Soil organic matter pools: Concepts, facts and insights

Dr Weijin Wang1

1Department Of Environment And Science, Brisabne, Australia

Dividing soil organic matter (SOM) into a number of pools with discrete bioavailability, size and turnover rates can help distinguish different components in terms of their roles in ecosystem function and enhance the accuracy of prediction on carbon and nitrogen dynamics with SOM models.  However, the delineation of SOM pools in different models is mostly based on qualitative attributes rather than measurable entities.  Many of the pools in mechanistic models are merely conceptual and hardly correspond to any experimentally measurable fractions.  This renders the models less transferable between different sites, soils types or cropping systems, although they may well mimic a specific experimental data set by iterative adjustment of the parameters.

There have been appeals to match the modelled pools with measurable organic fractions.  As the decomposition rate constants of synonymous pools vary greatly in different models, the pools in one model can hardly find their true equivalents in another model.  This makes it difficult to develop widely accepted analytical procedures to match the conceptual pools.

To avoid the difficulties in measuring the conceptual pools, one option is to revise and redefine the model structures, using measurable fractions of soil organic matter such as water soluble SOM, density or size fractions, microbial biomass and humus.  This might provide a solution for improving the transparency and transferability of models so that they can be used in different systems.  However, caution should be exercised to ensure that a measured fraction can be equated to a pool in the proposed model structures.  Since the characteristics of the same measurable fractions of soil organic matter may differ among soils, it may be necessary to derive soil-specific parameters for models based on measurable pools.

In my presentation, I will discuss the challenges related to the biological meaningfulness of conceptual and measured SOM pools.

Biography: Weijin is a principal soil scientist with the Department of Environment and Science, Queensland. He has been leading a number of projects on nitrogen cycling, nitrous oxide emissions and soil carbon and nutrients management.

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