Alkalinity and nitrate concentrations in calcareous watersheds: Are they linked, and is there an upper limit to alkalinity?

Abstract. Data from aquifers in calcareous watersheds in Switzerland demonstrate that alkalinity initially increases approximately in proportion to nitrate (NO₃⁻) concentration in the groundwater and eventually approaches an apparent maximum of approximately 8 mmol L⁻¹ at high NO₃⁻ concentrations. This close positive relationship between alkalinity and NO₃⁻ concentration appears to be predominantly a result of three processes: (i) mineralization of organic nitrogen (N) fertilizer, (ii) exchange of OH⁻ and H⁺ during the uptake of NO₃⁻ or ammonium (NH₄⁺), and (iii) CO₂ released by roots as a result of fertilizer-stimulated plant growth. Atmospheric deposition of N and strong acids (H₂SO₄ and HNO₃) play a minor role. We suggest that the asymptotic approach to a maximum groundwater alkalinity at NO₃⁻ concentrations exceeding 0.25 mmol L⁻¹ may be caused by (i) a maximum possible areal crop production at excessive N fertilization and (ii) an increasing CO₂ loss to the atmosphere due to the increasing CO₂ production in the soil. Thus, we estimate that the fertilizer-intensive agriculture of Switzerland generates an annual flux from the soil to the atmosphere of at least 0.26 Mt CO₂ a⁻¹. This analysis provides a general understanding and quantitative prediction of steady-state groundwater NO₃⁻ concentration; equilibrium groundwater alkalinity, pH, and pCO₂; and soil CO₂ emissions to the atmosphere based on quantitative and qualitative information on the supply of N and acidity to the soil by atmospheric deposition and N fertilization. The positive correlation between alkalinity and NO₃⁻ concentration in groundwaters persists in rivers and lakes. However, due to the diffusive loss of CO₂ to the atmosphere, subsequent precipitation of calcite, dilution with surface water, input of wastewater discharges and NO₃⁻ consumption by aquatic photoautotrophs, the correlation is less distinct.