华北山区典型人工林土壤水势动态和水分运移规律

大规模植树造林工程有效缓解了我国北方水土流失等问题，但伴随植被生长和降水格局变化，水循环过程发生明显改变。土壤水分运动是水循环的关键过程，研究变化环境下人工林植被土壤水分运移规律，对植被生态恢复具有重要意义。基于2014-2018年多时间尺度（半小时、天、月和年）华北山区崇陵流域典型人工侧柏林和荒草土壤剖面水势监测数据，阐明不同植被覆盖下土水势动态变化规律，提出土壤水分运移和植被水分利用模式。研究结果表明：侧柏林土壤水势日变幅显著低于荒草植被，但土水势日变幅随土壤深度增加而减小的速率90 a侧柏依次大于60 a侧柏和荒草；月、年尺度侧柏林不同深度土水势变化对降水的响应大于荒草地，其中60 a侧柏林年均土水势与年降雨量显著线性相关（P<0.05）。由水势梯度和零通量面多年平均变化可知，90 a侧柏林0-50 cm土壤水呈下渗趋势，根系水力提升促使50-100 cm土壤水向上蒸散；60 a侧柏林0-20 cm、70-100 cm以及枯水年30-70 cm土壤水均以蒸散为主，根系可同时吸收利用表层和深层土壤水分；荒草地0-20 cm土壤水分蒸发强烈，且为根系主要吸水深度，20-100 cm土壤水稳定下渗。相比60 a侧柏林和荒草，90 a侧柏林的土壤调蓄能力增强，与荒草互被可减少植被间水分竞争，充分利用土壤水，从而减少流域内地表径流和土壤侵蚀量。

Soil water potential dynamics and water utilization of typical planted forests in the mountain area of North China

Large scale afforestation alleviates soil erosion effectively in North China. The water cycle has been consequently modified under plant growth and changing precipitation regimes. Soil water dynamics is critical to investigate the water cycle of planted forests under changing environment for plant restoration. In this study, multi-time scale (i.e. half-hourly, daily, monthly, and annual scales) analysis of soil water potential (SWP) was conducted based on long-term measurement of the SWP between 2014 and 2018 under 60 a and 90 a arborvitae (Platycladus orientalis) planted forests as well as weeds as a reference in Chongling catchment in the mountain area of North China. The temporal variations in vertical gradients of SWP and zero flux planes (ZFPs) were analyzed to propose the patterns of soil water movement and plant water utilization for three representative vegetation types. Results showed that the diurnal variation of the SWP appeared as a unimodal curve with a peak occurring between 13:00 and 14:00. Daily variation in the SWP of arborvitae was significantly small compared to that of weeds. Daily changes of SWP decreased with the increase of soil depth, while the declining rate was larger under 90 a arborvitae than that under 60 a arborvitae and weeds. It was evident that the SWP decreased gradually from April to June, reaching a minimum value of -305.3 cm with the standard deviation (SD) of 58.3 cm. Nevertheless, the SWP increased rapidly from July to August ((-215.7±105.9) cm) and then remained high during September to October ((-210.6±51.9) cm). The responses of SWP at different soil depths to precipitation under arborvitae at monthly and annual scale were greater than those under weeds. There was a significant linear relationship between the annual mean SWP and annual precipitation under 60 a arborvitae (P<0.05). Based on the depth distributions of SWP gradients and ZFPs in the experimental years, soil water infiltration was estimated to mainly appear at upper 50 cm depth but root hydraulic lift facilitated soil moisture moving upwards in the 50-100 cm layer under 90 a arborvitae. The soil moisture in the 0-20 cm and 70-100 cm layers as well as that in the 30-70 cm layer in dry year mainly moved upwards and were taken up by plant roots under 60 a arborvitae. Strong evaporation and root water uptake primarily occurred at the top of 20 cm for weeds, while soil water infiltrated steadily in the 20-100 cm layer. It was found that soil water storage capacity under 90 a arborvitae was stronger than that under 60 a arborvitae and weeds, as it had smaller soil bulk density and increased the saturated soil water content. The planted forest of 90 a arborvitae coexisting with weeds could reduce water competition among different species and improve the water use efficiency. Surface runoff as well as soil erosion would be further reduced in the catchment. This study demonstrated the modifications of soil water dynamics and water utilization of the representative planted forests on the basis of long-term observations and multi-time scale analysis of SWP in Chongling catchment. It provided new insights into ecological restoration and water management in North China under changing environment.