A Nitrogen-led Model of Grass Growth

This model is built by considering the utilization rate of nitrogen as the first step in calculating the rate of growth of the various organs of a grass crop. The amount of carbohydrate determines whether there are sufficient carbon skeletons and sufficient energy available to support synthesis of new material. Growth of roots, tillers/stems and leaves is simulated with leaf divided into photosynthetic and non-photosynthetic structures. The model keeps account of soluble carbohydrate and nitrogen pools in each of the organs and storage pools of carbohydrate in the leaves and roots. The modelled crop has an age structure so that each plant organ has an age profile describing daily changes in growth; when the oldest tissue becomes senescent a fraction of its nitrogen is recycled. Seasonal changes in the percentage nitrogen content of the crop, when large amounts of soil nitrogen are available, are shown to be a consequence of changes in both soluble and stored carbohydrate. The contrast between high and low nitrogen treatments is shown to be a consequence of different allocation priorities for nitrogen. The model demonstrates that considering nitrogen as the primary element of synthesis provides an approach that predicts dry matter production successfully, as well as giving a different perspective of the growth processes, and suggesting that the capacity of the enzymatic processes governing synthesis ultimately limits crop yields. This different perspective may be most useful when trying to understand what controls growth and the relative influence of environmental changes on the physiology and morphology of the crop.