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.