In a watershed, wetlands may function either as a nutrient sink or as a source and net transformer of nutrients. One of the most widely recognized functions of wetlands is the ability to reduce or remove nutrients from surface water passing through the wetland. In order to compare nitrogen retention capacity, we investigated the ability of three wetlands to reduce or remove dissolved nitrogen from the surface water that passed through them. Although the three wetlands were located within one watershed, their surrounding land uses were significantly different. In this study, the surface water of each of the three wetlands was sampled from five points (the main inlet, outlet and three points inside the wetland) during the second week of every month, from December 2005 to December 2006, in order to measure dissolved nitrogen concentrations and their components (nitrate, nitrite, ammonium, and dissolved organic nitrogen). During the growing season (June-July), a vegetation census was conducted in each wetland that included an estimation of the percent of coverage and a survey of the diversity of vegetation. Investigation into the seasonality of the source or sink function indicated that wetland A had a source-role in three seasons (winter, spring, and summer) and a sink role during the autumn season. Wetland B had a sink-role during two seasons (winter and summer), a source-role in the spring, and a neutral role in autumn. Wetland C had a sink-role for dissolved nitrogen in surface water during all seasons of the study period. Results from the vegetation census indicated that Typha latifolia was the dominant species for wetland A, Potamogeton cristatus was dominant for wetland B, and Ischene globosa was dominant for wetland C. The percentage of vegetation cover was estimated as 83%, 35%, and 53% in wetlands A, B, and C, respectively. The results of this study indicated that the surrounding land use and human alterations to the environment had played a significant role in determining the function of each wetland as a sink, source or transformer for dissolved nitrogen in surface water passing through the wetlands. It emerges from this study that the seasonal changes in the function of the wetland for dissolved nitrogen as well as variations in vegetation cover (%) and dominant plant species, were affected by the composition of the surrounding lands. This study revealed not only that the role (as source, sink or transformer) that the wetland plays for dissolved nitrogen might change because of the above-mentioned factors, but also that this role could either be stable, or that it could change seasonally. Finally, an investigation of the components of total dissolved nitrogen in these three wetlands showed that under the influence of the surrounding land use, NH₄⁺ was the dominant form in wetland A (with a high percentage of urban area), and (NO₃⁻+NO₂⁻) were sub-dominant in wetland B (with a high percentage of agricultural area). Dissolved organic nitrogen was the dominant form in wetland C (with 0% of urban area and a high percentage of forest area). According to the results, dissolved organic nitrogen was not always the dominant component of the dissolved nitrogen, so that with an increase in the percentage of urban area, DIN was the dominant form and vice versa. Based on the results, the retention capacity of the wetlands for dissolved nitrogen in surface water increased in conjunction with a decrease in the level of urban land use.
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