Synergistic soil response to nitrogen plus phosphorus fertilization in hardwood forests
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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.
KeywordsCo-limitation Fertilization Nitrogen Northern hardwood forest Nutrient availability Phosphorus
We are indebted to Matt Vadeboncoeur and Steve Hamburg for field site selection and setup, and we thank Russell Auwae, Kikang Bae, and Craig See for outstanding work in the field. We also thank two anonymous reviewers whose comments helped to improve this manuscript. This work was supported by grants from NSF to Fisk and Yanai and this manuscript is a contribution of the Hubbard Brook Ecosystem Study. Hubbard Brook is part of the Long-Term Ecological Research (LTER) network, which is supported by the National Science Foundation. The Hubbard Brook Experimental Forest is operated and maintained by the USDA Forest Service, Northern Research Station, Newtown Square, PA, USA.
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