Hydrological extremes typically alter the hydrology of riparian zones, with associated effects on the functions of these ecosystems. Such changes may be particularly pronounced in urban areas due to the negative effects of human activities, which may reduce the buffering capacity and alter the biogeochemistry of riparian zones. In this study, we examined whether the effects of hydrological extreme events on carbon and nitrogen cycling in riparian zones of low-order watersheds varied with the degree of urbanization. Specifically, we measured changes in soil and stream C and N pools and fluxes during one rainfall and two drought events across three low-order watersheds along an urban-to-rural gradient in the New York Metropolitan area during 2015. Surface soil and stream water samples were collected before, during and after each event. Soil and stream water samples were analyzed for soil organic C, soil inorganic N, stream water dissolved and particulate organic C and stream water inorganic N.
During the two drought events, dissolved organic C in stream water increased, and this effect was larger in the urban watershed, though there was no change in soil organic C. The drought events were also associated with changes in the soil and stream N pools, though the magnitude of these changes varied with drought severity and the degree of urbanization. For example, soil NO3- level increased up to 5-fold with increasing drought and reduced soil water content. In contrast, drought generally was associated with decreases in inorganic N concentrations in stream water. The rainfall event was associated with decreased soil NO3- and NH4+ concentrations within a day after the event, while stream water NO3- concentrations sharply increased. Both soil and stream inorganic N returned to pre-rainfall concentrations within four days. The decrease in soil N and increase in stream water N was larger in the urban watershed compared with the suburban watershed. Our results are consistent with other studies suggesting that C and N cycles in riparian zones are affected by rainfall and drought events. Our results further suggest that the effects of these events on nutrient cycling in riparian zones differs with the degree of urbanization.