Synergistic soil response to nitrogen plus phosphorus fertilization in hardwood forests

Plant and microbial processes exert control on the stoichiometry of available nutrients, potentially influencing forest ecosystem responses to nitrogen enrichment and other perturbations that alter resource availability. We tested whether an excess of one nutrient influenced the available pool of another, to learn the net outcome of various feedbacks on mineralization and uptake processes. We examined nitrogen and phosphorus availability (assayed with buried ion-exchange resin strips) in the first year of fertilizing northern hardwood forests with 30 kg/ha N, 10 kg/ha P, or N and P together. Fertilizing with a single nutrient raised the availability of the added nutrient and had no detectable effect on availability of the other nutrient. However, resin-available N was raised substantially more by adding N+P than it was by adding N alone. This effect of N+P must be the result of either reduced biotic uptake of N or increased mineralization of N, and suggests that N loss following forest disturbances will be enhanced in cases where the availability of both N and P are increased. That P interacts with N to enhance N availability, by whatever mechanism, could help explain observations of N and P co-limitation in ecosystems and calls attention to the need to carefully elucidate mechanisms underlying co-limitation of forest productivity.