The effect of hysteresis on microbial activity in computer simulation models

Microbial activity in soils depends on the status of the soil water, which is expressed by pressure head (h) or water content (theta). There is no uni que relationship between theta and h because moisture relations exhibit hys teresis, For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This b ias was found to be most serious where soil is continually subject to sever e wetting and drying cycles such as can be found in irrigated and tropical agriculture.