Effects of precipitation on soil organic carbon fractions in three subtropical forests in southern China

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1 Journl of Plnt Ecology Volume 9, Numer 1, Pges 1 19 Ferury 216 doi:1.193/jpe/rtv27 Advnce Access puliction 6 Mrch 215 ville online t Astrct Aims The im of this study ws to investigte the effects of precipittion chnges on soil orgnic cron (SOC) frctions in sutropicl forests where the precipittion pttern hs een ltered for decdes. Methods We conducted field mnipultions of precipittion, including mient precipittion s control (CK), doule precipittion (DP) nd no precipittion (NP), for 3 yers in three forests with different stnd ges (rodlef forest [BF], mixed forest [MF] nd pine forest [PF]) in sutropicl Chin. At the end of the experiment, soil smples were collected to ssy SOC content, redily oxidizle orgnic cron (ROC) nd non-redily oxidizle orgnic cron (NROC), s well s soil microil iomss cron (MBC), ph nd totl nitrogen content. Smples from the forest floors were lso collected to nlyze cron (C) functionl groups (i.e. lkyl C, romtic C, O-lkyl C nd cronyl C). Furthermore, fine root iomss ws mesured periodiclly throughout the experiment. Importnt Findings Among the forests, ROC content did not exhiit ny notle differences, while NROC content incresed significntly with the stnd ge. This finding implied tht the SOC ccumultion oserved in these forests resulted from the ccumultion of NROC in the soil, Effects of precipittion on soil orgnic cron frctions in three sutropicl forests in southern Chin Xiomei Chen 1,, Deqing Zhng 2,, Guohu Ling 3, Qingyn Qiu 4, Juxiu Liu 2, Guoyi Zhou 2, Shizhong Liu 2, Guowei Chu 2 nd Junhu Yn 2, * 1 School of Geogrphicl Sciences, Gungzhou University, Gungzhou 516, Chin 2 South Chin Botnicl Grden, Chinese Acdemy of Sciences, Gungzhou 5165, Chin 3 Stte Key Lortory of Conservtion nd Utiliztion of Sutropicl Agro-Bioresources, South Chin Agriculturl University, Gungzhou 5165, Chin 4 University of Chinese Acdemy of Sciences, Beijing 139, Chin *Correspondence ddress. South Chin Botnicl Grden, Chinese Acdemy of Sciences, 723 Xingke Rod, Tinhe District, Gungzhou 5165, Chin. Tel: ; Fx: ; E-mil: jhyn@sci.c.cn Xiomei Chen nd Deqing Zhng contriuted eqully to this work. mechnism for SOC ccumultion in the mture forests of southern Chin. Moreover, NP tretment led to significnt reductions in oth ROC nd NROC content nd therefore reduced the totl SOC content in ll of the studied forests. Such decreses my e due to the lower plnt-derived C inputs (C quntity) nd to the chnges in SOC components (C qulity) indicted y C functionl groups nlyses under NP tretment. DP tretment in ll the forests lso tended to decrese the SOC content, lthough the decreses were not sttisticlly significnt with the exception of SOC nd ROC content in PF. This finding indicted tht soils in MF nd in BF my e more resistnt to precipittion increses, possily due to less wter limittions under nturl conditions in the two forests. Our results therefore highlight the different responses of SOC nd its frctions to precipittion chnges mong the forests nd suggest tht further studies re needed to improve our understnding of SOC dynmics in such n importnt C sink region. Keywords: rinfll pttern, lile orgnic cron, soil orgnic cron, stle orgnic cron, forest floor Received: 8 April 214, Revised: 26 Jnury 215, Accepted: 9 Ferury 215 Downloded from y guest on 2 Septemer 218 INTRODUCTION Chnges in the intensity nd ptterns of precipittion re occurring round the world (IPCC 213; Knpp et l. 28). In Chin, floods nd droughts hve een oserved to e more frequent during the pst 4 yers (Pio et l. 21), including in southern Chin (Zhou et l. 211), where the forest soil hs een identified s significnt C sink (Pio et l. 29; The Author 215. Pulished y Oxford University Press on ehlf of the Institute of Botny, Chinese Acdemy of Sciences nd the Botnicl Society of Chin. All rights reserved. For permissions, plese emil: journls.permissions@oup.com

2 Chen et l. Effects of precipittion on soil orgnic C frctions 11 Zhou et l. 26). Chnges in precipittion ptterns could consequently modify the mgnitude of the C sink in southern Chin, or even worse, trnsform the C sink into C source, s precipittion hs een commonly considered s modifier of soil C cycling (Anderud et l. 21; Goeel et l. 211; Young nd Ritz 2) y chnging soil moisture nd underground hydrologicl processes (Heisler nd Weltzin 26). However, the potentil effects of chnges in precipittion ptterns on soil orgnic cron (SOC), especilly on its different frctions, hve not yet een well ssessed in forest ecosystems. Becuse precipittion influences soil moisture nd hydrologicl processes such s surfce runoff nd ground wter infiltrtion (Heisler nd Weltzin 26), which re importnt controlling fctors in SOC cycling, chnges in precipittion ptterns hve gret potentil to influence SOC content nd its dynmics (Anderud et l. 21). For exmple, Meier nd Leuschner (21) oserved tht SOC decresed y ~25% more in eech forests with nnul precipittion >9 mm yer 1 thn in those with precipittion <6 mm yer 1. It hs lso een reported tht on the one hnd, soil moisture could ffect SOC ccumultion y influencing the quntity of plnts C input to soils (Zhou et l. 28), s well s the decomposition rte of those C inputs (O Brien et l. 21). On the other hnd, wter vilility nd its sptil distriution in soil mtrix cn ffect the sptil ccessiility nd degrdility of SOC for decomposers, nd then chnge the process of SOC decomposition (Goeel et l. 211; Young nd Ritz 2). Moreover, wter infiltrtion my trnsport sustrtes from the litter lyer to minerl soil, consequently chnging the fte of orgnic C (Lee et l. 24). A study of Mediterrnen woodlnd demonstrted tht enhnced soil moisture during the summer ccelertes C cycling through stimulted nnul stem primry production, litter fll, soil respirtion nd net nnul plnt-derived C input to the soil (Cotrufo et l. 211), of which ll re relted to SOC cycling. Little, however, is known out how SOC in sutropicl forests responds to precipittion chnges compred to the wellstudied temperte forests nd grsslnds (Brndo et l. 28). Sutropicl forest ecosystems store lrge proportions of living terrestril C nd soil C cross the world (Brown nd Lugo 1982). Most of these forests re nd will continue to experience chnges in precipittion ptterns due to glol wrming nd lnd use shifts (Knpp et l. 28). In the Dinghushn Nturl Reserve in southern Chin, for exmple, nnul norin dys hve significntly incresed while nnul lightrin dys hve decresed significntly since 198 (Zhou et l. 211). The chnged precipittion ptterns hve led to decrese in soil moisture of ~1%, ut n increse of 143% in surfce runoff in the wet seson from April to Septemer (Zhou et l. 211). Thus, there is n urgent need to understnd how precipittion chnges influence soil C dynmics in these sutropicl forests. SOC is continuum of C-contining frctions with different decomposition rtes (Brdford et l. 28; Kögel- Knner 22). Different SOC frctions my therefore respond differently to environmentl chnges (von Lützow et l. 27). For exmple, soil redily-oxidized cron (ROC) hs reltively higher turnover rte nd is more responsive to mngement prctices, wheres soil non-redily-oxidized cron (NROC) is reltively less responsive (Blir et l. 1995). The potentilly different responses of SOC frctions mke it importnt to how these frctions respond to environmentl chnges. Menwhile, litter is n importnt source of SOC, nd the nlysis of litter C functionl groups llows us to understnd the chnges in the orgnic C composition of forest litter nd cn therefore provide explntions for the different responses of SOC frctions (ROC nd NROC) to mnipulted precipittion tretments. The C frctions of litter input could influence the SOC composition significntly (Quideu et l. 2). Therefore, the nlysis of C functionl groups cn shed light on why SOC frctions respond differently to precipittion chnges (Kögel-Knner 22). Using this ssy, Ono et l. (211) showed tht different SOC compositions cross forests could e ttriuted to different supply rtes of liphtic nd romtic crons from the litter to the topsoil in the forests they studied. For ll of the ove resons, we conducted precipittion mnipultion experiment in three sutropicl forests in southern Chin, testing SOC content nd its frctions under three precipittion mnipultions in the forests. Moreover, fine root iomss nd C functionl groups on the forest floor were mesured to explore the potentil resons for oserved ptterns of SOC nd its frctions under different precipittion tretments. In doing so, we imed to test the following two ssumptions: (i) SOC content nd its frctions (ROC nd NROC) would vry cross the studied forests; nd (ii) the SOC frctions would exhiit different responses to precipittion tretments depending on the forest type, e.g. n erstz drought resulting from the no precipittion (NP) tretment my decrese ROC nd NROC, while high soil moisture resulting from doule precipittion (DP) tretment my stimulte the ccumultion of SOC. MATERIALS AND METHODS Site descriptions The study ws crried out in the Dinghushn Biosphere Reserve (112 1 E nd 23 1 N), Gungdong Province, South Chin. The reserve occupies n re of 12 h nd experiences typicl sutropicl humid monsoon climte, with men nnul temperture nd reltive ir humidity of 21.5 C nd 8%, respectively. The men nnul rinfll is 1956 mm with distinct sesonlity, i.e. >8% of precipittion flls in the wet seson from April to Septemer nd the remining 2% in the dry seson from Octoer to Mrch. The reserve comprises three types of typicl forests: n old-growth monsoon evergreen rodlef forest (BF), mixed pine nd rodlef forest (MF) nd pine forest (PF) (Deng et l. 212). The BF, t ~2 3 m ove se level, is distriuted Downloded from y guest on 2 Septemer 218

3 12 Journl of Plnt Ecology in the core re of the reserve nd hs een undistured for >4 yers. Mjor species in the BF include Cryptocry chinensis, Mchilus chinensis (Chmp. ex Benth.) Hemsl., Syzygium super, C. chinensis (Hnce) Hemsl., nd Syzygium rehderinum Merr. et Perry in the cnopy nd su-cnopy lyers. The MF is distriuted etween the core re nd the periphery of the reserve t n elevtion of 2 3 m. It originted from cler-cuts nd susequent pine plnttion in the 193s, nd then the grdul invsion of some pioneer rodlef species. The dominnt species in the cnopy lyer of the MF re Pinus mssonin, Schim super Grdn. et Chmp, Cstnopsis chinensis Hnce, nd Criiodendorn sclernthum vr. kwngtunense (S. Y. Hu) Judd. The PF is distriuted in the periphery of the reserve t n elevtion of ~2 m. It ws plnted in the 195s. The dominnt species in the PF is P. mssonin Lm. The mount of litter ws 981.8, 72.5 nd g m 2 yer 1 for the BF, MF nd PF, respectively (Chen et l. 212). Soils t ll the three forest sites re shllow ultisol overlying sndstone nd shle edrocks. The soil ulk density ws.91, 1.31 nd g cm 3 in the BF, MF nd PF, respectively. Other soil chrcteristics of the three forests re summrized in Tle 1. Experimentl design Experimentl tretments strted in Decemer 26 t the three forementioned sutropicl forests, with three qudrnts replicted in ech of the studied forests. In ech forest, we selected qudrnts with similr slope spect, slope degree, slope position nd plnt community to conduct precipittion mnipultions to minimize the heterogeneity derived from different environmentl fctors. In ech qudrnt, three plots were estlished to receive mient precipittion (CK), DP nd NP, respectively. Ech plot hd dimension of 3 3 m 2, nd the uffering distnce etween plots ws >1 m. Precipittion ws intercepted in the NP plots using cler polyvinyl chloride (PVC) pltes s roof t height of m ove the floor; precipittion ws redistriuted to the corresponding DP plots homogenously with PVC tues. Around ech NP plot, thick PVC pltes were inserted underground to depth of 15 cm to prevent outside surfce runoff from Tle 1: effects of precipittion on generl soil properties in the BF, MF nd PF flowing into the plot. Litter flling on the PVC roofs of the NP plots ws collected nd returned to the plots three times per month. Soil moisture t depth of 5 cm ws rndomly mesured five times within ech plot on hlf-month sis throughout the experiment using n MPKit (ICT, Austrli). Field smpling Soil smples from to 1 cm were collected in August 21, using circle soil uger with n inner dimeter of 2.5 cm. In ech plot, five rndom soil cores were mixed into one composite smple, resulting in totl of three smples for ech tretment re in ech forest. Finlly, 27 smples were otined for soil chemicl nlysis. Fresh soil smples were pssed through 2-mm sieve to remove rocks nd plnt roots. Su-smples were ir-dried nd ground to perform the nlyses of SOC, C frctions nd generl properties mesurements. Forest floor smples were collected from litter lyers, fermenttion lyers nd humus lyers in the estlished plots in August 21. In ech plot, three suplots of 2 2 cm were selected rndomly for smpling. The litter lyer ws smpled first, followed y fermenttion lyer nd then the humus lyer, following their ccessiility (Knerv nd Smolnder 27). The fresh litter lyer consisted of fresh or slightly decomposed litter from living plnts; the fermenttion lyer consisted of prtly decomposed litter, the origins of which were mostly identifile; nd the humus lyer consisted of decomposed orgnic mtter whose origins could not e identified (Knerv nd Smolnder 27). Nine smples from the sme lyer with the sme tretment in ech forest were homogenously mixed into one composite smple. A totl of 27 smples were collected from the forest floor. The smples were dried t 6 C to constnt weight, ground nd sieved through 149-µm mesh for the C functionl groups nlysis. Root smples were collected from to 2 cm depth using circle stinless steel uger with n inner dimeter of 1 cm in Ferury nd August of 27 nd Ferury, April, August, nd Octoer of 28 (Deng et l. 212). One smple ws collected in ech plot; thus, totl of 27 soil cores were collected for fine root iomss mesurements t ech smpling Forest type Tretment Soil moisture (%) ph Totl N (g kg 1 ) MBC (mg kg 1 ) BF NP ± ± ± ± 82 DP 3.36 ± ± ± ± 93 CK 27.4 ± 1.97 c 3.97 ± ± ± 89 MF NP 14.3 ± ± ± ± 74 DP 3.98 ± ± ± ± 149 CK ± 1.44 c 3.97 ± ± ± 2 c PF NP 7.77 ± ± ± ± 56 DP ± ± ± ± 1 CK ± 8.5 c 4.6 ± ± ± 87 c Downloded from y guest on 2 Septemer 218 Soil moisture (men ± stndrd devition in the tle) ws mesured from Jn 27 to Jn 29. Soil MBC ws ssyed in the 28 wet seson (Hung et l. 211). Within ech forest, vlues with different letters re significntly different t the P <.5 level y LSD test.

4 Chen et l. Effects of precipittion on soil orgnic C frctions 13 time. After sending the smples to the lortory, these soil cores were wshed crefully over 2-mm sieve to remove soil. Roots ( 3 mm) were hnd-picked nd dried t 6 C until they reched constnt weight. Soil nlyses Soil ph ws mesured in 1:2.5 soil/wter suspension. SOC ws determined y dichromte oxidtion nd titrtion with ferrous mmonium sulfte. Totl N ws mesured using the Kjeldhl method (Liu 1996). Soil ROC ws determined using the KMnO 4 oxidtion method, s descried y Blir et l. (1995). In detil, the ir-dried soil ws ground nd further pssed through 149- μm sieve. Su-smples of soil contining 15 3 mg C were weighed into 1 ml plstic centrifuge tues, nd then 25 ml of 333 mmol L 1 KMnO 4 solution were dded to ech tue. The tues were shken for 1 hour t 25 r min 1 nd centrifuged for 5 min t 2 r min 1. The superntnts were then diluted 25 times with deionized wter. Finlly, the sornce of the smples nd stndrds ws red on UV spectrophotometer t wvelength of 565 nm. Chnges in the concentrtion of KMnO 4 were used to clculte the soil ROC concentrtion, ssuming tht 1 mmol MnO 4 ws consumed in the oxidtion of 9 mg C. NROC ws mesured s the mount of C mount tht could not e oxidized y KMnO 4, s indicted y the differences etween the SOC nd ROC concentrtions of ech smple. Forest floor smples were nlyzed using cross-polriztion mgic-ngle-spinning (CPMAS) solid stte 13 C NMR spectroscopy (Avnce 3 MH) t frequency of 75.5 MHz. The importnt condition prmeters used in the NMR included spinning rte of 12 khz, contct time of 35 ms, recycle time of 5 s nd zero-filled of 2 dt points. Chemicl shift vlues were determined using glycocoll t ppm s n externl reference. To quntify the different types of C, the CPMAS 13 C NMR spectr were divided into four chemicl shift regions, i.e. lkyl C ( 5 ppm), O-lkyl C (5 11 ppm), romtic C (11 16 ppm) nd croxyl C (16 21 ppm). The integrtion of the peks within ech of the chemicl shift regions llowed us to estimte the reltive C contents, expressed s the percent rtio of the pek re of these groups to the totl pek re in ech composite smple. The reclcitrnce index ws determined s the rtio of (lkyl C + romtic C)/(O-lkyl C + cronyl C) (Ostertg et l. 28). Sttisticl nlyses Dt nlyses were crried out using SPSS 11.5 for Windows. A two-wy nlysis of vrince (ANOVA) ws used to study the effects of forest type, precipittion tretment nd their interction on soil ph, SOC, ROC nd NORC seprtely. Once significnt effects were detected, multiple comprisons with the LSD method were performed to ssess significnces mong different groups. A repeted mesures ANOVA ws used to exmine the effects of the different experimentl tretments on fine root iomss over time. Person correltion coefficients were clculted to show the reltionships etween soil prmeters. Differences were considered to e significnt when P <.5. RESULTS SOC nd its frctions Soil contined significntly higher SOC content in the CK plots in BF nd MF thn in PF, showing verges of ± 5.34, ± 2.95 nd 19.7 ±.43 g kg 1 in BF, MF, nd PF, respectively (Fig. 1 nd Tle 2). The NP tretment significntly decresed SOC content in ll the studied forests compred to the CK plots (P <.1, Fig. 1). The DP tretment tended to SOC (g kg -1 ) ROC (g kg -1 ) NROC (g kg -1 ) c BF MF PF NP DP CK Figure 1: effects of precipittion on SOC, ROC nd NROC in the BF, MF nd PF. Error rs re stndrd errors (n = 3). Within ech forest, vlues with different letters re significntly different t the P <.5 level y LSD test. Downloded from y guest on 2 Septemer 218

5 14 Journl of Plnt Ecology Tle 2: nlyses of vrince on the effects of forest type, precipittion tretment nd their interctions on soil ph, SOC, ROC nd NROC ph SOC ROC NROC Forest 3.56* ** 6.346** 56.81** Tretment 4.562* 17.45** 9.986** ** Forest tretment The numers in the cells re F-vlues nd sterisks indicte the level of significnce (*P <.5; **P <.1). decrese SOC content, ut the chnge ws significnt only in PF (P =.1 in BF, P =.14 in MF nd P =.1 in PF, Fig. 1). Consistent with the cler trend of SOC content, soil ROC in the CK plots ws lso significntly higher in oth BF nd MF thn in PF (Fig. 1 nd Tle 2) nd ws on verge 9.43 ± 1.32, 1.36 ± 1.64 nd 8.26 ±.47 g kg 1 in BF, MF nd PF, respectively. Similrly, the NP nd DP tretments ppered to decrese ROC content, yet the differences were only significnt etween the NP nd CK plots in MF (P =.1) nd etween the DP nd CK plots in PF (P =.4, Fig. 1). Soil NROC content ws significntly different mong the three forests (19.74 ± 3.8, ± 1.56 nd 1.81 ±.4 g kg 1 in the CK plots in BF, MF nd PF, respectively, Fig. 1c nd Tle 2). The NP tretment decresed NROC significntly in ll three forests (P <.5 in BF, P <.5 in MF nd P <.5 in PF, Fig. 1c); however, the difference ws not significnt etween the DP nd CK tretments in the forests studied (P =.9 in BF, P =.16 in MF nd P =.23 in PF, Fig. 1c). Moreover, forest types nd precipittion mnipultions significntly ffected the content nd frctions of SOC (P <.5, Fig. 1); however, the interctions etween the two were not significnt for SOC content or for the content of its frctions (P >.5, Fig. 1). Other soil physiochemicl properties In the CK plots, oth BF nd MF hd significntly higher soil moisture thn PF (P <.5, Tle 1). In ll the three forests, the NP tretment significntly decresed soil moisture (P <.5), while the DP tretment significntly incresed soil moisture (Tle 1). The NP tretment lso decresed soil ph significntly in BF (P <.5), ut did not ffect soil ph in either MF or PF (P >.5, Tle 1). The totl N content in BF nd in MF ws significntly higher thn the totl N content in PF under the CK tretment (P <.5), ut it ws not significntly different etween BF nd MF (Tle 1). None of our experimentl tretments chnged the soil totl N concentrtion significntly in either BF or PF, while the NP tretment significntly decresed the totl N in MF (P <.5, Tle 1). Soil MBC in BF ws significntly higher compred to PF or in MF (P <.5, Tle 1); it ws slightly higher in MF thn in PF ut the difference ws not significnt (P >.5, Tle 1). Moreover, under the NP tretment, MBC ws lower thn tht in the CK plots in ll three forest types. The DP tretment significntly incresed MBC in oth PF nd MF. Root iomss In CK plots, root iomss ws significntly higher in BF nd MF thn in PF (P <.5), ut there ws no significnt difference etween BF nd MF (P >.5, Fig. 2). In ll three forests studied, the NP tretment significntly decresed root iomss throughout the entire investigtion period (P <.5), while the DP tretment did not chnge root iomss significntly (P >.5), with the exception of PF in Ferury 28 (P <.5, Fig. 2). Forest floor lyer The reltive intensities of C functionl groups of the forest floor smples re shown in Fig. 3 nd Tle 3. Under the CK tretment, the smples totl orgnic C ws dominted y O-lkyl C in the three studied forests ( %, Fig. 3). In ech of Root iomss (g m -2 ) Root iomss (g m -2 ) Root iomss (g m -2 ) BF F = p <.1 MF F = p <.1 PF F = p <.1 Fe-7 Aug-7 Fe-8 Apr-8 Aug-8 Oct-8 Time NP DP CK Figure 2: root iomss ( 3 mm) under the precipittion tretments t the three forest sites. The F nd P vlues indicte the significnce of the tretment effect on fine root iomss over time y repeted mesures ANOVA. Downloded from y guest on 2 Septemer 218

6 Chen et l. Effects of precipittion on soil orgnic C frctions 15 BF - C functionl groups (%) MF - C functionl groups (%) PF - C functionl groups (%) L L L d g F F F h H H H DP Alkyl C O-lkyl C Aromtic C Cronyl C the forests, O-lkyl C intensity vried with the lyer, with the fresh litter lyer hving the highest content, followed y the fermenttion lyer nd then the humus lyer (Fig. 3). However, Alkyl C in BF nd in MF exhiited different trends from tht of O-lkyl C, i.e. the intensity in the fermenttion nd humus lyers ws higher compred to tht of the fresh litter lyer (Fig. 3 f). The intensity of romtic C ws not oviously different mong the lyers in BF, ut it ws higher in the fermenttion nd humus lyers thn in the fresh litter lyer in the other two forests (Fig. 3). The Cronyl C intensity ws the highest in the humus lyers (Fig. 3). The reclcitrnce index vried y lyer ut ws the sme for ll three forests: humus lyers > fermenttion lyers > fresh litter lyers (Tle 3). The reclcitrnce index in the humus lyers ws higher in BF nd MF thn PF (Tle 3). For BF, the intensities of lkyl C nd O-lkyl C in the fresh litter lyer nd in the fermenttion lyer under the NP nd CK Figure 3: distriution of C functionl groups from CPMSA 13 C-NMR spectr of forest floor lyers under different tretments mong the three forests. L = fresh litter lyer; F = fermenttion lyer; H = humus lyer. Tle 3: effects of precipittion on the reclcitrnce index in the forest floor in the BF, MF nd PF Litter lyers Tretment BF MF PF L NP DP CK F NP DP CK H NP DP CK The reclcitrnce index (%) ws determined s (lkyl C + romtic C)/(O-lkyl C + cronyl C). Arevitions: L = fresh litter lyer; F = fermenttion lyer; H = humus lyer. e i c f Downloded from y guest on 2 Septemer 218

7 16 Journl of Plnt Ecology DP tretments were higher compred to the CK tretment. Aromtic C nd cronyl C under the NP nd DP tretments hd reltively lower intensities thn tht under the CK tretment (Fig. 3 c). In fresh litter nd fermenttion lyers, the reclcitrnce indices incresed with the incresed precipittion, with the highest vlues in the DP plots nd the lowest in the NP plots, while DP tretment decresed the reclcitrnce index in the humus lyer (Tle 3). The humus lyers disppered under the NP tretment in this forest. For MF, the NP tretment incresed the intensity of lkyl C nd the reclcitrnce index, ut decresed the intensity of cronyl C in the fresh litter lyer (Fig. 3d f nd Tle 3). Aromtic C nd the reclcitrnce index in the fermenttion lyer were lower in the NP plots thn in the CK plots. In the humus lyer, the NP tretment incresed the intensities of lkyl C nd O-lkyl C, ut decresed the intensity of romtic C. Alkyl C nd O-lkyl C in the fresh litter lyer showed reltive increse under the DP tretment, while cronyl C ws lower here thn in the CK plots. The DP tretment incresed the O-lkyl C, while romtic C showed reltive decrese in the fermenttion lyer nd humus lyer. In oth the fermenttion nd humus lyers, the DP tretment decresed the reclcitrnce index. For PF, the NP tretment decresed the intensity of lkyl C nd incresed the intensity of romtic C in the fresh litter lyer (Fig. 3g). Compred to the CK control plots, lkyl C nd the reclcitrnce index in the fermenttion lyer were reltively higher under the NP tretment, nd romtic C ws reltively lower (Fig. 3h nd Tle 3). In humus lyer, the NP tretment incresed lkyl C nd the reclcitrnce index ut decresed romtic C nd cronyl C (Fig. 3i nd Tle 3). In oth the fermenttion nd humus lyers, O-lkyl C intensity incresed under the DP tretment. The DP tretment decresed romtic C nd cronyl C. In oth the fresh litter nd fermenttion lyers, the DP tretment incresed the reclcitrnce index; however, there ws no ovious difference in the humus lyer etween the DP nd CK tretments. DISCUSSION Effect of forest types on SOC nd its frctions Among our study sites, PF contined significntly lower SOC content thn the other two forests. On the one hnd, this my e ttriuted to the reltively lower litterfll nd root C inputs to the soil in PF compred to MF nd BF. On the other hnd, more forest gps in younger forests with developing cnopy limit the shded re nd therefore increse soil temperture while decresing soil moisture. The reltively lower soil moisture cn consequently contriute to the slower decomposition rtes of litterfll nd roots (Mrtin et l. 24; Ostertg et l. 28), possily due to limited microil ctivity nd lower sustrte trnsformtion cpcity when wter supply is limited. Likewise, lower forest cnopy, s well s lower soil moisture, ws lso detected in PF rther thn in MF nd in BF (Zhou et l. 26). The lower soil moisture is likely to restrict the decomposition of litter nd roots (Zhng et l. 28), nd then reduce the trnsfer of plnt-c into minerl soil in PF. The two possile decreses in C inputs in PF my result in the oserved pttern of SOC mong forests, compred with wht ws oserved in MF nd BF. Likewise, previous studies hve reported tht SOC content incresed with forest ge in different forest ecosystems (Hung et l. 211; Li et l. 213; Pregitzer nd Euskirchen 24), n oservtion tht is consistent with the present study. Underlying the cler increse in SOC with the forest development, we oserved tht different SOC frctions showed different dynmics within these forests. This is n importnt finding tht could improve our understnding of the mechnisms of SOC ccumultion in forests. For ROC, the proxy for lile SOC in our study (Blir et l. 1995), there ws no significnt difference mong the different forest types. The content of soil ROC is generlly determined y the lnce of plntderived C inputs nd outputs (Tirol-Pdre nd Ldh 24; Yng et l. 29). In our study sites, the mounts of litterfll nd root iomss ( 3mm) in BF nd MF were higher thn in PF (Chen et l. 212), indicting the higher plnt-derived C inputs in the former two forests. This seemed to e eneficil for ROC ccumultion in those res. Compred with PF, however, BF nd MF hd significntly higher soil moisture nd MBC, which could hve fcilitted the decomposition of ROC in considertion of lile SOC s preferle-utilized sustrtes y soil microorgnisms (Blir et l. 1995; Chen et l. 212). As result, the synchronism in the enhncements of ROC inputs nd outputs could e the explntion for why significntly higher ROC ccumultion ws not oserved in BF nd in MF versus in PF. Additionlly, in spite of the lower litterfll production in PF, the reltive intensity of O-lkyl C in fresh litter in PF ws higher thn tht in oth MF nd BF. The O-lkyl C group, minly comprising polyscchrides, ws commonly regrded s proxy for esily oxidizle crohydrtes (Alrcón- Gutiérrez et l. 28; Tirol-Pdre nd Ldh 24). The higher reltive intensity of O-lkyl C in PF implied higher percentge of ROC inputs to totl C inputs, nd could lso contriute to the similr ROC content in PF compred to the other two forests. The stle orgnic C frction, indicted y the NROC content in this study, ws significntly higher in BF nd MF thn in PF, pttern in line with the results from the C functionl group nlysis. The higher reclcitrnce index signified greter contriutions to SOC content from lkyl C nd romtic C groups, which re usully regrded s slower-decomposed orgnic C groups (Ostertg et l. 28). In our study, we oserved much higher reclcitrnce index in humus lyers in BF nd MF thn in PF, implying reltively higher percentges of these two reltively reclcitrnt orgnic C groups in oth BF nd MF. Comined with the oserved higher inputs of litterfll nd roots in these two forests, these fctors my contriute to the fster ccumultion of stle orgnic C in the minerl soil in MF nd BF. Downloded from y guest on 2 Septemer 218

8 Chen et l. Effects of precipittion on soil orgnic C frctions 17 Effects of decresed precipittion on SOC content nd its frctions Decresed soil moisture under the NP tretment hd negtive effect on soil ROC content in these forests, lthough the decreses in PF nd in BF were not significnt. At first this pttern surprised us ecuse, s discussed previously, lower soil moisture my e eneficil for ROC ccumultion due to the lower decomposition ctivity, which ws indicted y the decresed MBC content under the NP tretment in this study (Chen et l. 212; Deng et l. 21). However, reduction in ROC under the NP tretment ws nticipted when we counted the roots iomss. As the min input of soil ROC, the decreses in the production of this plnt-derived C offset the possiility of ROC ccumultion resulting from the lower decomposition ctivity (lower C output) under the NP tretment. This is in line with the oservtions of O Brien et l. (21). Moreover, the decresed root iomss my reduce the lile C input to minerl soil in the forms of root excretions, nd consequently decrese ROC content. In ddition, reduction in soil moisture could limit the downwrd movement of C from the litter lyer to the soil (Lee et l. 24), consequently decresing soil ROC in ll the three forest types. As proxy of stle orgnic C frctions, NROC is reltively less responsive to environmentl chnges, owing to the decomposition reclcitrnce of the C group to soil microorgnisms (Blir et l. 1995). Our results, however, showed tht NROC content decresed under the NP tretment in ll three forests, finding tht ppers to contrdict the forementioned trditionl interprettion. The reductions in NROC content likely resulted from the reduction in the production of litterfll nd roots, i.e. reductions in the NROC inputs led to the decrese in NROC content. Alterntively, chnges in the decomposility of litterfll under the NP tretment could lso contriute to such pttern, s residul OM ws more reclcitrnt in the wet nd wrm climte thn in the cold nd dry climte ccording to Hilli et l. (28), in spite of the supply of the sme stndrd litter. This is lso supported y our results from the CPMAS 13 C-NMR nlyses: from the fermenttion to humus lyers, romtic C under the NP tretment ws lower thn tht t the CK plots, ut the NP tretment ws not oserved to hve the sme influence in the fresh litter lyer. Aromtic C compounds re commonly considered to e less decomposle y microorgnisms (Ostertg et l. 28). A decrese in the reltive intensity of romtic C under the NP tretment could thus imply the decresed ccumultion of non-lile C in minerl soil. The SOC pool comprises the two frctions, i.e. ROC nd NROC in the present study. Compred with CK plots, the NP tretment decresed the contents of oth ROC nd NROC, susequently resulting in long-term decrese in SOC in ll three forests. Effects of precipittion increse on SOC content nd its frctions The DP tretment tended to decrese the contents of SOC, ROC nd NROC, ut it ws not significnt except for the decrese in the SOC nd ROC content in PF. Wter content is key fctor controlling SOC turnover in PF ecuse soil moisture is quite low in this forest (Zhou et l. 25). This is lso indicted y the incresed soil MBC content under DP tretment in PF, which likely implies tht soil microil growth ws impressed y the wter limit in this forest under nturl precipittion (compring the MBC content in the CK tretment with tht in the DP tretment). The incresed soil MBC under the DP tretment could e responsile for the decreses in the contents of ROC nd SOC ecuse of the higher microil decomposition cpcity under this condition (Li et l. 213). This is consistent with previous studies in the sme region (Chen et l. 212; Deng et l. 21, 212). Moreover, PF hs higher soil ulk density in 1 cm lyers. The higher ulk density my e nother reson for the decresed ROC nd SOC in this forest, s lower soil porosity would result in slow wter infiltrtion rte nd consequently enhnce surfce runoff (Levy et l. 1997), thus leding to more ROC nd SOC losses from soil erosion. Contrry to the SOC nd ROC contents in PF, the DP tretment did not exhiit significnt influence on the SOC or ROC content in BF nd MF. This ws likely due to the reltively higher soil moisture in these two forests even under CK tretment; therefore, wter limittion did not occur in BF nd MF. As result, incresed precipittion cnnot increse soil moisture dequtely to chnge the ROC nd SOC content significntly. Alterntively, the higher soil wter content under nturl conditions (CK tretment in this study) in oth BF nd MF itself my imply tht soils in these two forests re more resilient or persistent to precipittion increses (Deng et l. 211; Hung et l. 211), when the incresed soil wter content does not exceed the wter thresholds, over which oxygen limittion will occur in the soil system (Dvidson et l. 1998). This stronger resilience or persistence is lso indicted y our oservtions of the fine root iomss nd other soil properties including ph, nd totl N, s they were not significntly different etween the DP nd CK tretments in oth BF nd MF. Moreover, the DP tretment did not ffect the NROC content significntly in ny of the studied forests. This is lso consistent with the results of the C functionl group nlysis showing tht no significnt difference were oserved in lkyl C, romtic C nd the reclcitrnce index etween the DP nd CK tretments. The DP tretment in our study sites did not led to ny notle chnges in the fine roots iomss or in the percentge of reclcitrnt C functionl groups in the litter, resulting in the NROC ptterns oserved ove (Kögel- Knner 22; Ono et l. 211). In conclusion, NROC ut not ROC content incresed significntly from the young PF to the mture BF, implying tht n increse in totl SOC oserved mong these forests ws minly due to the increse of NROC. In ll three forests, the NP tretment exhiited negtive effects on totl SOC due to decreses in C inputs (including litterfll nd root productions) nd chnges in C qulity (SOC frctions nd C functionl groups). However, the DP tretment did not significntly chnge the Downloded from y guest on 2 Septemer 218

9 18 Journl of Plnt Ecology contents of totl SOC nd its frctions in oth BF nd MF, indicting tht the BF nd MF hd higher cpcity to resist the effects of precipittion increse on soil cron storge. Our results suggest tht the SOC frctions would respond differently to precipittion chnges in sutropicl forests in southern Chin depending on the forest types nd tht there my e mechnism underlying the oserved SOC ccumultion in mture forests in this region. Further studies re of high importnce to etter understnd the underlying mechnisms of the vrious responses of SOC frctions to chnges in precipittion. Such studies could improve our predictions of ecosystem C cycling under future precipittion chnges in southern Chin. 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