Resource allocation and nitrogen and phosphorus availability in northern hardwood forests

Background/Question/Methods

Nitrogen has long been accepted as the most limiting nutrient in temperate forest ecosystems, but more recent evidence suggests that we should expect colimitation by N and P. If colimitation indeed occurs in temperate forest ecosystems, this would support ideas of resource optimization whereby organisms allocate resources to minimize limitation by any one factor. The specific mechanisms underlying this process are of interest, especially if we are to understand responses to human influences related to nitrogen deposition and forest management. We studied northern hardwood forest stands of several successional stages to learn more about interactions between N and P availability and their implications for the nature of nutrient limitation in this ecosystem. We tested whether N and P availabilities vary in relation to one another or independently across our replicated forest stands.

Results/Conclusions

We found a clear positive relationship between N mineralization and resin-P availability that was somewhat surprising given the variation in forest age, species composition, and site characteristics that were encompassed by our study sites. This positive relationship contrasts with expectations under a strictly N limited scenario, in which we would expect consistently higher P availability relative to N. Stoichiometry of the decomposer microbial community might be expected to mediate this process, but we found no relationships between microbial N:P and nutrient availability. However, we did find a strong positive relationship between N mineralization and acid phosphatase activity, suggesting a mechanism by which organisms are using available N to maintain balance with P availability. A relative abundance of organic P in these forests suggests that this resource is potentially available in response to further N enrichment. Therefore, we propose that the biotic effort to maintain stoichiometric balance may alleviate increased demand for P due to N deposition in the northern hardwood forests as long as there is a large enough pool of recyclable P.