| Weibei rained highland,which has deep soil layers,are suitable for the growth of various crops due to excellent natural conditions such as light,heat,and gas.However,the rainfall is limited in this area,and the spatial and temporal distribution of rainfall is inconsistent.Therefore,this area was typical rain-fed agriculture area with severely limited water resources.Droughts frequently occur.and " Spring rain is as precious as oil are the true reflections of obstacles to agricultural production and ecological environment construction in this region.Since the implementation of the land contract responsibility policy,the Weibei region has been hailed as a high-quality,high-yield development area for apples because of its superior natural conditions in the process of adjusting the agricultural industry structure.Therefore,most of the farmlands were successively converted by orchards,and the land cover and natural landscape have undergone tremendous changes,and led to the rapid development of the local economy.However,there are always different opinions in among the researchers,due to the dry climate.it is not appropriate to develop the arbor forest(fruit)industry,and there is a risk of the soil drying inside and forming dry soil layers and this will be lead to soil desiccation.In view of the practical problems in the production and academic debates in the Weibei region,starting from the analysis of the feasibility goals of the sustainable development in the Weibei fruit industry is highly urgent.this study focused on the dryland farmlands and orchard soils in different orchard ages in Weibei region,site-specific monitored of soil moisture in the 0 to 150 cm range of apple tree growth,and used a variety of time scales,spatial scales,and multiple evaluation indicators to comparatively analyzed soil moisture characteristics and drought in farmlands,orchards and within different garden ages,at the same time,based on the soil moisture dynamics and processes,explored the movement,compensation mechanism and influencing factors of soil moisture in Weibei region,the following research results and progresses were obtained:(1)After coveted to orchards in the Weibei rainfed highland,the soil moisture content and the stability of the orchard soil are significantly better than that of the farmland because of changes in the ground covers.In the past,droughts of different degrees frequently occurred in farmland were no longer exist in the orchards.After positioning dynamic observation,there was no drought stress observed in orchard soil during the crop growth period,the soil water content in the range of 0 to 150 cm was kept at a level between the field capacity and the moisture content at capillary rupture(stress point).On the contrary,soil drought is more serious in the farmland from spring to early summer,and its affected in deep soil layer.Soil moisture stress(below the point of stress)at 20 to 40 cm was continuous for 78 days,and in the 40 to 60 cm range,the number of days that the soil moisture content was below the wilting coefficient lasted for 42 days.There is a clear aridity threat to the farmland from 0 to 120 cm in the soil from mid-April to mid-July,especially in the rainy season from mid-May to mid-July,water stress is most likely to occur at the soil depth of 20 to 60 cm.It is precisely because the evaporation inhibition effect of perennial fruit tree canopy cover that the soil moisture status is significantly better than that of farmland,which greatly relieves the degree of soil drought in the region.The research suggestthat the Weibei region,crops have been converted by perennial fruit trees,and ground covers replacement has significantly reduced the drought in the past,so the development of the fruit industry here is scientific and reasonable.Secondly,it was indirectly proven that the drought in Weibei region was mainly due to the soil moisture evaporation,but not mainly the transpiration of the vegetation canopy.(2)The orchards did not show a significant deterioration of the 0 to 150 cm soil moisture with the age of the orchards.According to the research on the soil water status of three orchards ages,the soil moisture content of the orchards remained stable above the soil water stress point during the entire fruit tree growth period.although on the time scale,the orchards ages ranged from mid-March to mid-June,and the soil water content at different depths showed a decreasing trend in the range of 0 to 150 cm,Among them,the moisture content of the orchards in the fruit-bearing period decreased the significantly at the 20 to 40 cm soil layer,reached 8.58%,the younger orchards have the lowest water content at 0 to 10 cm in soil layer,reached 0.4%.In the whole soil layer 0 to 150 cm soil profile range,the soil moisture content of young orchards decreased by an average of 2.72%,fruit-bearing period orchards decreased by an average of 5.40% and old orchards decreased by an average of 4.58% during the period of continuous drought from mid-March to mid-June.The results indicate that with the continuous growth of the age of the orchards age,in the old orchards above 25 years old,the presence of soil layers with accumulated effects of drought was not seen.(3)From the multi-indicator analysis,we can find that the orchards of different ages in Weibei region did not show soil water stress.The "effective water storage capacity(ASW)" indicator indicate he average age of young orchards was 187.97 mm,190.12 mm in the fruit-bearing period orchards and 176.95 mm in the old orchards of the apple growth period of 0 to 150 cm soil layer.The fluctuation of water storage on the time scale is the most stable in young orchards.soil water over-consumption(SWO)showed that only older orchards are positive in mid-June and the difference is minimal.and the "desiccation index(SDI)" illustrated the fruit-bearing period and older orchards had mild desiccation in June.From the "water consumption" and "day water consumption intensity" analyzed layer by layer,we can find that older orchards water depth of the section was smaller than that of the younger orchards,but the short-term water consumption can be effectively restored with the arrival of the rainy season.Older orchards was only slightly difficult to rehabilitate.From the analysis of multi-judgment indicators at time and space we can see that no obvious problem of soil desiccation in orchards.(4)During the growing period of fruit trees and farmlands in Weibei region have obvious stage characteristics and spatial characteristics.It is divided into two stages,loss stage and reclamation stage.In the 0 to 80 cm soil layers,the soil moisture in farmlands and orchards show “fluctuant-decreasing” stage,“significantly accumulated growth” stage,and “gradually decreasing” stage.In the 80 to 150 cm soil layers,only the characteristics of “low amplitude fluctuation decreasing” and “significantly increasing accumulation” stage are appeared.From the synchronicity of water changes point of view,in the range of 0 to 40 cm soil layers,basic synchronism between all soil layers,regardless of the loss stage or the reclamation stage.The loss below 40 cm soil layers was prolonged relative to the upper soil,the recovery period was relatively delayed by 1 month.From the analysis of water change drivers,the soil moisture in 0 to 40 cm is closely related to near-surface meteorological conditions and belongs to the "meteorological response-sensitive soil layer".The soil moisture in 40 to 80 cm has a certain relationship with the physiological water consumption of the fruit tree and the crown structure,which belongs to the “physiological water-consuming soil layer” and there is a short period of water stress discomfort.The 80 to 150 cm soil layers has small fluctuations in soil moisture and relatively stable soil space,belonging to the “compensation and compensation” soil layer.The results proved that spatial-temporal variability of profiles soil moisture in Weibei region and and its driving factors are related to the occurrence level of soil,the vegetation structure on the ground is also obvious for its effect,and the fruit canopy significantly weakens the degree of water change.(5)Based on the van Genuchten-Mualem model,and soil moisture characteristics curve and its parameters,soil water dynamic parameters,confirmed that the relevant water dynamic parameters between orchards and farmlands,even the orchards of different garden ages are basically similar.It was showed that the soil physical conditions between orchards and farmlands selected in this study were homogenous.Therefore,the soil moisture status between orchards and farmland,and different orchard ages was mainly due to ground covers rather than soil conditions.The dynamic process of soil profiles hydraulic potential and unsaturated hydraulic conductivity of different spatial and temporal during crop growth indicated that the soil moisture status and the process of water migration and compensation are mainly “state effect”,which induced by the ground covers,driven by soil hydraulic potential and hydraulic conductivity rather than the “nature effect” of the soil.In other words,the water movement and storage capacity of different soil layers at different time periods are mainly due to the water-blocking rate induced by the degree of desiccation and the water-blocking force induced by the thickness of desiccated soil layer.The farmlands were drought-prone at 20 to 60 cm soil layers,so the unsaturated hydraulic conductivity of the soil decreased drastically,increasing the water blocking capacity and making the crop vulnerable to drought.The soil moisture content of orchards maintained a relatively stable high level,the difference between soil layers was small.The rate of decline of unsaturated hydraulic conductivity was small and the spatial difference was small,so the capacity of soil reservoirs was stronger than that of farmlands,and the threat of fruit trees suffering from drought was small.In summary,the types of ground covers dominate the soil drought and planting alleviates drought in Weibei rainfed highland.In the process of strong biological transpiration,the soil drying tendency mainly occurs in farmland,its horizon is relatively shallow,and soil moisture content easily recovers in the rainy season.Recovering of older orchards during rainy season is slightly more difficult than young orchards,which was mechanism needs further study.As long as the orchards have proper soil management measures,adopt reasonable dense planting,remove soil every year,remove soil compaction,increase effective permeability of soil profiles and so on,soil moisture can be controlled and effectively restored. |
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