Long-Term Effects of High Nitrogen Loads on Cation and Carbon Riverine Export in Agricultural Catchments

The intensification of agriculture in recent decades has resulted in extremely high nitrogen inputs to ecosystems. One effect has been H+ release through NH4+ oxidation in soils, which increases rock weathering and leads to acidification processes such as base-cation leaching from the soil exchange complex. This study investigated the evolution of cation concentrations over the past 50 years in rivers from the Armorican crystalline shield (Brittany, western France). On a regional scale, acidification has resulted in increased base-cation riverine exports (Ca2+, Mg2+, Na+, K+) correlated with the increased NO3- concentration. The estimated cation increase is 0.7 mmol(+)/L for Ca2+ + Mg2+ and 0.85 mmol(+)/L for total cations. According to mass balance, cation loss represents >30% of the base-cation exchange capacity of soils. Long-term acidification thus contributes to a decline in soil productivity. Estimates of the total organic nitrogen annually produced worldwide indicate that acidification may also constitute an additional carbon source in crystalline catchments if compensated by liming practices.