Variations in soil nitrogen cycles in the riparian zone of low-order watersheds along an urban-to-rural gradient in the New York City Metropolitan area
Urbanization alters soil nitrogen cycles through increased deposition and effects on the soil microbial community. The riparian zone of low-order watersheds plays a crucial role in buffering the effects of changes in soil N cycling on the water quality, but riparian zones may lack the capacity to buffer changes in soil N cycling associated with urbanization, leading to overloading of N into stream water. Accordingly, studying the role of urbanization on soil N cycles in riparian zones could improve our understanding on human-nature interactions in urban ecosystems. In this study, we examined whether soil N pools and processes in riparian zones vary with urbanization by sampling riparian zones in the headwater, midflow and downstream portions of three low-order watersheds that differed in the degree of urbanization. All three watersheds (Bronx River, Mianus River, Wappingers Creek) are located in the New York City Metropolitan area, and vary from highly urbanized to comparatively undeveloped. All sites were sampled monthly during 2014. Soil inorganic and organic N pools were separately analyzed for the O and A horizons at each site. Soil N fluxes were assessed using the buried bag technique in situ.
Across the sampling period, soil NO3- and NH4+ concentrations did not clearly differ among watersheds or river positions. However, there was significant monthly variation in soil NH4+ concentration, with soil NH4+ concentration generally decreasing across the study period. Total inorganic N (TIN) concentrations exhibited similar temporal patterns as soil NH4+. Moreover, soil TIN concentration was significantly higher at 5 m from riverbank compared to 10 m from riverbank; and significantly higher in O horizon of soil than A horizon. Similar to soil NO3-, net soil NO3- flux, which is an estimate of net nitrification, did not clearly differ among watersheds. Net TIN flux, which is an estimate of net mineralization, did not clearly differ among watersheds, but varied between time periods. Net soil TIN flux was positive most months, but was negative from June to July, indicating net N immobilization. Our results suggest that soil inorganic N pools and fluxes vary greatly over time in these watersheds, while effects of urbanization are comparatively small. We suggest that site-specific factors may be more influential than urbanization effects on soil N cycles in these riparian zones.