Cold soil environments
The coldest environments on Earth are found at high latitudes
and/or high altitude (i.e. Polar Regions and alpine areas) and
cover a large proportion of the Earth’s land surface area. All
of these areas are undergoing significant changes in biodiversity
due to climate warming (see Section 5.1.3). The landscape in
the Polar Regions is dominated by tundra, bare soil and rocks,
or covered by snow or ice. Tundra is a vegetation type that is
most often associated with the Arctic, but vegetated areas of
Antarctica and some alpine areas also fit this classification (Fig.
3.41). This vegetation type is dominated by lichens, mosses,
grasses, sedges, herbs and some dwarf shrubs, and is associated
with cold annual temperatures, short growing seasons, high
frequency of freeze-thaw cycles and the presence of permafrost,
i.e. permanently frozen soil.
The plant community in the Arctic tundra is generally
specious and fairly productive, compared with other extreme
environments, providing a high input of organic matter into
the soil food web (Fig. 3.42). Therefore, despite the cold
temperatures, with annual average temperature below -10°C at
many sites, the Arctic tundra soils support more than 700 mite
species, 400 species of collembolans, 500 nematode species
and 70 species of enchytraeids and earthworms. However,
the species richness and density of invertebrates within a site
tend to be low compared with temperate soils. For example,
in the high Arctic it has been found that the communities of
oribatid mites and collembolans were plant specific and that
only 6-7 and 4-6 species of oribatid mites and collembolans,
respectively, were present in the soils associated with 6 different
plant species. Similar results have been found for plant species
on Svalbard. In contrast, nematode communities are generally
more diverse than microarthropod communities. For instance,
29 species of nematodes have been found in the top 3 cm soil
of a sub-alpine heath in northern Swedish Lapland, although this
is well below the diversity of nematodes found in non-extreme
ecosystem types. New molecular tools do, however, indicate
that the diversity of soil flora may be substantially greater.
The diversity of the microbial communities in the Arctic is
less known but molecular methods indicate that Arctic soils
also support a high diversity of microbial communities. For
example, in one study of Siberian tundra soils, where winter
temperatures often fall below -40°C, found 43 unique genetic
sequences, related to the Proteobacteria and Fibrobacter
groups. Furthermore, it has been demonstrated that bacterial
diversity in Arctic tundra soils can have over 2000 phylotypes, a
high proportion of which might not be found elsewhere.
In contrast to the Arctic, several factors contribute to a lower
terrestrial diversity in Antarctica, and especially in the polar
deserts of continental Antarctica. Colonisation of terrestrial
habitats in Antarctica is limited by the Southern Ocean combined
with predominant weather patterns, and so colonisation events
are relatively rare. This means that many of the terrestrial
inhabitants of Antarctica are endemic species that have had
to survive several glaciation events. Furthermore, the climate
is generally more severe than at comparable latitudes in the
northern hemisphere, and this harsh climate is a considerable
constraint to the Antarctic fauna and flora. Most of continental
Antarctica is covered by ice (only about 2% of the land mass is
ice-free) and hosts one of the most extreme soil environments,
with mean annual air temperatures below 0°C and very limited
precipitation (in some areas <100 mm year-1).
All of this contributes to a relatively low biodiversity, which is
very evident above ground with only 2 species of vascular plants
and 2 higher insects found in maritime Antarctica and none at all
in continental Antarctica! Despite this, overall the Antarctic soils
support at least 225 species of mites, 85 species of collembolans,
49 species of nematodes, 30 species of rotifers and 41 species
of tardigrades, of which about 170 are free-living endemics. This
is just the number of species of each group which have currently
been found, and so there is a possibility that these numbers will
increase further. Microbial communities are also fairly diverse
and show a high degree of endemism. For instance, 35 different
species representing 22 different genera of microfungi have been
found in the Windmill Island region, 28 fungal taxa representing
18 different genera in Victoria Land, and at least 24 species of
endoparasitic and nematode trapping fungi occur throughout
Antarctica. Cyanobacterial communities are widely distributed
throughout soils in Antarctica, even in the barren soils. For
example, 15 taxa have so far been isolated from 18 polygon
soils with a maximum of 12 taxa from one single soil sample at
34
Fig. 3.41: Autumn tundra near Red Dog Mine, Alaska. (JS)
Cierva Point on the Antarctic Peninsula, and 6 taxa from 124 soil
samples in the La Gorce Mountains, one of the most southern
ice-free areas of Antarctica. The species richness of bacteria is
still not as well described but recent studies suggests that there
is a considerable diversity of bacteria with a high proportion
of novel species. As in hot deserts, soil organisms tend to be
very unevenly distributed across the Antarctic desert landscape
with greater biomass and diversity in wetter microhabitats. The
biotic hotspots of Antarctic soils include vegetated soils and
soils beneath bird nests and moss beds. However, the species
richness of soil fauna in the most extreme parts of Antarctica
is much lower.
One of the most extreme cold deserts is the McMurdo Dry
Valleys of Antarctica (Fig. 3.43), where low precipitation (<100
mm per year) and average annual temperatures of about - 20°C
limit water availability to a very short time window during the
austral summer (25-75 days with temperatures above 0°C).
The Dry Valleys are dominated by soils with very low nutrient
availability and high salt concentrations, in addition to high daily
fluctuations in temperature, leading to frequent freeze-thaw
events. It, therefore, represents one of the most challenging
environments for life on Earth. The large expanses of very dry
soils (often <5% soil moisture) are dominated by the nematode
Scottnema lindsayae, a microbial feeder, which often represents
the only larger soil animal in these soils. Experimental evidence
suggests that a warming climate would decrease the extent of
these dry soils, and thereby reduce the range of S. lindsayae.
S. lindsayae is the most abundant invertebrate in the McMurdo
Dry Valleys, and it has been estimated to be responsible for
6-7% of soil organic carbon turnover, a significant amount,
indicating that climate changes may have critical impacts on
ecosystem processes. In areas with greater soil moisture the
below ground communities are generally more diverse. Here,
the nematode genera Plectus and Eudorylaimus occur in concert
with several species of tardigrades and rotifers and a few
species of microarthropods. The microbial communities in the
McMurdo Dry Valleys can be relatively diverse, but the diversity
of microbes, as with invertebrates, decreases with decreasing
soil moisture. In short, Antarctic soils harbour a high number of
novel microbial and animal taxa.
European Atlas of Soil Biodiversity | Chapter 3 Ecosystems and Biomes
Fig. 3.42: Life flourishing on a Hyperskeletic Leptosol in Northern
Canada in the form of the Arctic Poppy (Papaver radicadum) – one
of the hardiest plants on the planet. Even these poor soils are an
important component of the Earth's environment. (CT)
One dominant feature of cold environments is the presence of a
permafrost layer (i.e. ground that remains frozen for more than 2
years). Permafrost covers a large proportion of the Earth’s land
surface, and presents some adverse growing conditions for biota
including extreme cold, and frequent freeze-thaw cycles. Far from
being devoid of life, microbial communities of permafrost are very
diverse. For example, more than 30 bacterial genera have been
isolated from Arctic permafrost soil collected on Ellesmere Island
in Canada, and almost 50 strains of bacteria have been found
in a permafrost sample collected on the Qinghai-Tibet plateau.
Moreover, some microbes found in the permafrost are active
during cold periods and have been shown to be able to grow at
temperatures as low as -39°C. These examples demonstrate that
polar soil environments show substantial differences in their soil
communities and that even in the most extreme cold desert there
are more species than might be thought.