of information on the effect of disturbance on plant diversity. Regeneration of plant communities in postdisturbance boreal and subboreal ecosystems is discussed with particular reference to (i) current knowledge of buried seeds (soil seed banks) and vegetative propagation, and (ii) the importance of the fire regime on plant regeneration. In future, research should emphasize new methods to measure fire behaviour to predict the effect of fire on plant diversity. As well, a good understanding of plant succession should help the implementation of ecological management of industrial forests.

ISSN:1208-6053    

DOI:10.1139/a97-003

出版商:NRC Research Press    

年代:1997

数据来源: NRC