Monday, November 13, 2006

The Contributions of Bacteria and Fungi to Nitrogen Cycling in Forest Soils under Douglas fir and Red Alder.

Stephanie Boyle, Oregon State University, 3017 ALS Bldg, Corvallis, OR 97331, Peter Bottomley, OR St. Univ.-Dept. Microbiolgy, "220 Nash Hall, Campus Way", Corvallis, OR 97331-3804, United States of America, and David Myrold, 3017 ALS Bldg., Oregon State University, Oregon State University, Dept. of Crop & Soil Science, Corvallis, OR 97331-7306.

Previous studies have found that in soils with high N concentrations gross N cycling rates increase, nitrate (NO3-) accumulates, and N losses from soil are greater.  Presumably these differences result from changes in the functioning of soil microbes, but knowledge of the links between microbial community composition and N cycling rates remains limited.  A study was conducted to better understand the relationship between N-cycling and the soil microbial community by examining net and gross N transformation rates in conjunction with microbial community structure under pure stands of red alder and Douglas fir at two sites in Oregon. A combination of 15N-tracer experiments and community profiles were used to investigate the relative abundance and roles of bacteria and fungi in N cycling under different tree types and at sites with high vs. low productivity.  These experiments revealed that N cycles differently in low versus high N forest soils and the relative contributions of bacteria and fungi change in response to low and high productivity sites.  Community compositional differences include a lower fungal:bacterial ratio and an increase in the fraction of Gram‑positive bacteria at a highly productive site.  These observations suggest that in highly productive forest soils, bacteria may play a more significant role in the overall cycling of soil N and emphasize the importance of understanding soil microbial communities as a means to better understand biogeochemical processes.