Estimating denitrification in a large constructed wetland using stable nitrogen isotope ratios

Abstract

Nitrate losses and their relationships to nitrogen isotope fractionation were evaluated in a wetlands environment in southern California. This paper reports the first study to follow isotope ratio changes in a large natural system with extensive macrophyte growth. As NO 3 concentrations decreased during flow through the wetlands, progressive enrichment in 15NO 3 was found. This increase corresponded to an enrichment value of −2.5‰, a value much lower that those reported for laboratory studies of denitrification (−17 to −29‰), but closer to the range of enrichment factors attributed to denitrification in studies of groundwaters (−3.5 to −16‰). By considering a laboratory derived value of −17‰ as the enrichment factor strictly due to denitrification, 10–23% of the Prado losses could be attributed through isotope enrichment to denitrification. Other studies at Prado, using a mass balance approach which considered macrophyte growth, concluded that bacterial denitrification was the major loss mechanism (89–95%). A similar, but somewhat smaller discrepancy between estimates of denitrification was found in nearby wetlands with large shallow water bodies with dense phytoplankton growth but negligible macrophytes. These discrepancies suggest that enrichment or fractionation factors derived under laboratory conditions where all nitrogen losses are attributable to denitrification cannot be used directly as the denitrification end-member in field situations where organic matter decomposition and nitrogen recycling are occurring simultaneously or sequentially with denitrification. Further studies which consider entire growing and decomposition cycles in macrophyte-rich wetlands are needed to resolve the discrepancy in the various methods of estimating dentrification in natural wetlands.