Archived at http://orgprints.org/4662
Modelling short term carbon and nitrogen development
B. M. Petersen1, L. S. Jensen2, J. Berntsen1, S. Hansen2 and A. Pedersen2
1
2
Danish Institute of Agricultural Sciences, Dept. of Agroecology, Denmark
The Royal Veterinary and Agricultural University, Dept. of Agricultural Sciences, Denmark
Carbon (C) and nitrogen (N) transformations are highly interdependent, driven by the C
metabolism. Though many details of the transformations of C and N forms in soil are well
known, basic knowledge about the mechanisms determining substrate availability, and the
chemical and physical properties determining the stabilisation of organic matter are still
lacking. The soil biota furthermore constitutes an extremely complex food web, consisting of
a vast number of bacteria, fungi and fauna species, each having their own feeding preferences
and characteristic life cycle.
A good quantitative description of the main processes involved is required. This involves both
the long-term build-up and degradation of soil organic matter, and the more short-termed N
mineralisation-immobilisation turnover (MIT). The high complexity of the topic leads to the
need of mathematical models, which are usable for predicting the N availability for both
uptake and losses with a fine time scale resolution. Our aim was to develop a model, which
combines the ability to make satisfactory simulations of a number of laboratory experiments,
with the ability to simulate long-term field experiments (Petersen et al., 2003a). Much effort
has gone into limiting the number of required parameters, in order to obtain a robust model.
Furthermore a large data-set, covering a range of soil types and amendments, were utilised in
model development.
The novel computer model CN-SIM (Petersen et al., 2003b) describes the transformations of
C and N in the soil. The development has been divided into long-term soil organic C
simulation capabilities, and short-term simulations of C and N, the latter outlined in this
poster. A number of existing, independent laboratory experiments, covering a range of
amendments, have been utilised. The amendments include a variety of different crop residues
and animal manure.
Utilised experiments include measurements of 13C and 15N in various pools, and the model
facilitates the simulation of these isotopes.
Statistical methods were employed to estimate parameters, and obtain proximate confidence
intervals for these parameters. Cross-validation was used to assess how the model performed
on data not used for parameterisation.
6000
Kale (a)
Oilseed rape (b)
Pea (c)
a
4000
b
-1
CO2 (mg-C kg soil)
5000
3000
c
2000
1000
0
500
a
-1
N-min (mg-N kg soil)
400
300
200
c
100
b
0
0
20
40
60
80
Days
Figure 1. Measured replicates (symbols) and simulated (lines) CO2 evolution and mineral N
from amendment of kale leaves, oilseed rape haulm and pea haulm. Data originally reported
by Henriksen & Breland (1999)
1200
Sheep manure (A)
1000
Dairy cow manure (R)
Dairy cow manure (S)
Sheep manure (Q)
Sow manure (U)
Unamended (x)
A
-1
CO2 (mg-C kg soil)
U
800
R
S
600
Q
400
200
X
0
180
160
Q
R
120
U
S
-1
N-min (mg-N kg soil)
140
X
100
80
A
60
40
20
0
0
20
40
60
days
80
100
0
20
40
60
days
80
100
0
20
40
60
80
days
Figure 2. Measured (symbols) and simulated (lines) CO2 evolution and mineral N. Tretments
include unamended soil (X) , sheep manures “A” and “Q”, dairy cow manures “R” and “S”,
and sow manure “U”. All treatments were amended with ammonium sulphate. Data originally
reported by Kyvsgaard et al. (2000).
The simulations clearly indicate that considerable amounts of added N in some phase must
reside elsewhere than in microbes, mineral N or "humus". In the model this is represented by
a “soil microbial residue” pool, which may reflect a conglomerate of easily decomposable
microbial residues and soil fauna species with low fumigation extraction efficiency. Even
when utilising this pool, there is a tendency towards overestimating mineral N. This may be
due to the model constraints caused by fixed C/N ratios of the microbial pools SMB1 and
SMB2. Several measurements show that the microbial biomass actually can display a large
span in C/N ratio, dependant on N availability. Future versions might benefit from algorithms
mimicking this behaviour.
We argue that the combination of long- and short- term simulation capabilities and the use of
large data-sets will yield a maximum of constraints, all other equal leading to maximum
generality.
A model with a good generality was achieved. Few short-term data series were simulated very
closely though. This is often inherent when using a comparatively simple model to represent
highly complicated processes. If only a few data series were utilised for model development,
and constraints from the long-term simulation capabilities further were excluded, far more
impressive agreements between measurements and simulations could have been achieved, as
100
other studies have demonstrated. This might however have been at the cost of wider
applicability and biological realism.
References
Henriksen, T.M., Breland, T.A., 1999. Evaluation of criteria for describing crop residue
degradability in a model of carbon and nitrogen turnover in soil. Soil Biology & Biochemistry
31, 1135-1149.
Petersen, B. M., Berntsen, J., Jensen, L. S, Hansen, S. 2003a. CN-SIM – a model for the
turnover of soil organic matter. I: Long term carbon and radiocarbon development. Soil
Biology and Biochemistry (submitted).
Petersen, B. M., Jensen, L. S., Berntsen, J., Hansen, S., Pedersen, A., Henriksen, T.M.,
Sørensen, P. and Gattin, I. 2003b. CN-SIM - a model for the turnover of soil organic matter.
II: Short term carbon and nitrogen development. Soil Biology and Biochemistry (submitted).
Kyvsgaard, P., Sørensen, P., Møller, E., Magid, J., 2000. Nitrogen Mineralisation from sheep
faeces can be predicted from the apparent digestibility of the feed. Nutrient Cycling in
Agroecosystems 57, 207-214.