Recycling of Nitrate and Organic Matter by Plants in the Vadose Zone of a Saturated Riparian Buffer

In agricultural land-use regions, excess nitrate in the soil contributes to eutrophication and pollution of both surface and subsurface waters. This study examines the role of plant uptake within the vadose zone of a saturated riparian buffer (SRB) to reduce nitrate from the soil pore water. Two hypotheses were explored: (1) during the growing season, nitrate removal will be greater in the presence of plants than where plants are absent (barren), and (2) following the growing season, nitrate concentration in the soils underlying a barren plot will be less than in the soils underlying a plot with plants. Within the SRB, three experimental blocks each composed of two plots were established. One plot allowed the growth of plants, primarily switchgrass (Panicum virgatum L.); the other plot was barren. Statistical comparison among the treatments, pre-growing season, plot with plants, and barren plot, and among the different depths, 30 cm, 60 cm, and 90 cm, identified significantly different soil NO₃⁻-N concentrations. Plots with plants experienced a reduction in nitrate from the soil and vadose waters. Nitrate concentrations in the soils underlying the barren plot were higher than in the soils with plants, a result of no plant uptake and of the decomposition of the residual plants’ material returning nitrogen to the vadose. Sustained nitrate concentrations imply plants withdraw nitrate from the vadose zone, generating organic matter that is recycled year-to-year, suggesting that the plants serve as a short-term nitrate sink.