A Nitrogen-led Model of Grass Growth

This model is built by considering the utilization rate of nitrogenas the first step in calculating the rate of growth of the variousorgans of a grass crop. The amount of carbohydrate determineswhether there are sufficient carbon skeletons and sufficientenergy available to support synthesis of new material. Growthof roots, tillers/stems and leaves is simulated with leaf dividedinto photosynthetic and non-photosynthetic structures. The modelkeeps account of soluble carbohydrate and nitrogen pools ineach of the organs and storage pools of carbohydrate in theleaves and roots. The modelled crop has an age structure sothat each plant organ has an age profile describing daily changesin growth; when the oldest tissue becomes senescent a fractionof its nitrogen is recycled. Seasonal changes in the percentage nitrogen content of the crop,when large amounts of soil nitrogen are available, are shownto be a consequence of changes in both soluble and stored carbohydrate.The contrast between high and low nitrogen treatments is shownto be a consequence of different allocation priorities for nitrogen. The model demonstrates that considering nitrogen as the primaryelement of synthesis provides an approach that predicts drymatter production successfully, as well as giving a differentperspective of the growth processes, and suggesting that thecapacity of the enzymatic processes governing synthesis ultimatelylimits crop yields. This different perspective may be most usefulwhen trying to understand what controls growth and the relativeinfluence of environmental changes on the physiology and morphologyof the crop. C:N ratio; grass growth; mathematical model; nitrogen use; partitioning