Nitrate leaching from short-hydroperiod floodplain soils

Abstract

Abstract.Numerous studies have shown the importance of riparian zones to reduce nitrate (NO − 3 ) contamination coming from adjacent agricultural land. Much less is known about nitrogen (N) transformations and nitrate fluxes in riparian soils with short hydroperiods (1-3 days of inundation) and there is no study that could show whether these soils are a N sink or source.Within a restored section of the Thur River in NE Switzerland, we measured nitrate concentrations in soil solutions as an indicator of the net nitrate production. Samples were collected along a quasi-successional gradient from frequently inundated gravel bars to an alluvial forest, at three different depths (10, 50 and 100 cm) over a one-year period. Along this gradient we quantified N input (atmospheric deposition and sedimentation) and N output (leaching) to create a nitrogen balance and assess the risk of nitrate leaching from the unsaturated soil to the groundwater.Overall, the main factor explaining the differences in nitrate concentrations was the field capacity (FC). In subsoils with high FCs and VWC near FC, high nitrate concentrations were observed, often exceeding the Swiss and EU groundwater quality criterions of 400 and 800 µmol l −1 , respectively. High sedimentation rates of river-derived nitrogen led to apparent N retention up to 200 kg N ha −1 yr −1 in the frequently inundated zones. By contrast, in the mature alluvial forest, nitrate leaching exceeded total N input most of the time. As a result of the large soil N pools, high amounts of nitrate were produced by nitrification and up to 94 kg N-NO − 3 ha −1 yr −1 were leached into the groundwater. Thus, during flooding when water fluxes are high, nitrate from soils can contribute up to 11 % to the total nitrate load in groundwater.

Flood-Controlled Excess-Air Formation Favors Aerobic Respiration and Limits Denitrification Activity in Riparian Groundwater

“…In previous studies at this field site, it was shown that the riparian zones and water level changes are important constraints on the distribution and availability of OC and NO − 3 resulting in spatially and temporally highly variable biogeochemical activities (Huber et al, 2012;Peter et al, 2012a,b;Shrestha et al, 2014). We hypothesized that the formation of excess air in response to the hydraulic condition of the river additionally constrains these biogeochemical processes.…”

“…In the framework of an interdisciplinary study in a restored riparian section of the River Thur, Switzerland, pronounced hot spots and moments (McClain et al, 2003) for these biogeochemical processes were identified (Peter et al, 2012a,b). Especially flood events locally intensified OC turnover in the groundwater and impacted N transformation pattern in the riparian soils (Huber et al, 2012;Shrestha et al, 2014). Furthermore, during flood events, significant amounts of air can be delivered to groundwater by the entrapment and dissolution of air bubbles in response to water table fluctuations or groundwater recharge (Mächler et al, 2013).…”

River restoration: morphological, hydrological, biogeochemical and ecological changes and challenges

“…In both systems, we combined geochemical, biochemical and molecular-biological analysis to identify biogeochemical processes as well as the responsible organisms and to determine process rates and element fluxes (Huber et al, 2012b;Peter et al, 2012a;Samaritani et al, 2011;Shrestha et al, 2012Shrestha et al, , 2014. The results are briefly discussed in Sects.…”

Morphological, hydrological, biogeochemical and ecological changes and challenges in river restoration – the Thur River case study

“…First, hot spots of N 2 O (a major greenhouse gas) efflux can occur, in particular during the drying phase after major floods . Second, under unsaturated but sufficiently moist conditions, strong nitrification leads to the accumulation of high amounts of nitrate that are leached to the groundwater mainly during floods and in winter (Huber et al, 2012b). Hence, groundwater quality in near-river aquifers of restored river reaches could vary markedly because of the high spatial and temporal variability of both infiltration travel times and soil properties, in particular on gravel bars.…”

“…In both systems, we combined geochemical, biochemical and molecular-biological analysis to identify biogeochemical processes as well as the responsible organisms and to determine process rates and element fluxes (Huber et al, 2012b;Peter et al, 2012a;Samaritani et al, 2011;Shrestha et al, 2012Shrestha et al, , 2014. The results are briefly discussed in Sects.…”