HYDROLOGIC LOSSES AND CONCENTRATION-FLOW RELATIONSHIPS OF NUTRIENTS AND MAJOR IONS IN A SEMI-ARID AGRICULTURAL CATCHMENT

Mineral-derived nutrient cations are essential to fertility, and acidification of soils due to chemical fertilization may result in unsustainable loss fluxes from the soil exchange pool. Mineral weathering also contributes to nutrient cation loss fluxes. This study investigated nutrient cation and major ion fluxes for two years in drainage from a semi-arid, rain-fed catchment at the Cook Agronomy Farm (CAF) LTAR site in southeastern Washington. We measured flows, analyzed drainage samples, and estimated annual subsurface drain fluxes of Ca²⁺, Mg^2+, Na⁺, K⁺, HCO₃^-, NO₃^-, SO₄^2-, Cl^- and SiO₂ for water years 2015 - 2016 using LOADEST. The total dissolved solids (TDS) loss rate at CAF-LTAR ranged from 90 – 230 kg ha^-1 yr^-1, which is comparable to other catchments on silicate terranes. Drainage from this catchment is characterized as calcium-bicarbonate-nitrate dominant, which suggests that both H₂CO₃ and HNO₃ are important acids removing nutrient cations. Concentration-discharge dynamics show that minor dilution occurs in Ca²⁺, Mg²⁺, Na⁺, and Cl^-, major dilution occurs in HCO₃^-, chemostatic behavior occurs in SiO₂ and SO₄^2-, flushing occurs in K⁺, and NO₃^- behavior varies seasonally. Solute fluxes from the 2015 and 2016 water years are roughly equivalent when normalized to subsurface drain discharge, indicating that discharge exerts strong control on nutrient and major ion fluxes. The results of this study indicate that anthropogenic acids impart a discernible imprint on shallow groundwater chemistry and that the hydrologic loss vector must be considered in the long-term management of agroecosystem nutrient budgets.