PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Effects of Fraxinus uhdei invasion on nitrogen cycling in a Hawaiian rainforest.
Rothstein, David1, Vitousek, Peter2, 1 2
ABSTRACT- We studied the effects of Fraxinus uhdei invasion on nitrogen (N) cycling and soil N availability in a Hawaiian rainforest. Fraxinus is an exotic tree that grows faster and produces more labile litter than the dominant native tree, Metrosideros polymorpha. We hypothesized that Fraxinus invasion would accelerate rates of ecosystem-level N cycling, and increase soil N availability. We compared N cycling processes among three different forest types: Fraxinus plantation, native forest invaded by Fraxinus, and uninvaded native forest. We found that N cycled through the vegetation more rapidly in Fraxinus-dominated forests, that more N was returned in litterfall in Fraxinus forests, and that decomposing Fraxinus litter released N more rapidly than Metrosideros litter. However, we also found that several indices of soil N availability were significantly lower in Fraxinus forests. In order to better understand this surprising result, we analyzed the pool structure of soil organic matter and exoenzyme production by soil microbial communities in these three forests. We found a significantly greater pool of active C in soils from Fraxinus forests. In addition, we found that Fraxinus-forest soils had greater activities of enzymes involved in the breakdown of relatively labile compounds (e.g. peptides, chitin, cellulose) and lower activity of lignin degrading enzymes. These data suggest that large inputs of labile litter in Fraxinus stands are driving increased microbial growth, metabolism, and N immobilization.
KEY WORDS: invasion, nutrient cycling