Forecasting key response thresholds to nitrogen deposition is limited by a lack of synthesis of plant responses to soil chemistry across spatial scales and research approaches. To develop our best understanding of N deposition impact on tree growth and survival, our project analyzes over 100 plot- level studies that explicitly include data on plant function and soil chemistry in response to N inputs in the U.S. and Canada.  Plant measures included tissue chemistry, growth, health, survival, and stress indicators. Measures of soil chemistry included soil solution and exchangeable nutrients and metals available to plants. Threshold responses to soil solution or exchangeable chemistry were identified by species, ecosystem types, N deposition, and other modifying factors (soil type, elevation, latitude) where available.  Random forests, bivariate relationships, and general linear models with bootstrapping were used to relate plant and soil relationships and associated uncertainty.  Preliminary results show strong geographic bias across ecoregions.  Across all studies, soil base cations, soil P, and soil pH were the most important variables in predicting aboveground plant biomass.  Other plant functional and stress relationships showed non-linear behavior with significant variation across ecoregions and studies. Our overall goal is to identify thresholds that will directly inform critical load estimates used in management and policy.