Submitted 23 December 2019Accepted 21 July 2020Published 12 October 2020

Corresponding authorWeikai Bao baowkcibaccn

Academic editorStephen Livesley

Additional Information andDeclarations can be found onpage 12

DOI 107717peerj9702

Copyright2020 Qi et al

Distributed underCreative Commons CC-BY 40

OPEN ACCESS

Soil carbon nitrogen and phosphorusecological stoichiometry shifts with treespecies in subalpine plantationsKaibin Qi12 Xueyong Pang1 Bing Yang1 and Weikai Bao1

1CAS Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization amp EcologicalRestoration Biodiversity Conservation Key Laboratory of Sichuan Province Chengdu Institute of BiologyChinese Academy of Sciences Chengdu China

2University of Chinese Academy of Sciences Beijing China

ABSTRACTUnderstanding ecological stoichiometric characteristics of soil nutrient elementssuch as carbon (C) nitrogen (N) and phosphorus (P) is crucial to guide ecologicalrestoration of plantations in ecologically vulnerable areas such as alpine and subalpineregions However there has been only a few related studies and thus whether andhow different tree species would affect soil CNP ecological stoichiometry remainsunclear We compared soil CNP ecological stoichiometry of Pinus tabulaeformisLarix kaempferi and Cercidiphyllum japonicum to primary shrubland in a subalpineregion We observed strong tree-specific and depth-dependent effects on soil CNPstoichiometry in subalpine plantations In general the CN CP and NP of topsoil (0ndash10 cm) are higher than subsoil (gt10 cm) layer at 0ndash30 cmdepth profiles The differencesin CN NP and CP at the topsoil across target tree species were significantly linkedto standing litter stock tree biomasstotal aboveground biomass and Margalefrsquos indexof plant community respectively whereas the observed variations of CN NP andCP ratio among soil profiles are closely related to differences in soil bulk density soilmoisture the quantity and quality of aboveground litter inputs as well as undergroundfine root across plantations examined Our results highlight that soil nutrients inplantation depend on litter quantity and quality of selected tree species as well as soilphysical attributes Therefore matching site with trees is crucial to enhance ecologicalfunctioning in degraded regions resulting from human activity

Subjects Ecology Plant Science Climate Change Biology Natural Resource ManagementForestryKeywords Ecological stoichiometry Plantation in subalpine region Soil depth Litter Fine root

INTRODUCTIONEcological stoichiometry addresses the equilibrium or interactions of the main elementsas well as the correlations between elements and ecosystem functioning (Cambardellaamp Elliott 1992 Elser 2000 Gusewell 2004) Nitrogen (N) and Phosphorus (P) are themost critical nutrients limiting plant growth and their balance can regulate biologicalprocesses in terrestrial ecosystems (Elser et al 2007 Gusewell 2004 Reich amp Oleksyn 2004Vitousek amp Howarth 1991) such as the process of carbon (C) storage (Hessen et al 2004Yu et al 2015) Soil CN a sensitive indicator of the C and N reserves and also a soil

How to cite this article Qi K Pang X Yang B Bao W 2020 Soil carbon nitrogen and phosphorus ecological stoichiometry shifts withtree species in subalpine plantations PeerJ 8e9702 httpdoiorg107717peerj9702

quality in ecosystems (Tessier amp Raynal 2003) is bound up with the C allocation and Nmineralization rates of soil organic material in forest ecosystems Soil CP reflects the abilityof soil P mineralization a low soil CP favors microorganisms to decompose organic matterand desorb nutrients thus provides higher soil available P content (Tian et al 2010) SoilNP can measure the N saturation status and it is used to determine the thresholds forsoil nutrient limitation (Tessier amp Raynal 2003) The biogeochemical cycles of C N andP in terrestrial ecosystems are strongly interconnected through the biochemical reactionsduring primary production respiration and decomposition (Daufresne amp Loreau 2001Zechmeister-Boltenstern et al 2015) A deficiency or lower content of N and P leads toa higher CN and CP ratios and their excess triggers C deficiency (Gusewell Jewell ampEdwards 2005 Tessier amp Raynal 2003)

Since the CNP stoichiometry is regarded as an indicator of soil fertility and theexcesslimitation conditions of soil nutrients (Bing et al 2016) studying its response todisturbance resulting from human activities is essential for the enhancing of ecosystemfunctioning Although some studies report that soil CNP ecological stoichiometricratios are well-constrained (Cleveland amp Liptzin 2007) others found that soil CNPstoichiometric ratios can be affected by climate factors including temperature andprecipitation (Callesen et al 2007 Vesterdal et al 2008 Zhang Li amp Wang 2019)topography factors such as elevation and latitude (Moser et al 2011 Whitaker et al2014 Xu Thornton amp Post 2013 Zhang Li amp Wang 2019) soil texture and vegetationtypes (Cools et al 2014 Feng Bao amp Pang 2017 Tian et al 2010 Vesterdal et al 2008Xu Thornton amp Post 2013) In addition ecological stoichiometry can also be influencedby ecological restoration approaches and afforestation can be a major factor affectingsoil CNP stoichiometric ratio (eg Davis et al 2007 Deng Liu amp Shangguan 2014)Different tree species planted for restoration of afforestation can influence soil propertiesthrough multiple pathways However in alpine and subalpine regions whether thereare differences in soil CNP stoichiometric ratio between afforestation and reforestationremains largely unknown Moreover tree species differ from one another in the qualityand quantity of litter and root exudate (Aponte Garcia amp Maranon 2012 Paterson et al2007) and this in turn can influence soil C and N mineralization mediated by microbialcommunities (Alberti et al 2015 Prescott amp Grayston 2013) Finally the vertical patternsof C N and P stoichiometry vary with ecosystem type (Feng Bao amp Pang 2017) Althougha few studies have examined soil CNP stoichiometric ratios of forest ecosystems in alpineand subalpine region (Bing et al 2016 Feng amp Bao 2018 Feng Bao amp Pang 2017 Muelleret al 2017 Yang et al 2014 Zhang Li amp Wang 2019) studying how CNP stoichiometricratios respond to ecological restoration in depth should include more tree species andmore sites

The main objective of this study was to evaluate the effect of commonly used treespecies in plantations on soil CNP ecological stoichiometry Since soil stoichiometricratios vary with the species characteristics of plantations (Lawrence Fahey amp Zedler 2013Sardans Rivas-Ubach amp Penuelas 2012 Vinton amp Burke 1995) we hypothesized that (1)the soil CNP stoichiometry of plantations would be influenced by the tree species (2)the variations in soil C N P stoichiometric ratio across tree species decrease with increase

Qi et al (2020) PeerJ DOI 107717peerj9702 217

in soil depth because the influence of litter input on soil nutrient decreases with increasein soil layer (3) the fine root biomass (FRB) standing litter stock (LS) tree biomasstotalaboveground biomass (TBTAB) and Margalefrsquos index are major influencing factors of soilCNP ecological stoichiometry whereas the effect size depends on soil depth

MATERIAL AND METHODSStudy areaThe study was conducted at the Mao Country Mountain Ecosystem Research Station(3137primeN 10354primeE) the Chinese Academy of Sciences which located in Sichuan provinceChina The mean monthly temperature of the study area ranges from minus09 C in Januaryto 186 C in July with an annual mean temperature of 93 C The growing season is fromMay to September The mean annual precipitation is approximately 900 mm about 70falls during the growing season The annual accumulated temperature greater than orequal to 10 C is 26351 C The soil at the study site belongs to a Calcic Luvisol accordingto the IUSS Working Group WRB (2007) The soil texture was silt loam with 155 and153 of sand 625 and 633 of silt 219 and 215 of clay in the 0ndash10 cm and10ndash20 cm soil depths respectively (Jiang Pang amp Bao 2011)

Forest types and management activitiesIn August 2007 plantations of Pinus tabulaeformis (PT) Larix kaempferi (LAR)Cercidiphyllum japonicum (CJ) from three different sites were selected and native secondaryshrublands dominated by Corylus heterophylla and Quercus liaotungensis nearby werechosen as control resulting in 4 woodlands (12 plots) We chose PT LAR and CJ becausethey were commonly used when restoring or replacing native thicket in western Sichuanprovince as was also the case with the study area (Pang amp Bao 2011) These plantationswere established with terracing in the spring of 1987 on cutovers of primary thicketwhich were clear-fallen in the autumn of 1986 They have not been fertilized since theestablishment Prior to establishment the main soil properties in these plantations weresimilar to those of the native secondary coppice forest (Pang amp Bao 2011) In August2018 we sampled these plots again to examine the changes during the past 11 years Theunderstory species were dominated by native broad-leaved species including Quercusaliena Corylus heterophylla Rosa spp Spiraea spp Phlanis umbrosa Voila spp Anaphalissinica Potentilla spp without any species being absolutely dominant (Pang amp Bao 2011)The other basic information about the chosen forests was shown in the Table 1

Vegetation measurements soil sampling and analysisIn August 2007 and 2018 three 10 times 10 m standard plot was randomly set in an area ofabout 05 ha for each woodland The height (H) and the diameter at breast height (DBH)of trees in each plot were measured and stand density were calculated

In each plot the aboveground biomass of each layer of understory vegetation (includingshrubs and herbs) were recorded using destructive sampling in five 2 m times 2 m quadratesAll aboveground biomass within each sampling category was clipped and dried at 65 Cuntil the weight was constant The litter on the soil surface was collected from the same

Qi et al (2020) PeerJ DOI 107717peerj9702 317

Table 1 The basic information of plantation stands

Speciesidentity

Location Elevation (m) Aspect Slope () Canopydensity

Height (m) DBH (cm) SD (treesmiddothaminus1)

LAR 3141prime23primeprimeE 10353prime42primeprimeN 2070 NE 14 09 1169 1629 12003141prime23primeprimeE 10353prime43primeprimeN 2070 NE 15 08 1018 1131 22003141prime22primeprimeE 10353prime42primeprimeN 2081 N 21 098 1162 1366 1000

PT 3141prime27primeprimeE 10353prime41primeprimeN 2066 N 9 085 962 867 51003141prime26primeprimeE 10353prime42primeprimeN 2065 N 6 089 1168 1089 40003141prime26primeprimeE 10353prime43primeprimeN 2073 N 20 096 1065 829 2700

CJ 3141prime24primeprimeE 10353prime24primeprimeN 2056 NW 10 09 1184 1215 29003141prime26primeprimeE 10353prime39primeprimeN 2068 NW 19 093 1147 1197 34003141prime27primeprimeE 10353prime39primeprimeN 2020 NW 18 089 1317 116 3300

S 3141prime36primeprimeE 10353prime42primeprimeN 1933 N 17 094 595 387 151003141prime35primeprimeE 10353prime41primeprimeN 1948 NW 22 09 39 281 153003141prime35primeprimeE 10353prime42primeprimeN 1953 NW 18 086 591 347 12500

NotesLAR Larix kaempferi PT Pinus tabulaeformis CJ Cercidiphyllum japonicum S Shrubland Height Tree height DBH Diameter at breast height SD Stand density

quadrates mixed dried at 65 C and weighted Soil samples were collected to a depth of 30cm at three intervals of 0ndash10 10ndash20 and 20ndash30 cm from 10 random sampling sites alonga lsquolsquoWrsquorsquo shape with a soil auger (50 mm diameter) These sampling sites were at least 15 mapart from each other and 2 m away from the boundary The samples from each quadratwere pooled to give one composite sample per plot and depth The soil samples were takento the laboratory and soil moisture content were determined with 20 g soil each sample inoven drying method at 105 C for 24 h The soil bulk density (BD) was determined usingstainless steel cylinders (100 cm3) in triplicate for each plot before soil sampling (Qu etal 2016) The BD of soil was calculated by dividing the dry mass after oven drying at 105C for 24 h of each composite soil sample by the sample volume The soil samples wereair-dried after removing the gravel soil animals and plant debris and breaking the largefractions The air-dried soil sample was ground and then passed through 20-mesh (09mm) and 100-mesh (015 mm) nylon sieves respectively(Li et al 2018) The processedsamples were preserved for the determination of soil organic carbon (SOC) total nitrogen(TN) and total phosphorus (TP) SOC and TN were determined by combustion in a MacroElemental Analyser (varioMACRO Elementar Co Germany) The TPwasmeasured usingthe sulphuric acid-soluble perchlorate acid- molybdenum antimony colorimetric method(Bowman 1988) C N and P contents in leaves litter and soil samples were mass-basedThe atomic ratios were determined according to the formula

C N=Ccontent

12Ncontent

14

(1)

N P=Ncontent

14Pcontent31

(2)

C P=Ccontent

12Pcontent31

(3)

Qi et al (2020) PeerJ DOI 107717peerj9702 417

Statistical analysisThree-factor analysis of variance (ANOVA) followed by Tukey HSD post-hoc analysiswas used to determine differences in results for atomic ratios of CN CP and NP acrosstreatments with target tree species soil depths and sampling time (2007 and 2018) asfactors Additionally two-factor analysis of variance (ANOVA) followed by Tukey HSDpost-hoc analysis was used to determine differences in results for atomic ratios of CN CPand NP across treatments in the same soil depths with target tree species and samplingtime as factors Besides one-factor analysis of variance (ANOVA) followed by Tukey HSDpost-hoc analysis and was used to examine the differences in results for atomic ratiosof CN CP and NP across the same sampling times and soil depth between differenttarget tree species Before analysis the normality and the homogeneity of the residuals fordata were examined by ShapirondashWilk test and by KolmogorovndashSmirnov test in the lsquostatsrsquopackage respectively If assumption of ANOVA of a given variable was met we do ANOVAconsequently Otherwise the non-parametric Kruskal-Wallis test was performed and theWilcoxon test was performed in multiple comparisons For all analyses the significant levelwere set at α= 005 Besides the differences between the two sampling events (2018 versus2007) were compared with studentrsquos t -test or Wilcoxon test Pearson correlation analysiswas used to examine the correlations among TB understory plant biomass (UPB) TABTBTAB understory plant biomasstotal aboveground biomass (UPBTAB) FRB diversityindices of plant community (Richness index Margalersquof index Shannon-Wiener indexSimpson index and Pielou index) LS C N and P content CNP stoichiometric ratios inlitter and CNP stoichiometric ratios in topsoil Additionally the main influencing factorswere selected by multiple linear regression using lsquolsquostep-AICrsquorsquo function (R package MASS)(Venables amp Ripley 2002) in R version 352 Furthermore the corresponding contributionof selected factors were obtained by lsquolsquorelimporsquorsquo function (R package relimpo) (Groemping2006) in R version 352 Finally the determinant factors of soil CNP stoichiometry at the0ndash10 cm layers were examined with multiple regression with FRB LS TBTAB Margalefrsquosindex C and P contents in litter as independent variables

RESULTSSoil C N and P stoichiometrySoil C N and NP varied among soil depths and between sampling times (Table 2 Figs 1Andash1F) Soil CP was responsive to tree species soil depth sampling time and interactive effectof tree species by soil depth (Table 2 Figs 1Gndash1I)

Dynamics of Soil C N and P stoichiometryIn 2007 only the CN ratio of soil at the depth of 10ndash20 cm varied significantly withtree species (Fig 1B) Specifically the highest CN ratio was observed in soil of the PTplantation followed by CJ plantation and shrubland and the lowest CN ratio was observedin the soil of the LAR plantation (Fig 1B) In 2018 both the CN ratios of soil at the depthsof 0ndash10 cm and 10ndash20 cm varied significantly with tree species (Figs 1A amp 1B) At the

Qi et al (2020) PeerJ DOI 107717peerj9702 517

Table 2 Summary of the linear mixedmodel showing the effects of soil layer trees species and sampling time on the CN NP and CP of soil

Variables Depth (D) Tree species (TS) Time (T) Dtimes TS TStimes T Dtimes TStimes T

df F P df F P df F P df F P df F P df F P

CN 2 2685 lt0001 3 204 012 1 4188 lt0001 6 291 002 3 061 061 6 207 007

NP 2 5315 lt0001 3 2702 lt0001 1 000 099 6 718 lt0001 3 101 040 6 135 025

CP 2 10403 lt0001 3 2427 lt0001 1 2347 lt0001 6 718 lt0001 3 190 014 6 119 033

Qietal(2020)PeerJD

OI107717peerj9702

617

Figure 1 Soil CNP stoichiometric ratio across soil depth tree species and sampling times (AndashC) CN(DndashF) NP (GndashI) CP The capital and lowercase letters indicate significant differences across different treespecies (P lt 005) in 2018 and 2007 respectively NS and denote the differences between samplingtime (2018 versus 2007) based on t -test or Wilcoxon test are at P gt 005 001 le P le 005 and P lt 001respectively

Full-size DOI 107717peerj9702fig-1

depth of 0ndash10 cm the highest CN ratio in soil were observed in soil of the LAR plantationsfollowed by PT and CJ plantations and the lowest CN ratio occurred in the soil of theshrubland (Fig 1A) At the depth of 10ndash20 cm the highest CN ratio in soil was observedin soil of the PT plantation followed by shrubland and LAR plantations and the lowestCN ratio was observed in the soil of the CJ plantations (Fig 1B)

In 2007 the NP ratio of soil varied significantly with tree species for the depth of 0ndash10cm and 20-30 cm (Figs 1D amp 1F) At the depth of 0ndash10 cm the highest NP ratio of soil wasobserved in shrubland followed by LAR and PT plantations and the lowest NP ratio of soiloccurred in the CJ plantation (Fig 1D) At the depth of 20ndash30 cm the highest NP ratio insoil was observed in soil of the CJ plantation followed by shrubland and LAR plantationsand the lowest CN ratio was observed in the soil of the PT plantations (Fig 1F) In 2018only the NP ratio of soil at the depth of 0ndash10 cm varied significantly with tree species(Fig 1D) The trend was same as that at the depth of 0ndash10 cm in 2007 (Fig 1D)

In 2007 the CP of soil at the depth of 0ndash10 cm varied with tree species (Fig 1G)Specifically the highest CP of soil was observed in e shrubland followed by LAR and PTplantations and the lowest CP of soil was observed in the CJ plantation (Fig 1G) In 2018the CP ratios of soil at the depth of 0ndash10 cm and 10ndash20 cm have shown similar trend as

Qi et al (2020) PeerJ DOI 107717peerj9702 717

2007 however the soil at the depth of 20ndash30 cm has shown significant differences amongtree species (Figs 1Gndash1I) Specifically at the depth of 20ndash30 cm the highest CN ratio ofsoil were observed in soil of the shrubland followed by CJ and PT plantations and thelowest CN ratio of soil was observed in the LAR plantations (Fig 1I)

Correlations among soil CNP stoichiometric ratios soil propertiesand plant community attributes in 2018Across soil profiles the CN NP and CP significantly decreased with the increase in soilbulk density whereas significantly increased with the increase in soil moisture and FRB(Fig 2) At the topsoil the CN was significantly positively correlated to LS (P lt 0001)whereas negatively correlated to FRB and C content of litter (P lt 005) (Table 3) TheNP was significantly positively correlated to UPB (P lt 0001) UPBTAB (P lt 0001)tree amp shrub richness (P lt 0001) Margalefrsquos index (P lt 0001) Shannon-Wiener index(P lt 0001) Pielou evenness index (P lt 0001) but negatively correlated to TB (P lt 0001)TAB (P lt 0001) TBTAB (P lt 0001) P content of litter (P lt 005) and Simpsondominance index (P lt 0001) (Table 3) The CP was significantly positively correlatedto tree amp shrub richness (P lt 0001) Margalefrsquos index (P lt 0001) Shannon-Wienerindex (P lt 0001) Pielou evenness index (P lt 0001) but negatively correlated to TB(P lt 0001) TAB (P lt 0001) TBTAB (P lt 005) C content of litter (P lt 005) litterCN (P lt 005) and Simpson index (P lt 0001) (Table 3)

Soil CN at the topsoil was affected by LS and FRB (r2= 076 F = 1396 P = 0002) andthe LS contributed to 6731 of the variation (Fig 3) Soil NP at the topsoil was affected byP content of litter Margalefrsquos index and TBTAB (r2= 098 F = 12050 P lt 0001) andTBTAB and Margalefrsquos index contributed to 48 and 38 of the variation respectively(Fig 3) Soil CP at the topsoil was affected by C content of litter and Margalefrsquos index(r2 = 081 F = 1927 P lt 0001) and the Margalefrsquos index contributed to 75 of thevariation (Fig 3)

DISCUSSIONSoil CNP ecological stoichiometry for plantations in subalpineregionIn our experiment soil CN at the depth of 0ndash30 cm ranges from 145 to 155 in the examinedecosystems which is slightly higher than the global average CN of 143 (Yue et al 2017)and lower than the other subalpine average CN (Bing et al 2016) Soil CP at the depth of0ndash30 cm ranges from 184 to 299 in the examined ecosystems which is higher than Chinarsquosaverage of 136 (Tian et al 2010) lower than the global average and the other subalpine(Bing et al 2016 Yue et al 2017) Soil NP at the depth of 0ndash30 cm ranges from 129 to194 which is higher than that of global and Chinarsquos average (93 and131 respectively)(Tian et al 2010 Yue et al 2017) and lower than the other subalpine average CN (Bing etal 2016) What account for the discrepancy across studies are largely unknown

Soil CNP ecological stoichiometry between tree speciesIn accordance with our first hypothesis soil CNP stoichiometry varied significantly withtree species (Table 2 Fig 1) particularly for the topsoil where CN ratios in LAR and PT

Qi et al (2020) PeerJ DOI 107717peerj9702 817

Figure 2 Relationships between CNP stoichiometric ratio and (A D G) bulk density (B E H) fineroot biomass and (C F I) soil moisture at 0ndash30 cm of soil profiles in 2018

Full-size DOI 107717peerj9702fig-2

plantations are greater than CJ plantation and shrubland implying higher Nmineralizationrate in shrubland Three likely reasons account for this finding Firstly litter inputs andstocks differ across the examined plantations (Table 4) In the topsoil LS was the mostinfluencing factor of soil CN (Fig 3) Secondly the microclimate the quantity and qualityof root exudates and rhizodeposits as well as soil microbial community change with plantspecies (Aoki Fujii amp Kitayama 2012 Ohta amp Hiura 2016 Zhang et al 2011) which mayjointly influence soil nutrient status and its stoichiometric ratio Firstly as shown byprevious studies broadleaf litter is more decomposable than needle litter in boreal forests(Laganiere Pare amp Bradley 2010) Besides allocation of C to roots is directly proportionalto photosynthesis (Brzostek et al 2015) and understory shrubs generally have a lowerphotosynthetic capacity than overstory trees (Sakai et al 2005) Nevertheless the CNratio of conifer stands is greater than broadleaf stands may be related to the canopy densityand high light interception of conifers reduce the light efficiency on the forest floor (Liefferset al 1999) Furthermore the decomposition rates of broadleaf trees are commonly higherin comparison with conifer trees (Taylor Parkinson amp Parsons 1989 Zhang et al 2019)

Vertical pattern of soil CNP ecological stoichiometryConsistent with our second hypothesis the CN CP and NP of soil decreased withincrease in the soil depth (Fig 1) This finding is in agreement with previous studies

Qi et al (2020) PeerJ DOI 107717peerj9702 917

Table 3 Correlations among soil CNP stoichiometric ratios and plant community attributes in 2018

Variables CN NP CP

Tree biomass minus0002NS minus0887 minus0833

Understory plant biomass minus0348NS 0894 0516NS

Litter stock 0862 minus0212NS 0278NS

Total aboveground biomass minus0006NS minus0875 minus0825

Tree biomassTotal aboveground biomass 0371NS minus0954 minus0680

Understory plant biomassTotal aboveground biomass minus0502NS 0910 0563NS

Fine root biomass -632 0292NS minus0044NS

Litter carbon minus0595 minus0279NS minus0591

Litter nitrogen 0149NS 0271NS 0356NS

Litter phosphorus 003NS minus0587 minus0535NS

Litter CN minus0339NS minus0434NS minus0610

Litter NP 0063NS 0546NS 0561NS

Litter CP minus0455NS 0423NS 0143NS

Richness index minus0129NS 0967 0833

Margalef index 0054NS 0902 0878

Shannon-Wiever index 0193NS 0851 0906

Simpson index minus035NS minus0730 minus0876

Pielou index 042NS 0670 0860

NotesNS Not significantP lt 005P lt 0001

Figure 3 The gradient boost decision tree measuring the relative importance of factor influencing top-soil (0ndash10 cm) CNP stoichiometric ratio in 2018 (A) CN (B) NP (C) CP FRB Fine root biomass LSLitter stock TBTAB Tree biomassTotal aboveground biomass MI Margalef index L-P Litter phospho-rus L-C Litter carbon P lt 005 P lt 0001

Full-size DOI 107717peerj9702fig-3

addressing vertical pattern of soil CNP stoichiometry in forest soils (Feng Bao amp Pang2017 Li et al 2013 Qiao et al 2020 Tian et al 2010 Tischer Potthast amp Hamer 2014)These results are maybe related to the fact that soil nutrients decreased with soil depthBesides this could be due to the topsoil layer environmental factors being easily affected andthe return of nutrients from litters (Feng Bao amp Pang 2017) In addition soil nutrientsare usually first concentrated on the topsoil and then transferred to the subsoil layerwith water or soil animals Furthermore soil CN ratio decreased with the soil depth

Qi et al (2020) PeerJ DOI 107717peerj9702 1017

Table 4 The different plantations component of biomass in this study area

Speciesidentity

TB (tmiddothaminus1) UPB (tmiddothaminus1) LS (tmiddothaminus1) TAB (tmiddothaminus1) TBTAB UPBTAB

LAR 9163plusmn 1335bc 029plusmn 0095b 835plusmn 041a 10027plusmn 1368bc 091plusmn 0010c 0005plusmn 00004bPT 15716plusmn 3757ab 026plusmn 0037b 772plusmn 044a 16513plusmn 3803ab 095plusmn 0012b 0003plusmn 00009bCJ 21065plusmn 820a 025plusmn 0147b 394plusmn 017b 21484plusmn 829a 098plusmn 0001a 0002plusmn 00006bS 2926plusmn 253c 1069plusmn 1308a 387plusmn 089b 4381plusmn 341c 067plusmn 0012d 0075plusmn 00061a

NotesTB Tree biomass UPB Understory plant biomass LS Litter stock TAB Total aboveground biomass TBTAB Tree biomassTotal aboveground biomass UPBTAB Un-derstory plants biomassTotal aboveground biomassLowercase letters indicate significant differences between tree species (P lt 005)

among different plantations and this could be related to the different nutrient turnoverrates in decomposition process The easily decomposable material elapsed and N is fixedin decayed products and microbial biomass remaining durable materials had moreslowly decomposition rates and lower CN ratio (Yang et al 2010) Compared with thetopsoil layer the organic matter in subsoil layer is more humified and older result incontinuous decrease of the soil CN ratio with soil depth (Callesen et al 2007 Yang et al2010) Additionally difference in soil nutrient associated with changes in soil microbialdynamics litter decomposition microbial food web and soil nutrient accumulation andcirculation (Griffiths Spilles amp Bonkowski 2012 Zhao et al 2015) Besides the decrease insoil temperature with the increase of soil depth (Jackson et al 2000) may account for thedecreased soil CNP stoichiometric ratios in lower depth

Potential influencing factors of soil CNP stoichiometryIn partial agreement with our third hypothesis linkage among soil CNP ecologicalstoichiometry soil properties and plant community attributes varied with soil depth This isalso in agreement with our earlier result (Feng Bao amp Pang 2017) the relative contributionof factors varied among soil depths and the examined object Firstly bulk density increasedwith the increase in soil depth result in decreased soil porosity which further reduce soilmoisture and root penetration Besides the topsoil of trees is in direct contact with the plantcommunity Hence LS TBTAB and other plant community structure index will effect onit Additionally the correlations between environmental factors and stoichiometric ratiosdepended on the elements considered In summary the effects of tree species and soil depthon soil CNP stoichiometry associated with bulk density soil moisture the quantity andquality of aboveground litter inputs and underground fine root

CONCLUSIONSWe observed that tree-specific and depth-dependent have strong effects on soil CNPstoichiometry in subalpine plantations In general the CN CP and NP of topsoil arehigher in comparing with that of subsoil layer at 0ndash30 cm depth profiles The difference inCN NP and CP at the topsoil across target tree species significantly linked to standinglitter stock tree biomassaboveground biomass and Margalefrsquos index of plant communityrespectively whereas the observed variations of CN NP and CP ratio among soil profiles

Qi et al (2020) PeerJ DOI 107717peerj9702 1117

are closely related to differences in soil bulk density soil moisture the quantity and qualityof aboveground litter inputs and underground fine root across plantations examinedOur results highlight that soil nutrients status in plantation depend on litter quantity andquality of selected tree species as well as soil physical attributes Therefore matching sitewith trees is crucial to enhance ecological functioning in degraded regions resulting fromhuman activity

ACKNOWLEDGEMENTSWe thank De feng Feng and Xin Liu for helpful suggestions on the study and earlier versionof the manuscript We also highly appreciated Long Huang Shuang ping Peng and Zhongping Tang for their assistance and support provided in sample collection and analysis

ADDITIONAL INFORMATION AND DECLARATIONS

FundingThis study was jointly funded by the Major Science Technology Project of SichuanProvince (Grant numbers 2018SZDZXD035) The funders had no role in study designdata collection and analysis decision to publish or preparation of the manuscript

Grant DisclosuresThe following grant information was disclosed by the authorsMajor Science Technology Project of Sichuan Province 2018SZDZXD035

Competing InterestsThe authors declare there are no competing interests

Author Contributionsbull Kai binQi conceived and designed the experiments performed the experiments analyzedthe data prepared figures andor tables authored or reviewed drafts of the paper andapproved the final draftbull Xueyong Pang performed the experiments analyzed the data authored or revieweddrafts of the paper and approved the final draftbull Bing Yang analyzed the data authored or reviewed drafts of the paper and approvedthe final draftbull Weikai Bao conceived and designed the experiments performed the experimentsanalyzed the data authored or reviewed drafts of the paper and approved the final draft

Data AvailabilityThe following information was supplied regarding data availability

Raw data are available in the Supplemental File

Supplemental InformationSupplemental information for this article can be found online at httpdxdoiorg107717peerj9702supplemental-information

Qi et al (2020) PeerJ DOI 107717peerj9702 1217

REFERENCESAlberti G Vicca S Inglima I Belelli-Marchesini L Genesio L Miglietta F Marjanovic

H Martinez C Matteucci G DrsquoAndrea E Peressotti A Petrella F RodeghieroMCotrufoMF 2015 Soil CN stoichiometry controls carbon sink partitioning betweenabove-ground tree biomass and soil organic matter in high fertility forests Iforest-Biogeosciences and Forestry 8195ndash206 DOI 103832ifor1196-008

Aoki M Fujii K Kitayama K 2012 Environmental control of root exudation of low-molecular weight organic acids in tropical rainforests Ecosystems 151194ndash1203DOI 101007s10021-012-9575-6

Aponte C Garcia LV Maranon T 2012 Tree species effect on litter decompositionand nutrient release in mediterranean oak forests changes over time Ecosystems151204ndash1218 DOI 101007s10021-012-9577-4

Bing HWu Y Zhou J Sun H Luo J Wang J Yu D 2016 Stoichiometric variation ofcarbon nitrogen and phosphorus in soils and its implication for nutrient limitationin alpine ecosystem of Eastern Tibetan Plateau Journal of Soils and Sediments16405ndash416 DOI 101007s11368-015-1200-9

Bowman RA 1988 A rapid method to determine total phosphorus in soils Soil ScienceSociety of America Journal 521301ndash1304DOI 102136sssaj198803615995005200050016x

Brzostek ER Dragoni D Brown ZA Phillips RP 2015Mycorrhizal type determines themagnitude and direction of root-induced changes in decomposition in a temperateforest New Phytologist 2061274ndash1282 DOI 101111nph13303

Callesen I Raulund-Rasmussen KWestman CJ Tau-Strand L 2007 Nitrogen poolsand C N ratios in well-drained Nordic forest soils related to climate and soil textureBoreal Environment Research 12681ndash692

Cambardella CA Elliott ET 1992 Particulate soil organic-matter changes across agrassland cultivation sequence Soil Science Society of America Journal 56777ndash783DOI 102136sssaj199203615995005600030017x

Cleveland CC Liptzin D 2007 C N P stoichiometry in soil is there a Redfield ratio forthe microbial biomass Biogeochemistry 85235ndash252DOI 101007s10533-007-9132-0

Cools N Vesterdal L De Vos B Vanguelova E Hansen K 2014 Tree species is themajor factor explaining CN ratios in European forest soils Forest Ecology andManagement 3113ndash16 DOI 101016jforeco201306047

Daufresne T LoreauM 2001 Ecological stoichiometry primary producer-decomposerinteractions and ecosystem persistence Ecology 823069ndash3082DOI 1023072679835

Davis M Nordmeyer A Henley DWatt M 2007 Ecosystem carbon accretion 10 yearsafter afforestation of depleted subhumid grassland planted with three densities ofPinus nigra Global Change Biology 131414ndash1422DOI 101111j1365-2486200701372x

Qi et al (2020) PeerJ DOI 107717peerj9702 1317

Deng L Liu G-B Shangguan Z-P 2014 Land-use conversion and changing soil carbonstocks in Chinarsquos rsquoGrain-for-Greenrsquo Program a synthesis Global Change Biology203544ndash3556 DOI 101111gcb12508

Elser JJ 2000 Ecological stoichiometry from sea lake to land Trends in Ecology ampEvolution 15393ndash394 DOI 101016s0169-5347(00)01956-x

Elser JJ BrackenMES Cleland EE Gruner DS HarpoleWS Hillebrand H NgaiJT Seabloom EW Shurin JB Smith JE 2007 Global analysis of nitrogen andphosphorus limitation of primary producers in freshwater marine and terrestrialecosystems Ecology Letters 101135ndash1142DOI 101111j1461-0248200701113x

Feng D BaoW 2018 Shrub encroachment alters topsoil CNP stoichiometric ratiosin a high-altitude forest cutover Iforest-Biogeosciences and Forestry 11594ndash598DOI 103832ifor2803-011

Feng D BaoW Pang X 2017 Consistent profile pattern and spatial variation of soilCNP stoichiometric ratios in the subalpine forests Journal of Soils and Sediments172054ndash2065 DOI 101007s11368-017-1665-9

Griffiths BS Spilles A Bonkowski M 2012 CNP stoichiometry and nutrient limitationof the soil microbial biomass in a grazed grassland site under experimental Plimitation or excess Ecological Processes 11ndash6 DOI 1011862192-1709-1-1

Groemping U 2006 Relative importance for linear regression in R the packagerelaimpo Journal of Statistical Software 17925ndash933 DOI 1018637jssv017i01

Gusewell S 2004 N P ratios in terrestrial plants variation and functional significanceNew Phytologist 164243ndash266 DOI 101111j1469-8137200401192x

Gusewell S Jewell PL Edwards PJ 2005 Effects of heterogeneous habitat use by cattleon nutrient availability and litter decomposition in soils of an Alpine pasture Plantand Soil 268135ndash149 DOI 101007s11104-004-0304-6

Hessen DO Agren GI Anderson TR Elser JJ De Ruiter PC 2004 Carbon se-questration in ecosystems the role of stoichiometry Ecology 851179ndash1192DOI 10189002-0251

Jackson RB Schenk HJ Jobbagy EG Canadell J Colello GD Dickinson RE FieldCB Friedlingstein P HeimannM Hibbard K Kicklighter DW Kleidon ANeilson RP PartonWJ Sala OE Sykes MT 2000 Belowground consequences ofvegetation change and their treatment in models Ecological Applications 10470ndash483DOI 1018901051-0761(2000)010[0470bcovca]20co2

Jiang Y Pang X BaoW 2011 Soil microbial biomass and the influencing factors underPinus tabulaeformis and Picea asperata plantations in the upper Minjiang River ActaEcologica Sinica 31801ndash811

Laganiere J Pare D Bradley RL 2010How does a tree species influence litter decompo-sition Separating the relative contribution of litter quality litter mixing and forestfloor conditions Canadian Journal of Forest Research-Revue Canadienne De RechercheForestiere 40465ndash475 DOI 101139x09-208

Lawrence BA Fahey TJ Zedler JB 2013 Root dynamics of Carex stricta-dominatedtussock meadows Plant and Soil 364325ndash339 DOI 101007s11104-012-1360-y

Qi et al (2020) PeerJ DOI 107717peerj9702 1417

Li H Li J He Y Li S Liang Z Peng C Polle A Luo Z-B 2013 Changes in carbonnutrients and stoichiometric relations under different soil depths plant tissuesand ages in black locust plantations Acta Physiologiae Plantarum 352951ndash2964DOI 101007s11738-013-1326-6

Li R Kan S ZhuM Chen J Ai X Chen Z Zhang J Ai Y 2018 Effect of differentvegetation restoration types on fundamental parameters structural characteristicsand the soil quality index of artificial soil Soil amp Tillage Research 18411ndash23DOI 101016jstill201806010

Lieffers VJ Messier C Stadt KJ Gendron F Comeau PG 1999 Predicting and man-aging light in the understory of boreal forests Canadian Journal of Forest Research29796ndash811 DOI 101139cjfr-29-6-796

Moser G Leuschner C Hertel D Graefe S Soethe N Iost S 2011 Elevation ef-fects on the carbon budget of tropical mountain forests (S Ecuador) the roleof the belowground compartment Global Change Biology 172211ndash2226DOI 101111j1365-2486201002367x

Mueller M Oelmann Y Schickhoff U Boehner J Scholten T 2017Himalayan treelinesoil and foliar CNP stoichiometry indicate nutrient shortage with elevationGeoderma 29121ndash32 DOI 101016jgeoderma201612015

Ohta T Hiura T 2016 Root exudation of low-molecular-mass-organic acids by six treespecies alters the dynamics of calcium and magnesium in soil Canadian Journal ofSoil Science 96199ndash206 DOI 101139cjss-2015-0063

Pang X BaoW 2011 Effect of Substituting Plantation Species for Native Shrubs on theWater-holding Characteristics of the Forest Floor on the Eastern Tibetan PlateauJournal of Resources and Ecology 2217ndash224

Paterson E Gebbing T Abel C Sim A Telfer G 2007 Rhizodeposition shapes rhizo-sphere microbial community structure in organic soil New Phytologist 173600ndash610DOI 101111j1469-8137200601931x

Prescott CE Grayston SJ 2013 Tree species influence on microbial communities in lit-ter and soil Current knowledge and research needs Forest Ecology and Management30919ndash27 DOI 101016jforeco201302034

Qiao YWang J Liu HM Huang K Yang QS Lu RL Yan LMWang XH Xia JY 2020Depth-dependent soil C-N-P stoichiometry in a mature subtropical broadleaf forestGeoderma 370114357 DOI 101016jgeoderma2020114357

Qu L Huang Y Ma K Zhang Y Biere A 2016 Effects of plant cover on propertiesof rhizosphere and inter-plant soil in a semiarid valley SW China Soil Biology ampBiochemistry 941ndash9 DOI 101016jsoilbio201511004

Reich PB Oleksyn J 2004 Global patterns of plant leaf N and P in relation to tempera-ture and latitude Proceedings of the National Academy of Sciences of the United Statesof America 10111001ndash11006 DOI 101073pnas0403588101

Sakai T Saigusa N Yamamoto S Akiyama T 2005Microsite variation in light avail-ability and photosynthesis in a cool-temperate deciduous broadleaf forest in centralJapan Ecological Research 20537ndash545 DOI 101007s11284-005-0067-4

Qi et al (2020) PeerJ DOI 107717peerj9702 1517

Sardans J Rivas-Ubach A Penuelas J 2012 The CNP stoichiometry of organismsand ecosystems in a changing world a review and perspectives Perspectives in PlantEcology Evolution and Systematics 1433ndash47 DOI 101016jppees201108002

Taylor BR Parkinson D ParsonsWFJ 1989 Nitrogen and lignin content as predictorsof litter decay-ratesmdasha microcosm test Ecology 7097ndash104 DOI 1023071938416

Tessier JT Raynal DJ 2003 Vernal nitrogen and phosphorus retention by for-est understory vegetation and soil microbes Plant and Soil 256443ndash453DOI 101023a1026163313038

Tian H Chen G Zhang C Melillo JM Hall CAS 2010 Pattern and variation of CNPratios in Chinarsquos soils a synthesis of observational data Biogeochemistry 98139ndash151DOI 101007s10533-009-9382-0

Tischer A Potthast K Hamer U 2014 Land-use and soil depth affect resource andmicrobial stoichiometry in a tropical mountain rainforest region of southernEcuador Oecologia 175375ndash393 DOI 101007s00442-014-2894-x

VenablesWN Ripley BD 2002Modern applied statistics with S 4th edition New YorkSpringer DOI 101007978-0-387-21706-2

Vesterdal L Schmidt IK Callesen I Nilsson LO Gundersen P 2008 Carbon andnitrogen in forest floor and mineral soil under six common European tree speciesForest Ecology and Management 25535ndash48 DOI 101016jforeco200708015

VintonMA Burke IC 1995 Interactions between individual plant-species and soilnutrient status in shortgrass steppe Ecology 761116ndash1133 DOI 1023071940920

Vitousek PM Howarth RW 1991 Nitrogen limitation on land and in the seamdashhow canit occur Biogeochemistry 1387ndash115

Whitaker J Ostle N Nottingham AT Ccahuana A Salinas N Bardgett RD Meir PMcNamara NP 2014Microbial community composition explains soil respirationresponses to changing carbon inputs along an Andes-to-Amazon elevation gradientJournal of Ecology 1021058ndash1071 DOI 1011111365-274512247

Xu X Thornton PE PostWM 2013 A global analysis of soil microbial biomass carbonnitrogen and phosphorus in terrestrial ecosystems Global Ecology and Biogeography22737ndash749 DOI 101111geb12029

Yang YH Fang JY Guo DL Ji CJ MaWH 2010 Vertical patterns of soil carbonnitrogen and carbon nitrogen stoichiometry in Tibetan grasslands BiogeosciencesDiscuss 71ndash24 DOI 105194bgd-7-1-2010

Yang YWang G Shen H Yang Y Cui H Liu Q 2014 Dynamics of carbon and nitrogenaccumulation and CN stoichiometry in a deciduous broadleaf forest of deglaciatedterrain in the eastern Tibetan Plateau Forest Ecology and Management 31210ndash18DOI 101016jforeco201310028

YuQWilcox K La Pierre K nd Knapp AK Han X SmithMD 2015 Stoichiometrichomeostasis predicts plant species dominance temporal stability and responses toglobal change Ecology 962328ndash2335 DOI 10189014-18971

Yue K Fornara DA YangW Peng Y Li Z Wu F Peng C 2017 Effects of three globalchange drivers on terrestrial CNP stoichiometry a global synthesis Global ChangeBiology 232450ndash2463 DOI 101111gcb13569

Qi et al (2020) PeerJ DOI 107717peerj9702 1617

Zechmeister-Boltenstern S Keiblinger KMMooshammerM Penuelas J Richter ASardans J WanekW 2015 The application of ecological stoichiometry to plant-microbial-soil organic matter transformations Ecological Monographs 85133ndash155DOI 10189014-07771

Zhang C Liu G Xue S Song Z 2011 Rhizosphere soil microbial activity underdifferent vegetation types on the Loess Plateau China Geoderma 161115ndash125DOI 101016jgeoderma201012003

ZhangMH Cheng XL Geng QH Shi Z Luo YQ Xu X 2019 Leaf litter traits pre-dominantly control litter decomposition in streams worldwide Global Ecology andBiogeography 281469ndash1486 DOI 101111geb12966

Zhang Y Li CWangM 2019 Linkages of C N P stoichiometry between soil and leafand their response to climatic factors along altitudinal gradients Journal of Soils andSediments 191820ndash1829 DOI 101007s11368-018-2173-2

Zhao F Kang D Han X Yang G Yang G Feng Y Ren G 2015 Soil stoichiometry andcarbon storage in long-term afforestation soil affected by understory vegetationdiversity Ecological Engineering 74415ndash422 DOI 101016jecoleng201411010

Qi et al (2020) PeerJ DOI 107717peerj9702 1717