Effects of Human Activity on the Processing of Nitrogen in Riparian Wetlands: Implications for Watershed Water Quality

Abstract

Wetlands are critical ecosystems that make substantial contributions to ecosystem services. In this study, we asked how the delivery of an ecosystem service of interest (N processing such as denitrification and mineralization) is impacted by anthropogenic activity (as evidenced by land cover change). We identify relevant factors (hydrology, nitrogen, and carbon variables), select headwater wetland sites in Ohio and Pennsylvania USA to represent a gradient of anthropogenic disturbance as indicated by land cover characteristics (represented by the Land Development Index, or LDI), and determine if there are differences in the selected variables as a function of this gradient by categorizing sites into two groups representing high and low disturbance. We utilized Classification and Regression Trees (CART) to determine which variables best separated high from low disturbance sites, for each spatial scale at which land cover patterns were determined (100 m, 200 m, 1 km radius circles surrounding a site), and within each category of water quality variable (hydrology, nitrogen and carbon). Thresholds of LDI were determined via the CART analyses that separated sites into two general classes of high and low disturbance wetlands, with associated differences in Total Nitrogen, NH4+, Soil Accretion, C:N, Maximum Water Level, Minimum Water Level, and %Time in Upper 30 cm. Low Disturbance Sites represented forested settings, and exhibit relatively higher TN, lower NH4+, lower Soil Accretion, higher C:N, higher Maximum Water Level, shallower Minimum Water Level, and higher %Time in Upper 30 cm than the remaining sites. LDIs at 100 m and 200 m were best separated into groups of high and low disturbance sites by factors expected to be proximal or local in nature, while LDIs at 1000 m predicted factors that could be related to larger scale land cover patterns that are more distal in nature. We would expect a water quality process such as denitrification to be relatively lower in forested settings, due to the low available nitrogen (associated with high C:N) and constant and saturated conditions; conditions for maximum denitrification may be found in agricultural settings, where high nitrate groundwater can interact with surface soils through a wetting and drying pattern. The use of land cover patterns, as expressed by LDI, provided useful proxies for nitrogen, carbon, and hydrology characteristics related to provision of water quality services, and should be taken into account when creating, restoring, or managing these systems on a watershed scale.