The Boreal Nitrogen Gap: Size, fate and impacts of nitrogen fixation in Fennoscandia forest ecosystems

Lead Research Organisation: Bangor University
Department Name: Sch of Environment and Natural Resources


Nitrogen is a primary limiting nutrient in high boreal and subarctic forest ecosystems, unfortunately, our understanding of fundamental N cycling processes in these oligotrophic environs remains limited which in turn hinders our ability to effectively predict ecosystem carbon (C) turnover and greenhouse gas emissions. Over the past 10 years we have elucidated a major N input in high boreal and subarctic ecosystems (1, 2), specifically we described the occurrence of an association between cyanobacteria and the ubiquitous and dominant feathermosses Pleurozium schreberi and Hylocomium splendens (3, 4). These two mosses act as the primary sources of biologically fixed N in subarctic and boreal forest systems and are also important in Arctic alpine tundra ecosystems (5). Although we have developed a clear picture of the quantity of N fixed by this association (6), our understanding of the pathway for N release into the ecosystems from the feathermosses remains greatly incomplete. A thorough understanding of these pathways will lead to a better understanding of C cycling processes at high latitudes and benefit a range of researchers from botanists, to ecologists, to ecosystem modellers.

In this proposal we put forth several compelling hypotheses to further our understanding of fundamental processes in the N cycle of boreal Scots pine ecosystems and advance our general knowledge of the ecology of the feathermoss-cyanobacterial association in boreal forests of northern Sweden. To date most of our understanding of this association has surrounded field studies regarding the ecology of the feathermosses, little effort has been made to link the association to the belowground nutrient cycles and microbial community.

Studies have been designed to address the following objectives: The four primary objectives of this proposed research are therefore to: (1) Assess the relationship between feathermoss N fixation and site productivity along natural fertility gradients and N deposition gradients in northern Fennoscandia; (2) Determine the portion of fixed N that remains in an organic form in moss detritus and the role of disturbance in liberating this N; (3) Evaluate the role of mycorrhizas in the transfer of N (amino acid and peptide N) from feather mosses to plants; (4) Evaluate the relationship between fixed N and forest floor CO2, CH4, and N2O fluxes across N enrichment gradients (i.e. N deposition gradient and fire-chronosequence).

Experiments will be conducted on established field sites in northern Sweden including a fire chronosequence to address change in N turnover with natural forest succession; a total of five natural fertility gradients to assess the influence of variable N and P limitation on N fixation, turnover and greenhouse gas emissions; and an anthropogenic N deposition gradient from southern to northern Sweden along which we will assess the influence of increased N deposition on N fixation, turnover and loss.

These findings will have significant implications for those studying ecosystem processes at high latitudes including N fixation, N turnover, C fixation, and greenhouse gas emissions. It is our intent to work these results into existing biogeochemistry models that are designed to predict linked C and N dynamics as related to productivity and greenhouse gas emissions.

Literature Cited
1. T. H. DeLuca, O. Zackrisson, M.-C. Nilsson, A. Sellstedt, Nature 419, 917 (2002).
2. O. Zackrisson, T. H. DeLuca, M.-C. Nilsson, A. Sellstedt, L. Berglund, Ecology 85, 3327 (2004).
3. O. Zackrisson, T. H. DeLuca, F. Gentili, A. Sellstedt, A. Jäderlund, Oecologia In Press, (2009).
4. F. Gentili, M. C. Nilsson, O. Zackrisson, T. H. DeLuca, A. Sellstedt, Journal of Experimental Botany 56, 3121 (2005).
5. T. H. DeLuca, O. Zackrisson, Plant and Soil 294, 147 (2007).
6 T. H. DeLuca, O. Zackrisson, M.-C. Nilsson, M. J. Gundale, Science 320, 1181 (2008).

Planned Impact

The primary beneficiaries of the proposed research in the UK are those associated with the NERC Arctic Research Programme. This programme specifically requires a solid understanding of N cycling processes if effective carbon modeling efforts are to be made in this region. Other potential beneficiaries include ecologists, soil scientists, climate scientists, and modelers that are interested in ecosystem processes in high boreal and Arctic environs. Given the size of the Boreal and Arctic biomes and the total carbon storage in these ecosystems, it is highly important we understand the links between N fixation, ecosystem productivity, and carbon cycling. Although the work is focused in boreal regions, the fundamental science we hope to address is also highly relevant the bryophyte dominated polar environs and thus is of interest to the Scientific Committee on Antarctic Research (SCAR; Members, Officers and Secretariat), members attending the annual Antarctic Treaty Consultative Meetings and Antarctic Committee for Environmental Protection.

To achieve an impact, the main PDRA will interact directly with the Arctic Research Programme's outreach team. The purpose of this exchange will be to ensure effective knowledge transfer to those research projects funded within the Arctic ecosystems call and to build links with policy makers and understand their requirements for research knowledge exchange. We will also hold a single workshop (entitled Natural Nitrogen Capital) in month 24 which provides a forum for scientists working on N cycling in boreal and polar ecosystems (Arctic and Antarctic) to exchange ideas and learn new skills but importantly the interaction with key stakeholders. Day 1 will be an experimental training day designed to cater for PhD and PDRAs where they can learn new techniques and share experiences. Day 2 will be a series of formal talks and discussions. Day 3 will provide a forum modeling experts to assess the utility of the data and provide opinions on research gaps. This will then be followed by a discussion of knowledge gaps, future research funding priorities, and future partnerships.

We will deliver our findings to the greater scientific community through conventional channels of conference attendance and delivery of talks, publication of results in international journals of ecology and soil science and through scientific networking via electronic applications such as environmental and ecological web portals including the Knowledge Transfer Network for Environmental Sustainability.

A significant effort will be made to evaluate the management implications of our results and deliver those findings in a meaningful manner to stakeholders including government agencies (e.g. Forest Commission), charities, and policy makers via news releases, and publication in popular press. We have established a community outreach program at Treborth Botanic Garden in Bangor partially through a grant from Beacon for Wales in which we teach biology, phenology, climate change, and sustainability to citizens and specifically vulnerable adults. We can use this forum as a means of sharing our findings within the region. We have also begun working with Technicum CAST on visualization media as a means of making our work more tangible and meaningful to non-specialists. The process of building visualization materials also helps us to ask questions regarding our own work, as somewhat of a feedback loop.

We will publish articles for non-specialist publications such as NERC's Planet Earth, the Ecologist, and The National Trust. We will develop a web page on the University website with the PDRA that will include the 'Boreal Blog' intended for continual updates and feedback on our research findings and updated photos from the field.
Description Odd though it may seem for this cold and often snowy land, fire is a fundamental feature of the Boreal Forest. The apparent destruction of such a disturbance event is in fact an act of renewal, creating wildlife habitat and maintaining forest health. But fire also opens new dilemmas around nitrogen availability in the boreal forest. About 250 - 450 kg N ha-1 of this precious nutrient are volatilize in each forest fire, from the burning of needles, twigs, shrubs, and the forest floor. If N is to be sustained in these forests, N must be replenished within the natural fire return interval.

During the past decades ecologists has been asking the same question over and over again, where does the N come from in a forest without leguminous or woody N fixers? In 2002, Tom DeLuca and colleagues discover that a substantial amount of N can enter the system via biological N fixation in cyanobacteria living epiphytically on feather mosses that cover the forest floor. This finding opened new questions about N dynamics within the boreal ecosystem: Is the fixed N held in the moss? How is all that N transferred ultimately to the trees, and how long is that process taken? and, is there a relationship between forest N status and N fixation rates in the feather mosses? In summary, how all that system works? After three years of intense and exciting research, our research team has generated some remarkable results that we believe will open new ways to understand ecosystem function in the Boreal Forest. The following provides a brief summary of some of our conclusions.

The relationship between cyanobacteria and feather mosses is often considered an association rather than a symbiosis as it is not yet known whether cyanobacterial colonisation is of benefit to the moss. Our results from a 360-year fire chronosquence in northern Sweden suggest that the transfer of fixed N from cyanobacteria may have little direct impact on P. schreberi function [1] and occurs only under conditions that facilitate high cyanobacterial colonisation and N fixation rates (e.g. early successional stands without a tree canopy). We traced the fate of fixed N at nanoscale level and observed a spatial differentiation for fixed N acquisition by the moss cells where uptake occurs only by photosynthetically active moss cells [2]. This restricts the moss assimilation of fixed N to conditions when cyanobacteria are present in the green living part of the moss [2]. The newly fixed N assimilated by the moss is greatly conserved in the moss tissue and eventual occurs in the humus layer as moss decomposes [3]. On an annual basis, we suggest that disturbance events such as drying-rewetting cycles may serve as short-term drivers to liberate the fixed N from senescent moss tissue [4]. On a long-term basis, our results showed that fire disturbance could serves as a major driver to liberate the N that is fixed by the cyanobacteria and hold in the moss tissue and humus layer [3].
On the other hand, the highly diverse bacterial and fungal communities that live in association with the moss [5] appear to access the fixed N efficiently and across all sections of the moss shoot, both green and brown sections [2]. Over the same 360-year fire chronosquence we observed a shift in bacteria community composition associated with the moss layer from oligotrophic to copiotrophic bacteria as stand age, reflecting an ameliorating of growth conditions for bacteria where moss N2 fixation is greatest [4]. Heterotrophic organisms which feed on or interact with cyanobacteria within this complex systems called "the bryosphere" - the living and dead parts of the moss layer along with the associated biota- are likely to play a key role in the turnover and cycling of fixed N at an ecosystem level. In this light, we observed a positive correlation between feather moss N2 fixation and ecosystem N accumulation following stand age, as previously described, but also with soil dissolve organic N and gross turnover of inorganic N along the fire chronosequence studied here [6]. Interestingly, rather than a decline in N status over time our findings indicate that the nutrient status of stands approached equilibrium during late secondary succession after fire where moss N2 fixation is greatest [6].

1. Lorna E. Street, María Arróniz-Crespo, Leopoldo G. Sancho, Simon Oakley, Nick Ostle, Davey L. Jones and Thomas H DeLuca. Interactive effects of nitrogen deposition and nitrogen fixation on Boreal feather moss function. (Plant, Cell and Environment)

2. María Arróniz-Crespo, Jeremy Bougoure, Daniel Murphy, David L. Jones, Virginia Souza-Egipsy, Nick Cutler, Nick Ostle, Peta Clode, Stephen Clayton Wade, Thomas H. DeLuca. Nitrogen fixation in the boreal bryosphere: it takes a village. (Nature Plants)

3. Thomas H. DeLuca, Olle Zackrisson, Marie-Charlotte Nilsson, María Arróniz-Crespo. Tracing the Long-Term Fate of Nitrogen Fixed by Cyanobacteria in Pleurozium schreberi Brid (Mit.) Moss Carpets of Boreal Forests. (Ecosystems)

4. María Arróniz-Crespo, David L. Jones, Nick Ostle, Gwen Lancashire, Thomas H. DeLuca. Significant releases of organic nitrogen from Pleurozium schreberi Brid (Mit.) moss carpets of Boreal Forests: in situ monitoring by microdialysis probes. (Soil Biology & Biochemistry)

5. Nick A. Cutler, María Arróniz-Crespo, Lorna E. Street, David L. Jones, Dominique L. Chaput, Thomas H. DeLuca. Long-term recovery of microbial communities in the boreal bryosphere following fire disturbance (Ecology)

6. María Arróniz-Crespo, David L. Jones, Lorna E. Street, Nick Ostle, Simon Oakley, Olle Zackrisson, Kathrin Rousk, Johannes Rousk, Kelly Mason, Michael Gundale, Thomas H. DeLuca.. Long-term shifts in stand nutrient status through succession in fire-disturbed boreal forest (Functional Ecology)
Exploitation Route We have developed a range of techniques that can be taken forward by other researchers in he field.
Sectors Agriculture, Food and Drink,Environment

Description see outputs here
First Year Of Impact 2015
Sector Environment
Impact Types Cultural,Societal

Description Conference talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact talk at ETH Switzerland
Year(s) Of Engagement Activity 2016
Description Open science talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Talk on polar biology
Year(s) Of Engagement Activity 2016
Description Talk at Southampton Univ 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Talk with department. Discussion of new potential research areas
Year(s) Of Engagement Activity 2016
Description Talk to CAS China 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact Talk at CAS China on research.
Year(s) Of Engagement Activity 2015
Description conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Postgraduate students
Results and Impact Seminar at BOKU, Vienna
Year(s) Of Engagement Activity 2016