The sensitivity of water chemistry to climate in a forested,nitrogen-saturated catchment recovering from acidification

Fluxes of major ions and nutrients were measured in the N-saturated mountain forest catchment-lake system of ?ertovo Lake (Czech Republic) from 1998 to 2014. The lake has been rapidly recovering from atmospheric acidification due to a 90% decrease in sulphate (SO₄^2?) deposition since the late 1980s and nitrate (NO₃^?) contribution to the pool of strong acid anion and leaching of dissolved organic carbon (DOC) have increased. Present concentrations of base cations, phosphorus (P), total organic N (TON), and ionic (Al_i) and organically bound (Al_o) aluminium in tributaries are thus predominantly governed by NO₃^? and DOC leaching. Despite a continuing recovery lasting 25 years, the ?ertovo catchment is still a net source of protons (H⁺), producing 44 mmol m^?2 yr^?1 H⁺ on a catchment-area basis (corresponding to 35 μmol L^?1 on a concentration basis). Retention of the deposited inorganic N in the catchment averages 20%, and ammonium consumption (51 mmol m^?2 yr^?1) and net NO₃^? production (28 mol m^?2 yr^?1) are together the dominant terrestrial H⁺ generating processes. In contrast, the importance of SO₄^2? release from the soils on terrestrial H⁺ production is continuously decreasing, with an average of 47 mmol m^?2 yr^?1 during the study. The in-lake biogeochemical processes reduce the incoming acidity by ～40%, neutralizing 23 μmol L^?1 H⁺ (i.e., 225 mmol m^?2 yr^?1 on a lake-area basis). Denitrification and photochemical and microbial decomposition of DOC are the most important in-lake H⁺ consuming processes (50 and 39%, respectively), while hydrolysis of Al_i (from tributaries and photochemically liberated from Al_o) is the dominant in-lake H⁺ generating process. Because the trends in water chemistry and H⁺ balance in the catchment-lake system are increasingly related to variability in NO₃^? and DOC leaching, they have become sensitive to climate-related factors (drought, elevated runoff) and forest damage that significantly modify the leaching of these anions. During the study period, increased exports of NO₃^? (accompanied by Al_i and base cations) from the ?ertovo catchment occurred after a dry and hot summer, after forest damage, and during elevated winter runoff. Increasing DOC export due to decreasing acid deposition was further elevated during years with higher runoff (and especially during events with lateral flow), and was accompanied by P, TON, and Al_o leaching. The climate-related processes, which originally “only” confounded chemical trends in waters recovering from acidification, may soon become the dominant variables controlling water composition in N-saturated catchments.