Stoichiometry of base cations and silicon during weathering of a deep soil profile derived from granite

Yue ZHAO ,Jinling YANG,* ,Nan JIA ,Yufang SUN,4 ,Zhe XU and Ganlin ZHANG,3

1State Key Laboratory of Soil and Sustainable Agriculture,Institute of Soil Science,Chinese Academy of Sciences,Nanjing 210008(China)

2University of Chinese Academy of Sciences,Beijing 100081(China)

3State Key Laboratory of Lake Science and Environment,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences,Nanjing 210008(China)

4Soil&Environment Analysis Center,Institute of Soil Science,Chinese Academy of Sciences,Nanjing 210008(China)

ABSTRACT Evaluation of the stoichiometry of base cations(BCs,including K+,Na+,Ca2+,and Mg2+)and silicon(Si)(BCs:Si)during soil mineral weathering is essential to accurately quantify soil acidification rates.The aim of this study was to explore the differences and influencing factors of BCs:Si values of different soil genetic horizons in a deep soil profile derived from granite with different extents of mineral weathering.Soil type was typic acidi-udic Argosol.Soil samples were collected from Guangzhou,China,which is located in a subtropical region.To ensure that the BCs and Si originated from the mineral weathering process,soil exchangeable BCs were washed with an elution treatment.The BCs:Si values during weathering were obtained through a simulated acid rain leaching experiment using the batch method.Results showed that soil physical,chemical,and mineralogical properties varied from the surface horizon to saprolite in the soil profile.The BCs:Si values of soil genetic horizons during weathering were 0.3—3.7.The BCs:Si value was 1.7 in the surface horizon(A),1.1—3.7 in the argillic horizon(Bt),and 0.3—0.4 in the cambic(Bw)and transition(BC)horizons,as well as in horizon C(saprolite).The general pattern of BCs:Si values in the different horizons was as follows:Bt ＞A ＞Bw,BC,and C.Although BCs:Si values were influenced by weathering intensity,they did not correlate with the chemical index of alteration(CIA).The release amounts of Si and BCs are the joined impact of soil mineral composition and physical and chemical properties.A comprehensive analysis showed that the BCs:Si values of the soil derived from granite in this study were a combined result of the following factors:soil clay,feldspar,kaolinite,organic matter,pH,and CIA.The main controlling factors of BCs:Si in soils of different parent material types require extensive research.The wide variance of BCs:Si values in the deep soil profile indicated that H+ consumed by soil mineral weathering was very dissimilar in the soils with different weathering intensities derived from the same parent material.Therefore,the estimation of the soil acidification rate based on H+ biogeochemistry should consider the specific BCs:Si value.

Key Words: mineral weathering,soil acidification rate,soil evolution,soil genesis,soil geochemistry,weathered sequence

INTRODUCTION

Soil acidification rate is an important index reflecting the evolutionary trend of soil,and is related to the sustainable development of soil,as well as to the stability of the ecosystem(Molet al.,2003;Liuet al.,2010).Both cation exchange and soil mineral weathering buffer soil acidification.However,soil mineral weathering alone does not result in acidification.Therefore,if the effects of these two processes are not distinguished,the soil acidification rate will be overestimated(Van Breemenet al.,1984;Forsiuset al.,2005).Theoretically,soil mineral weathering involves Si release and is not influenced by cation exchange.Yanget al.(2013)proposed a method to accurately estimate the soil acidification rate by quantifying the amount of H+consumed and using the stoichiometry of base cations(BCs,including K+,Na+,Ca2+,and Mg2+)and Si(BCs:Si)in the process of mineral weathering.However,Yanget al.(2013) only provided a theoretical basis for accurate estimation of the soil acidification rate.Obtaining BCs:Si values during soil mineral weathering requires further research.

In the past,research on the stoichiometry involved in soil mineral weathering was mainly focused on fresh rocks(Kalinowski and Schweda,1996;White,2003).These studies revealed the mechanisms,processes,and characteristics of mineral weathering but could not be applied to mineral weathering in natural soils.Under natural conditions,soil contains various minerals with varying extents of weathering.In laboratory experiments,owing to sufficient reaction time and optimal conditions(such as high temperature and pressure,strong acid,and strong alkali)(West,2012),the weathering of a single mineral is mainly a congruent dissolution(Beig and Lüttge,2006;Gudbrandssonet al.,2014).In contrast,the weathering of natural soils is generally an incongruent dissolution(Kossoffet al.,2011;Brantley and Olsen,2014).Moreover,even for the soils with the same parent material,if the weathering intensities are different,the soil mineral weathering rates will also differ.For example,three types of soil are derived from granite:brown forest soil(Cambosols),yellow-brown soil(Alfisols),and laterite (Ferralosols),all of which contain potassium feldspar,plagioclase,kaolinite,and quartz.However,due to the different weathering intensities and mineral proportions,their weathering rates are 0.24,0.38,and 0.40 keq ha-1year-1,respectively(White and Brantley,1995;Chadwick and Chorover,2001;Houleet al.,2012).

Existing research on mineral weathering in soil mainly focuses on the element stoichiometry of K,Na,Al,O,and Si(Brantley and Olsen,2014;Lianet al.,2017),i.e.,K:Na:Si:Al=5.5:38.6:18.4:1 during congruent dissolution of microcline(Xiaoet al.,2003)and K:Al:O:Si=0.3:0.4:2.7:1 for congruent dissolution of alkaline feldspar(Fuet al.,2009).In addition,mineral weathering-related stoichiometric relationships include Mg:Si (Zhang and Bloom,1999) and Al:Si(West,2012).However,BCs:Si in the weathering process of soils with different weathering extents has not yet been studied.

Granite is the most abundant silicate mineral rock on earth,covering 25%of the entire land area(Cuiet al.,2015).Granite-derived soils are widely distributed in subtropical and tropical regions of China and have a high agricultural productivity potential.However,granite-derived soils have a weak acid-buffering capacity and a strong acid sensitivity,and are prone to irreversible soil acidification under acid rain(Senolet al.,2014),limiting the sustainable development of agriculture and security of ecosystems in granite regions.Granite is mainly composed of quartz,feldspar,and mica.Deep weathering crust and soil horizons can be formed after a long weathering process in high-temperature and rainy climates(Ehlen,1999;Scarcigliaet al.,2005),which is helpful for studying the BCs:Si of soils with different weathering extents.However,the BCs:Si values of granitederived soils in the current environment are few,and the factors influencing BCs:Si during weathering are not clear.

We performed a simulated acid rain leaching experiment on samples of a deep soil profile derived from granite with different genetic horizons as a weathering sequence using the batch method.The aims were to obtain BCs:Si values of natural soils with different extents of mineral weathering under normal temperature and pressure conditions and to explore the factors influencing BCs:Si in the process of soil mineral weathering.

MATERIALS AND METHODS

Site description and soil sampling

Soil samples were collected from Guangzhou,Guangdong Province,China(23°13′2′′N,113°30′19′′E).The area has a south subtropical maritime monsoon climate,with a mean annual temperature of 21.4—21.9°C and a mean annual precipitation of 1 623.6—1 899.8 mm(GSB-SONBSG,2020).The landscape is hilly.The land-use type is dominated by orchards with large areas of longan trees.There are pines,maples,and low shrubs around orchards.

Soil samples for this study were collected from different soil genetic horizons.The parent material is weathered granite,and the soil type is typic acidi-udic Argosol,which evolve slowly from the bottom to the top of the soil profile,representing a continuous soil-forming process from granite saprolite to surface soil.The soil sequence was divided into the surface horizon A(0—18 cm),the argillic horizons Bt1(18—32 cm),Bt2(32—43 cm),Bt3(43—59 cm),and Bt4(59—96cm),the cambic horizon Bw(96—150 cm),the transition horizon BC(150—216cm),and the parent material horizons C1(216—250 cm),C2(250—348 cm),and C3(348—700 cm).Horizon C(216—700 cm)is mainly granite saprolite.The soil samples used for physical and chemical analyses,as well as mineral identification,were sieved through 10,60,100,and 200 mesh screens,whereas the samples used for the simulated acid rain leaching experiment were not milled to maintain the actual condition of mixed minerals in the soil.

Elution and leaching experiments

To estimate the BCs:Si values of soil mineral weathering during simulated acid rain leaching,it is necessary to remove exchangeable BCs adsorbed on soil colloids using elution.Elution was performed according to the method of Yanget al.(2017).Three replicates from each horizon of the deep soil profile were set for elution by weighing 200 g of soil per replicate and placing it in a 500-mL centrifuge bottle.The elution procedure was as follows:i)200 mL of 1 mol L-1EDTA-ammonium acetate(CH3COONH4)solution(pH 7.0)was added to the centrifuge bottle with 200 g soil at a soil-toliquid ratio of 1:1 and stirred evenly to ensure that the soil was completely dispersed in the acid solution;ii)the centrifuge bottles with the samples were placed in a rotary shaker and shaken for 2 h;iii)the samples were centrifuged for 10 min at 7 000 r min-1;and iv)the supernatant was collected to determine the K+,Na+,Ca2+,and Mg2+contents using inductively coupled plasma optical emission spectroscopy(ICP-OES).This process was repeated until the contents of BCs in the supernatant did not change significantly for three consecutive cycles,which indicated complete elution of exchangeable BCs in the soil.The remaining eluent in the soil after elution was removed by repeated washing with anhydrous ethanol.

In the simulated acid rain leaching experiment,the 0.1 mol L-1acid solution used for leaching consisted of NH4Cl and diluted HCl.According to the average acidity of acid rain in the research area in recent years(Changet al.,2017),the pH of the acid solution was adjusted to 4.5.The leaching procedure was as follows:i)200 mL of acid solution was added to the centrifuge bottles with the base-eluted soil at a soil-to-liquid ratio of 1:1 and stirred evenly;ii)the centrifuge bottles with soil were placed in a rotary shaker and shaken for 24 h to ensure an adequate reaction between the soil and solution;iii)the samples were centrifuged for 10 min at 7 000 r min-1;iv) the supernatant was collected in volumetric flasks,and the contents of K+,Na+,Ca2+,Mg2+,and Si were determined by ICP-OES;and v) the soil samples in centrifuge tubes were left to stand for six days to ensure that the soil had an adequate reaction time and underwent alternating dry and wet processes.The above steps were a standard leaching cycle of the batch method and needed to be repeated.The simulated leaching experiment lasted 91 d,with a total of 13 leaching cycles.

Chemical analysis

Soil particle size was measured using the laser diffraction method (Zhang and Gong,2012).Soil pH was measured using a pH meter (soil:water ratio of 1:2.5,weight:volume).Soil organic matter(SOM)was determined using the potassium dichromate and sulfuric acid digestion method(Lu,2000).Exchangeable BCs were extracted using CH3COONH4(pH 7.0),and their contents were measured using ICP-OES.The cation exchange capacity(CEC)was determined using CH3COONH4(pH 7.0).Base saturation is the percentage of the total exchangeable base of soil in CEC.Total elements were determined by the melting method using ICP-OES;the dried soil sample and lithium metaborate(LiBO2·5H2O)were evenly mixed,placed in a crucible covered with graphite powder,and maintained at 900°C for 20 min.After the frit was completely dissolved in 4%(volume/volume)HNO3,part of the solution was transferred to a test tube to determine elemental content by ICP-OES(Zhang and Gong,2012).

To determine mineral composition of soil by X-ray diffraction(XRD),the soil sample was first saturated with 0.5 mol L-1MgCl2solution three times and adsorbed with 10%(volume/volume)glycerol solution twice.The samples were smeared on glass slides and identified using an X-ray diffractometer(Zhang and Gong,2012).The XRD patterns of the soil samples were compared with standard mineral diffraction patterns to identify the mineral types and calculate their relative percentages.The contents of K+,Na+,Ca2+,Mg2+,and Si in the eluent of the base-eluting and simulated acid rain leaching experiments were determined by ICP-OES.

Data analysis

Base cations are equivalent to the sum of K+,Na+,Ca2+,and Mg2+,and the chemical index of alteration (CIA) is the percentage of Al2O3molecules in the total number of Al2O3,CaO,Na2O,and K2O molecules.Pearson’s correlation analysis was used to test the correlations between the different soil properties and BCs:Si.All statistical analyses were performed using SPSS version 22.0 (IBM Corporation,USA).All charts were produced using Origin version 2018(OriginLab Corporation,USA)and Excel version 2019(Microsoft Corporation,USA).

RESULTS

Soil properties

Soil clay ranged from 5.7%to 18.5%,and it gradually increased with depth,that is,downward from horizon A.It reached a maximum at 59—96cm(horizon Bt4),and subsequently gradually decreased further downward.In horizon C,clay content was very low (Fig.1a).Soil sand ranged from 40.4%to 70.0%,and unlike the clay content,it initially decreased and then increased,reaching the maximum value in horizon C(Fig.1a).Soil silt was 24.4%—43.4%with low variance.

Soil organic matter content was the highest in horizon A(26.1 g kg-1)and decreased with increasing depth(Fig.1b).The fluctuations in soil CEC were similar to that of SOM content;it gradually decreased from 6.84 cmol(+)kg-1in horizon A to 3.43 cmol(+)kg-1in horizon C3(Fig.1c).Soil pH varied from 4.4 to 5.0,initially decreasing and then increasing along the soil profile,with a minimum pH measured in horizon Bt(Fig.1d).The contents of exchangeable BCs(K+,Na+,Ca2+,and Mg2+)initially decreased and then increased with increasing depth,and were relatively stable in the saprolite(horizon C,Fig.1e).The sum of exchangeable BCs was the highest in the surface soil(Fig.1e).The CIA value ranged from 81.2 to 93.3,showing a decreasing trend from horizon A to C,with the lowest value in horizon C(Fig.1f).Thus,some differences in the soil properties were observed among the different genetic horizons of the deep soil profile derived from granite.

The XRD analysis of the weathered soil sequence showed that feldspar,quartz,and kaolinite were the main minerals in the different horizons.However,owing to the different weathering processes of soil genetic horizons,the mineral content of each horizon differed,generating different secondary minerals(Fig.2).Gibbsite content was very low in the soil horizons above a depth of 96cm.Only a small amount of hydromica was present in the soil at 216—700 cm(horizons C1—C3)(Fig.2).Although the proportion of quartz in each horizon was high,the quartz content decreased from the surface to the bottom,whereas the feldspar content increased gradually.Kaolinite content increased gradually from 0 to 96cm and then decreased from 96to 700 cm.To a certain extent,the mineral composition and clay content could reflect the differences in weathering extent between different soil genetic horizons.

Fig.2 X-ray diffraction patterns of soil samples at different genetic horizon depths in a deep soil profile derived from granite.Gibbsite(G)can be observed only in horizons A(0—18 cm),Bt1(18—32 cm),Bt3(43—59 cm),and Bt4(59—96cm);hydromica(H)can be observed only in horizons C1(216—250 cm),C2(250—348 cm),and C3(348—700 cm).Q=quartz;K=kaolinite;F=feldspar.

Elution of exchangeable BCs in soil

As BCs in the soil colloids were eluted eight times at a soil-to-liquid ratio of 1:1,the K+,Na+,Ca2+and Mg2+contents in eluent would not change,indicating that exchangeable BCs of all soil samples were washed off.During elution,the leached contents of BCs in the eluent were related to the exchangeable BCs in the soil(Table I).The differences in BCs between the eluent and soil were only 0—0.15 cmol(+)kg-1,which further indicated that the exchangeable BCs in the soil had been completely eluted.

Release characteristics of BCs and Si

The cumulative release amounts of BCs(K+,Na+,Ca2+,and Mg2+)at a depth of 0—96cm were higher than those at 96—700 cm(Fig.3),indicating that the release amounts of BCs in horizons A and Bt were higher than those in horizons Bw,BC,and C during the simulated acid rain leaching.The order of cumulative release of BCs was K+＞Ca2+＞Na+＞Mg2+.Unexpectedly,there were two peaks in the cumulative release amounts of BCs,one at a depth of 0—18 cm(horizon A)and the other at 59—96cm(horizon Bt4) (Fig.3).The cumulative release amount of Si first decreased and then increased at 0—59 cm,sharply decreased at 59—96cm,and increased at 96—700 cm(Fig.3).

Fig.3 Cumulative release amounts of base cations(K+,Na+,Ca2+,and Mg2+) and Si at different genetic horizon depths in a deep soil profile derived from granite during the simulated acid rain leaching.

BCs:Si of soil during weathering

According to the released amounts of BCs and Si during the simulated acid rain leaching,the BCs:Si values of minerals in the soil ranged from 0.3 to 3.7 and fluctuated from horizons A to C(Fig.4).The BCs:Si value at a depth of 0—18 cm(horizon A)was high,with a value of 1.7.At a depth of 18—96cm(horizon Bt),the BCs:Si value increased gradually from 1.1 in horizon Bt1 to 3.7 in horizon Bt4.Ata depth of 96—700 cm(horizons Bw,BC,and C1—C3),the BCs:Si values were 0.3—0.4,sharply decreasing compared with those of the upper soil(Fig.4).

TABLE IContents of exchangeable base cations(BCs)in soil and leached BCs in eluent,observed at different genetic horizon depths in a deep soil profile derived from granite

Fig.4 Stoichiometry of basic cations(BCs,including K+,Na+,Ca2+,and Mg2+)and Si(BCs:Si)at different genetic horizon depths in a deep soil profile derived from granite during the simulated acid rain leaching.

DISCUSSION

Effect of Si on BCs:Si of soil during weathering

Silicon is a suitable indicator element of the process of soil mineral weathering because its chemical weathering reaction is simpler than that of other elements(White,2003).The order of the cumulative release amount of Si in different horizons was as follows:A＞C＞Bw＞BC＞Bt(Fig.3),whereas BCs:Si values in different horizons declined in the order Bt＞A＞C＞BC＞Bw (Fig.4).This difference was mainly due to the different mineral compositions and proportions of soil in different horizons.The XRD analysis revealed that only small amounts of gibbsite and hydromica existed in some horizons,whereas feldspar was present in all horizons but varied in proportion(Fig.2).In the natural environment,soil mineral weathering is incongruent(Rzepaet al.,2019),and the reaction intensity and intermediate products of mineral weathering differ at different weathering stages(Kossoffet al.,2011;Gudbrandssonet al.,2014).

Horizon A had the highest cumulative release amount of Si(Fig.3),which could be due to plant growth(Zhuet al.,2014;Yavittet al.,2021)and continuous dust deposition(Niet al.,2007),as well as the effect of incongruent dissolution.Far-source dust often contains weatherable minerals such as feldspar(Qiaoet al.,2014).In addition,the surface SOM may contain biological silica,which can be easily weathered(Derryet al.,2005).Although horizon A exhibited a high release amount of Si,the BCs:Si value was not the smallest(Fig.4).The main reason for this is that surface soil also releases numerous BCs during weathering.The release of high concentrations of K+and Ca2+in horizon A(Fig.3)was related to the absorption of K+and Ca2+from the bottom to the surface during plant growth (Yavittet al.,2021).Therefore,the amount of Si released affected the change in BCs:Si during soil mineral weathering,but it had no significant correlation with BCs:Si.

Effect of weathering extent on BCs:Si

The soil CIA reflects the extent of mineral weathering(Shao and Yang,2012).It showed a decreasing trend from horizon A to C in the deep soil profile(Fig.1f),indicating gradual weakening of the weathering intensity from the surface to bottom.The extent of soil mineral weathering affected the compositions and proportions of minerals(Fig.2),which could directly impact the amounts of BCs and Si released by mineral weathering and thereby help to determine BCs:Si.However,the BCs:Si and CIA values did not exhibit a consistent pattern with respect to the variation in characteristics(Figs.1f and 4).No significant correlation between them was observed(Fig.5a),which could be mainly because soil formation involves not only mineral weathering,but also the migration and transformation of materials.The mineral compositions and proportions in different soil genetic horizons are complex,and the extent of mineral weathering also varies.

Fig.5 Correlations of the stoichiometry of base cations(BCs)(K+,Na+,Ca2+,and Mg2+)and Si(BCs:Si)with soil chemical index of alteration(CIA,a),feldspar(b),kaolinite(c),clay(d),organic matter(SOM)(e),and pH(f)in a deep soil profile derived from granite during the simulated acid rain leaching.The feldspar and kaolinite contents are weight percentages.*Significant at P ＜0.05.

Factors influencing BCs:Si

Several factors affect the release of BCs and Si during soil mineral weathering.Feldspar is one of the main weathering minerals in granite-derived soils(Ehlen,1999;Molet al.,2003).If feldspar is directly weathered into kaolinite,the BCs:Si value is 1:2 (Yanget al.,2013).However,in the natural environment,soil mineral weathering involves incongruent dissolution(Rzepaet al.,2019),undergoing a complex transformation process.When feldspar is weathered into kaolinite,intermediates such as hydromica,chlorite,and vermiculite may be produced(Eqs.1—4)(Berner and Holdren,1977;Chigira and Oyama,1999).In addition,K+,Na+,Ca2+,Mg2+,and Si could be released in different proportions.When kaolinite hydrolyzes,only gibbsite and Si are produced,with no release of K+,Na+,Ca2+,and Mg2+(Eq.5)(Camaet al.,2002).Consequently,variations in the BCs:Si values(0.3—3.7)of the soil profile existed because of the influences of mineral compositions,proportions,and transformation processes.Correlation analysis showed a relationship between BCs:Si values and the content of kaolinite,while no correlation was observed between BCs:Si values and the content of feldspar(Fig.5b,c),indicating that there were other controlling factors.

The clay content was the highest in the argillic horizon(Bt)but relatively lower in saprolite(horizons C1—C3)(Fig.1a),similar to the distribution characteristics of BCs:Si(Fig.4).Correlation analysis showed a positive relationship between clay content and BCs:Si(Fig.5d),indicating that the particle composition is an important factor influencing BCs:Si.This correlation was related to the increase in mineral fragmentation and release of BCs during soil mineral weathering(Fig.3).

Soil organic matter affects soil structure,and soluble organic acids contribute to soil mineral weathering(Drever and Vance,1994;Ludwiget al.,1995).Soil organic matter(Fig.1b) as well as the cumulative release amounts of Si and BCs (Fig.3) were highest in horizon A of the deep soil profile.Thus,SOM affected BCs:Si to a certain level.However,there was no significant correlation between SOM and BCs:Si(Fig.5e).Additionally,as pH is the result of soil evolution,which in turn affects soil weathering and element transformation,it may impact the release of elements in the weathering process (Zhanget al.,1994).There was a negative correlation between pH and BCs:Si(Fig.5f).

Therefore,feldspar,SOM,and CIA had no significant influence on BCs:Si alone.However,they affected BCs:Si to some extent from different aspects.Regression analysis of the main soil properties and BCs:Si was performed,and the following equation was established:

whereyrepresents the BCs:Si value andx1,x2,x3,x4,x5,andx6represent clay,kaolinite,feldspar,pH,SOM,and CIA,respectively.TheR2of the equation is 0.875.Although the weathered soil sequence in this study is not the single main controlling factor affecting BCs:Si,the mineral composition and physical and chemical properties of soil influenced the BCs:Si value with their combined effects.

Application of BCs:Si for soil acidifcation

The input of H+into the soil by acid rain leads to loss of BCs and soil acidification.Soil buffers acidification through cation exchange and soil mineral weathering.Mineral weathering can compensate for the loss of BCs and inhibit acidification,whereas cation exchange leads to the continuous loss of BCs and exchangeable H+increase,which is a real process inducing soil acidification.Theoretically,the soil acidification rate can be estimated from the H+consumption of the soil acidification process,which can be calculated from the BC release of cation exchange.However,the observed BCs are released during both cation exchange and soil mineral weathering,both of which are difficult to distinguish.Therefore,it is challenging to accurately estimate the soil acidification rate.The release of Si comes from soil mineral weathering and is unrelated to the cation exchange process.Base cations released by mineral weathering can be distinguished from the cation exchange process contingent with the BCs:Si ratio and Si release.The soil acidification rate is then estimated based on the H+consumption of cation exchange(actual soil acidification process).

This study showed that soils with various extents of weathering have different BCs:Si values.The BCs:Si values at different soil depths can be used to estimate the acidification of the corresponding horizon in the soil profile.To illustrate this,we will consider an example of soil acidification estimation based on the H+budget method.If the H+consumption of soil buffering effects by mineral weathering and cation exchange is 3 000 mol ha-1year-1,the output of BCs is 3 000 mol(+)ha-1year-1.If the output of Si is 1 000 mol ha-1year-1,the release of BCs from mineral weathering is 1 700 mol(+)ha-1year-1,calculated according to the BCs:Si value(1.7)of horizon A.The consumed H+by soil mineral weathering is 1 700 mol ha-1year-1.Therefore,the soil acidification rate caused by cation exchange is 1 300 mol ha-1year-1,which is less than half of the H+consumption and is substantially lower than the rate with no distinction between H+consumption pathways.

The BCs:Si value of horizon Bt was 2.6(weighted average value of horizons Bt1—Bt4)in this study.If the same H+(3 000 mol ha-1year-1) is consumed in horizon Bt,the soil acidification rate is only 400 mol ha-1year-1when applying the same method.The BCs:Si value of horizon C was 0.4(weighted average value of horizons C1—C3),and the actual soil acidification rate is 2 600 mol ha-1year-1under the same H+input condition.This is because horizon C is hardly weathered granite saprolite with a large rock structure and large particle size.

Therefore,for different soil horizons or soils with different extents of weathering,even if they have the same parent material and H+input,their acidification rates are different.Hence,it is crucial to determine the BCs:Si value of the corresponding horizon when studying the soil acidification rate.

CONCLUSIONS

In this study,owing to weathering and matter migration,the genetic horizons of the deep soil profile derived from granite exhibited different physical,chemical,and mineralogical properties,affecting the change in weathering BCs:Si.Although Si is an indicator element of mineral weathering,it does not dominate the BCs:Si value in the soil genetic horizons during the weathering process.Horizon A was highly weathered and released a large amount of Si,including many BCs.Therefore,the BCs:Si value of horizon A was not the highest in the weathered soil sequence.Saprolite exhibited a weak weathering intensity and a low BCs:Si value,as it was influenced by the extent of breakage and large particle size.The BCs:Si values of different soil genetic horizons with the same parent material were different.The weathering extent of different horizons correlated with the BCs:Si value,which was influenced by clay,feldspar,kaolinite,SOM,pH,and CIA;moreover,it was the result of the synthesized influences of physical and chemical properties and mineral composition.The wide variance of BCs:Si values in the deep soil profile indicates that H+consumed by soil mineral weathering is significantly different in the soils with different weathering intensities derived from the same parent material.Therefore,in future studies,we need to investigate the main controlling factors of BCs:Si in soils with different parent materials and weathering extents to improve the stoichiometric relationship model of BCs:Si for accurate prediction of soil acidification rate.

ACKNOWLEDGEMENT

This study was supported by the National Natural Science Foundation of China(Nos.41877010 and U1901601).