Increases in forage legume biomass as a response to nitrogen input depend on temperature, soil characters and planting system: A meta‐analysis

Although the effects of nitrogen (N) on forage legume productivity have been well‐studied, there is still some debate as to whether the supply of additional N is essential to support higher forage legume yields. Here, a meta‐analysis was conducted to synthesize the effects of N fertilization on forage legume biomass across different climates, soil types and planting system. The results showed that forage legume biomass in pure stands generally increased after N addition and was positively correlated with plant accumulation of N, phosphorus and potassium. The direction and magnitude of forage legume biomass responses in pure stands were influenced by accumulated temperature during growth stage and soil characters including organic matter, pH and N availability. Soil pH was positively correlated while other factors were negatively correlated with forage legume biomass response. Increases in forage legume biomass were greater at sites where accumulated temperatures were lower than 2,000°C. N addition did not contribute to higher forage legume biomass in acidic soil and when soil N availability was high (e.g. soil total N content higher than 0.9 g/kg and N addition rate higher than 200 kg/ha). Moreover, forage legume biomass was negatively affected in mixed systems which was independent of N application rate. In conclusion, this meta‐analysis highlights the importance of considering temperature, soil characters and planting system when evaluating the necessity of N application during forage legume production.