Life in the Soil - Soil: Reflections on the Basis of our Existence - page 63

secretion is greatest. But this changes as the root gets older, and other species
gradually enter the scene as it grows. Conditions differ depending on the loca-
tion in the soil profile. The fresh leaf litter lying on the floor of a spruce forest is
high in carbon but low in nitrogen, making it a prime food resource for fungi that
require carbon to build their biomass. Carbon dioxide is released as the leaf litter
decomposes, and as the carbon content reduces so the nitrogen content increases.
Mycorrhizal fungi operate a little deeper, where the nitrogen content is sufficiently
high. Nitrogen is the most important tree nutrient; mycorrhizal fungi absorb it
from the humus and exchange it for carbohydrates that the tree creates through
photosynthesis. Farther down, other fungi absorb phosphorus and other mineral
nutrients released during the weathering process. Here, oxygen and humidity play
a role in determining which species are present. Some thrive in the airy passage-
ways left by worms or rotting roots, while others prefer damper soil.
Nitrogen availability is pivotal to fungal diversity. Some mycorrhizal fungi
absorb nitrogen through amino acids; others wait until fungi and bacteria have
released ammonium and nitrates during the decomposition of organic matter.
Nitrogen-poor forests are home to many species whose enzyme systems specialise
in releasing nitrogen from humus, but in nitrogen-rich forests these are an unnec-
essary cost and the few mycorrhizal fungi that grow there invest in rapid growth
instead.
Mushroom pickers looking for chanterelles do not like it when soil nitro-
gen content gets too high as this tasty fungus grows poorly in such areas. The
chanterelle is in decline in nitrogen-rich areas across Europe, especially in
the Netherlands, where nitrification is high due to intensive livestock farming.
Nitrogen-rich ammonia from farm manure rises into the air and precipitates on
nearby land.
Numerous small organisms eat soil bacteria and these in turn become food for
other animals such as predatory mites and beetles. The various food chains in the
soil mesh into a single food web in which animals like earthworms and termites
play an especially critical role by turning over and changing the soil infrastructure.
They eat decaying matter and let intestinal microorganisms work in their gut. The
decomposition process accelerates when the decaying matter breaks down into
small particles, allowing bacteria to work undisturbed and worms to benefit from
the nutrients released by the bacteria.
Earthworms on a Lawn in Tomelilla
I expect to encounter earthworms when digging in my vegetable patch but some-
times come across them in less familiar places, as for instance when I attended a
dance class at Tomelilla College of Further Education near my home in southern
Sweden.
“You've got to move your body up and down in time to the music,” explained
teacher Max Somah, from Guinea in West Africa, on the lawn outside the school.