Aya Tora Foged Permin:
Global change effects on moss-associated nitrogen fixation and microbial communities across pristine ecosystems

Date: 05-07-2022    Supervisor: Kathrin Rousk & Anders Priemé



Mosses are a major part of the ground-covering plant communities in high latitude ecosystems and are involved in ecosystem processes by e.g. affecting soil biogeochemistry and decomposition processes. Further, mosses are associated with bacteria that can fix atmospheric nitrogen (N2) and turn it into bioavailable N forms, and moss-associated N2 fixation is an important input of ‘new’ N in N-poor ecosystems. Thus, mosses are involved in element cycling in high latitude ecosystem, such as the N and carbon (C) cycle. Consequently, the response of mosses towards global change may determine ecosystem feedbacks, including C balance in N-poor ecosystems. Moss-associated N input may likewise be important for ecosystem processes in other N-poor ecosystems beyond high latitude ecosystems, but this remains  unexplored.

Here, we investigated moss-associated N2 fixation activity in mosses from a tropical montane cloud forest (TMCF) in Peru to expand the current ecosystem range where moss-associated N input is studied. We demonstrated that N input via ground-covering mosses is a relatively high N input contributing c. 2 Kg N ha-1 yr-1 at ecosystem level in TMCF.

Further, we show that moss-associated N2 fixation activity responded differently to global change manipulations across ecosystems. In a subarctic birch forest ecosystem, long-term warming decreased N2 fixation activity at the scale of ecosystem. This was a combined effect of lower moss cover and suppressed N2 fixation activity at the scale of moss shoot in respond to increase warming, which we speculate to be linked to lower moss-moisture in warmed plots. In contrast, short-term warming at moss shoot level promoted N2 fixation in the same moss species collected in a boreal forest and N2 fixation activity here was not constrained by lower moss-moisture. However, we showed that moss-associated N2 fixation in a TMCF was drastically reduced in response to low mossmoisture.

We demonstrated that moss-associated bacterial communities differ significantly between boreal and tropical forests, driven by contrasting factors such as moss-moisture and moss N-level. Further, we showed that cyanobacteria dominate the N2-fixing bacterial communities across boreal and tropical ecosystems. However, the relation between specific N2-fixing bacteria and N2 fixation activity differed between ecosystems and N2 fixation activity associated with boreal mosses seemed to be closer linked to specific N2-fixing genera. 

In conclusion, our results indicate that moss-associated N2-fixing bacterial communities are sensitive towards global change in regards to N2 fixation activity and community composition. Our findings further suggest that ecosystem N input via mosses may change in the future with possible impacts on ecosystem processes such as element cycling.