Study On Soil Moisture Dynamics And Infiltration Characteristics Of Robinia Pseudoacacia Plantations At Different Stand Ages In Loess Hilly Region

In the Loess Plateau region,due to low rainfall,and mostly concentrated in July to September,drought water shortage and soil erosion problems are prominent.Therefore,soil moisture is a key factor limiting the ecological restoration and reconstruction of the area.Studying the dynamic change law of soil moisture and soil infiltration characteristics of plantations with different restoration years is conducive to clarifying the differences in soil hydraulic properties,increasing the effective use of soil moisture,and providing insights for scientific management of plantations and vegetation restoration.in this study,the 10a,15a,25a and 40a Robinia pseudoacacia plantations in the loess hilly region were selected as the research object,with the farmland and grassland as controls.The long-term fixed point monitoring of soil moisture dynamic was conducted to explore the differences and spatio-temporal changes of soil moisture of Robinia pseudoacacia plantations in different stand ages,and the soil hydrophysical properties and soil moisture infiltration performance of Robinia pseudoacacia plantations in different stand ages were compared and analyzed.The main conclusions are as follows:（1）The soil separates of Robinia pseudoacacia plantations in different stand ages showed that silt was significantly higher than clay and sand（p<0.05）,and the clay and sand contents showed an increasing trend with the increase of stand age.In the vertical section,the change of silt with increasing soil depth was not significant but the sand increased.The soil bulk density of Robinia pseudoacacia plantations was remarkably lower than the farmland and grassland（p<0.05）,and that of 40a Robinia pseudoacacia plantation was the least,with a range of 1.18-1.41 g·cm^-3,and increased first and then stabilized with the increase of soil depth.The variation trends of soil porosity,saturated moisture content,field capacity and saturated hydraulic conductivity were basically the same,showing the minimum in farmland and the maximum in 40a Robinia pseudoacacia plantation,increasing with the increased stand age and decreasing with the increased soil depth.（2）Compared with precipitation,soil moisture dynamic change of Robinia pseudoacacia plantations in different stand ages showed a certain lag.The inter-annual variation of soil moisture in different places showed an increasing-decreasing-increasing fluctuation trend,and the soil moisture storage showed an increasing trend of fluctuation,with the lowest moisture storage in 2015 and the highest moisture storage in 2021.In the three seasons,the soil moisture content was in the order of autumn（20.69%）>spring（16.40%）>summer（15.10%）.Soil desiccation index（SDI）was the highest in summer,and strong dry layer appeared in 25a and 40a Robinia pseudoacacia plantations.In 0-1000 cm soil layer,the average soil moisture content of Robinia pseudoacacia plantations was significantly lower than that of farmland and grassland（p<0.05）,the maximum soil moisture content was found in farmland,and the minimum soil moisture content was found in 40a Robinia pseudoacacia plantation.The soil water deficit of 400-1000 cm soil layer was the most serious.（3）Under different waterheads,the infiltration rate of 40a Rbinia pseudoacacia plantation was the highest,while that of grassland was the lowest.In different soil layers,the infiltration rate decreased gradually with increasing depth,the infiltration rate of 40a Robinia pseudoacacia plantation in 0-10 cm and 30-40 cm soil layers was the largest,and the infiltration rate of 25a Robinia pseudoacacia plantation was the largest in 20-30 cm soil layer.Among the selected infiltration models,R² of Kostiakov model and general empirical model is higher than 0.90,and the fitting accuracy is higher.The R² range of Horton model is 0.50-0.85,which has the worst fitting effect.Among the factors affecting infiltration,bulk density,soil moisture content and sand were in a negative correlation with soil infiltration rate,soil moisture content was in a significant negative correlation with soil infiltration rate（p<0.05）,and organic matter content was in a significant positive correlation with initial infiltration rate and average infiltration rate（p<0.05）.In conclusion,planting Robinia pseudoacacia plantations has significant effects on improving soil physical properties,improving soil structure,increasing soil porosity and reducing soil bulk density,as well as helping to increase soil infiltration and reduce drainage.However,the growth of Robinia pseudoacacia can consume a lot of water,and the water consumption is gradually increased with the age of the plantations,resulting in a large range of soil moisture deficit,especially deep soil moisture,which is not conducive to the local ecological restoration and reconstruction.Therefore,when using artificial afforestation to restore ecology in the future,it is necessary to fully consider the consumption of soil water in its growth process,and formulate reasonable planting density and regeneration strategies.Meanwhile,some native tree species and shallow root herbs can be given priority.