Does Chronic N Enrichment Enhance Soil Microbial P Limitation? : Evidence from Several Long-Term Studies in the Eastern US.

Forest nitrogen (N) enrichment can alter key aspects of ecosystem biogeochemistry, including the bioavailability of limiting nutrients, and soil microbial processes that regulate nutrient cycling and carbon (C) storage.  Although recent declines in N deposition have been observed in the Eastern U.S., the legacy impacts of enhanced N loading in northeastern ecosystems are still a concern.  Recent evidence suggests that accelerated N cycling may enhance the role phosphorus (P) plays in regulating forest ecosystem processes, but the importance of P limitation on ecosystems processes in northeastern forests has not been well characterized.  This study capitalized on several long-term N-enrichment sites, including the Bear Brook Watershed in Maine, Harvard Forest, the Fernow Experimental Forest.  Collectively, control and treatment plots across these sites provide a cross-site study to examine the effects of elevated N loading on biological P limitation.  We conducted a series of experiments that included:1) measurements of phosphatase activities as an index of P demand and stress in rhizosphere and bulk soil, 2) experimental additions of P fertilizer to assess phosphatase and respiration response to short-term inorganic P addition.  Our findings suggest phosphatase suggest a difference in the magnitude of inorganic P limitation among soil microbial communities in the rhizosphere versus bulk soil.  Furthermore, we did not consistently observe a significant respone in soil respiration response to P fertilizer for N enriched soils, suggesting that in the short-term soil microbial activity is not always strongly regulated by inorganic P in these soils.