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

Abstract

Greenhouse vegetable cultivation is a substantial source of nitrous oxide (N₂O) 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 N₂O emissions without significant loss of vegetable yields. The primary aims of this study are to (i) quantify the variations in vegetable yields, N₂O emissions, nitrogen use efficiency (NUE) and crop water productivity (CWP) under greenhouse cultivation conditions; (ii) determine the major regulating factors of vegetable yields, N₂O emissions, emission factors (EFs), yield-scaled N₂O emissions, NUEs and CWPs; and (iii) evaluate the effectiveness of fertilization and irrigation strategies for N₂O mitigation. We compiled a comprehensive dataset from 44 peer-reviewed publications, which includes 408 determinations of seasonal N₂O 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 N₂O emissions against N input rates (r² = 0.480, n = 322, p < 0.001) for treatments that were unfertilized and fertilized with conventional fertilizers. For fruit vegetables, N₂O emissions responded to NUEs following a linear-plateau model (r² = 0.539, n = 115, p < 0.001), which suggests that management practices aiming to increase NUEs are effective for decreasing N₂O emissions at NUEs below 0.15 t kg^-1. The results of meta-analyses revealed that applications of biochar or enhanced efficiency fertilizers (EEFs) decreased EFs and yield-scaled N₂O 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 N₂O emissions (ranging from -68% to -59%), increasing NUEs (ranging from 76% to 157%), and subsequently reducing the reactive N released into the environment.

Keywords: Crop water productivity; Emission factor; Greenhouse vegetable; Nitrogen use efficiency; Nitrous oxide; Yield-scaled emission.