Global greenhouse vegetable production systems are hotspots of soil N2O emissions and nitrogen leaching: A meta-analysis

Vegetable production in greenhouses is often associated with the use of excessive amounts of nitrogen (N) fertilizers, low NUE (15–35%), and high N losses along gaseous and hydrological pathways. In this meta-analysis, we assess the effects of application rate, fertilizer type, irrigation, and soil properties on soil N₂O emissions and nitrogen leaching from greenhouse vegetable systems on the basis of 75 studies. Mean ± standard error (SE) N₂O emissions from unfertilized control plots (N₂Ocₒₙₜᵣₒₗ) and N leaching (NLcₒₙₜᵣₒₗ) of greenhouse vegetable systems were 3.2 ± 0.4 and 91 ± 20 kg N ha⁻¹ yr⁻¹, respectively, indicating legacy effects due to fertilization in preceding crop seasons. Soil organic carbon concentrations (SOC) and irrigation were significantly positively correlated with NLcₒₙₜᵣₒₗ losses, while other soil properties did not significantly affect N₂Ocₒₙₜᵣₒₗ or NLcₒₙₜᵣₒₗ. The annual mean soil N₂O emission from fertilized greenhouse vegetable systems was 12.0 ± 1.0 kg N₂O–N ha⁻¹ yr⁻¹ (global: 0.067 Tg N₂O–N yr⁻¹), with N₂O emissions increasing exponentially with fertilization. The mean EFN₂O was 0.85%. The mean annual nitrogen leaching (NL) was 297 ± 22 kg N ha⁻¹ yr⁻¹ (global: 1.66 Tg N yr⁻¹), with fertilization, irrigation, and SOC explaining 65% of the observed variation. The mean leaching factor across all fertilizer types was 11.9%, but 18.7% for chemical fertilizer. Crop NUE was highest, while N₂O emissions and N leaching were lowest, at fertilizer rates <500 kg N ha⁻¹ year⁻¹. Yield-scaled N₂O emissions (0.05 ± 0.01 kg N₂O–N Mg⁻¹ yr⁻¹) and nitrogen leaching (0.79 ± 0.08 kg N Mg⁻¹ yr⁻¹) were lowest at fertilizer rates <1000 kg N ha⁻¹ yr⁻¹. Vegetables are increasingly produced in greenhouses, often under management schemes of extreme fertilization (>1500 kg N ha⁻¹ yr⁻¹) and irrigation (>1200 mm yr⁻¹). Our study indicates that high environmental N₂O and N leaching losses can be mitigated by reducing fertilization rates to 500–1000 kg N ha⁻¹ yr⁻¹ (mean: ∼762 kg N ha⁻¹ yr⁻¹) without jeopardizing yields.