Water and nitrogen use efficiencies in citrus production: A meta-analysis

Water and nitrogen (N) are two key limiting factors for citrus production. Reported effects of water and N inputs on citrus yield, water use efficiency (WUE) and N use efficiency (NUE) vary greatly, mainly due to differences in cultivars, tree age, climate, soil types, and water and N input levels. So far, no systematic analysis has been performed, and as a result, the interactive effects of water and N inputs on yield, WUE and NUE of citrus orchards are unknown. Also, gaps between attainable and actual yields, WUE and NUE have not been established yet. Here, we report on a global meta-analysis of yields, WUE and NUE of citrus production systems, using 1009 observations from 55 studies, conducted in 11 countries. Median citrus yields ranged from 30 to 60 t ha^-1, which were in between average global yields (range 10-30 t ha^-1) and attainable yields (range 60-90 t ha^-1). Median WUE ranged from 2.5 to 5 kg m^-3 and median NUE from 150 to 350 kg kg^-1. Citrus yields were related to water and N inputs and tree age. Relationships between water and N inputs and yield, WUE and NUE were also analysed for sub-datasets and quantiles, to examine the relationships near the extremes. There were statistical significant interactions between water and N inputs in yield and NUE, but not in WUE. This indicates that studies aiming at the optimization of water and N inputs must consider interactions and optimize water and N inputs simultaneously. Based on our analyses, we estimated that reducing over-optimal irrigation to optimal irrigation may increase citrus yield by 20%, WUE by 30% and NUE by 15%. Similarly, reducing over-optimal N fertilization to optimal N fertilization may increase yield by 10%, WUE by 15% and NUE by 40%. We concluded that there is room for a significant increase in yield, WUE and NUE through the simultaneous optimization of water and fertilizer N inputs via precision fertigation.