Rapid decrease of soil carbon after abandonment of subtropical paddy fields

Transcription

1 Plnt Soil (2017) 415: DOI /s REGULAR ARTICLE Rpid decrese of soil crbon fter bndonment of subtropicl pddy fields Anlei Chen & Xioli Xie & Tid Ge & Hijun Hou & Wei Wng & Wenxue Wei & Ykov Kuzykov Received: 10 August 2016 /Accepted: 15 December 2016 /Published online: 26 December 2016 # Springer Interntionl Publishing Switzerlnd 2016 Abstrct Bckground nd ims Pddy field bndonment hs become mjor concern in Chin, prticulrly mong trditionl rice-cultivtion regions. Abndonment results in the ltertion of mny processes ffecting C sequestrtion nd turnover, but the finl effects on C stocks remin unknown. Methods To exmine the effects of pddy bndonment on topsoil orgnic C (SOC) content nd stocks, longterm experiment ws performed in subtropicl Chin, exmining n bndoned pddy field with different prebndonment fertiliztion history (from 1991 to 2006 yers) nd SOC grdients (from 19.2 to 22.0 g C kg 1 ). Results Pddy field bndonment significntly reduced the topsoil SOC content nd stock (0 20 cm). Eight yers fter cultivtion cesed, SOC content nd C stock hd decresed by % nd %, respectively, yielding men nnul loss rte of g C kg 1 yr. 1 nd t C h 1 yr. 1, respectively. Soils with higher initil SOC content were more sensitive to bndonment thn soils with low SOC levels. Dissolved orgnic C (DOC) ws more sensitive to bndonment, s evidenced by the fster decrese of DOC thn SOC. The rpid reduction of SOC content, combined with strong decrese in DOC, indictes tht post-bndonment C inputs into the soil were fr lower thn the concurrent SOC minerliztion. The SOC content decreses ws likely becuse of the shift from nerobic to erobic conditions. This chnge leds to fster litter decomposition nd SOC minerliztion, ccompnied with decresing SOC retention or stbiliztion by soil ggregtes, minerl or Fe redox processes. Conclusion Abndonment of pddy soils leds to switch from C sink to C source, resulting in high C losses. The succession of grsses in bndoned fields did not compenste for the losses of soil C stocks. Keywords Abndonment of pddy field. Soil orgnic crbon. Soil crbon stock. Weeds. Subtropicl humid region Responsible Editor: John Six. A. Chen (*): X. Xie : T. Ge : H. Hou : W. Wng : W. Wei Key Lbortory of Agro-ecologicl Processes in Subtropicl Region, Institute of Subtropicl Agriculture, Chinese Acdemy of Sciences, Chngsh, Hunn , Chin e-mil: lchen@is.c.cn Y. Kuzykov Deprtment of Soil Science of Temperte Ecosystem, Deprtment of Agriculturl Soil Science, University of Göttinggen, Göttinggen, Germny Introduction Lnd use chnge hs strongly ffected C cycles both regionlly nd globlly (Ll 2008; Poeplu nd Don 2013). Two reversible processes conversion of nturl vegettion to croplnd nd croplnd bndonment leding to succession of nturl vegettion both chnge soil orgnic crbon (SOC) stocks nd composition, exerting opposing effects on the SOC stocks (Guo nd Gifford 2002). Numerous studies cross climtic zones

2 204 Plnt Soil (2017) 415: hve shown tht croplnd bndonment nd succession of nturl vegettion results in C sequestrtion in soil (Guo nd Gifford 2002; Christopher et l. 2011; Kurgnov et l. 2015). Thus, croplnd bndonment is n effective wy to restore SOC nd reduce CO 2 concentrtion in the tmosphere (Ll 2004; Zhng 2010; Kurgnovetl.2014). Given tht the totl bndoned lnd re worldwide is pproximtely 220 million h (FAO 2013), these soils provide considerble potentil for long-term C sequestrtion. Pddy fields re rtificil wetlnds, with cultivtion under surfce irrigtion during rice production (Gong 1999). Rice (Oryz stiv L.) is the most importnt cerel crop in Chin, representing 22.7% of the totl re hrvested in 2013 (FAO 2013). Pddy cultivtion significntly lters the SOC pools (Pn et l. 2003; Hung nd Sun 2006; Wu 2011). In pddy soils, SOC ccumultion likely occurs becuse of intensive rice production, inputs of high level of plnt residues, coupled with their slowed decomposition under nerobic conditions (Kögel-Knbner et l. 2010). The mechnism underlying SOC stbiliztion is driven by occlusion into ggregtes nd phytoliths, s well s by interction with cly minerls nd iron oxides (Pn et l. 2008; Kögel-Knbner et l. 2010; Wissing et l. 2013). Although the protection of griculturl lnd is persistent priority in Chin, pddy field bndonment is becoming incresingly common (Shi nd Li 2013). This issue lso ffects other prts of Asi, including Jpn (Ygski nd Shirto 2014; Li et l. 2016). The cuses pper to be rpid urbniztion, economic growth, nd lbor shortge resulting from rurl-tourbn migrtion (Peng et l. 2009; Kong 2014; Xiong et l. 2014). For exmple, pddy field re hs declined noticebly from 33 to 29 million h during , with reduction of round 0.5% per yer (Xiong et l. 2014). Additionlly, the country hs seen south-tonorth shift of mjor rice-growing res (Liu et l. 2013; Kong 2014). This predominntly involves bndoning trditionl rice cultivtion regions in subtropicl Chin (Shi nd Li 2013; Xie et l. 2014; Xiong et l. 2014). This bndonment is especilly importnt in the low hilly regions, where pddy fields re spordic nd scttered mong other fields nd lnd use types. Here, bndoned pddy fields exist in ptches, surrounded by cultivted pddy nd non-pddy fields, which is different from the bndonment ptterns in other regions of the world (Rey Benys et l. 2007; Kurgnov et l. 2015). Pddy field bndonment lters the physicl (ggregte formtion/brekge), chemicl (Fe redox), nd biologicl (plnt, microbil ction) processes ffecting C sequestrtion nd turnover (Kögel-Knbner et l. 2010; Wissing et l. 2013). The ltertions re due to lrge shifts in the environment: first, periodic flooding stops; second, mngement prctices (tillge, fertiliztion, nd cultivtion) cese nd nturl vegettion succession occurs; third, the nerobic environment of pddy field trnsforms into n erobic environment. Becuse C input, SOM decomposition, nd SOC stbiliztion mechnisms re ll ffected, the bndoned soil differs considerbly from the originl pddy soil. Importntly, pddy field bndonment is distinct from the bndonment of croplnds tht hd been nd remin under erobic environments (Guo nd Gifford 2002; Christopher et l. 2011; Kurgnov et l. 2015). We hypothesized tht these differences should lso led to differences in the mechnisms of soil C ccumultion in the bndoned pddy fields. The conversion of pddy fields to vegetble fields, uplnd nd other croplnd, or forests leds to net losses in the SOC stock (Cui et l. 2012; Wng et l. 2014; Ygski nd Shirto 2014; Li et l. 2016). Furthermore, the nerobic to erobic conversion ccelertes SOC decomposition nd reduces residue C inputs to the soil. However, such trends re not consistent cross ll the bndoned sites. For exmple, Folgrit et l. (2003) reported tht surfce SOC (0 20 cm) decresed by 38% within 2 yers of bndonment, but others found significnt increse in soil C storge ner the surfce of former pddies (Zhng et l. 2007; Seiji nd Nobuhis 2013). The contrdictory results my be ttributble to the bndonment durtion, pre-bndonment mngement, belowground productivity, climte, nd pre-bndonment SOC (Zhng et l. 2007; Seiji nd Nobuhis 2013; Wng et l. 2014). Chronosequence nd pir pproches hve typiclly been used to investigte the post-bndonment SOC chnges. However, these two pproches re ssocited with uncertinties becuse detils bout historicl lnd use nd pre-bndonment SOC levels tend to be unknown, even with consistent lnd use, nd specific griculturl prctices hve chnged over time (Christopher et l. 2011; Spohn et l. 2015). Strict site selection nd smpling design when pplying the two techniques would enhnce their ccurcy in describing the post-bndonment SOC trends (Christopher et l.

3 Plnt Soil (2017) 415: ). In prticulr, long-term field experiments re idel for strict pir nd chronosequence studies. We, therefore, used long-term field pddy experiment ( , bndoned since 2007) on subtropicl pddy soil to exmine the pre- nd postbndonment trends in topsoil SOC content nd stocks. Three min questions nd hypotheses to ddress them were s follows: (i) How pddy bndonment lters soil C content nd stock? We hypothesized swift reduction in minerl soil C content; (ii) How do pre-bndonment SOC levels nd fertilizer history influence the subsequent C content? We hypothesized tht soils with greter pre-bndonment C levels would be more sensitive to bndonment; (iii) Wht re the min fctors driving the soil C response to bndonment? Mterils nd methods Site description The experimentl bndoned pddy field ws locted in the Toyun Agro-ecology Experimentl Sttion of the Chinese Acdemy of Sciences, Toyun County, Hunn Province (28 55 N, E). This region hs subtropicl humid monsoon climte, with men nnul precipittion of 1450 mm, men nnul temperture of 16.5 C, nd frost-free period of 283 dys. Pddy fields re the dominnt croplnd here. Abndonment experiment We modeled locl bndonment from 1991 to 2014 in pddy fields tht hd been cultivted by frmers for over 100 yers before the experiment begn. The design involved two stges: double rice cultivtion (common in humid subtropicl climte) from 1991 to 2006 nd bndonment from 2007 to 2014 (Fig. 1). During the cultivtion stge, ech field ws dministered one of the following four tretments: Con (control with no fertilizers), NK (N + K fertilizers), NP (N + P fertilizers), or NPK (N, P, K fertilizers). The fertilizers were pplied s ure for N (292.5 kg N h 1 yr. 1 ), clcium superphosphte for P (50.4 kg P h 1 yr. 1 ), nd potssium chloride for K (132.8 kg K h 1 yr. 1 ). Four replictes were estblished using rndomized block design. Ech 30- m 2 plot ws enclosed within cement ridges set up in During cultivtion, the order of one-yer crop rottion ws: 1) first rice, 2) second rice, nd 3) fllow; the crop seson lsted from the end of April to the end of October, with the remining months left for fllowing. During the bndoned stge, no field mngement occurred except the occsionl burning in few plots (replictions 1 nd 4) during 2008 nd The lck of significnt differences in the SOC content cross replictions (p < 0.05) indicted tht burning did not ffect this vrible. Furthermore, the cement ridges prevented most of the soil erosion nd the seling of hole in the cement when ploughing the gurd row nnully lso stopped ny long-term flooding. The bndoned pddy fields were wtered depending on the precipittion, nd were chrcterized by frequent wetting nd dry cycles during the wet seson (from April to July), nd by dryness during the dry seson. Weeds (Psplum psploides, Murdnni triquetr, nd Bidens frondos) dominted the bndoned pddy fields. Their soil ws derived from the quternry red cly. Initil soil smples (0 20 cm, in 1991) contined 16.5 g kg 1 orgnic C, 1.79 g kg 1 totl N, 0.45 g kg 1 totl P, 12.6 g kg 1 totl K, 4.88 mg kg 1 Frmer s pddy field t lest 100 yers Single or double rice Few nutrients Flooding Tillge Cultivtion stge Double rice Fertilizer ppliction Flooding Tillge Abndonment stge Weeds No fertilizer No flooding No tillge Fig. 1 Lnd use nd mngement prctices in experimentl pddy fields (Toyun, Agro-ecology Experimentl Sttion, CAS, Chin). The experimentl site hd been used s double-rice pddy fields by frmers for over 100 yers before the experiment ws Adjcent continued-cultivtion pddy fields ( ) estblished. The experiment included two stges: the double-rice cultivtion stge ( ) nd the bndonment stge ( )

4 206 Plnt Soil (2017) 415: vilble P (Olsen P), nd 59.7 mg kg 1 vilble K (mmonium cette extrctble). The bndoned pddy fields were compred with the djcent, continuously cultivted pddy fields to eliminte the effects of yer-to-yer vrition. These continued-cultivtion fields cultivted the sme rice vrieties s the bndoned fields nd included the sme four fertilizer tretments (Con, NK, PK, nd NPK), but rice ws grown throughout the entire experimentl period ( , nd for more thn 100 yers previously). The SOC content (0 20 cm) ws mesured every four yers from 1990 to The nnul fertilizer ppliction rtes were kg N h 1 ( ) or kg N h 1 ( ), 39.3 kg P h 1 ( ), nd kg K h 1 ( ) or kg K h 1 ( ). Smpling nd nlysis Smpling Soil smples from the surfce lyer (0 20 cm) were collected in April 1991, before the spring plowing, nd t every two yers from 1998 to Ech tested smple ws composite of six or nine subsmples tken t rndom points within single plot. During the bndoned stge, orgnic litter ws removed from the soil surfce before smpling. Soil smples were ir-dried nd pssed through 2-mm sieve before nlyzing the physicl properties. The subsmples were ground through 0.15-mm sieve for chemicl nlysis. The visible pieces of orgnic mteril were removed before nd fter grinding. Three soil columns (20 cm 20 cm re; 20 cm depth, or the depth of the pre- bndonment plow lyer) were collected t rndom points in ech plot to mesure the verticl distribution of weed roots in July nd August of 2012, 2013, nd Litter ws removed from the soil surfce in the bndoned pddy fields before collection. Ech column ws split into 0 5 nd 5 20 cm depths. Only totl root biomss ws mesured due to the difficulty in seprting ded nd live roots. The visible weed roots were wshed free of soil, dried t 75 C, nd weighed to clculte their dry mss. Anlyses left to stnd for 12 h. The ggregte size ws determined with the Mlvern MS2000 prticle nlyzer (Mlvern Compny, UK); volume percentges of mcroggregtes ( μm), smll ggregtes (2 50 μm), nd micro-ggregtes (< 2 μm) were clculted. To determine the soil specific surfce re (SSA), the root residues were crefully removed from the smples by pssing through 2-mm sieve (s described under BSmpling^) nd subjected to nitrogen dsorption t liquid-nitrogen temperture of 196 C in Qudrsorb SI (Quntchrome Compny, USA). The dissolved orgnic crbon (DOC) ws mesured in ir-dried soil smples, which were weighed nd plced in 100-mL polypropylene centrifuge tubes. They were extrcted in 1:10 soil-to-wter mixture for 5 h on n end-over-end shker t n pproximte shking speed of 200 r/min, were subsequently centrifuged for 10 min t 8000 rpm, nd then plced in TOC-Vwp nlyzer (Shimdzu Corportion, Jpn). The soil bulk density (BD) ws mesured using 100- cm 3 soil cores, collected from four rndom replictes in April Soil nd plnt orgnic C content ws determined using the wet digestion method with potssium dichromte (Nelson nd Sommers 1975). The C content of weed roots, rice stubble, nd rice roots ws 41.9, 43.0, nd 36.8%, respectively. Clcultions SOC sequestrtion or loss rte The rte of chnge in the SOC content ws clculted s the difference in the SOC content under given tretment cross the study yers, divided by the durtion (yers), s follows: ΔC rte ¼ ΔC cont = ði 2 i 1 Þ ð1þ where ΔC rte is the rte of chnge in SOC content (g C kg yr. 1 ), nd i is the time of mesurements (yers), nd ΔC cont is the difference in SOC content from yer i 2 to yer i 1. Soil C storge nd chnge rte To determine the soil ggregte size, the soil smples were dispersed by soking in (NPO 3 ) 6 for 2 h nd ultrsonic shking for 15 min; the smples were then Surfce soil C storge ws clculted s follows: C s ¼ conci B d H ð2þ

5 Plnt Soil (2017) 415: where C s is the surfce soil C storge (t h 1 ), conci is the SOC content (g kg 1 ), B d is the soil bulk density (t m 3 ), nd H is the soil thickness (0.20 m). In this study, we ssumed tht the bulk density ws stble throughout the bndonment stge becuse drstic soil disturbnce did not occur during this time. Therefore, B d mesured during April 2014 (1.01 t m 3 ) ws used to clculte C storge in ΔC s rte ¼ ΔC s = ði 2 i 1 Þ ð3þ where ΔC s rte is the rte of chnge in SOC storge (t h 1 yr. 1 ), i is the time of mesurements (yers), nd ΔC s is the difference in SOC storge from yer i 2 to yer i 1. Totl C stock of surfce lyer Totl C storge (C totl ) of the surfce lyer (including plnt root C nd SOC) (t h 1 ) ws clculted s follows: C totl ¼ C s þ C residue ð4þ where C s is the surfce soil C storge (t h 1 )nd C residue is the plnt biomss C (t h 1 ) remining in the surfce soil lyer (0 20 cm). Before bndonment, C residue included C from rice roots nd stubble (plowed into the soil yerly), but post-bndonment C residue only comprised weed root C. C from litter boveground (except for DOC) ws ssumed not to hve entered the soil becuse the decomposition ws fst nd no tillge occurred. The verge nnul rice yields ( ) were 5318, 6387, 6690, nd 8948 kg h 1 for Con, NK, NP, nd NPK tretments, respectively. In the djcent cultivted pddy fields, the verge rtios of rice stubble nd roots to rice yields were 27.6 nd 23.3%, respectively. Becuse negligible vribility exists in the reltionship between rice yield nd crop residue production (Evns nd Fischer 1999), rice yields were used s the bsis of clculting rice root nd stubble C. Sttisticl nlysis Dt were tested with one-wy nlysis of vrince (ANOVA) in SAS 6.1 (SAS Institute Inc. Cry, USA). Significnt differences between the mens were evluted t the 95% confidence intervl using Duncn s multiple rnge test (p < 0.05). Grphs were prepred in Origin 7.5 (Originlb, Northmpton, USA). The reltionships between the vribles were evluted using correltion nlysis. Results Soil orgnic crbon dynmics before nd fter bndonment During the period of cultivtion from 1991 to 2006, SOC incresed from the initil vlues by % in ll the tretments (Fig. 2). The pddy field bndonment led to significnt decrese in the SOC content (p < 0.05, Fig. 2). From 2006 to 2014, the SOC content decresed by %. The post-bndonment SOC loss rtes were clerly fster thn the ccumultion rtes during cultivtion. The Fig. 2 Dynmics of SOC (± SE) content in experimentl bndoned pddy fields () nd continued-cultivtion fields (b). The former contins two stges: the cultivtion stge ( ) nd the bndonment stge ( ). Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers (ll fertiliztion occurred during the cultivtion stge). During bndonment, Con, NK, NP, nd NPK represent the fertiliztion history during the cultivtion stge SOC content (g kg -1 ) b Cultivtion of double rice CK NK NP NPK Abndonment Continued-cultivtion Yer

6 208 Plnt Soil (2017) 415: control soil (without fertilizer) incresed the SOC contents (to 0.18 g kg 1 yr. 1 ) during the cultivtion period, wheres fertiliztion dditionlly incresed the sequestrtion rte (by g kg 1 yr. 1 ). During bndonment ( ), the SOC content decresed shrply t rtes of g kg 1 yr. 1. The highest SOC loss rte ( g kg 1 yr. 1 ) occurred 4 6 yers post-bndonment in previously fertilized soil, nd then the loss ws reduced to g kg 1 yr. 1 fter 6 8 yers of bndonment. In unfertilized control soil, SOC ccumulted t 0.11 g kg 1 yr. 1 from 2012 to 2014 (Tble 1). Soil DOC content(mg kg -1 ) b b b b b c c c c Con NK NP NPK Chnge in dissolved orgnic crbon The pddy bndonment significntly decresed the DOC concentrtions (Fig. 3) from mg kg 1 in the cultivted soil in 1998 nd 2006 to mg kg 1 in the bndoned soil in In 2006 nd 2014, the DOC concentrtion nd the SOC content were significntly correlted (p < 0.01, Tble 2). However, eight yers post-bndonment, DOC decresed much fster ( %) thn the totl SOC. On verge, the DOC proportions in SOC decresed post-bndonment, from 0.3% in 2006 to 0.1% in Chnge of soil C stocks Tretment Fig. 3 Dynmics of soil DOC (dissolved orgnic C) content (± SE). Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers (during the cultivtion stge). During the bndonment stge, Con, NK, NP, nd NPK represent the fertiliztion history. Lowercse letters indicte significnt differences t p < 0.05 cross ll tretments in 1998, 2006, nd 2014 The pddy soil bndonment significntly decresed the soil C stock (Fig. 4) by % in 2014 from the mount in In generl, the post-bndonment rte of C storge loss rnged from 0.50 to 1.15 t C h 1 yr. 1. We ssumed tht the soil bulk density would not chnge during bndonment. This ssumption ws supported by constnt soil bulk density in the continuedcultivtion pddy fields between the four tretments during 2006 nd 2014 (p > 0.05; dt not published). In these fields, the men bulk density during 2006 ws 1.04 t m 3, nerly identicl to the men in the bndoned pddy fields during Therefore, C storge in the bndoned pddy fields underwent similr decreses s the SOC content (Tble 1). The gretest losses occurred 4 6 yers post-bndonment ( ) with rtes of t h 1 yr. 1, which were much higher thn the ( t h 1 yr. 1 ) nd rtes ( t h 1 yr. 1 ). Likewise, soil C storge in 2014 did not differ cross fertiliztion histories (p > 0.05), despite long-term fertiliztion prctices (NK, NP, or NPK, or Con) exerting n effect on prebndonment C storge mounts (Fig. 4). Tble 1 SOC content (± SE), ccumultion, nd loss rte. Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers during cultivtion of double rice; post-bndonment Con, NK, NP, NPK represent the fertiliztion history Tre. SOC content (g C kg 1 ) SOC sequestrtion rte (g C kg 1 yr. 1 ) Rte of SOC chnge fter bndonment (g C kg 1 yr. 1 ) Con 17.2 ± 0.6e 19.2 ± 0.7 cd 17.3 ± 0.3e NK 17.6 ± 0.3e 20.3 ± 0.3bc 17.7 ± 0.5e NP 17.6 ± 0.4e 22.0 ± ± 0.6e NPK 18.0 ± 0.3de 21.2 ± 0.5b 17.6 ± 0.4e Lower-cse letters indicte significnt differences in SOC t p <0.05

7 Plnt Soil (2017) 415: Tble 2 Correltions mong mcro-ggregte, smll ggregte, micro-ggregte, SOC (soil orgnic C), nd DOC (dissolved orgnic C) in 2006 nd 2014 DOC Micro-ggregte Smll ggregte Mcro-ggregte SOC 0.896** ** 0.973** DOC ** 0.960** Micro-ggregte Smll ggregte ** ** indicte significnt differences of t p < 0.05 nd p < 0.01, respectively. Micro-ggregte: dimeter < 2 μm; Smll ggregte: dimeter 2 50 μm; Mcro-ggregte: dimeter μm Chnge in soil ggregte nd soil specific surfce re During rice cultivtion, the volume percentge of mcro-ggregtes or smll ggregtes in the totl ggregtes ws generlly stble cross ll the tretments nd yers (1998 nd 2006) (Tble 3). The volume percentge of micro-ggregtes significntly decresed (p < 0.05) from 1998 to 2006 for ll the tretments except control (Tble 3). Pddy field bndonment significntly decresed the mcro-ggregte percentge by 36.3%, 8 yers post-bndonment, nd the mcroggregtes were broken into smll- nd micro-ggregtes, strongly incresing the percentge of smllggregtes (p < 0.05, Tble 3). The proportion of mcro-ggregtes decresed similrly to the decrese in SOC content (p < 0.01, Tble 2). Consequently, the Soil C stock of surfce lyer (0-20 cm) (t h -1 ) c d bc d Con NK NP NPK Tretment Fig. 4 C stock (± SE) of the surfce soil lyer (0 20 cm) pre- nd post-bndonment. Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers (during the cultivtion stge). During the bndonment stge, Con, NK, NP, nd NPK represent the fertiliztion history under cultivtion. Lowercse letters indicte significnt differences t p < 0.05 cross ll tretments before (in 2006) nd fter (2014) bndonment d b d percentge of smll- nd micro-ggregtes incresed with the decrese in SOC (Tble 2). The brekge of mcro-ggregtes into smll- nd micro-ggregtes incresed the soil specific surfce re (SSA). The SSA in 2014 ws % greter thn in 2006, significnt chnge for soils with NP nd NPK history (p < 0.05, Fig. 5). Significnt positive correltions were found between SSA nd micro-ggregte volume percentge (r 2 =0.347,p <0.05,n =12).Additionlly, significnt negtive correltions were present between the SSA chnge nd volume percentge of smll- nd micro-ggregtes (p < 0.05 nd p < 0.01, respectively; dt not shown). The decresing SOC concentrtions my hve led to the wekening linkge cross the ggregtes, which in turn incresed the volume percentge of smller ggregtes. Significnt negtive correltions were found between SSA nd SOC (r 2 =0.379,p <0.05,n =12). Totl C stocks Compring the totl pre- nd post-bndonment C stocks (including minerl nd root C stock) indictes post-bndonment conversion of the soil from C sink to C source, becuse grss succession in bndoned fields contributes to the roots C. During , the verge root C in the bndoned pddy soil ws clerly higher thn before bndonment (Tble 4). However, 8 yers post-bndonment, weed root C did not compenste for the decreses in totl C stocks, including the decrese in soil C stocks below the O (litter) horizon. Thus, the totl C stocks in 2014 ws % lower thn the mount in 2006 (Tble 4). Overll, the totl C stock in the plow (Ah) horizon ment tht the pddy soil switched from C sink to C source, post-bndonment. This ws confirmed by the fct tht even greter

8 210 Plnt Soil (2017) 415: Tble 3 Volume percent (± SE) of soil ggregtion. Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers during cultivtion of double rice. After bndonment Con, NK, NP, NPK represent the fertiliztion history Tre. Volume of micro-ggregte (%) Volume of smll-ggregte (%) Volume of mcro-ggregte (%) Con 13.8 ± 0.3b 13.8 ± 0.8b 12.3 ± 0.6bc 65.9 ± 1.6c 69.1 ± 0.7b 75.3 ± ± 1.7b 17.2 ± 0.8b 12.5 ± 0.6c NK 13.5 ± 0.7b 11.5 ± 0.7c 12.1 ± 0.5bc 66.2 ± 1.2bc 68.3 ± 1.6bc 75.4 ± ± 1.7b 20.2 ± 1.5b 12.3 ± 1.3c NP 14.2 ± ± 0.9c 12.3 ± 0.3bc 66.6 ± 1.0bc 67.6 ± 0.5bc 75.9 ± ± 1.7b 21.0 ± ± 0.4c NPK 14.1 ± ± 0.5c 12.3 ± 0.3bc 66.7 ± 0.2bc 68.2 ± 0.9bc 74.7 ± ± 0.3b 20.1 ± 0.6b 13.0 ± 0.7c Different lowercse letters indicte significnt differences t p < Micro-ggregte: dimeter < 2 μm; Smll ggregte: dimeter 2 50 μm; Mcro-ggregte: dimeter μm losses of totl C stocks were found in soils with lrger pre-bndonment SOC (Tble 4). Discussion SOC ccumultion SOC ccumultion in pddies hs been lrgely ttributed to n increse in rice biomss production over the lst few decdes, which returns greter mount of root nd stubble residues to the soil (Hung nd Sun 2006; Sun et l. 2009; Wu2011). In the pst four decdes, rice yield in Chin hs more thn doubled: 3000 kg h 1 in 1970, 4300 kg h 1 in 1981, nd 6500 kg h 1 in 2010 (Grssini et l. 2013; Xiong et l. 2014). The incresed yield is likely due to the development of high-yield The soil specific surfce re (m 2 g -1 ) b b Con NK NP NPK Tretment Fig. 5 Dynmics of soil specific surfce re (± SE). Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers during the cultivtion. During the bndonment stge, Con, NK, NP, nd NPK represent the fertiliztion history. Lowercse letters indicte significnt differences t p < 0.05 cross ll tretments in 1998, 2006, nd 2014 b b vrieties nd improved crop mngement prctices, such s the use of minerl fertilizers, orgnic fertilizer (e.g. mnure) ppliction, nd irrigtion (Hung nd Sun 2006; Sun et l. 2009; Wu 2011). Higher crop productivity results in greter orgnic C input into the soil, dding to the sequestrted C in pddy systems (Liu et l. 2014b). These fctors could explin why C sequestrtion hs stedily incresed in subtropicl Chin recently, even in pddy soils with long irrigtion histories. SOC incresed fter the pddy fields were converted to our experimentl fields in 1991 (Fig. 2), pttern we lso ttributed to improvements in the locl (Hunn province) rice vrieties nd crop mngement. Hunn s men rice yield incresed from 2300 kg h 1 during to 3900 kg h 1 during Fertilizer ppliction lso rose mrkedly during this period: wheres there ws no ppliction in , it ws 13 kg h 1 in 1954 nd 270 kg h 1 in 1983 (Yerbook of Hunn Sttistics 1983). After our experimentl setup, we chnged the rice to n improved vriety every two or three yers from 1991 to 2006, in keeping with locl frming prctices. This procedure could explin the stedy incresed in C sequestrtion, despite century-long history of the pddy fields. Response of SOC content nd stock to bndonment Totl SOC content decresed quickly fter pddy field bndonment (Fig. 2), in support of our first hypothesis. The pre-bndonment SOC level ffected the rte of SOC loss in the bndoned pddy fields: the SOC response to bndonment ws more pronounced for soils with higher initil SOC contents. Moreover, soils with different fertilizer histories did not significntly vry in their SOC eight yers post-bndonment. Both

9 Plnt Soil (2017) 415: Tble 4 C storge (± SE) in the surfce soil lyer (0 20 cm) before nd fter bndonment. Con: no fertilizer; NK: N, K fertilizers; NP: N, P fertilizers; NPK: N, P, K fertilizers during cultivtion of double rice. After bndonment Con, NK, NP, NPK represent the fertiliztion history Tre. C storge in 2006(t h 1 ) C storge in 2014(t h 1 ) Difference from 2006 to 2014 (%) Residue C Soil C totl root C (0 5 cm) Root C (5 20 cm) Soil C totl Con 1.2 ± 0.0c 38.9 ± 1.4b 40.0 ± 1.4b 3.4 ± ± ± ± NK 1.3 ± 0.0b 41.0 ± 0.9b 42.3 ± 0.9b 4.2 ± ± ± ± NP 1.4 ± 0.0b 44.4 ± ± ± ± ± ± NPK 1.8 ± ± ± ± ± ± ± C storge of surfce lyer in 2006 included the minerl soil nd rice residue (stubble nd roots). In 2014, C storge included minerl soil nd weed roots. Different lower-cse letters indicte significnt differences t p <0.05 these results support our second hypothesis, stting tht pre-bndonment SOC nd fertilizer history would ffect the C loss rtes. The rpid SOC reduction ws primrily due to the pddy field bndonment, nd not becuse of other fctors, such s climte. Specificlly, the continuously cultivted pddy fields exhibited similr SOC ccumultion trends s the bndoned pddy fields during the rice cultivtion stge in the ltter. Moreover, SOC generlly remined stble in continuedcultivtion pddy fields even lter in the experiment, contrsting the rpid decrese in the bndoned pddy fields. Typiclly, SOC rpidly decreses when nturl vegettion is converted to croplnd, wheres croplnd bndonment to nturl vegettion enhnces the SOC ccumultion (Kurgnov et l. 2015). However, this stndrd pttern is reversed under pddy field bndonment or conversion to other lnd use types. Both the present nd previous studies (Cui et l. 2012; Wng et l.2014) hve found tht pddy bndonment (or conversion) rpidly decreses the SOC content. This decrese my be ttributble to the fct tht pddy fields in the humid res lredy contin SOC levels ner the mximum s compred to the other lnd use in the sme region (Wu 2011). Hence, the post-bndonment or post-conversion SOC in the pddy fields re expected to decrese in the humid regions. We note, however, tht pddy mngement does not necessrily enhnce the SOC sequestrtion due to the differences in residue mngement (OM inputs) between the pddy nd non-pddy sites (Winkler et l. 2016). This observtion suggests tht SOC chnge fter the pddy field bndonment might be diverse, minly becuse of the vrition in soil types, initil SOC level, bndonment durtion, nd locl mngement under distinct climtic conditions. Drivers of SOC reduction fter pddy bndonment Pddy bndonment ws the min driver of soil C loss, which ws cused by the nerobic-to-erobic conversion of soil nd subsequent differences from pddy soil in physicl, chemicl, nd biologicl processes (Kögel- Knbner et l. 2010). Abndonment significntly ltered the physicl processes in the former pddy fields, specificlly relted to the shift from periodic nerobic nd erobic conditions to the yer-round erobic conditions. Repeted puddling (tillge) over mny yers reduces the number of smll corse- nd meso-pores in soil, while incresing the number of micropores (Eickhorst nd Tippkötter 2009). After bndonment, the lck of flooding nd tillge leds to the increse of soil O 2 density through mcropore formtion by ded roots, soil orgnism (e.g., erthworm) ctivities, nd surfce-lyer crcking. Moreover, the nerobic-to-erobic conversion cuses the destruction of mcro-ggregtes nd increses in the specific surfce re of soil, both of which my further enhnce the oxidtion potentil. Thus, the reversl of physicl processes fter pddy bndonment fvors SOC decomposition. Post-bndonment physicl processes lso significntly ffect the chemicl processes, specific to the redox stte (Thompson et l. 2006; Kögel-Knbner et l. 2010). Retention of dissolved orgnic mtter (DOM) on minerl phse nd subsequent stbiliztion ginst the microbil decy lrgely depend on the redox stte. High concentrtions of DOM in post-flooding pddy soils enhnce the chnges nd the relese of structurl iron in cly minerls, nd support the formtion of ferrihydrite (Kögel-Knbner et l. 2010). DOM stbiliztion in pddy soils occurs when precipitted by ferrihydrite (Fe 3+ ). These Fe oxyhydroxides re less susceptible to nerobic

10 212 Plnt Soil (2017) 415: dissolution, mintining their cpcity to retin nd stbilize the DOM (Kögel-Knbner et l. 2010). After pddy bndonment nd incresed erobic conditions, the decrese in the relese of structurl iron in cly minerls nd the formtion of ferrihydrite, s well s dehydrtion of ferrihydrite weken the DOM retention nd stbiliztion. Becuse red soil in subtropicl Chin is iron-rich, the post-bndonment nerobic-to-erobic conversion leds to Fe 3+ becoming the most importnt element (fter oxygen) in the former pddy soils for oxidizing the orgnic mtter (Yo et l. 1999). The finl fctor ffecting C sequestrtion in the bndoned pddy soil ws the chnge of biologicl processes. Most C inputs into the bndoned pddy soil were from weed roots; very little boveground litter entered the soil due to the lck of tillge nd fst decomposition cused by high temperture, humidity, nd erobic environment. The greter mounts of weed-root C nd lower SOC content in the bndoned fields suggest tht the conversion rte of weed-root C to SOC ws fr lower thn the C decomposition rte, even tking into ccount the fct tht weed-root C ws higher thn the rice residue C. Under similr climtic conditions nd the sme soil types, the conversion rtio of the totl strw-c input to SOC ws lower in n uplnd (6.6% of strw-c conversion to SOC) nd pddy-uplnd rottion tril (6.5%) thn in flooded pddy (9.1%) (Liu et l. 2014). In the present study, over 75% of weed-root C ws present in the top 5 cm of soil, llowing for ccelerted decomposition of ded roots nd mintennce of low C conversion rtio. Additionlly, the decresing C/N rtio of litter input to soil, from 69 in the rice residue to 50 in the weeds roots (dt not shown), lso enhnced the SOC nd OM decomposition. In summry, chnges to ll the three process groups under erobic conditions ffected the blnce between C input nd output through dmpening of the SOC stbiliztion nd enhncement of the SOC decomposition. Compring with the other lnd use types, the pddy SOC is more lbile nd vulnerble to relese under erobic conditions due to high DOC content nd microbil biomss (Cui et l. 2012). Generlly, the incresed oxidtion potentil, post-bndonment, enhnces the SOC decomposition; in this study, we demonstrted tht the lbile SOC ws indeed more sensitive to bndonment, given the fster decrese of DOC compred to SOC. The greter ggregte stbility nd the enrichment of C in the mcro-ggregtes re lso the mjor resons for higher C sequestrtion cpcity of the pddy soil (Pn et l. 2008; Wissing et l. 2013). Residue retention increses the ggregtion, which depends on the residue quntity nd qulity (Chivenge et l. 2011). Under erobic conditions, coupled with incresed weed-root C litter, the mcro-ggregtes were destroyed nd the ggregte stbility ws wekened. This pttern suggests tht both the decresed SOC content nd input could drive mcro-ggregte destruction nd the reduction of mcro-ggregte formtion. Becuse broken mcroggregtes form smller ggregtes, the destruction lso ccelertes the decomposition of encpsulted prticulte orgnics by exposing the smll ggregtes to incresed O 2 content (or prtil pressure). This conclusion is demonstrted by post-conversion C decrese in pddy, which ws minly ttributed to C loss from the brekdown of wter-stble mcro-ggregtes (Wng et l. 2014). However, becuse mcro-ggregte destruction lowers the SOC, nd decresed SOC cn led to reduced mcro-ggregte formtion, it is still uncler which of these fctors drive C loss nd which is n outcome of C loss during both the interctions. In generl, incresed decomposition rther thn decresing orgnic C input ws the likely driver of SOC reduction in the bndoned pddy soils, ided by the decresing SOC interction with cly minerls nd iron oxyhydroxides. These chnges were due to the nerobic-to-erobic shift fter the bndonment, which cused ltertions in the mjor physicl, chemicl, nd biologicl processes. In contrst, croplnd bndonment in rid nd semirid regions re ssocited with SOC increse, phenomenon driven by decresed SOC decomposition, reduction of soil erosion, incresing root C input, nd enhnced SOC stbiliztion in ggregtes (Guo nd Gifford 2002; Christopher et l. 2011; Novr et l. 2014). Conclusions Pddy field bndonment in humid subtropicl climte rpidly decresed the SOC content by % of its initil content within eight yers of cesing the cultivtion. The men nnul rte of SOC content nd C stock losses were g C kg 1 yr. 1 nd t C h 1 yr. 1, respectively. The rte of SOC loss ws times higher thn the C sequestrtion rte during cultivtion. The DOC decresed much fster ( %) thn the SOC.

11 Plnt Soil (2017) 415: The rpid decrese in C stock ws minly cused by the nerobic-to-erobic conversion, which incresed the decomposition nd reduced the SOC input. This chnge lso reduced the interctions of SOC with cly minerls nd iron oxyhydroxides, s well s decresed the SOC stbiliztion within the soil ggregtes. Pddy field bndonment crries high risk of soil C loss, nd soils with lrger initil SOC content re more sensitive to bndonment. Weeds estblished on bndoned fields do not compenste for the decrese in SOC, resulting in pddy fields shifting from C sink to C source. Therefore, the continued cultivtion of pddy fields with high soil C sequestrtion is criticl for mintining the SOC stocks. Acknowledgements We thnk Dr. Pul Hill of Bngor University nd Prof. Ron McLren of Lincoln University for providing input on n erlier drft of this mnuscript. 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