Effects of nitrogen deposition on soil organic carbon fractions in the subtropical forest ecosystems of S China

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1 J. Plnt Nutr. Soil Sci. 212, 175, DOI: 1.12/jpln Effects of nitrogen deposition on soil orgnic cron frctions in the sutropicl forest ecosystems of S Chin Xiomei Chen 1,2, Yuelin Li 1, Jingming Mo 1, Dennis Otieno 1,3, John Tenhunen 3, Junhu Yn 1, Juxiu Liu 1, nd Deqing Zhng 1 * 1 Key Lortory of Vegettion Restortion nd Mngement of Degrded Ecosystems, South Chin Botnicl Grden, Chinese Acdemy of Sciences, Gungzhou 5165, Chin 2 Grdute University of Chinese Acdemy of Sciences, Beijing, 139, Chin 3 Deprtment of Plnt Ecology, University of Byreuth, 9544 Byreuth, Germny Astrct Experiments were conducted etween 23 nd 28 to exmine how N dditions influence soil orgnic C (SOC) nd its frctions in forests t different succession stges in the sutropicl Chin. The succession stges included pine forest, pine nd rodlef mixed forest, nd oldgrowth monsoon evergreen rodlef forest. Three levels of N (NH 4 NO 3 )-ddition tretments comprising control, low-n (5 kg N h 1 y 1 ), nd medium-n (1 kg N h 1 y 1 ) were estlished. An dditionl tretment of high-n (15 kg N h 1 y 1 ) ws estlished in the rodlef mixed forest. Soil smples were otined in July 28 for nlysis. Totl orgnic C (TOC), prticulte orgnic C (POC, > 53 lm), redily oxidizle orgnic C (ROC), nonredily oxidizle orgnic C (NROC), microil iomss C (MBC), nd soil properties were nlyzed. Nitrogen ddition ffected the TOC nd its frctions significntly. Lile orgnic-c frctions (POC nd ROC) in the topsoil ( 1 cm) incresed in ll the three forests in response to the N-ddition tretments. NROC within the topsoil ws higher in the medium-n nd high-n tretments thn in the controls. In the topsoil profiles of the rodlef forest, N ddition decresed MBC nd incresed TOC, while no significnt effect on MBC nd TOC occurred in the pine nd mixed forests. Overll, elevted N deposition incresed the vilility of lile orgnic C (POC nd ROC) nd the ccumultion of NROC within the topsoil irrespective of the forest succession stge, nd might enhnce the C-storge cpcity of the forest soils. Key words: Chin / forest succession stges / N deposition / soil orgnic cron ccumultion / sutropicl forest Accepted Mrch 29, Introduction Anthropogenic N cretion glolly hs incresed from 15 Tg Ny 1 in 186s to 156 Tg N y 1 in the erly 199s, nd is expected to e 27 Tg N y 1 in 25 (Gllowy et l., 24). Studies show tht N deposition is one of the ovious glol chnges with significnt influence on C storge in forests. Nitrogen ddition to soils ccelerte decomposition of recent soil C nd retrd decomposition of older soil orgnic (SOC), resulting in net long-term enhncement of C storge (Neff et l., 22; Hgedorn et l., 23; Swnston et l., 24). Sutropicl nd tropicl forest ecosystems store 46% nd 11% of the world s living terrestril nd soil C pools, respectively (Brown nd Lugo, 1982). Most of these forests re lso prone to N deposition, rising from the rpid industriliztion of the countries in these regions. In S Chin, for exmple, the current N-deposition rte in some of the forests is estimted t 3 73 kg h 1 y 1 (Ren et l., 2; Zhou nd Yn, 21), compred to 25 3 kg h 1 y 1 in mny forests in Europe which re prtly suject to N sturtion (Aer et l., 1998; McDonld et l., 22). Nonetheless, little is known regrding how the tropicl nd sutropicl forests respond to the incresing N-deposition rtes. Thus, there is n urgent need to understnd how N deposition influences C dynmics in the tropicl nd sutropicl forests in order to predict how rising tmospheric N levels will ffect their role in the glol C cycle. The chrcteristics of SOC vry with forest structure nd ge nd re lso modified y the level of soil N concentrtion (Hgedorn et l., 23). Most studies on the effect of N deposition on soil C dynmic, however, hve een restricted to temperte forests which re often N-limited (Pregitzer et l., 28). Those studies revel tht elevted N deposition modifies C dynmics in these forests y modulting the soil microil enzymtic ctivity (Wldrop et l., 24) nd litter iochemicl chrcteristics (Gllo et l., 25). Preliminry results from sutropicl forests show tht N ddition influences SOC y decresing the microil iomss C nd incresing wter-solule orgnic C in the surfce soils (Fng et l., 29). Reduction in respirtion fter N ddition in the rodlef forests hve lso een reported for these forests (Mo et l., 28). According to Fng et l. (27), N ddition significntly depresses litter decomposition ut the mgnitude vries with forest type. How nturl sutropicl forest ecosystems respond to N ddition is still not well understood * Correspondence: D. Zhng; e-mil: zhngdeq@scg.c.cn 212 WILEY-VCH Verlg GmH & Co. KGA, Weinheim

2 948 Chen, Li, Mo, Otieno, Tenhunen, Yn, Liu, Zhng J. Plnt Nutr. Soil Sci. 212, 175, nd clls for more investigtions in order to define the future role of these forests in C storge. Soil orgnic C is conglomertion of C-contining frctions t different cycling rtes (Ågren nd Bostt, 22; Knorr et l., 25; Brdford et l., 28). Different SOC frctions, therefore, respond differently to the physicl nd iologicl environmentl chnges (Lützow et l., 27, 28). In the pst, numerous frctiontion methods including physicl nd chemicl frctions hve een developed to seprte SOC frctions (Lützow et l., 27). Finding the pproprite methods for seprting the SOC frctions is key to studying how the individul SOC frctions respond to environmentl chnges. Prticulte orgnic C (POC) ssocited with sndy soils (> 53 lm) is rich source of iologiclly ville C nd energy for soil microorgnisms nd hs een recommended y Yng et l. (29) for SOM investigtions in the sutropicl forests of Chin. Redily oxidizle orgnic C (ROC) hs reltively high turnover, nd is responsive to mngement prctices (Dtt et l., 21). Soil microil iomss C (MBC) cn indirectly indicte soil microil ctivity (Fng et l., 29). Exmining the chnges in these lile C frctions is more sensitive tool to mesure the chnges in SOC dynmics in response to environmentl chnges, ut the long-term C storge is criticlly dependent on the stle SOC frctions (Neff et l., 22). For exmple, nonredily oxidizle orgnic C (NROC) shows reltively wek response to environmentl chnges (Blir et l., 1995) nd cn e used to monitor the SOC stle over time. Therefore, we study the SOC dynmic in response to N deposition through comintion of lile nd stle C. In this study, we investigted the SOC dynmics in response to N deposition in the sutropicl forests of S Chin through comintion of lile nd stle C. Experimentl N dditions were set in forests t different succession stges nmely: pine forest (), pine nd rodlef mixed forest (MF), nd old-growth monsoon evergreen rodlef forest (BF) in the Dinghushn Biosphere reserve. Totl orgnic C (TOC), POC, ROC, NROC, MBC, nd soil properties were nlyzed. We hypothesized tht (1) elevted N deposition increses TOC through ccumulted lile orgnic C (POC nd ROC) nd retrded minerliztion of nonredily oxidizle orgnic C nd (2) stimultion of TOC ccumultion in sutropicl forest soils s result of N deposition is dependent on the ge of the forest. 2 Mterils nd methods 2.1 Site descriptions The study ws crried out in Dinghushn Biosphere Reserve (Gungdong Province, sutropicl S Chin: E, 23 1 N). The reserve occupies n re of 12 h nd experiences typicl monsoon nd sutropicl humid climte, with men nnul temperture nd reltive humidity of 21.5 C nd 8%, respectively. The men nnul rinfll is 1956 mm nd hs distinct sesonlity, with 8% of it flling from April to Septemer (Yng et l., 21). The reserve comprises three typicl forest ecosystem types t different succession stges, nmely: pine forest, mixed pine nd rodlef forest (herefter mixed forest), nd n old-growth monsoon evergreen rodlef forest (herefter rodlef forest), representing erly, middle, nd dvnced succession stges, respectively (Peng nd Wng, 1993). The dominnt species in ech forest re given in T. 1. The density of pine decreses with incresing forest ge so tht in the rodlef forest, pine is nturlly replced y rodlef species. The pine forest originted from 195s cler-cutting nd susequent pine-plnttion estlishment. It represents forest t the erly-succession stges. The mixed forest consists of pine nd rodlef trees, which originted from plnted or nturl pine forests, fter self-seeding nd invsion y rodlef trees. It represents the mid-succession stges. The rodlef forest represents climx stge of succession. All the sites occur t similr elevtion (1 25 m sl) nd hve similr slope conditions (18 25, S-fcing spect). Soils t ll the three forest sites re shllow Ultisols (USDA Soil Txonomy) overlying sndstone nd shle edrocks, with depth down to 4 6 cm. Chrcteristics of the three forest types re summrized in T. 1. The nnul litter fll (g m 2 y 1 ) ws 981.8, 72.5, nd 587.5, with initil verge lef-litter N concentrtion (mg g 1 ) of 15.7, 14.2, nd 12.2 for rodlef, mixed, nd pine forests, respectively (cf. Mo et l., 26). 2.2 Experimentl design nd tretments The experiments were conducted etween 23 nd 28. Three levels of N (NH 4 NO 3 )-ddition tretments, comprising (1) control, (2) low-n ddition (5 kg N h 1 y 1 ), nd (3) medium-n ddition (1 kg N h 1 y 1 ), were estlished in the pine, mixed, nd rodlef forest sites, while n dditionl tretment of high-n (15 kg N h 1 y 1 ) ws estlished in Tle 1: Site chrcteristics of the three forests under study. Forest type Forest coverge /% Forest cnopy density /% Stnd ge /y Litterfll /gm 2 y 1 Dominnt species Pine forest ± ± Mixed forest ± 6.1 c ± 79.3 Brodlef forest ± 3.1 c ± 12 Pinus mssonin, Rhodomyrtus tomentos, Ficus vriolos, Becr frutescens P. mssonin, Schim super, Cstnopsis chinensis, Psychotri rur, F.vriolos, Evodi lept, Ghni tristis, Adintum cpillus-veneris C. chinensis, S. super, Cryptocr concinn, Mchilus chinensis, Cryptocry chinensis, Blstus cochinchinensis, Psychotri rur, Hemigrmm decurrens 212 WILEY-VCH Verlg GmH & Co. KGA, Weinheim

3 J. Plnt Nutr. Soil Sci. 212, 175, Effects of N deposition on SOC frctions 949 the rodlef-forest site. In ech forest type, three replicte plots, ech mesuring 1 m 2 m, surrounded y 1 m wide uffer strip, were rndomly selected for ech level of N tretment. A totl of 3 plots were estlished (9 in the pine, 9 in the mixed, nd 12 in the rodlef forests). A totl re of.7 h ws demrcted for ech forest type. The low-n tretment (5 kg N h 1 y 1 ) corresponded to vlue higher thn the current tmospheric N deposition (3 73 kg N h 1 y 1 ), ut ws lower thn the projected N deposition ( kg N h 1 y 1 ) for S Chin in 23 (Zheng et l., 22). Decisions on the other levels of N ddition (1 kg N h 1 y 1 nd 15 kg N h 1 y 1 ) were sed on these levels. The ville N nd totl N of rodlef-forest soils were twice s high s those of pine nd mixed forests (T. 2), s reported previously y Mo et l. (26). For ech fertiliztion tretment, solid fertilizer (NH 4 NO 3 ) ws weighed, mixed with 2 L of wter, nd evenly pplied onto the plots, elow the cnopy nd close to the soil surfce, using ckpck spryer. The solution ws spryed s 12 equl monthly pplictions over the entire yer, eginning July 23 nd continuing throughout the study period Soil smpling Soil smples from two depths, 1 nd 1 2 cm, respectively, were otined from ech of the replicte plots etween 21 nd 22 July 28, using soil corer ( 2.5 cm). From ech depth in every plot, we collected five cores rndomly nd mixed them into one smple. At ny smpling session, totl of 6 smples were otined for nlysis. The fresh soil smples were pssed through 2 mm sieve to remove rocks nd plnt roots. Susmples were ir-dried nd ground for TOC- nd SOC-frctions mesurements SOC frctions Prticulte orgnic C (POC 53 2 lm) ws mesured using the method descried y Cmrdell nd Elliot (1992). Redily oxidizle orgnic C (ROC) nd nonredily oxidizle orgnic C (NROC) were determined with 333 mmol L 1 KMnO 4 oxidtion (Blir et l., 1995) nd MBC with the chloroform fumigtion-extrction method (Joergensen nd Brookes, 199) Soil mesurements Soil chrcteristics including soil ph (glss electrode), texture (Pge et l., 1982), soil wter content, totl N, nd oveground litter input were mesured. Soil wter content (SWC) ws determined grvimetriclly. Totl N ws mesured using the Kjeldhl method (Liu, 1996). In the study period 23 to 28, MBC in the three forests were investigted every 2 y. 2.4 Sttisticl nlysis Differences in soil TOC concentrtion nd orgnic-c frctions mong the three different tretments in the three forests were ssessed with two-wy nlysis of vrince (ANOVA). Onewy ANOVA with LSD tests ws performed to ssess the effects of N ddition within forest types. We pplied multivrite liner mixed-effects models in order to correct for the repeted mesures in the different plots. p vlues were clculted using likelihood-rtio tests sed on chnges in devince when ech term ws dropped from the full model. Generlized Liner Model (GLM) ws employed in order to check for interctions etween fctors. We checked the normlity of the model residuls visully y norml proility plots, nd we ssured the homogeneity of vrinces nd goodness of fit of the models y plotting residuls vs. fitted vlues. In some cses, regression nlysis ws employed to compre prmeters. All the tests were performed using SPSS softwre (version 11.5). Differences for ll tests were considered to e significnt t p <.5. 3 Results 3.1 Soil properties In the control plots, soil ph decresed with forest ge, i.e., pine > mixed > rodlef forests (Figs. 1 A C). Soil ph within the shllow soil profile of the three forest types sujected to medium-n nd high-n tretments were lower thn in the sme profile in the control plots (Fig. 1). SWC t the pine nd mixed forests were lower thn tht in the rodlef forest, especilly during winter. Men TN in the control rodlef forest ws higher thn in the mixed nd pine forests (T. 2). Monthly soil MBC dynmics etween 23 nd 28 in the three forest sites showed tht the highest MBC levels occurred in My nd June. MBC ws highest in rodlef forest compred to the pine nd mixed forests. 3.2 Sptil pttern of soil TOC response to N-ddition tretment Numers of TOC within the shllow ( 1, 1 2 cm) soil profiles of the control plots were significntly (T. 3; Tle 2: Topsoil ( 1 cm) properties of the three forest types under study (Vlues re men ± stndrd devition). Soil properties Pine forest Mixed forest Brodlef forest Soil order Ultisol Ultisol Ultisol ph 4.12 ± ± ±.7 Soil wter content / % 25.1 ± ± ± 3. SOC / g kg ± ± ± 3.4 Totl ville N ( 1 cm) / mg kg ± ± ± 1.5 Totl N / g kg ± ± ± WILEY-VCH Verlg GmH & Co. KGA, Weinheim

4 95 Chen, Li, Mo, Otieno, Tenhunen, Yn, Liu, Zhng J. Plnt Nutr. Soil Sci. 212, 175, ph A MF B BF c C CN LN MN HN 4. d 3.8 TOC / g kg D MF E BF F POC / g kg G MF H BF I 1 J MF K BF L ROC / g kg c c NROC / g kg M MF N BF O Figure 1: Effects of N ddition on soil ph (upper pnel), totl orgnic C (TOC) (second pnel), prticulte orgnic C (POC) (third pnel), redily oxidizle orgnic C (ROC) (fourth pnel), nd nonredily oxidizle orgnic C (NROC) (ottom pnel) in different soil profiles in the tree forests under study. LN: low-n tretment; MN: medium-n tretment; HN: high-n tretment; CN: control tretment. Pine forest, mixed forest, nd rodlef forest, revited for ll figures s, MF, nd BF, respectively. Error rs re stndrd errors (n = 3). 212 WILEY-VCH Verlg GmH & Co. KGA, Weinheim

5 J. Plnt Nutr. Soil Sci. 212, 175, Effects of N deposition on SOC frctions 951 Tle 3: Anlyses of vrince on the effects of forest type, N-ddition tretment, nd their interctions on soil ph, totl orgnic C (TOC), prticulte orgnic C (POC), redily oxidizle orgnic C (ROC), nonredily oxidizle orgnic C (NROC). Numers re F vlues. Strs indicted the level of significnce (*: p <.5; **: p <.1). Soil depth /cm ph TOC /gkg 1 POC /gkg 1 ROC /gkg 1 NROC /gkg 1 1 forest ** * 44.2** tretment 11.74** 4.62* 12.3** 13.98** 12.51** forest tretment ** ** ** ** ** 1 2 forest ** 8.97** 27.3** 4.18** tretment **.9 forest tretment ** ** ** 377.8** 5.15** p =.6 nd.15 for 1 nd 1 2 cm, respectively) higher in the rodlef forest thn in the pine nd mixed forests (Figs. 1 D F). No significnt difference occurred etween pine nd mixed forests. In the rodlef-forest plots, TOC in the top 1 cm profiles followed the order: high ([3.97 ± 2.3] g kg 1 ) > medium ([3.53 ± 2.5] g kg 1 ) > control ([25.42 ± 3.38] g kg 1 ) > low ([23.76 ± 2.98] g kg 1 ) N pplictions. Differences mong tretments were significnt in the rodlef forest, ut not in the other two forest types (Figs. 1 D E). No significnt differences in TOC concentrtions occurred in the deeper (1 2 cm) soil lyers in ll the three forest types (Figs. 1 D F). 3.3 POC response to N-ddition tretment Concentrtions of POC significntly incresed within the top 1 cm soil profile in oth the mixed nd rodlef forests in response to medium- nd high-n dditions (Figs. 1 H I). Averge POC concentrtions within this profile in the pine forest were not significntly different (Fig. 1 G). Eqully, POC concentrtions within the 1 2 cm soil lyers in the rodlef, mixed, nd pine forests were not significntly ffected y N tretments (T. 3). 3.4 ROC nd NROC vrition with soil lyer in different tretments There ws significnt increse in the concentrtions of ROC within the 1 nd 1 2 cm soil profiles of the three forest types in response to N ddition (Figs. 1 J L). In the rodlef forest, ROC concentrtions within the topsoil ( 1 cm) of medium- nd high-n-ddition plots were significntly higher thn in the control tretments (Fig. 1 L). For mixed forests, ROC concentrtions t the 2 cm depth in the low- nd medium-n tretments were significntly higher thn in the controls (Fig. 1 K). Concentrtions of NROC in the top 1 cm soil lyers of control plots in the rodlef forest were significntly higher thn in the pine nd mixed forests (T. 3; Figs. 1 M O). No significnt difference occurred etween pine nd mixed forests. NROC concentrtions within the top 1 cm soil profile responded differently to N ddition in ech forest type (Figs. 1 M O). For the old-growth monsoon evergreen rodlef forest, high-n tretments incresed nonoxidizle orgnic-c concentrtions within the top 1 cm soil profile (Fig. 1 O). For mixed forests, nonoxidizle orgnic-c concentrtions in medium-n tretments were significntly higher thn in low-n nd control tretments (Fig. 1 N). In the 1 2 cm lyer, there were no significnt differences resulting from N tretments in ny of the forests (T. 3). 3.5 Response of MBC to N tretments There ws no significnt difference in MBC mong the three tretments in pine nd mixed forests. For rodlef forest, high-n tretment decresed the MBC significntly (Fig. 2). Microil iomss cron / mg kg Discussion CN LN MN HN MF BF Forest types Effects of N ddition on lile orgnic-c frctions in the topsoil We oserved n increse in POC nd ROC within the top 1 cm soil lyer in ll the three forest types treted with medium- nd high-n ddition. POC nd ROC ccount for the lrgest frction of litter-ltered plnt-derived C, especilly lignin derivtives (Tirol-Pdre nd Ldh, 24; Grndy et l., 28). Their increse my e due to the lower decomposition of litter under medium- nd high-n dditions (Mo et l., Figure 2: Microil iomss C (MBC) t the 1 cm soil lyer under the three N-ddition tretments nd controls. Different letters denote significnt differences (p <.5) etween tretment plots within forest. Error rs re stndrd errors (n = 3). 212 WILEY-VCH Verlg GmH & Co. KGA, Weinheim

6 952 Chen, Li, Mo, Otieno, Tenhunen, Yn, Liu, Zhng J. Plnt Nutr. Soil Sci. 212, 175, ). According to Crreiro et l. (2) nd Sinsugh et l. (22), N sturtion inhiits the ctivities of extrcellulr ligninolytic enzymes, resulting in reduction in the rte of litter decomposition. A previous study (Mo et l., 28) in the rodlef forest showed depressed soil respirtion rtes s result of medium- nd high-n dditions, n indiction tht these levels of N inhiit decomposition, most likely s result of N sturtion. Thus, medium- nd high-n tretments led to excess soil N vilility nd lowering N demnd y the litter decomposers (Mo et l., 28). The high-n tretment ws lso ssocited with incresed soil cidity (Fig. 1), which my lso inhiit microil ctivity (Wldrop et l., 24), potentilly lowering the microil iomss (Fig. 2) nd resulting in POC nd ROC ccumultion in the topsoil. 4.2 Effects of N ddition on nonredily oxidizle orgnic C Nitrogen ddition promoted n ccumultion of NROC in the topsoil s demonstrted y its higher concentrtion in the surfce lyer ( 1 cm) in the rodlef mixed nd pine forests sujected to the medium- or high-n tretment, compred to the controls (Fig. 1). Preferentil sustrte use could e one explntion for these oservtions (Hgedorn et l., 23; DeForest et l., 24). Avilility of lile orgnic C (POC nd ROC) ws enhnced under elevted N deposition, which could led to decresed decomposition of nonlile orgnic C. In our results, the lile frctions were significntly nd positively correlted with the nonlile frction (NROC) (p <.1). Microes in soil cn first use the ctive C frction, decresing their demnd on reclcitrnt orgnic C. The dditionl N (medium-n nd high-n tretments) lso cn e used y microorgnisms, reducing their dependency on older SOM decomposition for N. This, therefore, indirectly slows down the microil ctivities on SOM, inhiiting SOC decomposition. Our results revel tht N ddition incresed soil cidity (Fig. 1), nd NROC nd soil ph were negtively correlted (p <.1). This my e one of the resons for the ccumultion of NROC. Soil cidifiction not only reduces microil ctivity (Berg et l., 1999), ut lso helps soil humic colloids to form ggregtes (Goh et l., 1987). SOC in the ggregte is protected from microil decomposition, thus improving the stility of the nonlile orgnic C. the rodlef-forest soils were lredy N-rich t the eginning of the experiments (Mo et l., 28) nd further medium- nd high-n dditions for 5 yers significntly lowered decomposition of litter, soil respirtion (Mo et l., 26, 28), nd soil microil iomss C, thus fvoring C storge. The pine nd mixed forests, on the other hnd, were still in erly nd middle stges, nd their soil N vilility ws lower thn tht of the rodlef forest (T. 2). This resulted in no significnt effect on SOC storge in pine nd mixed forests under N- ddition tretments. Previous studies hd shown tht more thn 6 y of N ddition my significntly ffect MBC in soils of temperte forests tht re N-deficient (Mo et l., 28). TOC nd its frctions (POC, ROC, NROC) were ll positively correlted (p <.1), indicting tht the effects of N deposition on TOC my e investigted through the frction vritions. However, predicting the effect of N deposition on SOC dynmic needs to consider the long-term mesurement. 5 Conclusions Elevted N deposition incresed the vilility of lile orgnic cron (POC nd ROC) nd stimulted the ccumultion of nonredily oxidizle orgnic C (NROC) in the topsoil of the three forest ecosystems t different succession stges. The 5-yer N-ddition experiment indicted tht the ccumultion of NROC my e process ssocited with n increse in soil TOC in the forest ecosystems in sutropicl S Chin. The old-growth forests demonstrted higher cpcity for SOC ccumultion. Although our results indicte tht incresing tmospheric N deposition my promote SOC-storge cpcity of mture forest ecosystems of sutropicl S Chin, more long-term investigtions re needed in order to vlidte the role of nitrogen deposition on soil C dynmics. Acknowledgments This work ws jointly funded y Ntionl Bsic Reserch Progrm of Chin (29BADC6B7 nd 29CB42111), Knowledge Innovtion Progrm of the Chinese Acdemy of Sciences (Grnts No. KZCX2-YW-GJ1 nd KSCX2-EW-J- 28), nd Gungdong Provincil Nturl Science Foundtion of Chin ( ), nd Chinese Acdemy of Sciences Fellowship for Young Interntionl Scientists (Grnt No. 29Y2AS5). 4.3 Effects of N ddition on soil TOC Elevted N deposition (medium-n nd high-n) incresed the vilility of the lile orgnic C nd enhnced the ccumultion of nonlile orgnic C in the top 1 cm soil profile, which might e process to led to incresed TOC ccumultion in the long term (Hgedorn et l., 23). Significnt differences were oserved mong the forest types, with TOC concentrtion in the surfce lyers ( 1 cm) in the rodlef forest eing higher in the medium-n nd high-n tretments thn in the control (Fig. 1), ut no influence of these tretments in the pine nd mixed forest. The different responses cn e ttriuted to differences in soil N ville in the three forest ecosystems. Compred to the other two forest types, References Aer, J. D., McDowell, W., Ndelhoffer, K., Mgill, A., Berntson, G., Kmke, M., McNulty, S., Currie, W., Rustd, L., Fernndez, I. (1998): Nitrogen sturtion in temperte forest ecosystems. Bioscience 8, Ågren, G. I., Bostt, E. (22): Reconciling differences in predictions of temperture response of soil orgnic mtter. Soil Biol. Biochem. 34, Berg, M. P., Kniese, J. P., Zoomer, R., Verhoef, H. A. (1999): Longterm decomposition of successive orgnic strt in nitrogen sturted Scots pine forest soil. Forest. Ecol. Mnge. 17, Blir, G. J., Lefroy, R. D. B., Lisle, L. (1995): Soil cron frctions sed on their degree of oxidtion, nd the development of 212 WILEY-VCH Verlg GmH & Co. KGA, Weinheim

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