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https://doi.org/10.5194/bg-2018-461
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-461
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

  06 Nov 2018

06 Nov 2018

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This preprint has been withdrawn by the authors.

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

Beat Müller1, Joseph S. Meyer2,3, and René Gächter1 Beat Müller et al.
  • 1Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland
  • 2Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, USA
  • 3Applied Limnology Professionals LLC, Golden, Colorado 80401, USA

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.

This preprint has been withdrawn.

Beat Müller et al.

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Beat Müller et al.

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Short summary
Data from aquifers, rivers and lakes in calcareous watersheds in Switzerland display a distinct correlation between alkalinity and nitrate concentrations. Analysis of long-term datasets suggests that this close positive relationship is a result of three processes: (i) mineralization of organic fertilizer, (ii) exchange of acid or base ions during the uptake of ammonium or nitrate by crop plants, and (iii) carbon dioxide release by roots as a result of fertilizer-stimulated plant growth.
Data from aquifers, rivers and lakes in calcareous watersheds in Switzerland display a distinct...
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