Agriculture Reference
In-Depth Information
M.F. O'Sullivanet al.
CN Losses after Ploughing out Grass and Grass-CloverSwards
3.5
Carbon and Nitrogen Losses
after Ploughing out Grass and
Grass-Clover Swards
M.F. O'S
ULLIVAN
, A.J.A. V
INTEN AND
B.C. B
ALL
Environment Division, Scottish Agricultural College, West Mains
Road, Edinburgh EH9 3JG, UK
Introduction
Growing clover can be an environmentally friendly way of increasing
the nitrogen supply in soils because bacteria in the clover root system fix
atmospheric nitrogen. Potentially, this biologically fixed nitrogen could
reduce reliance on energy-intensive fertilizers. However, we need to be sure
that this nitrogen is available to subsequent crops and does not contribute
to pollution of air or water. The process of mineralization releases
ammonium from nitrogen-rich crop residues and soil organic matter. In
well-aerated soils, ammonium undergoes nitrification to nitrate, which con-
tributes to the pollution of surface and ground water. Denitrification, which
occurs under anaerobic soil conditions, converts nitrogen compounds to
nitrous oxide and nitrogen gas. Nitrous oxide emitted from soil contributes
to the greenhouse effect.
The nitrogen cycle in soil is inseparable from the carbon cycle. Storage
of carbon within the soil system is beneficial to agriculture and the
environment. Soil organic matter (SOM) provides the largest terrestrial
sink of carbon and affects the atmospheric content of carbon dioxide,
carbon monoxide, methane and other greenhouse gases. It is a source of
crop nutrients and is one of the dominant factors influencing soil structure.
Structure in turn affects many processes that occur in soil. Examples are the
rate at which heat and material move through the soil, its suitability as a
medium for root growth and its resistance to degradation. Therefore, any
changes to SOM status will affect crop productivity and its sustainability,
environmental pollution and soil quality.
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