Effects of intercropping time and planting density on nitrogen use efficiencyof melon-sunflower intercropping system

A field experiment comprising of monoculture and intercropping of two crops (melon and sunflower) was conducted at three sunflower sowing times (at vine running, flowering and fruit expansion stages of melon) at three intercropping densities of each crop high (24 975 plant·hm^-2), medium (22 200 plant·hm^-2) and low (19 980 plant·hm^-2) to study plant nitrogen accumulation and nitrogen use efficiency and to determine the relationship between solar energy utilization efficiency and nitrogen use efficiency. The results showed that intercropping significantly increased nitrogen accumulation and nitrogen use efficiency of melon, but reduced those of the sunflower. Under intercropping conditions, nitrogen accumulation of above-ground parts of melon was 195.08 kg·hm^-2, increasing by 13.0% compared with that of monocultured melon (172.61 kg·hm^-2). Nitrogen use efficiency and partial nitrogen productivity of intercropped melon significantly increased, respectively, by 40.5% and 55.4%, over those of monocultured melon. Nitrogen use efficiency and nitrogen partial productivity of sunflower under intercropping decreased, respectively, by 8.2% and 58.4% compared with those of monocultured sunflower, but nitrogen harvest index increased by 4.9%. Nitrogen use efficiency of intercropping system at three sunflower intercropping times (vine running, flowering and fruit expansion stages of melon) increased, respectively, by 43.5%, 12.5% and 59.8% over that of monoculture sunflower of the same sowing times. Also nitrogen use efficiency of intercropping system intercropped at fruit expansion period of melon increased by 6.7%, compared with average nitrogen use efficiency of monocultured melon. Partial nitrogen productivity of intercropped system with three intercropping times increased, respectively, by 6.5%, 32.1% and 40.4% compared with monocultured sunflower of the same sowing time, but decreased, respectively, by 22.5%, 10.1% and 34.3% compared with the average value for monoculturd melon. Nitrogen harvest index of intercropping system at three sunflower intercropping times decreased by 7.2%, 7.7% and 12.5%, respectively, compared with that of monocultured sunflower. Nitrogen utilization efficiencies of intercropping system at three intercropping densities reduced by 14.2% (high density), 20.4% (middle density) and 13.9% (low density), respectively, compared with that of monocultured melon at the same corresponding density, but increased, respectively, by 25.2%, 20.0% and 9.5% compared with the average value of monocultured sunflower. Partial nitrogen productivity of intercropping system with three intercropping densities decreased, respectively, by 29.6%, 15.6% and 21.1% compared with the corresponding treatments of monocultured melon. Nitrogen harvest index of intercropping system at high and low intercropping densities increased, respectively, by 2.7% and 1.4% compared with average nitrogen harvest index of monocultured sunflower, but decreased by 7.6% at medium density. There was significantly positive correlation between nitrogen use efficiency and light use efficiency of melon under intercropping system, but not with sunflower. The results suggested that fruit expansion period of melon was the most suitable intercropping time of sunflower and 40 cm plant spacing was the optimum planting density in melon-sunflower intercropping system, which resulting in high nitrogen use efficiency.