Study On The Relationship Between Root Water Uptake And Soil Carbon Sequestration In Deep Soils Of Typical Plantations On The Loess Plateau

Drought,water shortage and soil erosion are the main limiting factors for ecological construction and sustainable development of the economy and society on the Loess Plateau.Revegetation is an effective measure to control soil erosion,increase soil carbon sequestration and improve ecological environment.Since the implementation of the ‘Grain-for-Green’Programme(GFGP),the ecological construction of the Loess Plateau has produced a remarkable effect,the vegetation coverage has nearly doubled,soil erosion has been effectively controlled,and the ecological environment has been significantly improved.Nowadays,the coexistence of ecological plantation and economic plantation has become an important landscape feature of the GFGP on the Loess Plateau.The unique physiographic conditions(drought and water shortages,deep vadose zone)lead to the formation of deep root systems in plants.The developed deep root system is not only beneficial for plantations to utilize deep soil water resources,but also provides a large amount of plant-derived carbon for deep soil microorganisms.And there is great potential for carbon sequestration in deep soil.However,deep soil water is a fragile water sources with limited reserves,and once excessively consumed,it will take decades to recover.On the Loess Plateau with drought and water shortage,large-scale cultivation of plantations will lead to negative balance of soil water,resulting in deep soil drying.What are the root water uptake characteristics of plantations?And what is the relationship between deep root water absorption and soil carbon sequestration?Answering these scientific questions is of great scientific value to ascertain the water cycle characteristics and evaluate the potential of carbon sequestration on the Loess Plateau.This paper combined isotopic labeling in deep soils with natural stable isotope signatures,studying the deep soil water utilization strategy,the characteristics of deep soil water deficit and the relationship between root water uptake and soil carbon sequestration in deep soils of plantations(black locust and apple tree)under different precipitation gradients.The main results were listed as follows.(1)The vertical distribution characteristics of root systems for plantations under different precipitation gradients are clarified,elucidating the relationship between root distribution and soil water.The two plantations had both developed deep root systems,and their rooting depth reached 10 m under different precipitation gradients.The proportion of deep roots in apple orchards increased significantly with increasing mean annual precipitation(MAP),while the proportion in black locust always remained at around 60% under different precipitation gradients.Fine root length density(FRLD)exhibited the best correlation with soil water among all root parameters.As MAP increased,the relationship between FRLD and soil water in black locust changed from negative correlation to positive correlation and then to negative correlation,while that of apple orchard gradually changed from positive correlation to negative correlation.(2)A new method had been proposed to accurately quantify the contribution of deep soil water,clarifying the seasonal variations in water uptake depth of apple tree with different stand ages.We combined isotopic labeling in deep soils with natural stable isotope signatures,which breaking through the drawback of the natural abundance of stable water isotopes method that can not accurately determine the root water uptake depth of plants and improving the quantification accuracy of deep soil water.Isotopic labeling showed that the water uptake depth of 11-year-old apple trees reached 300 cm in the blossom and young fruit(BYF)stage and only 100 cm in the fruit swelling stage,whereas 17-year-old trees always consumed water from the 0–320 cm soil layer.Overall,apple trees absorbed the most water from deep soils during the BYF stage,and 17-year-old trees consumed more water in these layers than 11-year-old trees throughout the growing season.In addition,the natural abundance of stable water isotopes method overestimated the contribution of deep soil water,especially in the 320–500 cm soil layer.(3)The proportion of root water absorption of plantations with different precipitation gradients was quantified,revealing the important contribution of deep soil water to plantations on the Loess Plateau.Based on the range of root water uptake depth of apple trees determined by results(1),the deep soil water use strategy of plantations on the Loess Plateau was quantified.The results showed that the two plantations had similar water uptake patterns.The water sources of plantations in semi-humid area were relatively stable,the contribution of deep soil water to plantations in Fufeng and Changwu reached to 49.8% and 41.0% respectively,while the root water uptake patterns of plantations in semi-arid area was sensitive to precipitation input and had strong plasticity.Overall,deep soil water made great contributions to the transpiration of plantations on the Loess Plateau,and the average contribution of deep soil water could reach more than 40%.In addition,there were significant isotopic offsets between xylem and soil water in two plantations with different precipitation gradients,and the offsets were lower in drier areas.The isotopic offsets between xylem and soil water resulted in an obvious overestimation of the contribution of deep soil water,especially in dry season.(4)The characteristics of deep soil water deficit on the Loess Plateau were clarified,revealing the response of deep soil water consumption to MAP.The relative difference in soil water storage between plantations and arable land(the difference in soil water storage between plantations and arable land / water storage in arable land)was used to characterize the deep soil water deficit effect of plantations.The results showed that both two plantations consumed a large number of deep soil water,the deep soil water deficit effect in black locust and apple orchard reached-0.7 and-0.6,in the areas with MAP less than 500 mm,deep soil water deficit was severe.Two plantations exhibited different deep soil water consumption patterns.The deep soil water consumption of black locust increased with increasing MAP,while there were no significant differences in deep soil water consumption of apple orchards among different precipitation gradients.(5)The index of carbon sequestration efficiency in deep soils was established,elucidating the relationship between root water uptake and soil carbon sequestration in deep soils of plantations.The carbon sequestration effect of plantations with different precipitation gradients was significant,and the black locust was significantly higher than apple orchards.There was a significant negative correlation between soil carbon sequestration in deep soils of plantations and MAP,and the maximum carbon sequestration in black locust and apple orchard was 1.39 and 1.9 times of the minimum carbon sequestration,respectively.The carbon sequestration efficiency was used to evaluate the water–carbon relationship in deep soils of plantations.The results showed that there was no significant difference in deep soil carbon sequestration efficiency in black locust with different precipitation gradients.While the soil carbon sequestration efficiency in deep soils of apple orchard significantly increased as MAP decreased.This indicates that the efficiency of SOC sequestration in deep soils is enhanced in a drier climate and the cultivation of apple trees in drier areas may bring higher carbon sequestration benefits.In addition,although the SOC sequestration in deep soils of black locust was significantly higher than that in apple orchards in semi-arid areas,there was no significant difference in deep SOC sequestration efficiency between two plantations.In summary,plantations on the Loess Plateau can make a significant contribution to deep SOC sequestration through root water uptake.The results not only deepen the systematic understanding of deep soil water–carbon relationship in deep vadose zone,but also provide a scientific basis for vegetation construction and water resources management on the Loess Plateau and other similar regions,which is of great significance to ecological conservation and high-quality development of the Yellow River Basin and the realization of the goal of “carbon neutrality”.