Root Water Uptake Process In Deep Soil For Forest Growing On The Loess Plateauand Its Effect On Water Stress And Soil Carbon Input

Forestation has been written into the 2030 sustainable development agenda by the United Nations for its great potential in climate change mitigation,soil erosion control and eco-restoration.The Loess Plateau was once the region with the world's most severe soil erosion and fragile ecological environment.To control soil erosion,the Government of China conducted the Grain to Green Program to convert farmland to perennial forest.However,compared with farmland and grassland,forest usually has higher water demand and greater rooting depth.Therefore,exploring trees'deep soil water extraction process and the response of forest water stress to deep soil water status are of great significance for clarifying the water demand of forest,understanding the water consumption strategy of forest and formulating a reasonable afforestation plan.At the same time,as the hub of the water-carbon cycle,root water absorption directly affects the soil carbon storage.Therefore,clarifying the relationship between deep water consumption and soil carbon input in forest is of great significance for accurately assessing the carbon stocks of forest ecosystems and their significance in global climate change mitigation.Based on Grain to Green Program,this study takes root water uptake in deep soil of forest as the core of the research.We investigated the long-term changing process of forest water stress,carbon sequestration,root water uptake and root development in deep soil,and the interactions among them based on regional scale paired experiment,space-for-time substitution method,carbon isotope technique,numerical model and meta-analysis.The main conclusions are as follows:?1?On the Loess Plateau,the extracted deep soil water cannot be replenished effectively by precipitation after forestation,which stimulated trees to extract water in deeper,moister soil,resulting in the water depletion depth finally exceeding 25 m below soil surface.Therefore,with increasing stand age,deep soil water uptake decreases soil water storage.This phenomenon was termed as one-way root water uptake in deep soil in this study.Under one-way deep soil water mining,soil water storage in deep soil decreased with increasing stand age:from 6 to 25 years of stand ages,forest extracted 83 mm water from deep soil every year,which account for 16%of regional annual precipitation.For forest within 21 to 25 years old,soil water storage loss in deep soil reached 1622±186 mm,which was more than three times of annual precipitation in forestation region.?2?Among many factors such as planting density,rainfall,and initial soil water content,soil water storage loss in deep soil explained the vertical variation of forest rooting depth to the greatest extent?R²=0.86?.Under one-way root water uptake,soil water storage in deep soil decreased with increasing stand age,which stimulated trees rooting to deeper soil with a rate of 1.00±0.28 m yr^-1.The maximum rooting depth reached 24.9±0.74 m for trees within21 to 25 years old.61%±20%of fine roots?<2 mm in diameter?of forest were found in deep soil?>1 m?,and the proportion of fine roots in deep soil increased with increasing stand age.As the depleted deep soil water can hardly be replenished,the total available water in a certain deep soil layer is fixed,and the value was based on the difference between the initial soil water content before afforestation and wilting point.The relative uniform available soil water in deep soil promoted forest root distribution converted from exponential pattern in young forest to the relative uniform root profile in old forest.?3?After afforestation,most of the precipitation was consumed before infiltrating to deep soil.Therefore,the water extracted in deep soil is old water that infiltrated many years ago.For apple trees growing in the Wangdonggou watershed,76%of the extracted deep soil water was from the precipitation before 1963.The shallow infiltration depth made most of the fertilizer existing in shallow soil,decoupling the zone of water with the zone of nutrients in the root zone.For the 18-and 22-years old apple orchard,more than 60%of fine roots were beyond the fertilizer zone.?4?In Wangdonggou watershed,apple trees transpiration and leaf area index?LAI?increased with increasing tree age,peaked at 16 years old,and then decreased with stand age.The growing season transpiration of the 23 years orchard only accounted for 77%of that in16 years old,although the diameter at breast height of the 23 years trees was greater than that of 16 years old trees.Water content within 0¹ m had no statistical difference during the growing season because the soil water in this layer can be replenished by precipitation.However,the soil water content in deep soil decreased significantly with tree age as precipitation can hardly infiltrate to deep strata.The average soil water content in deep soil was found significantly correlated with the ratio between actual transpiration and potential transpiration?T_a/T_p?.The results above suggested that it is deep soil water rather than shallow soil water controls apple trees final transpiration and water stress condition.?5?Annual ring?¹³C was depleted with increasing stand age for all of the five sampling sites.This result suggested that although the soil water content in deep soil decreased with tree age,the water stress of trees didn't increase as expected,but decreased in general instead of increase.In apple orchards with different ages in Changwu state,we found that both LAI and stomatal density decreased with tree age,which might result in high water consumption of a single stoma although stand level transpiration was small in old forest,and finally lead to the depleted?¹³C.The results above suggested that future research on forest water stress should pay attention to scale effect:small transpiration of stand level is not equal to high water stress at micro scale,for instance,stomatal level.Meanwhile,these results revealed that although the“old small”trees on the Loess Plateau have small transpiration,they probably survive via physiological adjustment,which is of great significance for the sustainability of the Grain to Green Program.?6?Following afforestation,the root biomass in deep soil increased with tree age,and on average,51%±4%of root biomass was found in deep soil.The mean carbon input rate of root biomass to deep soil was 0.17±0.04 t C ha^-1 yr^-1.Moreover,the carbon input rate was positively correlated with soil water storage loss in deep soil,which can be viewed as water for carbon trade in deep soil.However,the roots of forest did not significantly change the soil organic carbon content in the root zone possibly because there the deep initial carbon stock before afforestation was too great.For instance,the organic carbon stock of 1 to 23.2 m soil in Changwu state reached 788±10 t ha^-1,and the root biomass in the 22 years old orchard only account for 1.1%of the organic carbon stock in farmland.Therefore,limited root biomass can hardly change soil organic carbon to a detectable level.Furthermore,the decreased soil water content in the root might also restrict fine root turnover and the release of root exudates,and thus inhibit roots alter soil organic carbon content.?7?Compared with the measured relative uniform root distribution,when using traditional exponential root distribution to simulate water transport in SPAC in apple orchard?using Hydrus-1D?,forest transpiration was underestimated in the early stage of simulation?before13 years of forest age?.This is mainly due to that for exponential root distribution,most of the forest roots were concentrated in shallow soil,while the shallow soil suffered intermittent water stress under the influence of evapotranspiration,which will reduce the transpiration of forest.However,under relatively uniform root distribution,trees can absorb water in deeper soil layer with higher water content,and finally resulting in higher transpiration in the early stage.However,when the deep soil moisture is reduced to a certain extent,the root water uptake will be permanently stressed,then the exponential root distribution will start to overestimate transpiration.For deep soil with high variations in soil hydraulic properties in the vertical direction,reducing soil layer amount during simulation will underestimate soil water holding capacity and overestimate groundwater recharge.Therefore,more attention should be paid to root distribution and soil layer characteristics when analyzing and simulating hydrological processes in deep soil.In summary,on the Loess Plateau,root water uptake in deep soil is the key component of water cycle in forest ecosystem,which has great impact on forest transpiration,soil water storage,forest water stress and soil carbon input,and therefore,should be given more attention in future study of forest ecohydrology.