Canopy Exchange and Modification of Nitrogen Fluxes in Forest Ecosystems

Abstract

Purpose of Review We provide an overview of the main processes occurring during the interactions between atmospheric nitrogen and forest canopies, by bringing together what we have learned in recent decades, identifying knowledge gaps, and how they can be addressed with future research thanks to new technologies and approaches. Recent Findings There is mounting evidence that tree canopies retain a significant percentage of incoming atmospheric nitrogen, a process involving not only foliage, but also branches, microbes, and epiphytes (and their associated micro-environments). A number of studies have demonstrated that some of the retained nitrogen can be assimilated by foliage, but more studies are needed to better quantify its contribution to plant metabolism and how these fluxes vary across different forest types. By merging different approaches (e.g., next-generation sequence analyzes and stable isotopes, particularly oxygen isotope ratios) it is now possible to unveil the highly diverse microbial communities hidden in forest canopies and their ability to process atmospheric nitrogen through processes such as nitrification and nitrogen fixation. Future work should address the contribution of both foliar nitrogen uptake and biological transformations within forest canopies to whole ecosystem nitrogen cycling budgets. Summary Scientists have studied for decades the role of forest canopies in altering nitrogen derived from atmospheric inputs before they reach the forest floor, showing that tree canopies are not just passive filters for precipitation water and dissolved nutrients. We now have the technological capability to go beyond an understanding of tree canopy itself to better elucidate its role as sink or source of nutrients, as well as the epiphytes and microbial communities hidden within them.

Nitrogen deposition in forests: Statistical modeling of total deposition from throughfall loads

“…To our knowledge, the leaching of inorganic nitrogen from forest canopies has not been reported in the literature until now. On the contrary, numerous reports indicate that inorganic nitrogen may be taken up by canopy foliage, epiphytic lichens or other microflora (Sparks, 2009;Butterbach-Bahl et al, 2011;Woods et al, 2012;Guerrieri et al, 2021). In view of the large scatter of modeled TD at high TF rates, we hypothesize that the data points with TD < TF (i.e., data points below the 1:1 line in Figure 4, at TF values > 20 kg N ha −1 yr −1 ) reflect the uncertainty inherent in both the TF measurements and the TD estimations.…”

Relative importance of soil properties and functional diversity to the spatial pattern of the forest soil nitrogen

“…The surface soil had a higher capacity for NH 4 + sorption than the sub-surface layer (Manirakiza and Seker 2020). Organic nitrogen undergoes nitrification to form nitrate and nitrite compounds, a process that requires sufficient oxygen and carbon sources as well as suitable temperature and humidity (Guerrieri et al, 2021, Rollinson et al, 2021. Nitrate nitrogen was more mobile in the soil and could more easily enter the sub-surface soil (Klimas et al, 2020).…”

Conceptual analysis: What signals might plant canopies send via stemflow?

“…Canopies have multiple opportunities for microbiallymediated N retention (Guerrieri et al, 2021), including in canopy soils-a compartment of this ecosystem recently gaining research attention (Gotsch et al, 2016;Tatsumi et al, 2021). Monitoring for known saprotrophic signals in stemflow, for instance, may provide insights into canopy soil processes and how they differ from processes in ground soils (Matson et al, 2014;Looby et al, 2020).…”