Agriculture Reference
In-Depth Information
J.L. Gauntet al.
Procedure for IsolatingSOMFractions
2.6
A Procedure for Isolating Soil
Organic Matter Fractions
Suitable for Modelling
J.L. G
AUNT
1
, S.P. S
OHI
1
, H. Y
ANG
1,2
, N. M
AHIEU
3
AND
J.R.M. A
RAH
4
1
Department of Soil Science,IACR-Rothamsted,Harpenden,
Hertfordshire AL5 2JQ, UK;
2
International Rice Research Institute,
PO Box 933 Manila, Philippines;
3
Department of Chemistry, Queen
Mary and Westfield College, University of London, London E1 4NS;
and
4
AAT Consultants, 15 Clerk Street, Edinburgh EH8 9JH, UK
Introduction
Jenkinson and Rayner (1977), and many others since, have modelled the
turnover of soil organic matter (SOM) in terms of discrete conceptual
pools, each with different characteristic properties and a measure of reactiv-
ity (e.g. the first-order reaction constant
k
). Although the decomposition
of plant material in soil is a complex process, such simple models have
successfully described the long-term dynamics of SOM.
If a SOM turnover model is to predict nitrogen (N) transformations
as well as carbon (C) fluxes, it is necessary to include a functional charac-
teristic of organic matter that determines N mineralized or immobilized
during turnover of C. A typical example of such a functional characteristic
is the C : N ratio of a conceptual pool. The application of models based on
conceptual pools with a first-order reaction constant and a fixed definition
of function make for easy modelling but parameters (e.g. initial pool sizes)
which are unmeasurable. The overall performance can be optimized against
system outputs such as soil C content.
There are drawbacks to such models. Parameterization against system-
level outputs alone cannot guarantee process-level accuracy. Perhaps the
most important drawback is that such models are not directly testable: it is
impossible to devise a procedure that will reliably extract all and only that
part of the SOM with a first-order reaction constant
k
. For the same reason,
it is not possible to make a measurement in an unknown field and predict
future SOM decomposition. Recognition of these limitations has fuelled a
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