Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 60

Materials and Methods

Soil samples of the A horizon were collected on several arable sites in two
German catchments. They represent a series characterized by variable time
elapsed since grassland ploughing (GP, 1-33 years), soil texture (sandy
loam-clay loam) and SOM (h2-h4) class. Arable (A) sites were assumed to
be under cultivation for at least 115 years, setting the long-term equilibrium
content (Y[A]). C org and N t (Kjeldahl) were measured by dry combustion.
Assuming a ploughing depth of 0.25 m and a uniform dry bulk density of
the soil (1.5 g cm −3 ), the storage of C and N in the GP soils ranged from
200 to 90 t C ha −1 and18to7tNha −1 , actually representing two different
SOM classes. The decomposable and recalcitrant fractions of potentially
mineralizable N (N 0 =N dpm +N rpm ) were determined by incubation
(Richter et al ., 1989). A sub-sample was used to estimate carbon and
nitrogen in the microbial biomass (C mic ,N mic ) by chloroform fumigation
extraction (CFE; Brookes et al ., 1985; Vance et al ., 1987).

Modelling

Two approaches were used to explain the dynamics of SOM fractions after
grassland ploughing in the sub-samples of sandy and clay loam. A static
model (Equation 1) was used to quantify the total amount of C, N and N 0
in SOM derived from grassland ( Y[G] ) at any time ( t GPA , years) under
arable cultivation before reaching an equilibrium content ( Y[A] ). The
half-life, t 1/2 , is estimated from the rate constant k (Equation 2).
Y = Y[A] + Y[G]

× exp( − k × t GPA ) (1)

t ½ = ln 2/ k (2)

The dynamic model SUNDIAL was chosen, which is based on the
Rothamsted carbon model (Roth-C; Jenkinson and Coleman, 1994)
extended to include N turnover (Bradbury et al ., 1993). As major inputs,
the model considers C and N from fertilizer (mineral and organic) and
crops (stubble + straw and debris), which are partitioned between mineral
(CO 2 ,NH 4 and NO 3 ) and organic pools (humus and microbial biomass) in
the soil. Mineralization and immobilization of N due to decomposition of
crop residues, biomass and humus are calculated from the C/N ratio of the
decomposing material on a weekly basis. For the simulations of C and N
turnover after grassland ploughing, humus C and fraction of microbial
biomass C (C mic /C act ) were initialized according to measurements in single
samples of loam soils (Widmer, 1993; Table 2.3.1). The fraction of C mic
was changed in the range of low (0.028; arable) to high (0.046) to account
for different mixing ratios (ploughing depth, sod depth and litter incorpora-
tion). Absolute values for C mic were high compared with other locations