Irrigation leads to greater maize yield at higher water productivity and lower environmental costs: a global meta-analysis

Irrigated maize production has increased in the world; however, a comprehensive assessment on how irrigation affects yield, water productivity (WP), and the environment under agricultural intensification on a global scale is paid little attention. In this study, we report the trade-offs among grain yield, WP, reactive nitrogen (N-r) losses, and greenhouse gas (GHG) emissions in response to irrigation management using a global meta-analysis of field experimental data published in peer-reviewed journals prior to May 2018. A total of 1490 paired data points from 162 publications and 21 counties showed that grain yield increased by 30.35% from 7357 kg ha(-1) in non irrigated systems to 9512 kg ha(-1) in irrigation systems (146 mm). Similarly, WP increased by 9.91% from 19.1 to 20.5 kg ha(-1) mm(-1). These increases in yield and WP varied depending on seasonal irrigation amounts, precipitation levels, annual average temperature, N application, soil organic matter, and bulk density. Based on 270 peer-reviewed publications, the emission factors (EFs) of N-r losses in irrigation systems via NH3 emissions decreased by 9.3%, N2O emissions decreased by 42.3%, and NO3- leaching increased by 36.1%, compared with non-irrigated systems. Calculated yield-scaled N-r losses decreased by 10.3% from 5.94 kg N Mg-1 in non-irrigated systems to 5.32 kg N Mg-1 in irrigated systems. Correspondingly, yield-scaled GHG emissions including field emissions; fossil fuel from irrigation pumping, N, P, K fertilizer production and transportation; and field management were 22-28% lower under irrigated conditions than non-irrigated system. This finding highlights the multiple benefits of protecting environments and improving WP through irrigation to attain higher maize yield. However, there are trade-offs to this approach, including increased leaching and the risk of a future water resource crisis.