Global meta-analysis of nitrogen fertilizer use efficiency in rice, wheat and maize

Almost half of global nitrogen (N) fertilizer is applied to rice, wheat and maize, but a large proportion is not directly taken up causing detrimental effects on the environment. To understand how fertilizer management, climate and soil factors influence N fertilizer use efficiency (REN) in these three crops, we conducted a meta -analysis with 3586 observations from 261 peer-reviewed papers. Across all observations, the average REN was (39 % (95 % confidence intervals 38-40 %) and varied with crop type. Modelled REN was higher for rice (42 %, 95 % CIs 39-45 %) and wheat (42 %, 95 % CIs 40-45 %) than for maize (36 %, 95 % CIs 34-39 %). Fertilizer type and frequency were important factors influencing REN with generally low values for organic fertilizer and single applications. REN significantly increased with increasing growing season temperature for rice and wheat, but it decreased for maize, possibly due to crop differences in plant N demand and uptake responses to temperature. Soil factors such as organic carbon (SOC) content, pH and clay content also showed important effects on REN, but this depended on crop type. For instance, an increase in SOC content resulted in a decrease in REN for wheat, but an increase for maize. On the other hand, REN decreased with increasing soil clay content for all three crops. We observed that a doubling of the global average of the REN (or roughly 80 %) can be achieved under a wide range of N fertilizer application rates (20-300 kg ha-1) and there are therefore opportunities to improve REN through fertilizer management when adjusted to the right environmental and soil conditions for each specific crop.