Soil water effects of agroforestry in rainfed jujube (Ziziphus jujube Mill.) orchards on loess hillslopes in Northwest China

Soil water is the most critical factor influencing the growth and productivity of crops on the Loess Plateau of China. The popular clean-cultivation soil management practice in rainfed jujube orchards in this region causes a variety of environmental problems, including serious soil erosion and soil quality degradation. Agroforestry is a promising avenue to cope with this problem but its effect on soil water in the root zone is yet to be elucidated. In this study, two different agroforestry systems were established in jujube orchards on the hillslopes of the Loess Plateau, to test the effects on root-zone soil water. The Hemerocallis fulva and fodder Brassica napes were planted between jujube rows. The results showed that both agroforestry systems clearly improved soil water at depths of 0-20 cm and 20-60 cm under jujube trees, however, they apparently reduced the inter-row soil water at depths of 60-120 cm and 120-180 cm in different years compared to the control. A simple method, based on the difference of soil water content between inter-row crops and jujube trees (SWDR), was introduced to judge the possibility of water competition happening. The analyses based on this method showed that in the jujube- H. fulva intercropping system, H. fulva frequently competed soil water with jujube trees in the 0-120 cm depth primarily in the relatively dry period (May to July). However, there was almost no water competition occurred in the jujube-B. napes treatment. Overall, fodder B. napes may be more appropriate than Hemerocailis fulva for intercropping with jujube trees. The results here can provide insights into ground management practice of sloping orchards in water-limited areas.