Score one for the little guys: cryptogam contributions to global carbon and nitrogen cycles

A recent paper by Elbert et al. in Nature Geoscience estimates the global contribution of "cryptogamic covers" to nitrogen fixation and net primary productivity. One of the authors, Bettina Weber, recently presented this work at the Ecological Society of America meeting. The discussion about how carbon can be sequestered almost always revolves around carbon intense ecosystems, those which store a lot of carbon per unit area, or ecosystems which can produce a lot of biomass very quickly. Cryptogams -- a catchall referring to bryophytes, lichens, and cyanobacteria-- are small and have a reputation for being slow growers (sometimes they are, sometimes they aren't). Because they are small and because they are perceived as slow-growing, they get little attention as carbon repositories or players in the carbon cycle. This study investigates the global impacts of cryptogamic communities such as soil biocrusts, rock biocrusts, boreal moss & lichen "carpets", and epiphytic mosses and lichens growing in tree canopies (Figure 1). 

Figure 1 (Elbert et al. 2012). a, Ground cover in the Namib lichen fields (Teloschistes capensis, Xanthoparmelia walteri, Ramalina spp.), Alexander Bay, South Africa. b, Soil crust with cyanobacteria (black) and chlorolichen (Psora decipiens), Nama Karoo semi-desert, Northern Cape, South Africa. c, Rock crust with chlorolichen (Rhizocarpon geographicum aggr.), Sadnig, Eastern Alps, Austria. d, Rock crust with chlorolichens (Chrysothrix chlorina, yellow, Leproloma membranaceum, whitish-grey) and mosses (Dicranum scoparium, Hypnum cupressiforme var. filiforme), Spessart, Germany. e, Plant cover with cyanolichen (Physma byrsaeum) on rainforest tree, northeast Queensland, Australia. f, Plant cover with chlorolichens (Evernia prunastri, Parmelia sulcata, P. subrudecta and others) and a bryophyte (Orthotrichum affine) on maple tree, Trier, Germany.


The very first terrestrial communities were likely communities of cryptogams. Today, to a large degree, these communities occupy the leftovers -- habitat not occupied by the vascular plants, or in some cases the actual surfaces of vascular plants. In some places it seems that the Earth is wearing a fuzzy cryptogram sweater....have you ever seem a temperate rainforest? If so you know what I mean. Can these forgotten botanical panhandlers, vagabonds and mendicants have any role to play in something so grand and large as the carbon and nitrogen cycles of Planet Earth? Yeah, of course (and stop calling them "lower" plants, it's just plain offensive).

Carbon-wise, it is the ground covers in temperate and boreal forests that are taking up the most carbon. Again, if you've ever been to one, you'll understand. A black spruce taiga up in Alaska might have half a meter of water-logged moss and lichen tissue accumulated on the ground.  I notice in Figure 3 from the paper, that the deserts are showing some modest values due to soil and rock covers. Across the planet the authors estimate cryptogamic communities are comprising about 7% of the primary productivity.

Nitrogen-wise, the most intense fixation rate estimates are in desert cryptogamic soil and rock covers...a.k.a. biocrusts. Cryptogamic plant covers in extratropical forests are a distant second place. Across the planet the authors estimate cryptogamic communities are conducting almost half of the biological nitrogen fixation.

There are 2 reasons why we should take these communities seriously as part of the carbon sequestration equation. First, comprising 7% of the global NPP may not seem like a lot, but its actually similar in magnitude to the flux we generate by burning fossil fuels according to the authors. The second reason is that primary production in most ecosystems is limited by nitrogen. Most of the Earth's nitrogen is in the atmosphere in a form that's useless to primary producers. A small minority of organisms have the ability to convert this atmospheres nitrogen to essentially a plant fertilizer (nitrogen fixation). This study suggests that nearly half of the biologically fixed nitrogen in terrestrial ecosystems is fixed by these cryptogamic communities. So, in other words cryptogams are largely in charge of the key missing ingredient that would allow for greater plant production and carbon sinking.

In figure 3 from the paper, the authors map where on earth these cryptogam-mediated porcesses are most intense. For carbon (left side), note the importance of the boreal forest. Because we have so little land in the southern hemisphere at similar latitudes, this is primarily a northern hemisphere phenomenon. For nitrogen (right side), look at the drylands coming into play.

Figure 3 (Elbert et al. 2012) Geographic distribution of CO2 uptake and N2 fixation by cryptogamic covers. a–f, The colour coding indicates the flux intensity of carbon net uptake (a,c,e) and nitrogen fixation (b,d,f) by CGC (a,b), CPC (c,d) and their sum (CGC + CPC, e,f). The flux units are g m−2 yr−1 ; note that the scale bars for carbon (e) and nitrogen (f) differ by two orders of magnitude. White areas indicate ecosystems for which no data are available; hashed areas were excluded from global budget calculations (annual mean precipitation <75 mm yr−1 and desert areas designated as dune sand/shifting sands and rock outcrops). 

 

 

One observation I have is that the authors seem to be trying to estimate current rates of carbon and nitrogen fixation, not the potential. Since I am so used to seeing soil biocrusts compromised by disturbance, I wonder how much higher these rates would be without such disturbances...twice as high, three times? How much C could we sink worldwide if we stopped chronically disturbing soil biocrusts?

Wolfgang Elbert,, Bettina Weber,, Susannah Burrows,, Jörg Steinkamp,, Burkhard Büdel,, Meinrat O. Andreae, & Ulrich Pöschl (2012). Contribution of cryptogamic covers to the global cycles of carbon and nitrogen Nature Geoscience DOI: 10.1038/ngeo1486