LAUSR

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.