Nitrogen fertilization of soils fuels carbonate weathering and translocation in calcareous watersheds

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In calcareous watersheds, groundwater alkalinity results largely from dissolution of carbonate minerals in soils. The alkalinity increases initially approximately in proportion to nitrate (NO3−) concentration 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 fertilizer, (ii) exchange of OH− and H+ during the uptake of NO3− or ammonium by crop plants, and (iii) CO2 released by roots as a result of fertilizer-stimulated plant growth. 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. Our analysis provides a general understanding and quantitative prediction of steady-state groundwater NO3− concentration, alkalinity, pH, the degree of CO2 supersaturation in the soil, and soil CO2 emissions to the atmosphere. The positive correlation between alkalinity and NO3− concentration observed in groundwaters persists in rivers and lakes. We conclude that an economically efficient agricultural practice that avoids over-fertilization might accelerate the in-soil carbonate weathering rate up to approximately threefold compared to unfertilized soils, but it will not jeopardize the use of aquifers for drinking water.