Multiple Element Limitation in Northern Hardwood Ecosystems (MELNHE).

Although temperate forests have long been thought to be primarily nitrogen limited, resource optimization theory suggests that ecosystem productivity should be co-limited by multiple nutrients. Mechanisms for maintaining stochiometric balance in ecosystems include nutrient resorption in senescing tissues, uptake by plants or microbes, and feedbacks on soil nutrient mineralization. We examined foliar resorption of N and P in 6 northern hardwood stands with differing native soil fertility and tested whether resorption of one nutrient depended on the availability of the other. We also began a long-term nutrient manipulation experiment, fertilizing 13 stands with 30 kg/ha N, 10 kg/ha P, or N and P together. We examined nitrogen and phosphorus availability (assayed with buried ion-exchange resin strips) in the first year of fertilization.  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 interactive effect of N and P is due to either reduced biotic uptake of N or increased mineralization, or both. Foliar P resorption efficiency was not strongly related to native soil P availability (p>0.20) but was highest where soil N content was high (p=0.02).  There was no relationship between N resorption efficiency and soil N (p=0.24), but high soil P led to higher N resorption efficiency (p<0.10).  The presence of multiple-element effects on nutrient resorption helps explain the absence of single-element effects in many studies. The interactive effects of N and P on both soil availability and plant nutrient conservation may help maintain N and P co-limitation in terrestrial ecosystems.  Additional mechanisms underlying the N and P co-limitation of forest productivity will be tested as we continue to observe the effects of our experimental nutrient manipulation.