Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 406

structural stability. Chenu et al . showed that soil structural stability is
largely dependent on complexes of clay and organic matter. In protecting
soil from physical degradation, the quality of soil organic matter is probably
more important than the overall content. Loveland et al . reviewed many
research papers, and found that there was little consistent evidence for
'critical thresholds' of soil organic carbon above or below which soil
physical properties change significantly. This does not imply that the role
of soil organic matter is any less important. However, it creates more
difficulties for those responsible for devising policies of soil protection,
and suggests that evaluation of soil quality on a case by case basis may be
required in order to ensure that soil functions are maintained adequately.
Soil organic matter modelling provides a valuable opportunity to
explore ways of managing the terrestrial carbon cycle. Modelling at the
regional scale is important for climate change issues. At this scale, Paustian
advocated a whole ecosystem approach where the interactions between soil
organic matter, crop yields, economic returns and subsequent changes in
management feed back to determine organic matter and crop responses.
This modelling has, amongst other things, highlighted the close linkage
between C and N cycling processes. For example, Franko found that the
organic matter content in some German soils has reached an optimum
level. Above this level, nitrogen losses can exceed inputs resulting in a
net loss to the environment. At a similar scale, organic matter modelling
was applied by Gaunt et al . to predict the dynamics of soil nitrogen
supply required for making more precise fertilizer recommendations. They
suggested that measurable fractions of soil organic matter can be used to
define the potentially available nutrient pools, making the models of greater
practical value. Tillage has a large effect on carbon and nitrogen dynamics,
with C and N lost after ploughing out grassland. Richter et al . used a
modelling approach to show that these losses resulted in a decrease of net
mineralization and a widening of the C : N ratio. They also showed the
importance of microbial carbon and nitrogen associated with litter and crop
debris in specifying the soil microbial biomass. Paustian identified a need
for an increased collaboration between modellers. However, he stressed
the continued need for long-term experimentation and for closer corre-
spondence between theoretical and measured organic matter fractions. The
availability of a richer set of field experimental data, and the use of isotopic
tracers should in future allow more robust and constrained testing of
conceptually based models.
Soil Organic Matter and Land Management
The sequence and type of crop rotations have been widely shown to
influence plant productivity by affecting physical, chemical and biological