Spatiotemporal Distribution And Influencing Factors Of Soil Water Content In Cropland-apple Orchard Mosaic Pattern In The Loess Tableland Of China

As an important component of terrestrial water resources,soil water is also a critical factor that restricts the plant growth in dry-farming regions.In the past thirty years,the continuous development of apple orchard in the southern loess tableland has formed an orchard-cropland mosaic vegetation pattern,the characteristics of soil water use of crops and fruit trees were different.The deep soil water consumption characteristics and soil water resource utilization pattern attracting people's attention,the solution of the problem is of great significance to improve soil reservoir function,promote the sustainable and healthy development of cropland-apple orchard mosaic layout.In this study,we selected apple orchards with different stand ages and adjacent croplands in Qingyang,Changwu and Luochuan tablelands.Based on field sampling,determination of indoor,combined with the mathematical statistics and literature retrieval,this study illustrates the spatiotemporal distribution and influencing factors of soil water content(SWC)in cropland-apple orchard mosaic pattern in the loess tableland of China.The main conclusions are as follows:(1)The characteristics of soil desiccation in deep vadose zone in apple orchard.The soil texture of Qingyang,Changwu and Luochuan showed similar tendencies over the 0–23 m profile,with obvious stratification characteristics.In the shallow 0–3 m soil layer,climate factors(precipitation)were significantly correlated with SWC,and the relative contribution rate for variation in SWC was 46.9%.Below 3 m soil layer,the dominant factors affecting the variation in SWC and the corresponding soil layers were as follows: tree age(3–13 m)and soil texture(> 13 m).The formation depth of dried soil layer in orchard ranged from 2 to3 m,which mainly affected by the precipitation,while the thickness of dried soil layer increased significantly with increasing tree age.The root water consumption depth of apple trees and soil water deficit(compared with cropland)showed an "S" shaped with tree age.After about 20 years,the roots water uptake depth of apple trees no longer increased,the annual water consumption was gradually equal to the annual precipitation,and the yield also decreased.(2)Changes of deep soil water in the conversion of cropland to orchard at regional scale.After converting cropland into orchards,apple trees continue to absorb and utilize deep soil water,thus the SWC in deep soil layer decreased with increasing tree age after the young age,and then remained stable after 20 years.Affected by the spatial variation of annual precipitation,deep(> 2 m)SWC in orchards significantly increased with mean annual precipitation from 500 to 650 mm.However,the effect of planting density on SWC varied with tree age.In the young orchard,the deep SWC in apple orchards with different planting densities had significant difference(P < 0.05),but in old orchards,the effect of planting density on SWC was weakened,which was significantly lower than that of the control(farmland).Changes in deep SWC in orchards depended on tree age,planting density and annual precipitation.Therefore,we suggest that the planting density should be regulated on the local climate conditions,and the relationship between fruit tree productivity and soil moisture should be balanced to coordinate the sustainable utilization of regional water resources,food security and economic development.(3)Spatial distribution of the SWC under the cropland-orchard interaction zone.In the boundary zone between the 24 years old orchard and the adjacent cropland,the influence of apple trees on the SWC of adjacent cropland decreased with the increase of measuring distance from the orchard.The distribution of SWC in the cropland-orchard interaction zone showed obvious boundary effect: apple trees could absorb soil water below 2 m depth from the adjacent cropland to meet their own needs,and the 24 years old orchard could absorb soil water within 7 m,showing dependence on the adjacent cropland,but the 10 years old orchard did not affect the SWC in adjacent cropland.Therefore,a suitable areal proportion between croplands and orchards should be maintained,and the width of cropland should be more than 14 m,so as to sustain the synergetic utilization of soil water resources and optimization of regional agricultural production in the cropland-orchard ecosystem.(4)Changes and influencing factors of SOC in cropland and orchards.Affected by the loess-paleosol sequence distribution,the croplands and orchards has similar SOC distribution patterns,and the planting of apple trees mainly affecting the SOC content in the0–40 cm soil layer.Compared with the cropland,the SOC storage in orchard first decreased and then increased with tree age,and exceeded that in cropland,but there was no significant difference among tree ages.The precipitation,mean annual temperature and tree age had great effects on SOC in shallow soil layer,while soil texture can better explain the variations in SOC in deep layer.In addition,the soil carbon storage of 0–1 m only accounted for 8.83%± 1.88% of the total soil carbon storage,indicating that the deep soil has a high carbon storage,which has great implications for future land use management and assessment of the terrestrial carbon cycle.Overall,based on a large number of field samples and literature integration analysis,this study clarified spatiotemporal dynamic of SWC in typical sample plots under mosaic pattern of cropland and orchard,reveals the changes and influencing factors of SOC and its relationship with soil water in croplands and orchards.This study also provides some suggestions for sustainable utilization of soil water resources,sustainable development of grain and fruit industry considering grain self-sufficiency.This study is of great significance for the rational adjustment of land use structure,coordinated promotion of regional food security,fruit production and economic development in the loess tableland.