Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 62

6

0.93 + 1.57 × exp ( − 0.0306 × year)

5

4

3

( )

2

1

0

0 50 100 150

Years after grassland ploughing

Fig. 2.3.1. Spatial and temporal variation of total N in the A horizon after ploughing grassland
within two catchments; w loam soil (h2-h3), v humic loam soil (< h3) and T clay soils (h4).

Table 2.3.2. Mean parameters ( ± SE ) and approximate half-life, t 1/2 , after grassland ploughing in
loam soils (h2-h3); exponential decay function: Y = Y[A] + Y[G] × EXP( − k × t GPA ).

Parameter
variable

Y[A] Y[G] k(year − 1 ) r 2 t 1/2 (years)

C org 0.85 1.72 0.0217 0.887 32

(g 100 g − 1 ) (0.05) (0.11) (0.004)

N t 0.93 1.57 0.0306 0.884 22

(g kg − 1 ) (0.11) (0.11) (0.006)

N 0 125 192 0.071 0.771 10

(mg kg − 1 ) (16) (25) (0.020)

N 0 , which may be derived from microbial N, decreased by 35% and had
a half-life of ~10 years. This is longer than predicted for N mic in the
SUNDIAL model (Bradbury et al ., 1993).

Simulations

The SUNDIAL simulation results presented here include the active
organic matter fractions, humus and biomass, and net mineralization.
Nitrogen in the active humus fraction initially is underestimated by
~1000 kg N ha −1 (Fig. 2.3.2a; humus N + N mic ). Depending on the C/N
ratio, this corresponds to ~10-15 t C ha −1 , which can be attributed to the