Combined climate factors alleviate changes in gross soil nitrogen dynamics in heathlands

The ongoing climate change affects biogeochemical cycling in terrestrial ecosystems, but the magnitude and direction of this impact is yet unclear. To shed further light on the climate change impact, we investigated alterations in the soil nitrogen (N) cycling in a Danish heathland after 5 years of exposure to three climate change factors, i.e. warming, elevated CO₂ (eCO₂) and summer drought, applied both in isolation and in combination. By conducting laboratory ¹⁵N tracing experiments we show that warming increased both gross N mineralization and nitrification rates. In contrast, gross nitrification was decreased by eCO₂, an effect that was more pronounced when eCO₂ was combined with warming and drought. Moreover, there was an interactive effect between the warming and CO₂ treatment, especially for N mineralization: rates increased at warming alone but decreased at warming combined with eCO₂. In the full treatment combination, simulating the predicted climate for the year 2075, gross N transformations were only moderately affected compared to control, suggesting a minor alteration of the N cycle due to climate change. Overall, our study confirms the importance of multifactorial field experiments for a better understanding of N cycling in a changing climate, which is a prerequisite for more reliable model predictions of ecosystems responses to climate change.