MODELING ROOT-GROWTH OF WHEAT AS THE LINKAGE BETWEEN CROP AND SOIL

The simulation of crop - soil systems with a model requires an appropr iate description of the root dynamics. An empirical root growth model that simulates root-shoot relations, root distribution and a dynamic r esponse to environmental conditions is presented. The root model exten ds an existing crop model and links it to a soil model to calculate dr y matter accumulation, water and nitrogen dynamics of a wheat crop. Si mulated roots are distributed over soil layers according to carbon sup ply from the shoots by using a 'top down principle'. This principle fa vours the top layers for root growth by first providing all available carbon to the first layer. Under unfavourable soil conditions in that layer, carbon is given to the next deeper soil layer. This procedure i s repeated until a separately calculated rooting depth is reached. At that depth all available carbon is used for root growth regardless of current soil conditions. Under most simulated conditions the 'top down principle' results in a negative exponential function of a monotone d ecrease of root distribution with soil depth. However, it can also acc ount for larger root densities deeper in the profile when water or nit rogen deficiency occurs in soil. In addition to soil water and soil ni trogen supply the root model considers soil compaction, aeration and r oot distribution history for root growth simulation. The new model, co nsisting of an existing crop and soil model and linked through a new d eveloped root model, was calibrated and tested using two independent f ield experiments. A sensitivity analysis was carried out by varying pa rameters, initial soil conditions and hypothetic weather patterns as p art of the validation process. Root length density distribution (r(2)( 1:1)=0.65), shoot, grain and total root biomass (r(2)(1:1)=0.87) were predicted satisfactorily, thus providing a useful tool for specific si mulation studies on that site.