Riparian Lowlands in Clay Till Landscapes Part II: Nitrogen Reduction and Release Along Variable Flow Paths

Abstract

Riparian lowlands are known to control catchment nitrogen (N) balances. This study examined the role of agricultural tile drainage systems, often present in clay till landscapes, on the transport, transformation, and mass balance of N species in four riparian peat lowland transects receiving agricultural tile drainage water. Monitoring of N speciation of drain, stream, and groundwater, combined with a previously established water balance, enabled the determination of N mass balances for different flow paths including groundwater, subsurface drain water, and overland flow for each piezometer transect. The type of overland flow largely affected nitrate-N (NO 3 -N) removal efficiency, as determined by the total N output from a transect relative to the NO 3 -N loading (%). Infiltration and subsurface flow followed by exfiltration (short return flow) allowed an efficient removal of NO 3 -N (71-94%), while direct overland flow strongly lowered NO 3 -N removal (25%) in one transect. The hydraulic loading rate versus the lowland infiltration capacity determined the transport pathways and thus the resulting NO 3 -N removal efficiency. For all transects there was a net export of organic N and/or ammonium, associated with in situ N release from peat decomposition, through overland flow and groundwater discharge. These exports partly counterbalanced NO 3 -N removal and significantly reduced the overall total N removal for the riparian lowlands. However, the N removal efficiencies remained positive (1-56%). The study indicates that N budgets for riparian lowlands need to account for overland flow as a transport pathway for N.

Riparian Lowlands in Clay Till Landscapes: Part I—Heterogeneity of Flow Paths and Water Balances

“…The top layer consisted of 0.5–2 m of peat or clay. Generally, clay content in this top layer was higher toward the edge of the riparian lowland (0–3% TOC) and peat content was higher toward the center of the valley (13–41% TOC) as seen in Petersen et al (2019).…”

“…While the focus was to identify and quantify the pathways of outflowing water, subsequent studies are needed to further quantify and determine parameters controlling the internal flow dynamics and the quantitative significance of direct overland flow versus short return flow or return flow . Nitrogen dynamics along the variable flow paths is described in a companion paper (Petersen et al, 2019).…”

“…Special focus was here on the differentiation and dynamics of overland and subsurface flow; (ii) to elucidate the influence of subsurface drains on the relative distribution of overland and subsurface flow; and (iii) to document the heterogeneity of flow paths and water balances within a small headwater lowland and reveal the major factors controlling the distribution of overland versus subsurface flow. The influence of these flow paths on the transport and transformation of N within the riparian lowland was investigated in a companion paper (Petersen et al, 2020).…”

Groundwater–surface water interaction in Denmark

“…Anthropogenic effects on the chemical characteristics of water were studied, for example, the implementation of controlled drainage to generate anoxic conditions to minimize the impact on harvest yields (Carstensen et al, 2019), the implications of human‐made drain management on stream‐riparian zones (Steiness et al, 2019), and the bypass effect of drains or geological conditions (Steiness et al, 2021). Studies specifically focusing on the transport of nutrients and geological characteristics have also demonstrated the preferred methodologies in clay till areas (Petersen, Prinds, Iversen, et al, 2020; Petersen, Prinds, Jessen, et al, 2020).…”