Improving nitrogen and irrigation water use efficiency through adaptive management: A case study using annual ryegrass

Nitrogen is often poorly managed in irrigated agro-ecosystems. Accumulation and leaching of N can occur due to excessive fertiliser N, high soil inorganic N carryover between seasons, rapid mineralisation in spring and poor irrigation scheduling. This can reduce forage yield, quality and N-use efficiency, and lead to pollution of soil and water resources. Experiments were conducted to test whether adaptive nitrogen and irrigation management approaches using ryegrass as a case study could (1) reduce N application without compromising yield, (2) maintain or improve forage quality, (3) improve water use efficiency, and (4) minimise potential for nitrate leaching, using the current local recommended fertiliser rates as a baseline. Adaptive management strategies based on the concentration of nitrate measured in a wetting front detector at different depths reduced fertiliser N application by 28–32% compared to the baseline recommendation, reduced residual soil N that is potentially leachable, and improved forage quality without reduction in forage yield. The essence of the adaptive approach is to set thresholds for action that are relatively easy to monitor, based on a simple conceptualisation of the system. The thresholds were defined for the depth that a strong wetting front could be passively detected under field conditions, and for the concentration of nitrate in the percolating water. These thresholds were chosen as simple integral measures of the water and N cycles. Results suggest that a good adaptive manager would improve the thresholds for action as more experience is gained.