Wheat grain yield and nitrogen use characteristics under monoculture and intercropping with different nitrogen fertilization rates

Wheat/maize relay-intercropping system is an important rainfed planting pattern in Sichuan Province, China. It is therefore important to investigate rational nitrogen management strategies to improve not only productivity and nutrient use efficiency, but also to explore the yield advantage mechanism of the interaction between intercropped wheat and maize. In this direction, a field experiment was conducted in 2014 involving four nitrogen application rates (0 kg.hm^-2, 60 kg.hm^-2, 120 kg.hm^-2 and 180 kg.hm^-2, denoted by N1, N2, N3 and N4, respectively) under three planting patterns. The planting patterns included wheat monoculture (W), wheat/maize strip relay-intercropping (W/M) and wheat/empty strip (W/E) at an experimental research base in Chongzhou, Sichuan. Grain yield, biomass, nitrogen uptake, nitrogen use efficiency and partial factor productivity of nitrogen (PFP-N) of wheat were calculated. The results showed that 1) intercropped wheat had yield advantage over monoculture wheat under all the nitrogen application rates. The aboveground biomass and grain yield of intercropped wheat in W/M and W/E treatments were on average 15.7% and 17.8% higher than that of monoculture wheat. 2) Border row wheat had advantage in terms of productivity, nitrogen uptake and nutrient use efficiency. Aboveground biomass, yield, nitrogen uptake and PFP-N of the border row wheat were 23.8%, 27.3%, 48.9% and 19.1% higher than those of monoculture wheat, respectively. It suggested that intercropped wheat had higher nitrogen use efficiency with higher grain yield compared to monoculture wheat. 3) Compared to W/E pattern, aboveground biomass and grain yield of wheat under W/M pattern dropped by 6.5% and 5.7% under the zero and 60 kg.hm^-2 nitrogen application treatments, respectively. However, under the medium nitrogen application rate (N3, 120 kg.hm^-2) grain yield, aboveground biomass, shoot nitrogen uptake and PFP-N of wheat under W/M were 14.1%, 5.0%, 6.8% and 4.5% higher than those under W/E, respectively. These results indicated that intercropped wheat may be inhibited by intercropping maize under low nitrogen application rate (such as low than 60 kg.hm^-2), in contrast wheat development in intercropping could be promoted by sufficient nitrogen application. Furthermore, intercropped wheat had advantages in grain yield and nutrient use efficiency, especially for border row plants. Although maize and wheat interaction enhanced this process, sufficient nitrogen fertilizer application in maize and wheat was critical for promoting growth of relay-intercropped wheat.