Water movement via adventitious roots of the prostrate shrub Juniperus sabina in semiarid areas of China

The prostrate evergreen shrub Juniperus sabina L. is an important tree species for revegetation in semi-arid areas of China. The root system of J. sabina comprises a main root system, which can reach deeper soil, and adventitious roots that mainly extend in the surface soil. In our previous studies, water movement between different soil layers via these root systems associated with rainfall-induced vertical heterogeneity in soil moisture has been suggested by sap flow analysis. In this study, to confirm the existence of water movement from the deeper soil to the surface soil (i.e., hydraulic redistribution) under the progression of drying after rainfall, we controlled soil moisture in six potted J. sabina plants. On the assumption of drying after rain, the main roots continued to receive irrigation and only the irrigation on the adventitious root part was stopped. When drying had sufficiently progressed and the decline in soil moisture had stabilized, ¹⁸O-labeled water was irrigated at the main roots for 7-9 days. The 18O-labeled water was detected from the adventitious roots and soil on the adventitious root through the analysis of the oxygen stable isotope ratio of water. Sap flow was also measured during the experiment. Two sensors were implanted on each studied plant: one in the base of the decumbent stem, located between the main root system and adventitious roots, and the other distal to the decumbent stem. Sap flow occurred at nighttime only in the base of the decumbent stem during drying of the adventitious root. These results suggest the existence of water movement from moist soil to dry soil via the main and adventitious roots (hydraulic redistribution), which increased with the progression of drought.