Nitrogen fixation in Boreal feathermoss communities

Transcription

1 Nitrogen fixation in Boreal feathermoss communities N 2 Marie-Charlotte Nilsson NH 4 + Swedish University of Agricultural Sciences (SLU) Faculty of Forest Sciences Umeå, Sweden

2 With contributions from: Tom DeLuca Michael Gundale Karolina Iningbergs Anita Sellstedt Olle Zackrisson David Wardle Fuijo Hyodo Ulla Rasmussen Fransesco Gentili Ellen Dorrepaal PhD-students

3 Cover 65-80% Biomass g/m 2 Moss biomass, productivity Time since last fire Pleurozium schreberi Hylocomium splendens

4 Previously thought: lack of wide-spread N 2 -fixers insignificant N 2, NOx Organic N accumulation (3 kg N/ha/yr) Loss of N (consumption of vegetation and humus) succession Low N availability

5 Calothrix sp. Epiphytic cyanobacteria on feather mosses 25 µm 25 µm Nostoc sp. Represents the main form of N input to old boreal forests Cylindrospermum sp. Stigonema sp. 25 µm 25µm DeLuca et al. Nature (2002) Gentili et al. (2005)

6 Main topics 1. Biological N 2 -fixation (BNF) patterns of feather mosses 2. Fundamental biotic and abiotic controls on BNF and implications for global change 3. The significance of BNF for the boreal forest

7 Study system Fire chronosequences short-term ( yrs) long-term ( yrs) Arctic circle N Umeå Stockholm

8 1. Biological N 2 -fixation (BNF) patterns of feather mosses kg N/ha/yr < 10% of internal N cycling rates N 2 -fixation vs succession > NH 4 + and NO 3 - deposition Equivalent to mean annual accumulation rates of N Meet N requirements to support NPP of mosses Acetylene reduction (µmol m 2 day -1 ) Time since fire (yr)

9 Hylocomium splendens

10 Seasonal pattern of BNF Early-summer Autumn Mid-Summer Months of the year DeLuca et al. Nature (2002)

11 Species differences in BNF Hylocomium Pleurozium 5 (µg N/moss g/day) Late Mid Early 0 June September Lagerström et al. (2007) Prel data Guillaume Bay

12 2. Abiotic and biotic controls on BNF (global change) Stigonema sp. Time since fire (yr)

13 Transplanting experiment Effects of site Presence of cyanobacteria Early Scots pine Late Norway spruce

14 Acetyleene reduction (P<0.01) Early to late Late to early 100 (P<0.05) 0 Simplified from DeLuca et al. Oecologia (2007)

15 N deposition via canopy throughfall - demand driven? 9 kg N/ha/yr Resin lysimeters (n=12) A= 4.9 cm 2 P appears not limiting Other co-limiting factors such as moisture DeLuca et al. Nature (2008)

16 Cyanobacteria presence/absence No of cyanobacterial cells/shoot Early Late P<0.05 P<0.01 Simplified from DeLuca et al. Oecologia (2007)

17 Cyanobacterial diversity and composition (nifh gene sequences)? Pleurozium (6 specific) Hylocomium (17 specific) n=10 35 unique DGGE bands ( species /phylotypes) Most types found in all successions High degree of host specificity (only 12 shared) Early Mid Late Iningbergs et al. New Phyt (2011) Early Mid Late Moss species determined phylotypes (not succession)

18 Pleurozium Pleurozium schreberi Hylocomium splendens Nitrogen fixation µg g -1 moss d r = ; P = r = ; P < Number of phylotypes Number of phylotypes Nitrogen fixation µg g -1 moss d r = ; P = r = ; P < PC1 PC1 Community composition (DGGE band data) Iningbergs et al. New Phyt (2011)

19 Studies in relation to global change - Background Plant productivity is expected to increase in boreal forests (climate warming and CO 2 ) Some models predict that productivity may be constrained if a corresponding increase in BNF does not co-occur. Changes in N availability and N supply rates are of interest, because it may impact on the boreal forest to serve as a C sink.

20 Fertilization experiment (ongoing) N C P C*N C*P N*P C*N*P Control N= 5 kg/ha C= 10 kg/ha P= 10 kg/ha

21 + 2 deg C and a 3-fold frequency reduction Warming and changed precipitation ( ) + recovery (2010) 116% of normal 7% of normal 29% of normal Ambient (control)

22 Recovery Gundale et al. Biol Letters (2012)

23 Quantity and frequency N 2 fixation (mg N m -2 day -1 ) normal Stronger response in late successions Gundale et al. CJFR (2010) Jackson et al. Oikos (2010)

24 Open top chambers Run by Ellen Dorrepaal

25 Changes in vegetation: Removal experiment 13 years prior to measurements (ericaceous dwarf shrubs and tree roots)

26 Empetrum hermaphroditum Vaccinium vitis-idaea Vaccinium myrtillus Tree roots Removal of: HYLOCOMIUM: N 2 -fixation (m 2 ) N 2 -fixation (g moss) Moss biomass Decline No change Decline Decline No change Decline No change No change No change Decline Decline Decline N 2 -fixation (m 2 ) N 2 -fixation (g moss) PLEUROZIUM: No change No change No change No change No change No change No change No change Moss biomass Increase Increase No change No change Gundale et al. Ecology (2010)

27 3. Significance for the boreal forest Nitrogen in atmosphere (N 2 ) Ammonium NH 4 + Key question: Faith of N? (supply rate to plants) Transfer to moss (prel data)

28 Stable isotope analysis (origin of N) δ 15 N ( ) δ 15 N increase with time Successional stage Hyodo & Wardle Rapid Commun Mass Spectrom (2009)

29 δ 15 N decrease with time Successional stage Hyodo et al. Plant and Soil (2012)

30 Meet N requirements to support NPP of mosses Tissue %N and decomposition of mosses is low Equivalent to mean annual accumulation rates of N Maintenance of evergreen ericaceous dwarf shrubs (not trees)?

31 Conclusions High spatial and temporal variability of N 2 -fixation Moss species, cyanobacteria diversity and composition has an important role Environmental conditions and interactions between shrubs and mosses are important for determining N 2 - fixation rates The significance for boreal forests needs to be further evaluated