THE INFLUENCE OF MEDITERRANEAN RIPARIAN FORESTS ON STREAM NITROGEN DYNAMICS: A REVIEW FROM A CATCHMENT PERSPECTIVE RUNNING TITLE: RIPARIAN INFLUENCES ON STREAM N DYNAMICS
Riparian zones are considered natural filters of nitrogen (N) within catchments because they can substantially diminish the exports of N from terrestrial to aquatic ecosystems. However, understanding the influence of riparian zones on regulating N export at the catchment scale still remains a big challenge in ecology, mainly because upscaling plot scale results is difficult, as it is disentangling the effects of riparian, upland, and in-stream processes on stream water chemistry. In this review, we summarize previous studies examining key hydrological and biogeochemical processes by which Mediterranean riparian zones regulate catchment water and N exports. We focus on Mediterranean regions because they experience a marked climatic seasonality that facilities disentangling the close link between climate, riparian hydrology, and stream N exports. We show that Mediterranean riparian soils can be hot spots of N mineralization and nitrification within catchments given their relatively moist conditions and large stocks of N-rich leaf litter. Extremely large nitrification rates can occur during short-time periods (i.e. hot moments) and lead to increases in stream N loads, suggesting that riparian soils can be a potential source of N to adjacent aquatic systems. Moreover, riparian trees can contribute to decrease riparian groundwater level during the vegetative period, and promote reverse fluxes from the stream to the riparian zone. During periods of high hydrological retention, stream water exports to downstream ecosystem decrease, while stream water chemistry is mostly determined by in-stream processes. Riparian tree phenology can also affect catchment N exports by shaping the temporal pattern of both light and litter inputs into the stream. In spring, light enhances in-stream photoautotrophic N uptake before riparian leaf-out, while riparian leaf litter inputs promote in-stream N mineralization in summer and fall. Finally, we illustrate that the impact of Mediterranean riparian zones on stream hydrology and biogeochemistry increases along the stream continuum, and can ultimately influence catchment N exports to downstream ecosystems. Overall, findings gathered in this review question the well-established idea that riparian zones are efficient N buffers, at least for Mediterranean regions, and stress that an integrated view of upland, riparian, and stream ecosystems is essential for advancing our understanding of catchment hydrology and biogeochemistry.