Soil processes, ectomycorrhizal fungi, and atmospheric nitrogen deposition: What are the connections?

Over the past century, human activities have resulted in a substantial increase in atmospheric nitrogen (N) deposition throughout eastern North America, effectively doubling the amount of reactive N entering terrestrial ecosystems.  There remains considerable uncertainty, however, in the potential response of soil C and N dynamics and the related soil microbial community to the rapid global increase in reactive N.  We examined peroxidase, phenol oxidase, general proteolytic activity, and N mineralization, all soil processes important in C and N storage and turnover, across a well-known atmospheric N deposition gradient in the northeastern United States.  We also morphologically examined the community structure of ectomycorrhizal fungi, one of the soil organisms principally involved in all of these soil processes, across the N deposition gradient.  Peroxidase and phenol oxidase activities significantly decline with increasing N deposition (r² = 0.441 and 0.253, p = 0.0032 and 0.0242 respectively).  N mineralization also significantly declines with increasing N deposition (r² = 0.421, p = 0.0001).  Surprisingly, general proteolytic activity increases with increasing N deposition (r² = 0.331, p = 0.0081).  Using detrended correspondence analysis, we found significant differences in ectomycorrhizal communities across the gradient with the first three axes describing 48% of the observed variation.  Our data support the hypotheses that atmospheric N deposition significantly alters ectomycorrhizal community structure and accelerates decomposition of light soil C fractions with short turnover times while further stabilizing more recalcitrant soil C fractions with multidecadal turnover times.