Englemann spruce nitrogen dynamics across anitrogen deposition gradient inColorado, USA

We asked whether nitrogen (N) deposition has altered Englemann spruce (Pice a englemannii) biogeochemistry along the east side of the Colorado Front Ra nge, USA. Twelve similar old-growth Englemann spruce stands were sampled, s ix with low (1-2 kg N ha(-1) yr(-1)) and six with higher (3-5 kg N ha(-1) y r(-1)) N deposition inputs. Species composition, elevation, aspect, parent material, site history and climate were matched as closely as possible acro ss all sites. High N deposition sites had significantly lower organic horiz on C:N and lignin:N ratios, and foliar lignin:N and C:N ratios, as well as greater %N and N:Mg ratios, and potential net mineralization rates. The rel ationship between organic horizon %N and potential net mineralization rates suggests a threshold of 1.2% N, above which mineralization increases linea rly. Studies in the Northeastern US and Europe have found changes in forest biogeochemistry in response to nitrogen (N) deposition inputs between 3-60 kg N ha(-1) yr(-1). Our results suggest that current levels of N depositio n (3-5 kg N ha(-1) yr(-1)) along the Colorado Front Range may be altering E nglemann spruce biogeochemistry. The results indicate even relatively low N inputs may cause measurable changes in forest biogeochemistry.