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Compton, J.E., L. Watrud, L.A. Porteous, and S. DeGrood. 2004. Response of soil microbial biomass and community composition to chronic nitrogen additions at Harvard Forest. For. Ecol. & Mgt. 196:143-158. WED-03-140

Soil microbial communities may respond to anthropogenic increases in ecosystem, nitrogen (N) availability, and the microbial response may ultimately feed back 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 stands. N additions decreased microbial biomass C as determined by chloroform fumigation-extraction. Utilization of N-containing substrates was lower in N-treated pine soils than in the controls, suggesting that N additions reduced potential microbial activity in the pine stand. Counts of fungi and bacteria as determined by direct microscopy and culture techniques did not show a clear response to N additions. Nitrogen additions, however, strongly influenced microbial community DNA profiles. The ammonia monooxygenase gene (amoA) generally was found in high N-treated soils, but not in control soils. The nifH gene for N2-fixation was generally found in all soils, but was more difficult to amplify in the pine N-treated soil than the controls, suggesting that the population of N2-fixers was altered by N additions. The 16S rDNA gene for Nitrobacter was found in all samples, hut distinct differences among DNA profiles were observed in the pine B horizon in the control, low N, and high N-treated plots. Our findings indicate that chronic N additions decreased chloroform microbial carbon and altered microbial community profile. These changes in microbial community structure may be an important component of the response of terrestrial ecosystems to human-accelerated N supply.

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