Spatial distribution of roots in a dense jujube plantation in the semiarid hilly region of the Chinese Loess Plateau

Aims

Planting density is a major factor during the conversion of cropland to woodland in the semiarid hilly region of the Chinese Loess Plateau. Jujube (cv. Lizao on Ziziphus rootstock) tree plantations have been planted densely to increase income, with a spacing of 3?m between tree rows and 2?m between trees in rows. This practice could lead to soil water depletion. To estimate these risks, water budgets should be calculated accurately, based on a realistic characterization of root distribution. The objective of this study was to determine the spatial root distribution (vertical and horizontal) in a dense jujube plantation under different water management practices in the hilly region of the Chinese Loess Plateau.

Methods

Spatial root distribution in densely planted 8-year-old jujube trees was investigated using a trench-profile method down to 1?m. Four treatments were tested: sloping land (W1), mini-catchment (W2), drip-irrigation (W3), and drip-irrigation plus mini-catchment (W4). Furthermore, mechanical excavation combined with a water spraying method was used to determine the maximum rooting depth.

Results

Spatial root distribution was most affected by drip-irrigation. Horizontally, fine roots were concentrated in the area soaked by irrigation in treatments W3 (69.8%) and W4 (73.8%). Similarly, in the vertical direction, there were significantly more fine roots within the 0–1?m profile in the W4 and W3 treatments compared with the W2 and W1 treatments. Fine root intersects were even more abundant in top 0.4?m of W4 treatments compared with the whole soil profile (0–1?m depth) in the W2 and W1 treatments. The W2 and W4 treatments with mini-catchments did not result in a significantly higher number of fine root intersects relative to the treatments without catchments (W1 and W3, respectively). The correlation between fine root intersects and maximum rooting depth was analyzed and the results indicated that drip-irrigation and mini-catchment treatments increased fine root intersects, but decreased the maximum rooting depth.

Conclusions

This study found that root distribution was most affected by the drip-irrigation treatment, which led to higher root densities close to water emitters on the surface, down to a depth of 0.6?m in the soil. Thus, drip-irrigation could have a significant impact on the root development of densely planted jujube trees in semiarid regions and it might potentially avoid the formation of dry soil layers. These findings provide significant support for the dense planting of jujube trees in the semiarid region.