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

Abstract

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.