Water-Nitrogen Colimitation in Grain Crops

On historical timescales, enhanced availability of water and nitrogen has been a major driver of improved crop yield worldwide. The interaction between water and nitrogen has received attention, but theories and conceptual tools lag behind our fast-increasing capacity to monitor crop nitrogen and water status using remote sensing. Theory, mostly developed in ecology, predicts that plant growth is maximized when all resources are equally limiting. Therefore, for a given level of stress, growth is maximized under colimitation of resources. The concept of colimitation is less developed in agriculture, where the notion of sequentially, single limiting factors has been dominating since von Liebig law of minimum. During the last two decades, the concept of colimitation of resources has been used to explain yield gaps in wheat, barley, and canola in Mediterranean-type environments of Australia and Europe, and in maize in the Pampas of Argentina. These studies conform to theory: high yield is usually associated with high water and N colimitation. In view of the increasing capacity to measure crop water and N status of crops using remote sensing, we reviewed N-water colimitation as this concept provides a robust framework to integrate otherwise dispersed measurements. First, we revise the concept of colimitation in terms of timescale, level of organization, and thropic level; an agronomic definition of colimitation is advanced. Second, we outline the methods to quantify colimitation between two resources in a generalized crop model, and between water and nitrogen in particular. Third, we summarize studies providing experimental and modeling evidence of colimitation. A notion of crop yield at the biophysical " boundaries" of nitrogen and water use efficiency is presented within the colimitation context. Finally, we propose future directions to develop tools to quantify waternitrogen colimitation using remote sensing.