Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 61

Table 2.3.1. Parameters selected for model initialization.

Management/soil type Humus C (t ha − 1 ) C mic /C act fraction C/N ratio

A/sand 25 0.028 12-17

A/loam 36 0.028 11 .4

A/clay 44 0.028 10 .4

GP/loam 90 0.032 10 .4

GP/humic loam 204 0.012 11.4

Grass/loam 94 0.035 9.4

(Lavahun et al ., 1996), but the relative difference of microbial C in grass-
land and arable soil was similar (20%).
Management scenarios for the simulation were based on a standard
3-year rotation, consisting of winter oilseed rape, winter wheat and winter
barley (RWB), with and without residue (straw; Y/N) incorporation. Two
different yield potentials were considered for the cereals (miN/maX), which
were correlated with a difference in N fertilizer addition (+50 kg N ha −1 ).
Generally, a trend was accounted for in N fertilization rising between 1965
and 1985 in cereals, starting at 120 kg N ha −1 and reaching a plateau of 180
and 230 kg N ha −1 , respectively. Simulations were also performed for 'con-
tinuous arable' (A) to check the effect of fertilization on SOM equilibria.

Results and Discussion

Measurements

Soils from areas of high water table had about twice the amount of SOM;
clay soils were too variable to fit an empirical relationship (Fig. 2.3.1). For
C org and N t measured in humic soils, the hypothetical equilibrium content
will not be reached until 115 years of arable cultivation. Some decline can
be derived from the measured N t contents in clay soils if one identifies two
different soil types (Humic Gley and Gley). For the sub-sample of loess
soils (sandy loam), typically low in SOM (h2-h3), exponential decay
functions could be parameterized (Table 2.3.2) as shown for total nitrogen
(Fig. 2.3.1). We assumed that equilibrium was reached after 115 years.
Quantifying the half-life of either fraction under arable conditions (Table
2.3.2) shows that N t reaches its equilibrium level faster than C org . The
drop in carbon content is ~10% larger compared with nitrogen, in spite of
returning carbon-rich residues, such as straw, with a wide C/N ratio and a
comparatively slow turnover rate. Overall, mineralization of the grass sod
goes along with a narrowing C/N ratio. Potentially mineralizable nitrogen,