Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 54

investigation area is characterized by a decrease of area for root crops from
24 to 13% which is now used for oilseed rape, sunflower and as setaside
land. Two percent of the former cereal area (now 60%) is now used for
legume crops. The yield has increased for winter wheat from 0.64 kg m −2 in
the 1980s to 0.73 kg m −2

in the 1990s and for sugar beet from 3.5 to

5.2 kg m −2 .
The simulated scenario starts in August 1980. Observed values for the
required initial conditions of the model were not available. The initial con-
tent of SOM was calculated using site-specific statistical information about
yields and animal concentration before 1981 assuming an even application
of the organic inputs and that the organic matter content had reached a
steady state. Parameters and algorithms of the necessary calculations have
been published (Franko, 1997). Initial soil water content was assumed to
be 60% of field capacity, and soil mineral nitrogen has been assigned
according to internal standard parameters of the model that are related to
the different soil units.

Evaluation of Soil Organic Matter Level

It is known from long-term experiments that land use is only responsible
for changes in the decomposable part of SOM whereas the inert SOM is
dependent of the site conditions. Therefore, the evaluation is restricted to
the decomposable SOM pool. Recently published results of Körschens
(1999) show that the effect of SOM on yield is limited to a certain level and
that in different long-term experiments a SOM range of ~0.14-0.51% of
decomposable carbon provides the best conditions for high-quality yields.
Could higher inputs of organic material to the soil and the subsequent
increase in the carbon storage in the soil be of benefit for the global carbon
cycle? A simple calculation shows that there are better solutions for the
reduction of carbon dioxide emissions. Under the conditions of Chernozem
in central Germany, an additional straw input of 400 g m −2 per year will
eventually lead to an increase in the SOM carbon of ~1.2 kg m −2 with a
maximum rate of 60 g m −2 year −1 . This rate decreases with time because of
the remineralization of the newly formed SOM. It takes ~100 years to reach
the final level. Afterwards, no further increase will occur. At this steady
state, the total input is equal to the mineralization. However, the same
amount of straw can be used as an energy source in order to replace fossil
fuels. The energy equivalent of 400 g of straw is ~5.6 MJ (Brenndörfer
et al ., 1994) - an amount equivalent to the energy gain from ~120 g of
carbon in fossil fuels. This is twice as high than the maximum accumulation
rate in SOM. Because of its regenerative character, the consumption of the
straw as an energy source will relieve the global carbon balance per square