Responses of mature forests to nitrogen deposition: insights from a nitrogen manipulation experiment in Italy

The ability of forests to continue absorbing atmospheric CO₂, and hence mitigating climate change, is constrained by global change drivers, such as increasing nitrogen deposition induced by anthropogenic activities. N input from atmospheric deposition can positively affect forest productivity in N-limited temperate and boreal ecosystems. However, under N saturation conditions, a cascade of negative effects can be observed, leading to tree growth decline, increase in soil acidification and N loss pathways, and nutrient imbalance. Experimental simulations of N deposition have been extensively used to directly determine whether atmospheric N input contributes to alleviating N limitation and to understand ecosystem responses. However, the majority of these experiments have considered soil N applications, often applying N doses several order of magnitude higher than ambient deposition, thus not mimicking realistic changes in atmospheric deposition. Moreover, soil applications exclude a priori atmosphere-canopy exchange processes, including direct canopy N uptake. In this context, the experiment established in a mature and eutrophic Fagus sylvatica L. forest in Italy represents a unique resource for advancing understanding on forest responses to global change. At this site, four different treatments have been carried out since 2015: control, canopy (30 kg ha^-1 yr^-1) and soil (30 and 60 kg ha^-1 yr^-1) N applications, with doses reflecting more realistic changes in atmospheric deposition (about twice and three times as much as ambient deposition). We asked whether the two experimental approaches (canopy vs. soil applications) can lead to different responses in terms of i) tree growth and intrinsic water-use efficiency (the ratio between photosynthesis and stomatal conductance) and ii) ecosystem nitrogen processes (including plant-microbe interactions). For this purpose, we combined dendroecological analyses (the measure of ring widths and stable carbon isotope composition, δ¹³C) with the measure of foliar nutrient concentrations and stable nitrogen isotope composition (δ¹⁵N) in different forest samples, including foliar, litter, soil main root and ectomycorrhizal root tips. On-going δ¹³C analyses will provide insight regarding the physiological mechanisms underpinning growth changes in relation to different treatments. Whereas, δ¹⁵N in different forest samples will help to elucidate differences in ecosystem N processes between the two approaches, i.e., N retention within the system or increasing N loss pathways and other nutrient limitations due to N saturation.