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Feather Mosses, Nitrogen Fixation and the Boreal Biome 27 Feather Mosses, Nitrogen Fixation and the Boreal Biome

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Feather Mosses, Nitrogen Fixation and the Boreal Biome 27 Feather Mosses, Nitrogen Fixation and the Boreal Biome 26 Feather mosses, nitrogen fixation and the boreal biome 27 Feather mosses, nitrogen fixation and the boreal biome Feather mosses, nitrogen fixation and the boreal biome > T. H. DeLuca School of the Environment and Natural Resources, Bangor University, Bangor, UK [email protected] Introduction Nitrogen (N) is the primary limiting nutrient for both productivity and decomposition in boreal forests (Tamm, 1991) yet we lack a clear understanding of the influence of N inputs from nitrogen fixation on forest ecosystem dynamics, productivity and carbon (C) cycling. Although our atmosphere is 78% N2 gas, none of that N is available Figure 1. A late succession Scots pine, Norway spruce forest in directly to land plants or animals; it must first be converted northern Sweden. Feather mosses make up the light green carpet on the forest floor. to ammoniacal (NH3) N by nitrogen-fixing microbes, often acting in symbiosis with plant species, and then further transformed to useful amino acids in plants or bacteria. As there is a general lack of woody or herbaceous N-fixing plant-microbe associations in upland boreal forests, the primary source of useful N supply in these ecosystems has until recently remained a mystery. There is currently a great deal of interest in boreal forest ecosystems and the vital role they play in global N and C balance. The boreal ecosystem is the second largest biome in the world and one of the largest terrestrial C sinks on Earth, storing approximately 30% of total terrestrial carbon stocks. Feather mosses are an ever-present component of the boreal forest floor (see Figs 1, 2), accounting for as Figure 2. A close up view of Pleurozium schreberi, pic: © Uwe much as 95% of ground cover, and a major contributor to the Drehwald annual productivity of the boreal biome. While the ecology formed from both feather mosses and sphagnum mosses, and ecosystems of these bryophytes within the boreal forest this dense insulting layer controls both heat flux from have been widely described by numerous researchers, the incoming radiation and losses from the largely mineral boreal contribution of bryophytes and, in particular, feather mosses, soils (Beringer et al., 2001), thereby directly controlling soil to global ecosystem models often goes unobserved. temperatures and indirectly regulating surface processes such as carbon cycling. The feather moss ‘bottom layer’ is a vital organ of the boreal forest, functioning to filter water, insulate against Over the past 10 years, we have investigated the notion temperature change, regulate nutrient uptake and minimise that feather mosses also serve as hosts for N-fixing nutrient losses. It provides the breathing, protective skin cyanobacteria, aiming to expand our knowledge of their of the forest floor (Lindo and Gonzeles, 2010). Potentially beneficial properties. Our results show that, for the biota 28 Feather mosses, nitrogen fixation and the boreal biome of the relatively pristine (low N deposition) environments in creating bioavailable N in the boreal forest. As stated of northern Scandinavia, feather mosses are a dominant previously, the conspicuous lack of any significant quantity source of N (DeLuca et al., 2002b; Zackrisson et al., 2004; of herbaceous or woody N-fixing plant-microbe associations DeLuca et al., 2007; Zackrisson et al., 2009), thus providing a across much of the boreal forest had long puzzled forest first step towards a holistic understanding of the role which ecologists, who therefore questioned the source of available natural N fixation plays in these high latitude boreal forests. N supply in fire-maintained or harvested boreal forest This research has also provided an insight into the means by ecosystems (Tamm, 1991). which N subsequently accumulates in such pristine forest ecosystems and the following paragraphs describe some of Repeated efforts to measure N fixation associated with the history of our studies and raise some future questions feather mosses have failed because most researchers would that have yet to be addressed. take their samples from high latitude ecosystems in late June to mid-July, a time of uniquely low N fixation activity Feather moss carpets in forest feather moss ecosystems (DeLuca et al., 2002b; Zackrisson et al., 2004; Zackrisson et al., 2009). Also, the Upland moss communities of the boreal forest contain spatial variation of N fixation in feather mosses obligates hundreds of species of bryophytes but are dominated an intensive sampling strategy, which was not the norm in by two pleurocarpous species (in which the female surveys of bryophyte N fixation. Our work has therefore archegonium reproductive structures are on a short side opened up the possibility that feather mosses are actually branch rather than on the main stalk): Pleurozium schreberi responsible for a great deal of the N accumulation resident and Hylocomium splendens, and one acrocarpous species in boreal forests and has underscored the importance of (in which the archegonia are at the top of the stem), feather mosses in the total N economy of the boreal biome Polytrichum commune. The photosynthetic capacity of these (DeLuca et al., 2002b; Zackrisson et al., 2004). feather mosses is reflected in an ecosystem productivity that rivals, and at times (during early to mid-succession) We have found cyanobacteria (see Fig. 3) residing in the can surpass, that of the overstorey (Bond-Lamberty et al., leaf incurve of some of the pleurocarpous moss species, 2007). Hylocomium and Pleurozium often account for 30- leaving them both difficult to observe and difficult to 95% of the average cover of the boreal forest floor and yield culture. Nevertheless, we have commonly retrieved three a net primary productivity (NPP) of 200 to >400 kg ha-1 yr-1, genera of cyanobacteria (Nostoc, Stigonema and Calothryx) comprising a total biomass of between 0.1–2.0 Mg C ha-1 associated with Pleurozium schreberi alone (Gentili et al., as live moss and 2.0–4.0 Mg C ha-1 as dead moss (Vogel 2005). These cyanobacteria demonstrate several unique and Gower, 1998; Turetsky, 2003). Their photosynthetic properties, including having biphasic temperature optima contribution to C fixation capacity has been found to be of for productivity at 13oC and 22oC, which makes them ideally particular importance when the overstorey is under stress, suited to N fixation in boreal ecosystems. The feather moss as, for example, in the large-scale bark beetle outbreak that carpets are thus a uniquely convenient system for studying recently raged across the Canadian boreal forest, where N fixation, as either individual shoots or whole moss carpets bryophyte productivity remained relatively consistent with can be placed in a vessel for manipulation or measurement the opening of the forest canopy in the absence of fire of fixation activity and this has allowed us to conduct (Olsson and Staff, 1995). With the dip in photosynthetic C intensive monitoring. fixation by trees and the net C emissions associated with dead tree decay, the carbon assimilation associated with Distribution of N fixation in Pleurozium the moss bottom layer becomes much more important for schreberi in Fennoscandia ecosystem maintenance. The carbon fixed by the mosses is slowly incorporated into the fibric materials that comprise Over the past 10 years, we have established sampling plots the dense humus and debris mat often associated with for Pleurozium schreberi and Hylocomium splendans at boreal Spodosol soils (Beringer et al., 2001). about 50 sites across northern Sweden, Finland and Norway (Fig.4). The majority of samples have been collected from Although feather mosses have long been known to intercept natural forest preserves to minimise site disturbance and and retain nutrients from forest throughfall, there has all sites occur within the mid- to northern boreal zone of previously been little understanding of the role they play Fennoscandia (59o–69oN; 17o–19oE). When samples from 500 ) -1 d Pleurozium 400 -2 Hylocomium m 29 Feather mosses, nitrogen fixation mol and the boreal biome μ 300 ( ! 200 these plots were assessed for N fixation activity using the acetylene reduction technique, there was a clear relationship 100 between the latitude and longitude of the sample collected Acetylene reduction and N fixation rate, wherein fixation rates were greater in the north, away from the Botinian Bay section of the Baltic Sea, likely as a result of the high rates of N deposition along 0 this more populated coast. In comparison, the boreal forests 59 59 59 60 61 64 j 65 Ty Br 59Bj So 59Fi Va 60In 60Gr Ab60 Ed60 60Lo o Ta 62 Ku Vi 64 Ki 64Vi 65 Ja 65 T Ku 65u Su65 66 2 Ad67 67 a An67 69 of northern Sweden exhibit exceptionally low rates of N H SonVat 61 61Lon Ro63 64 Mo 64 De 66 Ru Gu65 65 N Ai1Ai2 66Ai3 66Ai 66v3Aiv1 66Ai 67v Ru N67 Site and latitude (name and degrees north) deposition, both compared to southern Sweden and to other ! more industrial regions of the world (Gundale et al., 2011). Indeed, nitrogen deposition in Sweden ranges from about 20 kg N ha-1 yr-1 in southern Sweden to less than 1.0 kg ha-1 yr-1 in Northern Sweden where most of our studies have been performed. Nitrogen deposition can interfere greatly with N fixation in cyanobacteria and this may partially account for the geographic variation in our results. Successional influence on N-fixation rates Figure 3. The cyanobacteria, Stigonema sp., taken from the leaf incurve of Pleurozium schreberi. Fire is the primary form of chronic disturbance in the boreal forest ecosystems of northern Europe and is a potent driver of shifts in nutrient cycling and plant community composition. The recovery of organic N resources lost Pleurozium during severe wildfire is dependent upon biological N fixation 400 (Smithwick et al., 2005) and, in the boreal forest, this is primarily accomplished by the feather moss-cyanobacterial 300 associations (Fig.
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