Deep Soil Water Consumption Process And Its Eco-hydrological Impacts Of Apple Orchard On The Loess Hilly Plateau

The Loess Plateau is one of the optimal planting zones of apple and had world’s largest apple tree cultivation zone,with coverage of 1.31 million ha and 23 million tons,accounting for more than one-quarter of global coverage and production.In recent years,apple orchards have been increasing fast on the Loess Hilly region owing to the policy of “Expansion into West and North”,and apple industry has become the leading industry for farmers to get rid of poverty.However,the fast expansion of apple orchard will change the balance of the regional water cycle and aggravate the contradiction between water supple and demand due to water scarcity and strong evapotranspiration.This in turn could undermine the health development of apple orchards.Deep soil water is an important water resource for vegetation in the Loess Plateau to cope with extreme climatic events such as drought,but its reserves are limited.And once consumed,it is difficult to recover.In the Loess hilly region where the contradiction between water supply and demand is prominent,the large-scale development of deep-rooted apple trees will inevitably result in the deep soil desiccation.However,what is the specific development processes of deep soil water supply and consumption? How,and to what extent,will apple trees respond physiologically growth and production if they cannot access deep soil resource? How does the hydrological processes of orchard ecosystems respond to deep soil water deficit? Answering these questions are of great scientific significances to the retaining of deep soil water and sustainable development of rainfed apple orchard in the hilly region of the Loess Plateau where precipitation is limited.In this study,fruit trees in full-stage of rainfed apple orchard were selected as the research object.Through long-term(2018?2022)high-frequency continuous situ monitoring of deep-profile soil water and stem flow and isolating of shallow and deep soils below 200 cm depth and model simulation,this study systematically analyses the specific process of soil water recharge and consumption of deep soil water,and clarifies the effect of deep soil water on the tree transpiration,physiological growth,yield and orchard hydrological processes.This study mainly includes two treatments: control(continuation of normal conditions)and partitioned(separated the deep roots from the shallow layers)treatments.The main progress are as follows:(1)This study clarified the specific processes of soil water recharge and consumption.The deep soil water consumption of apple trees in typical loess area has obvious seasonal variation.The deep soil water(200?450 cm)in the control plots clearly decreased in the dry seasons(May to July,that is,the fruit expansion period)and the proportion of deep soil water use was as high as 70% during the extreme drought period,resulting in serious deep soil desiccation.There were significant(p < 0.05)lower soil water storage at the upper 200 cm in the partitioned plots than in the control plots.This could be attributed to the lack of access to deep soil stimulating trees to develop shallow root systems,thereby increasing the proportion of water take up from the upper 200 cm.After cutting down trees,the soil water in the shallow layers can be effectively restored in an extremely wet year(2022).Specifically,compared with the end of 2021,the average shallow soil water in the control and partitioned plots increased by 42% and 92%,respectively,reaching 76% and 72%of the farmland.However,in the control plots,the deep soil water recovery was slow.In particular,the effect of precipitation infiltration was weak for that soil water below 350 cm layers,and the average deep soil water content is only 36% of the farmland.(2)This study quantified the effect of deep root wateruptake on transpiration.Transpiration of the partitioned plots in 2020(annual precipitation: 488 mm,normal year)and 2021(annual precipitation: 431 mm,dry year)were significantly lower(p < 0.05)than that in the control plots,by 37% and 24% on average,respectively.However,there were no significant difference between them in 2019(annula precipitation: 560 mm,wet year).These results indicate that deep soil water deficit would severely inhibit the transpiration when precipitation is limited.Deep soil water deficit leaded to a lower critical values of transpiration rate in the dry season compared with the control plots,but increased the sensitivity of transpiration rate to meteorological factors in the wet seasons.In addition,the linear regression between transpiration rate and deep soil water in dry season showed a significant(p < 0.01)relationship,and transpiration rate decreased with deep soil water decreased.In the wet seasons,the relationship decreased.(3)This study revealed the response of physiological growth and productivity of apple trees to the lack of deep root water uptake.Deep soil water deficit significantly impacted stomatal limitation by imposing average reductions of stomatal conductance,photosynthesis rate,and transpiration rate by 38%,26%,and 34% while increasing leaf water use efficiency by 36%,during the dry seasons;these parameters recovered well in the wet season.In contrast,no significant differences in nonstomatal limitation occurred between treatments.In the partitioned plots,midday leaf water potential(14%?44%)and leaf hydraulic conductance(28%?58%)significantly decreased without adjustment,even in the wet season.Consequently,yield and quality were significantly(p <0.05)lower in partitioned plots by 31% and 15%,respectively,than that in the control plots.(4)This study predicted the response of hydrologic process in apple orchard to different degrees of deep soil water desiccation in different precipitation years.Based on the Hydrus-2D,the soil water in root zone were simulated in different precipitation years(extremely dry years,normal years and extremely wet years).Results showed that apple trees with deep root systems clearly used soil water and the proportions of deep soil water uptake increased with the decrease of precipitation.the proportion of deep root water uptake in extremely dry years was 9%?12% and 19%?26% higher than that in normal years and extremely wet years,respectively.Deep soil desiccation clearly inhibited cumulative actual transpiration and cumulative actual evapotranspiration,and the negative effect was exacerbated by an increase of deep soil desiccation and a decrease of precipitation.Severe deep desiccation and lacking of DSW resource,respectively,caused clear reduction of cumulative actual T(by 19.3-23.8% and 37.5-42.5%),cumulative actual ET(by 13.0-14.4%and 11.2-23.5%)and T/ET(7%-12% and 10%-25%)across precipitation.T/ET was constrained by rainfall and increased with increasing of degree of deep soil desiccation.Precipitation dominated the change of T/ET once deep soil water was not available.This study combines long-term observation fata with model simulation,quantifying the effect of deep soil water deficit on the physiological growth and production level of apple trees,and exploring the responses of hydrological processes to different degrees of deep soil desiccation in different precipitation years.This study demonstrate that deep soil resources play key roles in trees’ responses to drought,and provide scientific basis for deep soil water conservation and sustainable development of dryland ecosystem in area with limited precipitation resources.