High N availability decreases N uptake and yield under limited water availability in maize

Water and nitrogen (N) are the most limiting factors to plant productivity globally, but we lack a critical understanding of how water availability impacts plant N requirements and ecosystem losses. Plant N requirements are particularly uncertain when water is limited because of the interactive effect of water and N on plant growth, plant N demand, and plant N uptake. We investigated impacts of N application and water availability on plant and N metrics, including above and belowground growth, water productivity, N productivity, N uptake, N recovery, and greenhouse gas emissions within a semi-arid system in northeastern Colorado, USA. High soil N availability depressed grain yield and shoot growth under limited water availability, with similar trends also under full water availability, despite no indication of physical toxicity. Under low N availability, plant N concentrations in aboveground tissues showed greater recovery of N than what was applied in the low N treatments under both full and limited water availability. This enhanced recovery underscores the need to better understand both plant soil foraging and processes governing resource availability under these conditions. Finally, limited water availability reduced N uptake across all N treatments and left 30% more soil N-NO 3 - deep in the soil profile at the end of the season than under full water availability. Our results show that plant N use is not linearly related to water use and emphasize the need for an integrated understanding of water and N interactions, plant foraging for these resources, and the dynamics of processes that make N available to plants.