Abstract: A field experiment was carried out to explore the effects of water-nitrogen coupling on root growth and yield of pepper（Capsicum frutescens L.） in the condition of drip irrigation under film mulching. Different degrees of deficient irrigation with mild deficit （W1, 3 656 m³/hm²） and moderate deficit （W2, 3 082 m³/hm²） during the flowering and fruit setting stage and full fruit stage of pepper were set up, and three nitrogen application levels of 325 kg/hm² （N1, high nitrogen）, 250 kg/hm² （N2, medium nitrogen） and 175 kg/hm² （N3, low nitrogen） were set up, with the traditional mode （irrigation rate of 4 230 m³/hm², nitrogen application rate of 325 kg/hm²） as the control treatment （CKN1）. The effects of different combinations of irrigation amount and nitrogen application rate on root growth and yield of pepper, irrigation water use efficiency and nitrogen partial factor productivity were studied.The results showed that the amount of irrigation, nitrogen application rate and their coupling effects had significant effects on yield and root growth of pepper, irrigation water use efficiency and nitrogen partial factor productivity （P<0.05）. The yield of pepper under W1N2 treatment was the highest, which was 35 189 kg/hm². Compared with other treatments, it increased by 4.37%~55.73%. And W1N2 treatment had the highest irrigation water use efficiency and high nitrogen partial factor productivity. The root growth of pepper was the highest in CKN1 treatment followed by W1N2 treatment, and there was no significant difference between the two treatments. There was no positive correlation between single irrigation amount or single nitrogen application rate and distribution parameters of the pepper root system, but the more obvious the coupling effect of water and nitrogen, the more developed the pepper root growth was. It was concluded that the optimal combination of water and nitrogen for pepper cultivation in northern Xinjiang was the irrigation amount of 3 656 m³/hm² and the nitrogen application amount of 250 kg/hm².

Key words: water and nitrogen coupling, pepper（Capsicum frutescens L.）, root system, yield, deficit irrigation, nitrogen reduction, water and nitrogen use efficiency