Holm oak decline and mortality exacerbates drought effects on soil biogeochemical cycling and soil microbial communities across a climatic gradient

Abstract

The extent to which the increasingly frequent episodes of drought-induced tree decline and mortality could alter key soil biogeochemical cycles is unclear. Understanding this connection between tree decline and mortality and soils is important because forested ecosystems serve as important long-term sinks for carbon (C) and essential nutrients (e.g., nitrogen and phosphorus). In order to fill in this knowledge gap, we conducted a study on 13 sites distributed across the Spanish Iberian Peninsula where the dominant tree species was the Mediterranean evergreen Holm oak (Quercus ilex L. subsp. ballota [Desf.] Samp), a species that has shown important drought-induced crown defoliation and mortality rates in recent decades. Our study covered different climatic, soil, land-use type (forests, dehesas, and open woodlands), and crown defoliation (healthy, affected, and dead Holm oaks) gradients that characterize this species distribution within the Spanish Iberian Peninsula. Specifically, the soil C and nutrient content (nitrogen, N; phosphorus, P; magnesium, Mg), several functional parameters (heterotrophic respiration (RH); N mineralization (i.e., N ammonification, Ramm; and N nitrification, Rnit)), and relative abundances of key microbial soil functional groups (nitrifiers and ectomycorrhizal fungi (ECM)) were studied. Our results showed that aside from the potential effects associated with the climatic gradient, Holm oak decline and mortality resulted in soil stoichiometric imbalances triggered by net losses of essential oligonutrients (e.g., Mg) and the accumulation of very mobile forms of nitrogen (NO3- - N) and available phosphorus (Av P). Changes in the abundance of key microbial soil functional groups (nitrifiers and ECM) co-occurred with observed nitrate and available P accumulation. Therefore, we conclude that the potential vulnerability of soil C and nutrient stocks to ongoing changes in climate may strongly depend on tree vulnerability to climate change, its effect on soil-plant relationships, and how this may impact the ecology and functioning of key soil functional groups and key metabolic pathways. © 2020 Elsevier Ltd