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RESPONSE OF SOIL MICROBIAL BIOMASS AND COMMUNITY COMPOSITION TO CHRONIC NITROGEN ADDITIONS AT HARVARD FOREST
Compton, J E., L S. Watrud, L A. Porteous, AND S. DeGrood. RESPONSE OF SOIL MICROBIAL BIOMASS AND COMMUNITY COMPOSITION TO CHRONIC NITROGEN ADDITIONS AT HARVARD FOREST. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science BV, Amsterdam, Netherlands, 196:143-158, (2004).
To examine the long-term effects of chronic N additions on soil microbes
Soil microbial communities may respond to anthropogenic increases in ecosystem nitrogen (N) availability, and their response may ultimately feedback on ecosystem carbon and N dynamics. We examined the long-term effects of chronic N additions on soil microbes by measuring soil microbial biomass, composition and substrate utilization patterns in pine and hardwood forests at the Harvard Forest Chronic N Amendment Study. Functional and structural genes for important N cycling processes were studied using DNA community profiles. In the O horizon soil of both stand types, N additions decreased microbial biomass as determined by chloroform fumigation-extraction. Utilization of N-containing substrates was much lower in N-treated pine soils than in the controls, indicating that N additions reduced potential microbial activity in the pine stand. Counts of fungi and bacteria as determined by direct and culture techniques were quite variable and did not show a clear response to N additions. Nitrogen additions, however, strongly influenced microbial community DNA profiles. The ammonium oxidizing gene amoA generally was found in high N treated soils, but not in control soils. The nifH gene for N2-fixation was more difficult to amplify in the pine N treated soil than the controls, suggesting that the potential for nitrogen fixation was depressed after chronic N additions. Our findings indicate that chronic N additions decreased microbial biomass and altered DNA community profiles. These changes in microbial community structure and function may be an important component of the response of terrestrial ecosystems to increased N supply due to human activities.