Fertilization and storage of inorganic and organic nitrogen and phosphorus in Hawaiian forest plants

Hawaiian wet forests have previously been shown to contain examples of both N- and P-limitation, as demonstrated by fertilization experiments.  When fertilized, a variety of native and alien Hawaiian plants, take up much larger quantities of P than N, even when P has been shown not to limit growth rates.  To determine if this greater P accumulation may be due to greater storage capacity of inorganic P than inorganic N, I surveyed 8 abundant species of trees, shrubs, vines, and ferns at a 300-yr-old N-limited site and 9 species at a 4.1-myr-old P-limited site.  Sun leaves were sampled from individuals growing in long-term fertilization plots (control, +N, +P, and +NP).  In general, foliar samples contained about 1% inorganic N or less, and there was little change in the percentage of N in inorganic forms after N-fertilization.  However, foliar samples often contained more than 50% of P in inorganic forms, and this percentage increased dramatically with fertilization.  The consistency of the larger P response across species of different life forms, even at the young site where P additions have been demonstrated not to increase litterfall or tree diameter growth, suggests that Hawaiian wet forest plants routinely take up phosphorus in excess of current demand.  This research supports the hypothesis that this accumulation is possible due to the ability of plants to easily store P in inorganic forms, while inorganic N storage is less feasible for plants.  This P storage has important implications for overcoming P-limitation, which commonly limits tropical forests worldwide.