Influence of nested groundwater systems on reduction–oxidation and alkalinity gradients with implications for plant nutrient availability in four New York fens

Summary This study compares hydrologic investigations of four New York fens designed to characterize spatial variability in groundwater flow (GWF) and the resulting hydrochemical patterns that influence plant nutrient availability. At the marl fen and two rich fen sites, hydrometric data and pore water samples collected from nested piezometers set along the GWF-path showed spatial patterns consistent with a mixing gradient established by the intersection of local (i.e., hill slope), relatively dilute groundwater discharge and mineral-rich groundwater discharged from a larger-scaled (i.e., intermediate) system. Distinct redox and carbonate gradients along the GWF-paths of three rich fens reflected strong interactions between groundwater supply of terminal electron acceptors and microbial processes affecting speciation of iron, nitrogen and phosphorus. Decreased nitrate concentrations, increased dissolved iron, and removal of sulfate along flowpaths indicated that redox conditions became more reduced as groundwater moved down-gradient. Nitrate removal occurred across the interface between the upland mineral soil and the organic-rich fen peat. Lack of ammonium accumulation indicated that denitrification limits N-supply to plants in down-gradient fen areas. Further down-gradient, sulfate-enriched groundwater discharge sustained sub-oxic conditions and enhanced phosphorus mobility through effects on iron–sulfur–phosphorus dynamics, alkalinity generation, and dissolution of mineral-bound P. In contrast, the poor fen was sustained by an isolated surficial groundwater system; a thick lacustrine clay aquitard limited intermediate or regional-scaled groundwater discharge. As a result, water table fluctuations were more responsive to short-term weather events and imposed the strongest influence on redox status. A redox gradient, evidenced by increasing iron and ammonium concentrations, extended downward with depth from the aerated peat surface. The dilute water supply and lack of additional internal alkalinity generation limited buffer capacity and decomposition. Overall, ammonium accumulation increased N-availability whereas reduced decomposition rates further limited P availability. Our results show that interaction of multiple GWF systems within the fens was the critical determinant of water table stability (rich fens) or fluctuation (poor fen), as well as the supply of dissolved ions that influence geochemical processes affecting plant nutrient availability. Together these factors accounted directly or indirectly for observed spatial patterns in redox and alkalinity gradients, which in turn controlled speciation of phosphorus-reactive minerals. Differences among fens in spatial patterns could be accounted for by differences in the geomorphometry of the basins within which the fens developed and the stratigraphy and hydraulic characteristics of underlying sediments. Results suggest that Fe–S dynamics, rather than carbonate precipitation, likely influences inorganic phosphorus pools and release mechanisms.