Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 73

Contrary to the situation in the Mackay Early experiment, simulated
yields (Fig. 2.4.2b) and water balance (Table 2.4.2) in the Mackay Late
experiment were little affected by the different decomposition of residues.
The difference in the impact of the parameter schemes between the two
experiments can be attributed to the timing of residue decomposition in
relation to climate at the sites and, primarily, its impact on soil evaporation.
Differences in residue mass simulated with the two parameter schemes
were greatest from ~100 days after harvest (Fig. 2.4.1). In the Mackay Late
experiment (harvested in November each year), this time occurred in the
cooler, drier months of late autumn and winter, when potential soil
evaporation was low in the Mackay Late experiment, and so the presence
(or absence) of residue would have the least impact on soil evaporation.
However, the situation was reversed in the Mackay Early experiment
(harvested in July each year). There the greatest differences in simulated
residue mass occurred in the hotter, wetter months of late spring and
summer when potential soil evaporation was high. It is under these
conditions that the presence of residue has the greatest impact on soil
water balances, and hence yields.
The difference in the simulation results between the two experiments
highlights the interactions between climate, crops and soil processes and
their impact on cropping system characteristics. Successful use of cropping
system models to extrapolate the results of agronomic experiments to
different soil, management and/or climatic conditions requires that all
these interactions are captured and described adequately.
Acknowledgements
The authors thank Drs Merv Probert and Brian Keating for their advice
and constructive comments during the study, members of the Bureau
of Sugar Experimental Stations, Mackay, for access to information on
the long-term experiments, and Dr Graham Kingston for facilitating the
experimental work at Harwood. This study was partially funded by the
Sugar Research and Development Corporation, Australia.
References
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Poulton, P.L. and Dalgleish, N.P. (1996) Simulation of a legume ley farming
system in northern Australia using the Agricultural Production Systems
Simulator. Australian Journal of Experimental Agriculture 36, 1037-1048.
Fylstra, D., Ladson, L., Watson, J. and Waren, A. (1998) Design and use of the
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