Principles of terrestrial ecosystem ecology.pdf

222 9. Terrestrial Nutrient Cycling
therefore not always the dominant input of
minerals to ecosystems.
Summary
Nutrients enter ecosystems through the chemical
weathering of rocks, the biological fixation
of atmospheric nitrogen, and the deposition of
nutrients from the atmosphere in rain, windblown
particles, or gases. Human activities have
greatly increased these inputs, particularly of
nitrogen and sulfur, through combustion of
fossil fuels, addition of fertilizers, and planting
of nitrogen-fixing crops. Unlike carbon, the
internal recycling of essential plant nutrients is
much larger than the annual inputs and losses
from the ecosystem, producing relatively closed
nutrient cycles.
Most nutrients that are essential to plant
production become available to plants due to
the microbial release of elements from dead
organic matter during decomposition. Microbial
exoenzymes break down the large polymers
in particulate dead organic matter into
soluble compounds and ions that can be
absorbed by microbes or plant roots. The net
mineralization of nutrients depends on the
balance between the microbial immobilization
of nutrients to support microbial growth and
the secretion of nutrients that exceed microbial
requirements for growth. The first product of
nitrogen mineralization is ammonium. Ammonium
can be converted to nitrate by autotrophic
nitrifiers that use ammonium as a source of
reducing power or by heterotrophic nitrifiers.
Both plants and microbes use DON, ammonium,
and nitrate in varying proportions as
nitrogen sources, when their growth is nitrogen
limited. Soil minerals and organic matter also
influence nutrient availability to plants and
microbes through exchange reactions (primarily
with soil cations, except in some tropical
soils that have a substantial anion exchange
capacity), the precipitation of phosphorus with
soil minerals, and the incorporation of nitrogen
into humus.
Nutrients are lost from ecosystems through
the leaching of elements out of the ecosystem
in solution, emissions of gases, loss of nutrients
adsorbed on soil particles in wind or water
erosion, and the removal of materials in
harvest. Human activities, as with nutrient
inputs, often increase nutrient losses from terrestrial
ecosystems.
Review Questions
1. What are the relative magnitudes of atmospheric
inputs and mineralization from dead
organic matter in supplying the annual
nitrogen uptake by vegetation?
2. If Earth is bathed in dinitrogen gas, why is
the productivity of so many ecosystems
limited by availability of nitrogen? What is
biological nitrogen fixation? What factors
influence when and where it occurs?
3. What are the mechanisms by which nitrogen
is deposited from the atmosphere into
terrestrial ecosystems?
4. What are the major steps in the mineralization
of litter nitrogen to inorganic
forms? What microbial processes mediate
each step and what are the products of each
step? Which of these processes are extracellular
and which are intracellular?
5. What ecological factors account for differences
among ecosystems in annual net
nitrogen mineralization? How does each of
these factors influence microbial activity?
6. What determines the balance between
nitrogen mineralization and nitrogen immobilization
in soils?
7. What factors determine the balance
between plant uptake and microbial uptake
of dissolved organic and inorganic nitrogen
in soils?
8. How do ammonium and nitrate differ in
mobility in the soil? Why? How does this
influence plant uptake and susceptibility to
leaching loss?
9. What is denitrification and what regulates
it? What are the gases that can be produced,
and what are their roles in the atmosphere?
10. What is the main mechanism by which
phosphorus enters ecosystems?
11. What factors control availability of phosphorus
for plant uptake? Why is phosphorus
availability low in many tropical soils?