Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 407

aspects of soils, as well as the prevalence of weeds, pests and diseases
(Sumner, 1982). Rotations comprising only annual crops may cause a
decline in soil organic matter by leaving relatively few plant residues (above
and below ground) compared with perennial crops. In contrast, perennial
forages have been shown to build up soil organic nitrogen (Clement and
Williams, 1967), improve soil physical properties (Tisdall and Oades,
1982) and reduce the risk of soil erosion. Management of crop rotations
by manipulating application rates of crop residues and manures, tillage
and treatment of crop residues (e.g. mulching or incorporation) can have a
significant effect on soil organic matter dynamics.
Organic farming systems represent an increasingly important land use
in Europe and beyond. The land area farmed organically in Europe has
grown from 100,000 ha in 1985 to 2.8 Mha in 1998 (N. Lampkin, personal
communication). The principles of organic agriculture as defined by the
International Federation of Organic Agricultural Movements (IFOAM,
1998) specifically include the maintenance of long-term soil fertility as a
prerequisite. Increases in soil organic matter in soils under organic
management are widely reported (e.g. Reganold et al ., 1993, Clark et al .,
1998). Wander and Traina (1994) have also measured higher levels of
carbon in the 'light fraction' of soils under organic management which
is thought to be an indication of a more biologically active pool. As the
number of organic farmers grows, there are increasing numbers of
conventional, specialist, arable farmers wanting to convert to organic
production. This poses particular challenges for soil organic matter
management. Grass-clover leys are traditionally the 'engine' of organic
systems in Western Europe, but grass-clover leys are not an economic
option without livestock to utilize their productivity. Philipps has shown
that with less than 25% of nitrogen-fixing crops in the rotation (i.e. a
legume grown 1 year in every 4), organic matter declines over a 10-year
period. However, working with a rotation with a similar proportion of
legumes, but comparing the use of mineral fertilizers and farmyard manure
with or without biodynamic preparations, Raupp found that soil organic
matter was higher in the manured + biodynamic treatment, and lowest in
the treatment which received only mineral fertilizer.
The sustainable management of crop residues in agroecosystems is a
global challenge. Lal (1995) calculated that on an area of 1 × 10 9 ha of agri-

cultural land, ~3.5 10 9 Mg of crop residues are produced. Approximately

74% of this originated from cereals while the next biggest contributors were
sugar crops at 11%. Crop residues have an important role not only in
building soil organic matter and in conservation of soil and water, but also
in supplying nutrients to subsequent crops in rotations and to simultaneous
crops in, for example, agroforestry systems. The magnitude of benefits
from crop residues depends on the quantity and quality of the residues, as
well as the following crop, climatic, edaphic and management factors. Until

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