Impact of Phosphorus Enrichment on Wetland Ecosystem Processes: Nutrient Resorption, Above and Belowground Decomposition, and Microbial Activities.

摘要

Phosphorus (P) is an essential element for plant mineral nutrition, and as a result, can commonly limit plant growth in terrestrial as well as wetland systems. Naturally low nutrient P limited marshes are widespread in northern Belize. Experimental P applications have demonstrated changes in marsh plant (macrophyte) growth and community structure. Marshes once dominated by the sparse macrophyte Eleocharis cellulosa Torr. and cyanobacteria mats, are replaced with monotypic stands of the robust macrophyte Typha domingensis Pers. This dissertation examines the effect of P enrichment on the growth, P use and conservation strategies, and decomposition of these two species; and how these responses impact system microbiota and nutrient cycling. Under natural low P conditions, Eleocharis was found to have high root phosphatase activities, increasing it's ability to acquire P. Eleocharis also had higher P in senescent tissues, demonstrating the retranslocation and conservation of P. The ability to acquire and conserve P, however, was reduced with the addition of P. Following P enrichment, both species immobilized a high amount of P in their tissues. During senescence, both were able to retranslocate P in leaves, roots, and rhizomes, immediately following the initiation of senescence. This led to drastic differences in the ratio of carbon (C) to P and total P within senescent litter. P enrichment enhanced the decay of leaves for both species, but had no effect on coarse root and rhizome decay. The senescence of P enriched tissues released a large amount of P, stimulated microbial growth and activities, which lead to faster rates of decay in the leaves. Given that P enrichment did not impact root and rhizome decay, but enhanced root production, the addition of P may increase accretion rates. The enhanced root biomass of macrophytes was found to control heterotrophic N-fixation, although the quality of litter also appeared to be important. Eleocharis litter supported N-fixation more than Typha litter. In the absence of autotrophic N-fixation, heterotrophic N-fixation appears to supply enough N to meet demands within the system. Higher rates of microbial activities (nitrogenase activity, N mineralization, and immobilization, methanogenesis) were also found following P enrichment. Microbial activities varied with macrophyte species, due to their litter stoichiometry and nutrient use strategies.