Why are the benefits of enhanced-efficiency fertilizers inconsistent in the field? Prerequisite conditions identified from simulation analyses

Enhanced-efficiency nitrogen fertilizers provide an opportunity for sustainable intensification of agricultural industries. However, field experiments evaluating yield and nitrogen loss benefits from enhanced-efficiency fertilizers often fail to obtain statistically significant treatment differences. Agricultural systems modeling provides a means to perform thousands of virtual response trials, allowing us to unravel the complex interactions between crop, management, and seasonal climate that determine the efficacy of these fertilizers. Here, we present simulations of controlled-release fertilizer use in Australian sugarcane (Saccharum sp. L.) production in a wet tropical climate. To quantify the agronomic and environmental benefits, we analyze the yield and nitrogen loss responses to nitrogen rate (response curves), comparing those for urea and controlled-release fertilizers. We also evaluate the impacts of soil type, crop start times, and controlled-release patterns as they interact with seasonal climate and rainfall distribution. The simulation results showed that nitrogen loss and yield benefits were highly variable, and their likelihood determined by three prerequisite conditions: (1) sufficient longevity of protection of the fertilizer nitrogen, (2) occurrence of a nitrogen loss event during this period of protection and before the nitrogen is taken up by the crop, and (3) the crop being responsive to the fertilizer nitrogen. These prerequisite conditions extend to other cropping systems as well as to other enhanced-efficiency nitrogen fertilizers such as nitrification inhibitors. Here, for the first time, we provide a framework for understanding the inconsistent results in field experiments testing the efficacy of enhanced-efficiency nitrogen fertilizers. This understanding will help improve the setting, design, and interpretation of experiments for better demonstration of benefits as well as identify where enhanced-efficiency fertilizers can best increase sustainability of nitrogen management.