Plant and Microbial Mechanisms of Nitrogen Retention in Northern Hardwood Forests Receiving Atmospheric NO3 Deposition

[摘要] ABSTRACTAnthropogenic N deposition in industrialized areas in Europe and North America can exceed rates of 30-55 kg N ha-1 y-1 This study examines plant and microbial mechanisms for N retention in northern hardwood forests in Michigan that have received experimental NO3- deposition at the rate of 30 kg N ha-1 y-1 for 10 years.Sugar maple (Acer saccharum Marsh.), the dominant overstory tree species in northern hardwood forests of the Lake States, has been shown to have a limited capacity to take up and assimilate NO3- (Rothstein et al. 1996).The first objective of this study is to determine if chronic NO3- addition has induced sugar maple to uptake and assimilate NO3-. The rates of NO3- and NH4+ uptake and the rate of NO3- reduction did not differ between the ambient and experimental N deposition treatments.Furthermore, the rate of NO3- uptake was much lower than the rates of NH4+ uptake, at the same time that the rates of NO3- reduction remained low.Together, these results indicate that sugar maple is not inducible as a direct sink for NO3- deposition by chronic NO3- additions.The incapacity of sugar maple to utilize NO3-, indicates that this dominant tree species cannot be a direct sink for NO3- deposition.This finding may partially explain the high rates of NO3- leaching in these forests in response to experimental NO3- deposition.Soil microbial community function can alter the capacity of an ecosystem to retain N deposition by controlling the cycling of N into stable and accumulating N pools, such as soil organic matter.The second objective of this study is to examine the in situ flow and fate of N in northern hardwood forest soils receiving experimental NO3- deposition.Microbial assimilation of added NO3- appeared to be greater in the treatment receiving only ambient N deposition..Furthermore, net DON production was greater, and NH4+ production less, under experimental NO3- deposition.Although the results of the tracer experience indicated no difference in N retention in soil between N deposition treatments, the recovery of the tracer in more readily leached soil pools (i.e. NO3- and DON) in the chronic N deposition treatment is consistent with observations of high rates of NO3- and DON loss in this treatment (Pregitzer et al. 2004).

[发布机构] University of Michigan

[效力级别] Science

[关键词] Natural Resources and Environment;Science;Natural Resources and Environment