Study On Soil Water Effects Of Returning Farmland To Forest On The Loess Plateau Of China

In recent years,the dramatic changes in vegetation has been made by large-scale farmland to forest on the Loess Plateau of China.However,the negative feedback between vegetation growth and soil water capacity has been developed due to irrational vegetation construction,which makes the formation of dried soil layers(DSL),the emergence of vegetation degradation and other ecological problems.Meanwhile,warming-drying trend exacerbated the contradiction between water use of forest and soil water supply,and caused safe supply of water resources in the region.To use the limited-water resources maintaining the sustainable development of artificial forest ecosystem on the Loess Plateau becomes an important goal of ecological environment construction in the region,and the core of the goal is soil water.Therefore,studying soil water effects of returning farmland to forest on the Loess Plateau of China,it is helpful to study the mechanism of soil-water conservation of artificial forest ecosystem in theory,and it is of great practical significance for the limited-water resources utilization and the management of forest in practice.In this paper,the soil water content(SWC)of 0 ~ 5.0 m soil layer was studied for the artificial forest on the Loess Plateau at large scale.The factors affecting the distribution of soil water in different soil layers were quantitatively analyzed.The characteristics of DSL and its influencing factors were analyzed along the transect scale.The characteristics and influencing factors of soil water storage(SWS)for understory vegetation and understory vegetation removal were dynamically monitored and analyzed at plot scale.Based on the analysis of the change of SWC in the process of redistribution of overstory(refer to the net input of soil water without canopy intercept),surface runoff and changes in SWS in the growing season,the characteristics of understory evapotranspiration(Eu)and its influencing factors were studied according to the principle of water balance.At the same time,the soil water balance of artificial forest ecosystem was studied under different thinning treatments.The main conclusions are as follows:1.Evaluation of soil water effects of returning cropland to forest on the Loess Plateau.SWC in artificial forest is generally lower than the agricultural land,SWS decreased by an average of 169.6 mm.The SWS capacity of 3.0 ~ 4.0 and 4.0 ~ 5.0 m soil layer is larger than 1.0 ~ 2.0,2.0 ~ 3.0 m soil layer,and the losses of soil water increases with the depth of soil layer.SWS in different rainfall areas is different.Rainfall in the area of > 550 mm,the highest loss of soil water is 214.7 mm,the loss of soil water in the area of 450-550 mm is 195.7 mm,and in the area <450 mm is the least,only 36.0 mm.There was a significant positive correlation between soil water losses and initial SWS,rainfall,temperature and slope.2.Evaluating the dominant factors affecting the spatial pattern of soil water in different soil layers.SWC has great variation in different types of artificial forest different.The results showed economic forest(ENF)> ecological forest(ELF)> shrub land(SL),variability of deep SWC was great.Rainfall and clay content had the greatest influence on soil water distribution in different soil layers.With the increase of soil depth,the local factors had a great influence on the soil water distribution of ELF,while the meteorological factors had a great influence on the soil water distribution of ENF,but there was no significant difference between the local influence and the meteorological factors for SL.3.Spatial pattern and influencing factors of DSL along a south-north transect scale.SWC gradually reduced in the vertical direction,soil desiccation serious.The average thickness of DSL was 289.1 cm,the average water content of DSL was 7.7%,and the formed depth of DSL was 151.9 cm.There was a large variation of DSL in different rainfall areas.From dry area to arid area,SWC in DSL decreased and then increased.The contribution of environmental factors explained variability to DSL variation was 38.0%.Local factors alone explained the percentage of variability(16.5%)was greater than the climatic factors alone(4.2%).4.Soil water effects of returning cropland to forest at hill-slope scale.The more distance to the gully,the greater SWS.The soil sampling site at distance of 300 cm could represent the average of the SWS at the measured scale.In the vertical direction,SWC had a great variability,which controlled by gully.Under the different vegetation types,spatial distribution pattern of SWC was dominated by vegetation types,gully,however,slope position,micro-terrain and other factors had a lesser influence.5.The effects of undergrowth vegetation on soil water balance in artificial forest on the Loess Plateau.In the artificial forest of Robinia pseudoacacia,the percentage of throughfall accounted for about 87.3% of the rainfall outside the forest,interception by the canopy was followed,which accounted for about 10.4% of the rainfall,the percentage of stemflow accounted for about 2.3% of the rainfall.During the observation period,water use of Eo was about 265.0 mm,the water use of Eu was about 120.5 mm,and the percentage of the Eu was about 31.3% accounted for the water use of whole forest during the growing season.6.The effect of ecological thinning on soil water balance in artificial forest.During the observation period,soil water deficit were 82.4 mm and 38.5 mm for thinning 40% and 90%,respectively.Soil water deficit was less with increased thinning.With the increasing of the thinning for artificial forest,the transpiration of understory vegetation increased by 52.5 mm.The increased in transpiration of understory vegetation offset the increased part of evapotranspiration of the thinning.In summary,extensive afforestation on the Chinese Loess Plateau has led to decreases in deep SWC,and formed a persistent DSL due to soil water deficit,which has a profound effect on the artificial forest ecosystem,while the Eu couldn‘t be neglected,which is a great water loss part of artificial forest ecosystem.The ecological thinning for artificial forest can significantly change soil hydrological cycle process.