To identify if N enrichment elicits consistent
responses in microbial activity and community composition
across ecosystems and to determine what
mechanisms may be responsible for the observed
responses, soils were collected from 28 sites throughout
North America representing a broad range of biomes
and soil types. Soils were amended with inorganic N
and incubated for 1-year. By incubating the soils under
standardized, laboratory conditions we eliminated the
indirect effects of N additions on plant C inputs and
instead focused only on direct N responses across the
soils. We examined effects of N amendments on the soil
bacterial community composition, but did not examine
effects on the fungal communities, even though such
effects may be important, particularly with regard to
the process responses examined here (Allison et al.,
2008; Edwards et al., 2011; Fontaine, 2011). We tested
the validity of the various hypotheses outlined above
by assessing N effects on soil respiration, microbial biomass,
bacterial community composition, extracellular
enzyme activities and soil temperature sensitivity after
the 1-year incubation under controlled conditions