Microbial Indicators of Nutrient Enrichment: A Mesocosm Study

Microbial communities are in close contact with the wetland soil microenvironment and can therefore function effectively as monitors of soil pollution. The objective of this studywas to determine changes in the functional responses of microbial communities as a result of an external input of nutrients, while controlling for vegetation. A controlled experiment was performed at the mesocosm scale, consisting of two 1 m by 13 m raceways containing an organic peat soil, each planted with Cladium sp. and Typha sp. communities. One of the mesocosms was loaded with N (2 g N m yr) and P (1 g P m yr) for 18 mo. Nutrient loading resulted in increases in the soil and detritus labile nutrient pools, however, insufficient N and P where added to significantly alter their total levels. Over the experimental period, the extracellular enzyme acid phosphatase showed a significant decrease in activity across both plant communities (P , 0.01) in contrast to bglucosidase activity, which varied primarily by plant community. Other microbial response variables such as the microbial activities (CO2 and CH4production,P5 0.0016 and 0.0213, respectively),microbial biomass (P 5 0.0018) also varied primarily by vegetation type, with Typha sp. dominated areas exhibiting the highest level of activities. The nutrient dosing experiment indicated that the most immediate microbial response measures to nutrient enrichment are those directly associated to specific nutrients, such as P or N, while other measures showed a more complex response involving C source (e.g., vegetation type). WETLANDS are important components in the landscape in that they are often the receiving bodies of point and nonpoint source contaminants. Major changes in wetland ecosystem structure can affect their capacity to function as contaminant sinks and transformers, rendering them less effective as sinks in the overall watershed. Nutrient enrichment has been shown to generate significant alterations to gross-scale wetland ecosystem structure and function (DeBusk et al., 1994; Davis et al., 2003) and induce changes in soil physicochemical and microbiological characteristics that then may serve as indicators of nutrient enrichment. In natural systems, microbial communities have been shown to respond to N and P enrichment, such as increases in litter decomposition rates, observed in streams (Alexander, 1977; Qualls, 1984) and in wetlands (Davis, 1991; DeBusk and Reddy, 1998). Microbial responses to enhanced nutrient levels are also reflected in changes in indices of microbial activity, such as respiratory activities (Wright and Reddy, 2001) and extracellular activities (Sinsabaugh and Moorhead, 1994). Eutrophication in marsh systems has also been associated with increases in microbially mediated N, C, and P turnover rates (Reddy et al., 1999), as well as increases in soil microbial biomass content (Qualls and Richardson, 1995). Experimental addition of N and P to natural systems (Elwood et al., 1981; Newbold et al., 1983) or to enrichment mesocosms (Qualls and Richardson, 2000; Newman et al., 2001) has produced significant microbial responses to the P and N additions, primarily as a function of their status as limiting factors in the systems under observation. However, many of these studies were conducted on systems that had already undergone significant changes in plant community composition, affecting the litter quality and hence possibly the microbial community response. Litter quality has been significantly correlated with the microbial response measures. Factors such as the lignocellulose composition (DeBusk and Reddy, 1998) and the nutrient content of the plant litter material (Kögel-Knaber, 2002) all determine the response of the microbial communities in concert with a direct response to enhanced levels of nutrients. Enclosed experimental systems (mesocosms) have been used extensively as a means to achieve controlled experiments at ecosystem level conditions (Kemp et al., 1980; Odum, 1984; Ives et al., 1996). Mesocosms have been used in aquatic environments to assess planktonic responses (Peterson et al., 1997, 1999) and they have also been used in wetlands to evaluate soil microbial community responses (Newman et al., 2001) as well as benthic responses to nutrient enrichment (McCormick et al., 1996). The objectives of this study was to experimentally determine the response of microbial communities to external N and P inputs in two different plant communities, Typha and Cladium sp. MATERIALS AND METHODS