Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 75

J.R.M.Arahand J.L. Gaunt Questionable Assumptions in CurrentSOMTransformation Models

2.5

Questionable Assumptions in
Current Soil Organic Matter
Transformation Models

J.R.M. A RAH 1

AND J.L. G AUNT 2

1
AAT Consultants, 15 Clerk Street, Edinburgh EH8 9JH; and
2
IACR-RothamstedExperimental Station,Harpenden, Hertfordshire
AL5 2JQ, UK

Introduction

Cycling of soil organic matter (SOM) lies at the heart of all terrestrial
(agro)ecosystems, controlling the direction and nature of soil-atmosphere
trace gas exchange, modulating nutrient availability, and thereby influenc-
ing productivity. Current SOM transformation models, especially those
designed for incorporation within larger ecosystem-scale models, almost
universally assume (i) a soil which is areally homogeneous with respect to
all relevant processes and (ii) first-order decomposition kinetics. In this
chapter, we analyse these assumptions and examine the conditions which
must be satisfied for them to hold. We go on to suggest that in order to
test these assumptions it is necessary to depart from a third universal
supposition, that (iii) of the existence within the soil of well-defined and
functionally discrete SOM pools. Measurements of the transformations of
measurable (extractable) SOM fractions are essential in order to secure the
foundations on which existing transformation models are built.
It will be clear from the notation that Fig. 2.5.1 represents no published
model. It is a caricature, intended to highlight the arbitrary and question-
able aspects of the type of model it portrays. Allowing for a few pools more
or less, and a little more sophisticated treatment of flux partitioning, nearly
all existing SOM transformation models can be represented by a diagram
similar to Fig. 2.5.1. We have no space here to mention all the major
current contenders; the reader is referred to Powlson et al . (1996) for a
review. The major apparent exception is the cohort model of Ågren and
Bosatta (1987), in which individual additions of organic matter are tracked