BGD

Biogeosciences Discussions

BGD

Biogeosciences Discuss.

1810-6285

Göttingen, Germany

10.5194/bg-2018-461

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

Müller

Beat

https://orcid.org/0000-0003-3696-9035

¹ Meyer

Joseph S.

² ³ Gächter

René

Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland

Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, USA

Applied Limnology Professionals LLC, Golden, Colorado 80401, USA

06 11 2018

2018 1 19

2018

This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

This article is available from https://bg.copernicus.org/preprints/bg-2018-461/

The full text article is available as a PDF file from https://bg.copernicus.org/preprints/bg-2018-461/bg-2018-461.pdf

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.