Difference Of Fine Roots Function Traits And Influencing Factors Of Larix Gmelinii Rupr. Between Litter Layer And Soil Layer

Fine roots are the most important component of the tree root system,which have an important role in absorbing nutrients and moisture,and affecting the carbon(C)allocation and nutrient cycle in forest ecosystem.In some forest ecosystems,fine roots tend to grow into the litter layer.However,in previous studies,their functional traits of these fine roots in litter layer have been overlooked.Compared with the studies on fine roots of soil layer,we are still unclear about how the fine root anatomy,morphology and histochemistry change among different layers,and which physicochemical properties of the substrate influence these functional traits.Therefore,we selected the 29-year-old Larix gmelinii Rupr.plantation in Maoershan,and used the soil core to collect the fine roots(including first five orders,where the distal root tip is named as 1-order)from the litter layer and surface soil layer(0-10 cm),subsurface soil layer(10-20 cm,sampling only in July)and deep soil layer(20-30 cm,sampling in July only)in May,July and September of 2016.At the same time,we measured the physical and chemical properties of soil and litter.The main purposes of this study were:1)to explore the anatomical,morphological,chemical(C,N(nitrogen)concentration)and physiological(respiration rate)variation patterns of fine roots in litter layer and soil layer;2)and to demonstrate the seasonal dynamics of these fine roots function traits;3)and also to analysis the relationship between the physicochemical properties of the substrate and fine root function traits,along with vertical depth and seasonal changes.Our results showed:(1)With the increase of root order,the anatomy of the fine roots in the litter layer and the soil layer exhibited consistency,the radius of stele diameter,the average diameter of conduit,and the proportion of the total conduit area per vascular area increased.Besides,root cortex thickness and the ratio of cortical thickness to stele radius decreased.However,the anatomy traits of first five order roots in litter layer showed large differences from other soil layers.For example,the stele radius of the 2-and 3-order fine roots in the litter layer was lower than that of the soil layer;the cortex thickness of the 1-to 3-order roots was higher than that of the subsurface soil layer and the bottom soil layer.However,the ratio of total conduit cross-sectional area per stele area was smaller than that of the surface soil and the sub-surface soil layer;and the mean diameter of conduit of the 1-to 3-order fine roots was lower than that of the soil surface layer,but higher than that of subsurface soil layer.In addition,fine root anatomy showed different response to season in different vertical layers.For example,in litter layer,the radius of stele decreased with the seasons,while,in the soil layer,the radius stele of 1-to 3-order roots in the surface soil were the highest in July(59.53± 1.49 ?m-165.03±27.97?m),but the lowest in September(54.48±8.20 ?m?127.68±6.54 ?m).(2)With the increase of the vertical layer,the diameter of fine roots increased,and the specific root length(SRL)decreased.Additionally,the high order(3-to 5-order)roots showed relatively more distinct variations.By contrast,the difference in tissue density among vertical layers was not significant,and the fine roots in subsurface layer exhibited the greatest tissue density.In terms of seasonal dynamics,the root diameter(3-to 5-order)of litter and surface layers increased with season,whereas the SRL decreased.(3)The vertical depth and root order had a significant effect on the total C,total N content,and C/N ratio of fine roots.The total C and N content of the fine roots decreased with the increase of soil depth,but the ratio of C to N increased.Additionally,the seasonal changes of the fine root histochemistry in both litter and the surface soil layer were consistent.The total N content of fine roots decreased with the season and the litter layer,the average of the second to fifth order roots changed from 15.29± 1.95 mg · g-1 in May to 14.16± 1.78 mg · g-1 in September,which was generally higher than those of surface soil,the corresponding values were 14.73±0.55 mg · g-1 and 13.29±0.29 mg · g-1,respectively.Although the total C content of fine roots showed no significant variations with seasons,the C content in litter layer were also generally higher than that of surface soil layer.(4)The vertical and seasonal changes of fine roots function traits are closely related to the physicochemical properties of the substrate.The vertical changes of cortex thickness,cortical thickness,and center column radius of 1-to 2-order roots were positively correlated with the physicochemical properties of the matrix.The vertical density of conduit density,the ratio of the total conduit cross-sectional area to the proportion of the center column area,and the physical and chemical properties of the matrix were negative Correlation;The vertical variation of the roots diameter of 1-to 4-order was negatively correlated with the physicochemical properties of the substrate,and the root length of the 2-to 3-order roots was positively correlated with the physical and chemical properties of the substrate.In addition,the seasonal variation of the fine roots function traits in response to changes in the physicochemical properties of the substrate is also related to the order.For example,compared with roots of 1-to 2-order,the anatomical characteristics of 3-order roots are more closely related to the physical and chemical properties of the matrix.In different vertical layers,the environmental factors that mainly affect the fine root functional properties are different.For example,in the root histochemical characteristics,the total N content of the fine roots in the soil surface is most affected by the vertical variation of water content,while in the litter layer,the total root N content does not have a significant relationship with the water content.Compared to the litter layer,the fine roots of the soil layer are more susceptible to environmental factors.In summary,fine roots in the litter and soil layer exhibited obvious differences in anatomy,morphology,chemistry and physiology,and they also showed different seasonal dynamics.These variation 'patterns of fine root function traits were related to the change of physicochemical properties of the growth matrix,and some traits linked to the physicochemical properties of the substrate,but some did not.Our results revealed the functional traits of fine roots in litter layers and their associations with physicochemical properties of the substrate and improved the understanding of the potential mechanisms of nutrient uptake in the root system of the L.gmelini.These findings would provide a theoretical basis for the nutrition management of L.gmelinii plantation in the future.