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Mosses as a gateway of nitrogen into northern ecosystems

Periodic Reporting for period 1 - MYCOMOSS (Mosses as a gateway of nitrogen into northern ecosystems)

Reporting period: 2018-03-01 to 2020-02-29

Climate change is one of the major challenges of our time and the socio-economic consequences are alarming. High latitude ecosystems experience especially drastic changes in temperatures. In cold ecosystems organic matter decomposition is slow, which restricts recycling of nitrogen (N), which here is the primary limiting nutrient for plant growth . However, new N can enter ecosystems via biological N2 fixation taking place in e.g. mosses or through atmospheric deposition. N2 fixation is highly temperature dependent and increased N input via N2 fixation could therefore be a consequence of climate warming. Input of N via atmospheric N deposition is also increasing as a result of increased anthropogenic use of synthetic fertilisers. Increased N availability has large effects on plant production, community composition , and plant and soil carbon stocks, which can ultimately feedback to the global carbon cycle and thus influence climate change. Understanding N cycling in northern latitude ecosystems is therefore essential for understanding consequences of climate change.

Mosses are primitive plant and are a major component of high latitude and high elevation ecosystems where they often cover most of the ground. Mosses are known to be important contributors to primary production and through their insulation and water retention capacity they control soil biogeochemical processes such as decomposition of soil organic matter. However, mosses are also key players for N input to terrestrial ecosystems as they readily take up nutrients entering the ecosystem from deposition through the cell walls of their thin leaves and through their association with bacteria fixing atmospheric N2. In this way, mosses work as a gateway and conduit for new N to enter these N-limited ecosystems. Mosses are considered efficient in holding on to newly incorporated N in the short term and old moss tissue is highly recalcitrant and therefore decomposing slowly. However, mosses may loose N via leakage e.g. upon drying and re-wetting , while old parts of the moss shoot eventually die and turn into litter, which then becomes subject to decomposition. Mosses thus likely play a two-faced role in the cycling of N but the mechanisms of and extent to which N bound in mosses becomes available to the decomposer sub-system and to vascular plants is largely unknown, despite the crucial role of N in these ecosystems.

The overall objective of MYCOMOSS was to develop mechanistic and quantitative understanding of the role of mosses as providers of new nitrogen to nitrogen-limited ecosystems under climate change.
I estimated atmospheric input of C (photosynthesis) and N (N2 fixation) in three common tundra moss species through one growing season under experimental climate warming and evaluated possible effects this could gave on the ecosystem. N2 fixation and photosynthesis were collected 5 times during the growing season at eight field sites above the alpine treeline. I also assessed moss growth, nitrogen and phosphorous leaching and C and N in moss tissue content (as a measure of decomposability). The main finding of these results are that moss water content determines the magnitude of the warming response. With some deviations, mosses that hold on to moisture well, mosses in wet habitats and mosses during wet season increased their, N2 fixation, growth or photosynthesis under warming. This has implications for our predictions of climate change effects on the C and N cycle and shows that moisture is a key factor in regulating warming responses. These results are under preparation for publishing

As nitrogen is highly limiting for growth in cold ecosystems, it is possible that other organisms, eg. plants and fungi, will try to get hold of nitrogen entering the ecosystem via mosses. To describe fungal community composition in the moss layer, I collected moss shoots from 2 experiments in Sweden. One in Boreal and one in subarctic forest. I analysed shoots for 15N and 13C content and for fungal community composition and total fungal biomass. Fungal community analysis revealed that plant-associated fungi are present all the way up into the living moss tissue. These results are currently being remade with some methodological adjustments. The planned manuscript is therefore still under preparation.

To directly trace nitrogen from the moss into the rest of the ecosystem, I used labeled N (15N) added to the moss. I created 144 controlled mini-ecosystems containing soil, one of two plant species and moss covering the soil. Soil and plants were separated from the moss with one of 3 meshes of varying sizes. In this way, 15N in mosses could enter the soil-plant compartment 1) only through free water transport, 2) through both water transport and mycorrhizal colonisation of the moss or 3) through any mechanism also including plant roots growing into the moss layer. Plants established well but did not grow as fast as anticipated. I therefore decided to postpone the planned harvest of pots in 2019 to 2020. Instead, in 2019 preliminary samples of plant leaves were taken and analysed for 15N. These showed that we could trace 15N which confirms that the concentrations added are sufficient for our purpose. However, after only one year we did not see any difference in plant 15N concentration between plant with different access to moss N. This year I and MSc will finalise the project. She will include the results in her MSc thesis and I will synthesise the result in a manuscript for publication.
This research has consequences for our understanding of nitrogen and carbon storage and release, which control the natural contribution to CO2 concentrations in the atmosphere and thus climate warming. This will give rise to better predictions of climate change by e.g. the IPCC (Intergovernmental Panel of Climate Change). Further, better understanding of the functional diversity of moss species will highlight the importance of including mosses when predicting effects of global change Arctic ecosystems.
Signe Lett during field campaign measuring moss associated N2 fixation in the field in June 2018.
Mini-ecosystems with dwarf birch and bog-rosemary, and mixed moss cover.
three mosses included in WP1. Hylocomium spendens, Pleurozium schreberi, and Sphganum app.
Boreal forest trenching experiment in Jadraås, SE, belonging to Ass. Prof. Karina Clemmensen