Consequences of the 2022 drought on nutrient dynamics in forest soil solutions of a declining spruce plot in the Strengbach catchment (Vosges Mountains, France)

Abstract. The year 2022 was the hottest and driest year ever recorded in France, including within the Strengbach catchment, a Critical Zone Observatory (http://ohge.unistra.fr, last access: 18 September 2025) located in a forested watershed of the Vosges Mountains and characterized by declining Norway spruce (Picea abies) stands. During and following the summer drought of 2022, an unusual chemical signature was detected in soil solutions, marked by elevated concentrations of K+, Ca2+, Mg2+, NO3+, NH4+, Al3+, and Dissolved Organic Carbon (DOC) with significant variations of fluorescence indices (HIX, BIX and FI). The long-term monitoring of soil solution chemistry exhibited that drought affects soil nutrient dynamics at the soil-water-plant interface through different mechanisms: (1) lower mineral dissolution, (2) reduced plant nutrient uptake, (3) increased concentrations in throughfall, (4) organic matter accumulation during the dry period and subsequent release upon rewetting, (5) changes in dissolved organic matter quality suggesting soil microbial stress, (6) disruption of the nitrogen cycle, with ammonium accumulation during drought followed by intense nitrification after rainfall resumed, and (7) acidification of the soil solution, enhancing the desorption of both nutrient cations and toxic Al3+. The decline in soil fertility during and after drought is especially concerning in forest ecosystems already subject to nutrient deficiency, such as those in the Strengbach catchment. Understanding these drought-induced biogeochemical disturbances is essential for predicting ecosystem responses to extreme climatic events, whose intensity and frequency are expected to increase in the Vosges Mountains under ongoing climate change.