Factors affecting agricultural nitrogen removal in riparian strips: Examples from groundwater-dependent ecosystems of the Po Valley (Northern Italy)

The role of riparian ecosystems in mitigating the effects of diffuse agricultural sources is recognized in several regulatory measures and public policy initiatives in many parts of the world. This study aimed to evaluate the N buffering capacity of semi-natural riparian zones associated with spring-fed lowland streams, also known as “fontanili”, representing the most important groundwater-dependent ecosystems in Northern Italy. Monitoring parcels were set up in nine riparian sites selected to cover a range of different soil properties and hydrogeological settings, and to sustain the evaluation of the main drivers affecting their N removal efficiency. Subsurface water level, nutrient concentrations and the main hydro-chemical parameters were monitored along transects of piezometers installed from crop fields to the spring channels. On selected samples from two sites stable isotopes of the water molecule were also determined. Median NO3− input concentrations from adjacent cropland to the riparian sites ranged from 0.10 to 21 mg N L−1, with maximum values exceeding the drinking water limit recorded during the summer and winter fertilization periods. Highly variable groundwater nitrate patterns were found in the riparian areas, including short nitrate plumes extending from the adjacent cropland into some riparian zones, or in others, small patches where NO3− declined at variable distance from the stream. Some chemical indicators (e.g., NO3−/Cl− ratio, O2, DOC) suggested that NO3− attenuation was mostly due to the denitrifying activity occurring in the subsurface aquifers in specific conditions (hot spots and moments), although, in some cases, physical processes such as dilution also contributed. The overall N removal efficiency was greater than 90% in four sites, 74%, 34% and 30% in three sites, and zero in the remaining two sites. Useful predictors of the nitrate removal capacity were factors linked to the water residence time, such as the hydraulic conductivity, the soil texture and the slope of the riparian profile, together with the water table depth and soil organic carbon. A combination by standardized averaging of these five factors supported a clear discrimination of sites with zero or low N removal effectiveness from those with high efficiency.