The role of phosphorus availability: has nitrogen deposition pushed the upland ecosystem to the limit?

Reactive nitrogen (N) deposition can significantly affect ecosystem processes, particularly in the oligotrophic upland ecosystem. Phosphorus (P) limitation may significantly constrain ecosystem productivity, yet ecosystem responses to P availability in instances of high N deposition are poorly understood. The release from P limitation can increase vegetation biomass production, enabling ecosystems to sequester greater reactive N inputs. We hypothesised that this increased biomass production would positively influence ecosystem carbon (C) storage potential. We observed interactions between vegetation species composition, soil nutrient pools, soil microbial activity, soil respiration (CO2 and CH4 flux) and soil C storage potential in response to N and P availability (to alleviate the potential limitation), using both short term (1 year) and long term (1996 - 2012) nutrient addition experiments. Exposure to nutrient enrichment was responsible for significant changes in the structure and composition of vegetation species, above-ground vegetation biomass and vegetation diversity. Shifts in vegetation species coverage had strong cascading effects on nutrient cycling rates, with significant reductions observed in soil C and N pool sizes, particularly after long term exposure, suggesting P limitation may have a larger than expected effect on below-ground ecosystem processes. Soil respiration response was variable between CO2 and CH4, but with higher emissions of C with nutrient addition, particularly where N and P were added in combination, both in the short and long term. This study demonstrated the significant effect of P availability on upland ecosystem processes and highlights the long term effect of P addition on vegetation species composition and therefore the long term influence on ecosystem function and C storage; which is essential information for management strategies considering P addition as a method to negate the effects of chronic N enrichment from deposition