Nitrogen use efficiency, crop water productivity and nitrous oxide emissions from Chinese greenhouse vegetables: A meta-analysis.

Greenhouse vegetable cultivation is a substantial source of nitrous oxide (NO) emissions in China due to intensive managements with nitrogen (N) fertilizers and irrigation water. We hypothesize that reducing input rates of N fertilizers or irrigation water to optimal levels would mitigate NO emissions without significant loss of vegetable yields. The primary aims of this study are to (i) quantify the variations in vegetable yields, NO emissions, nitrogen use efficiency (NUE) and crop water productivity (CWP) under greenhouse cultivation conditions; (ii) determine the major regulating factors of vegetable yields, NO emissions, emission factors (EFs), yield-scaled NO emissions, NUEs and CWPs; and (iii) evaluate the effectiveness of fertilization and irrigation strategies for NO mitigation. We compiled a comprehensive dataset from 44 peer-reviewed publications, which includes 408 determinations of seasonal NO emissions from greenhouse vegetable systems across China between 2006 and 2019. An emission coefficient of 0.95% was obtained as the slope of the linear regression between NO emissions against N input rates (r = 0.480, n = 322, p < 0.001) for treatments that were unfertilized and fertilized with conventional fertilizers. For fruit vegetables, NO emissions responded to NUEs following a linear-plateau model (r = 0.539, n = 115, p < 0.001), which suggests that management practices aiming to increase NUEs are effective for decreasing NO emissions at NUEs below 0.15 t kg. The results of meta-analyses revealed that applications of biochar or enhanced efficiency fertilizers (EEFs) decreased EFs and yield-scaled NO emissions (ranging from -42% to -34%) but did not significantly increase NUEs. Reducing the input rates of N fertilizers (approximately 50% of the total N input) or irrigation water (approximately 20% of the full irrigation water) provided advantages for maintaining vegetable yields (ranging from -7% to 3%), decreasing NO emissions (ranging from -68% to -59%), increasing NUEs (ranging from 76% to 157%), and subsequently reducing the reactive N released into the environment.