## 3 Results and Discussion
### Temporal Variation in Water Flux at the Bottom Outlets  
The temporal variations of water flux at the bottom of containers (\(I_{out}\)) without or with living and dead roots of poplar were calculated and shown in Figure 3. This was related to soil permeability and groundwater recharge [38; 39]. Similar trends of \(I_{out}\) in containers without or with living and dead roots were observed and divided into three stages. Compared to the container without roots, the existence of living and dead roots can accelerate the water movement in soil.  
In the first stage (0-9 h), the soil was unsaturated, and the soil pores were not filled with water (\(I_{out}\) increased). The value of \(I_{out}\) at the beginning was 0 cm d\({}^{-1}\) and increased with the continued infiltration of water. The time for the first drop of water to flow out of the container bottom was recorded and shown in Table 2. Compared to the container without roots, the existence of living and dead roots accelerated the water movement in soil. Also, a larger effect of dead roots on the time of the first drop of water flow out of the bottom was observed.  
In the second stage (9-21 h), the soil was saturated, and the flux at the top the container was balanced with the water flux at bottom outlet (\(I_{out}\) stabilized). The steady-state water fluxes at the bottom of the container were calculated and are shown in Table 2. Compared to the container without roots, the steady-state water flux at the bottom of containers with living roots and dead roots increased by 12.0% and 2.2%, respectively. The existence of living and dead roots accelerated the water infiltration in soil, and a larger effect of living roots on the steady-state water flux was observed. The effect of living roots on soil permeability was similar to the effect observed by [PERSON] et al. [7], where they reported that \(M.\)_baccata_ with fibrous roots could increase infiltration rate by 19%. [PERSON] et al. [5] reported that black oak and red maple could increase the infiltration rate by an average of 153% in their greenhouse study. The different impact levels on soil permeability are related to the different soil densities, soil pores, and root conditions in each test [8; 12; 13; 14]. The observed effect of dead roots on soil permeability was similar to the effect observed by [PERSON] et al. [25], where they reported that dead alfalfa with taproot growth could improve water infiltration rates in soil. Many spaces were left to become macropores after root decay, which could increase the water infiltration rate [25; 27; 28]. This finding provided stronger evidence for the effect of dead roots on soil permeability.  
In the third stage (21-30 h), water flow was terminated, and the soil became unsaturated again (\(I_{out}\) decreased). When the water flux at the bottom of the container was terminated (0 cm d\({}^{-1}\)), some water was still stored in soil. The water storage capacities of soil were calculated according to Figure 2 and are shown in Table 2. Compared to the container without roots, the water storage capacities of soil with living roots and dead roots increased by 13.7% and 15.4%, respectively. The effect of living roots on the  
Figure 3: Water flux at the bottom of containers without or with living and dead roots.  
soil water storage capacity was similar to the effect observed by [PERSON] and [PERSON] [6], where they reported that plant roots could increase the soil water storage capacity by 11.6%. Living roots increased soil pores [1, 3, 12, 13, 14] and could store water in soil. Additionally, the results of dead roots about water storage capacity of soil agree with the hypothesis that dead roots could increase water storage capacity of soil, which contribute to the macropores left by dead roots.