| The spatiotemporally varying physiochemical properties of the forest soils in the mountainous regions play essential roles in supplying or affecting numerous ecosystem services in the dryland regions of northwest China.Due to the implementation of the long-term and massive forest restoration and natural forest protection projects and the increasing climate change effects in the last decades,the soil properties have significantly changed.Detection of the vertical variation of soil properties and soil depth and the difference between forest types is essential for assessing and managing the forest ecosystem services.Therefore,this study was conducted in northwest China’s two distinct mountainous forest areas.The first was Liupan Mountains in southern Ningxia province,with a semi-humid and semi-arid climate,and the second was the Qilian Mountains in western Gansu province,with an arid climate;both are ecological and hydrological essential areas for the surrounding regions.In this study,the soil physicochemical properties,including soil bulk density（SBD）,porosity,pH-value,electrical conductivity（EC）,cation exchange capacity（CEC）,base saturation（BS）,available potassium（AK）,available phosphorus（AP）,total soil potassium（STK）,soil organic carbon（SOC）,total soil nitrogen（STN）,total soil phosphorus（STP）,and their stoichiometry were analyzed based on the soil samples collected from typical forest stands.These properties are fundamental parameters of soil fertility and site productivity.In this study,the soil samples were collected at five soil layers（0-20,20-40,40-60,60-80,and 80-100 cm）from 34 sample plots of pure（Larix principis-rupprechtii）plantations（of tree species）with a size 30 m x 30 m in the Liupan Mountains.Similarly,the soil samples were collected at six soil layers（0-10,10-20,20-40,40-60,60-80,80-100 cm）from 46 sample plots of natural forests（NF）and plantation forests（PF）of Picea crassifolia and Sabina przewalskii with a plot size 20 m x 20 m in the Qilian Mountains.This study aimed to quantify the variation in soil physical and chemical properties among soil depths and their differences between forest types because this was not well investigated and assessed in the study regions before.The main results are described below.1)In Liupan Mountains,the study results presented that the soil physiochemical properties varied in a wide range among soil layers.It showed a significant variation in soil BD,porosity,pH values,CEC,BS,and EC values.The higher soil BD(1.41 g cm-3)and pH-value（7.05）were in the deepest soil layer（80-100 cm）,while the lowest values(0.92 g cm-3,6.43)were in the topsoil layer（0-20 cm）,respectively.The highest values of total soil porosity（60.33%）and CEC(29.36 cmol kg-1)were in the topsoil layer（0-20 cm）,while the lowest values(43.87%,16.04 cmol kg-1)were in the deepest soil layer（80-100 cm）,respectively.The maximum soil EC(0.0817 mS cm-1)was in the topsoil layer,while the lowest(0.0466 mS cm-1)was at the 60-80 cm soil layer,which was not significantly different from the 40-60 and 80-100 cm soil layers.The maximum BS（96.83%）was in the deepest soil layer（80-100）,while the minimum BS（92.51%）was in the topsoil layer（0-20 cm）,which was similar to those at the soil layers of 20-40,40-60 and 60-80 cm.Furthermore,the highest contents of SOC(48.31 g kg-1),STP(0.855 g kg-1),and AP(10.0 mg kg-1)were in the topsoil layer（0-20 cm）,while the minimum contents(12.81 g kg-1,0.611 g kg-1,and 1.95 mg kg-1)were in the deepest soil layer（80-100 cm）respectively,which were similar to those in the soil layer of 60-80 cm.The nutrients contents showed a decreasing trend with rising soil depth;whereas the STK showed an increasing trend with rising soil depth,i.e.,the lowest STK(20.58 g kg-1)was in the topsoil layer（0-20 cm）,and the highest one(21.79 g kg-1)was in the deepest soil layer.The highest STN and AK(4.14 g kg-1,216.5 mg kg-1)were in the topsoil layer（0-20 cm）,and the lowest STN and AK(1.02 g kg-1 and 68.75 mg kg-1)were in the deepest soil layer（80-100 cm）,while similar AK contents were observed in the soil layers of 40-60 and 60-80 cm.2)In Liupan Mountains,the element stocks and stoichiometry of forest soils in pure plantations varied significantly with soil depth.The magnitude and variation range of SOC stocks（SOCs）and STN stocks（STNs）in different soil layers followed the order of 0-20 cm>20-40 cm>40-60 cm≥ 60-80 cm ≥80-100 cm.The STP stocks（STPs）varied non-significantly across the 0-100 cm soil profile.The highest percent contribution of SOCs and STNs was from the topsoil layer of 0-20 cm,while the highest percent contribution of STPs was from the deepest soil layer of 80-100 cm,accounting for 29.88%,31.06%,and 20.64%,respectively.Moreover,the soil C:P and N:P ratios decreased with rising soil depth and stabilized at the soil layer of 60-80 and 80-100 cm.The highest soil C:N-ratio（13.54）was in the deepest soil layer（80-100 cm）,while the lowest C:N-ratio（11.58）was in the soil layer of 40-60 cm,which was statistically similar to those in the soil layers of 0-20,20-40,and 60-80 cm.The highest C:P and N:P ratios（58.37 and 4.84 ratios）were in the topsoil layer（0-20 cm）,while their minimum ratios（20.62 and 1.64）were in the deepest soil layer.Overall,the soil C:P and N:P ratios decreased quickly from the topsoil layer to the soil depth of 20-40 cm and thereafter gently within the soil depth from 40-100 cm,except for C:N ratios.3)The investigation in the Qilian Mountains showed that the physicochemical soil properties in NF differed from those in PF.The soil BD increased from the lowest ones(0.79 and 0.78 g cm-3 for PF and NF)in the topsoil layer（0-10 cm）to the highest ones(1.37 and 1.10 g cm-3)in the deeper soil layer（80-100 cm）.Similarly,the soil pH value increased from the lowest（8.11 and 7.66 for PF and NF）in the topsoil layer to the highest（8.41 and 8.23 for PF and NF）in the deepest soil layer.The average soil BD of 0-100 cm soil layer for PF(1.06 g cm-3)was higher significantly（p-0.05）than that for NF(0.95 g cm-3).Similarly,the average soil pH value of 0-100 cm soil layer for PF（8.28）was significantly（p≤0.05）higher than that for NF（8.05）.The PF showed higher SBD and pH values in all soil layers compared to NF.The highest average contents of SOC,STN,and STP(60.12,4.42,and 0.73 g kg-1)of both forest types were in the topsoil layer,while the lowest values(25.26,1.68,and 0.60 g kg-1)were in the deepest soil layer（80-100 cm）,with average contents of the total soi layer of 0-100 cm of 41.54,2.85,and 0.66 g kg-1,respectively.The SOC and STN contents decreased suddenly with rising soil depth and then stabilized at the soil layer of 60-100 cm,while the STP contents showed a slight variation along with soil depth except for the 60-100 cm soil layer.The means of SOC,STN,and STP contents of 0-100 cm soil layer for NF(47.4,3.33,and 0.68 g kg-1)were significantly（p<0.05）higher than those for PF(35.0,2.38,and 0.64 g kg-1),respectively.4)In Qilian Mountains,the SOC and STN stocks per 10 cm soil thickness decreased,but the STP stocks increased,with rising soil depth significantly（p-0.05）for both PF and NF.However,no significant difference existed between 0-10 and 10-20 cm for SOC and STN stocks and between 40-60 and 60-80 cm for the STP stocks.The highest averages of SOC and STN stocks per 10 cm soil thickness for both forest types(45.71 and 3.38 Mg ha-1)were in the topsoil layer（0-10 cm）,while the lowest values(28.79,1.94 Mg ha-1)were in the deepest soil layer（80-100 cm）.In contrast,the highest STPs(0.74 Mg ha-1)were in the deepest soil layer,and the lowest(0.58 Mg ha-1)was in the topsoil layer,with non-significant variations among the middle soil layers.Similar to the variation of stocks,the SOC and STN contents decreased significantly with rising soil depth up to the depth of 60 cm and thereafter stabilized in deeper soil layers（60-80,80-100 cm）.The average SOCs and STNs of the total soil profile of 0-100 cm were 67.6 and 4.77 Mg ha-1 for NF,and they were significantly（p≤0.05）higher than those for PF(53.6 and 3.60 Mg ha-1),while no significant difference existed for the average STPs of the total soil profile(1.12 and 1.11 Mg ha-1)between PF and NF.The average soil C:P and N:P ratios of the two forest types decreased significantly（p≤0.05）with rising soil depth,from the highest values（85.28 and 6.15）in the topsoil layer（0-10 cm）to the lowest ones（41.18 and 2.74）in the deepest soil layer（80-100 cm）;while the average soil C:N ratios was similar among all soil layers within the range of 0-100 cm.The averages of C:P and N:P ratios of the total soil profile of 0-100 cm for NF（71.04 and 4.88）were higher than those for PF（55.15 and 3.65）.But the average soil C:N-ratio of 0-100 cm for PF（16.35）was higher than that for NF（14.5）.The overall results in both study regions suggested that the variations of soil physicochemical properties largely depend on soil depth and forest types.There is a strong positive correlation between SOC and nutrient contents except for a negative correlation with STK.The stocks of SOC and STN were significantly correlated with the C:P and N:P ratios in all soil layers.The results in Qilian Mountains proposed that the NF has higher contents,stocks,and stoichiometry of SOC,N,and P.Our results also show that physicochemical soil properties in terms of soil fertility in plantation forests are unlikely to be restored in natural forests,suggesting that PF can be avoided to replace NF to maintain ecosystem sustainability.However,further studies are needed to draw more profound findings on the contents,stocks,stoichiometry,and dynamics of nutrient elements of forest soils in the study regions. |
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