Carbon and nitrogen pools in different aggregates of a Chinese Mollisol as influenced by long-term fertilization

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1 DOI /s SOILS, SEC 2 GLOBAL CHANGE, ENVIRON RISK ASSESS, SUSTAINABLE LAND USE RESEARCH ARTICLE Crbon nd nitrogen pools in different ggregtes of Chinese Mollisol s influened by long-term fertiliztion Ying Chen & Xudong Zhng & Hongbo He & Hongtu Xie & Ying Yn & Ping Zhu & Jun Ren & Lihun Wng Reeived: 3 Mrh 2009 /Aepted: 10 July 2009 # Springer-Verlg 2009 Abstrt Purpose It is known tht soil orgni mtter (SOM) dynmis re sensitive to fertiliztions, but it is different from soil to soil. It is unler how the long-term pplitions of orgni mnure nd minerl fertilizers impt the umultion nd distribution of soil orgni rbon (SOC) nd totl nitrogen (TN) espeilly in soil ggregte frtions of Chinese Mollisols, whih hve been intensively ultivted for dedes under mize monoropping nd onventionl tillge wys. Thereby, the reserh of this kind is very importnt for the sustinble use of griulturl lnd in Chin, where lnd resoures re extremely limited for its huge popultion. The objetives of this study were to identify how the long-term fertiliztion tretments would ffet the ggregte, SOC nd TN distribution pttern in the Chinese Mollisol, nd how soil ggregtion ontribute to the storge nd stbiliztion proess of SOC nd TN. Responsible editor: Chengrong Chen Y. Chen : X. Zhng (*) : H. He : H. Xie : Y. Yn Key Lbortory of Terrestril Eologil Proess, Institute of Applied Eology, Chinese Ademy of Sienes, Shenyng , Chin e-mil: xdzhng@ie..n X. Zhng Ntionl Field Reserh Sttion of Shenyng Agroeosystems, Chinese Ademy of Sienes, Shenyng , Chin Y. Chen Lioning Rie Reserh Institute, Shenyng , Chin P. Zhu : J. Ren : L. Wng Jilin Ademy of Agriulturl Sienes, Gongzhuling , Chin Mterils nd methods Soils were smpled in April 2005 from long-term experiment site estblished in Six tretments were seleted s follows: CK (zero fertilizer), NPK (minerl fertilizers), M1 (orgni mnure t lower level), M1NPK (ombintion of M1 nd NPK), M2 (orgni mnure t higher level) nd M2NPK (ombintion of M2 nd NPK). Four ggregte frtions, i.e., lrge mroggregte (>2,000 µm), smll mroggregte (250 2,000 µm), miroggregte ( µm) nd silt + ly frtion (<53 µm), respetively, were obtined by wet sieving. SOC nd TN in bulk soils nd ggregtes were nlyzed using C/N/H/S-nlyzer (Vrio ElementrIII, Germny). Results nd disussion The smll mroggregte ws the most bundnt omponent in the soils, ounting for % of the dry soil mss nd bout 50% of SOC nd TN. The smll mroggregte proportion deresed while lrge mroggregte perentge inresed signifintly fter orgni mnure tretment. However, minerl fertilizers pplition enlrged the proportion of miroggregte nd silt + ly frtion. Consequently, SOC nd TN ontents in ll the four ggregtes inresed remrkbly with orgni mnure pplition, wheres mere minerl fertilizers ddition did not ffet SOC nd TN ontent signifintly. The ombintion of NPK with the higher level of orgni mnure ws more effetive in improving SOC nd TN umultion thn orgni mnure pplition lone. Moreover, orgni mnure pplition tended to derese C/N rtios, in generl, nd higher C/N rtios in >53 µm frtions thn in others, suggesting tht SOC in the lrger frtions is reltively fresh or little ltered by miroorgnisms, wheres, the SOC ssoited with <53 µm frtion is more deomposed nd reltive ged. However, the higher enrihment ftor for N (E N ) thn for C (E C ) in <53 µm frtion indites preferentil depletion of lbile SOC nd reltive umultion of stble SOC pools in the finest frtion.

2 Conlusions The long-term pplition of orgni mnure generlly filitted ggregtion proesses nd then improved the quntity nd qulity of SOM in the Mollisol, wheres the long-term minerl fertilizers pplition generlly resulted in the de-ggregtion effet. The ombintion of orgni mnure nd minerl fertilizers ws better wy to further enhne the sequestrtion of SOC nd improve SOM qulity by ggregtion ompred to the pure orgni mnure. It is onluded tht when inresing orgni input, physil protetion plys mjor role for the sequestrtion of the inresed SOC in the Chinese Mollisol. However, when SOC is depleted under ertin mngement prtie, it will hppen preferentilly in the physil proteted SOC in lrge ggregtes. This study hs gret implitions for better understnding SOC nd N dynmis s ffeted by long-term fertiliztion. Keywords Aggregte. Fertiliztion. Mollisol. Nitrogen. Soil orgni rbon 1 Introdution Mollisols in Chin re minly distributed in the northestern region, where lmost ll the Mollisols hve been ultivted for food prodution sine the soils, by nture, were fertile nd produtive; hene, they ply very importnt role in food supply in Chin (Xing et l. 2004). Sine the relmtion of the Mollisols lnd, espeilly in the lst 50 yers, the soils hve been intensively used in n improper wy, i.e., Chinese onventionl ultivtion system (Yng et l. 2003). With this ultivtion system, ll the bove-ground rop residues re removed from the lnd fter hrvest nd used for heting nd ooking beuse rop residues were lmost the unique energy soure in the ountryside of the Mollisols region. Consequently, the qulity of the Mollisols hs being deteriorted due to the hroni output of soil orgni mtter (SOM). Deline in SOM resulted in poor soil struture, whih, in turn, led to losses of wter nd nutrients by runoff (Liu et l. 2003). This hs beome serious problem in the region in terms of sustinbility. Studies hve shown tht SOM n be restored by inresing orgni residue input (Aoym et l. 1999; Blir et l. 2006; Jones et l. 2006; Rudrpp et l. 2006) nd lso mngement is very ruil for SOM dynmis, but we still do not know how the long-term pplition of inorgni nd/or orgni fertilizers ffets SOM umultion in the Chinese Mollisols. It is known tht soil struture is key ftor in the funtioning of soil, beuse it medites mny biologil nd physil proesses in soils (Six et l. 2000; Bronik nd Ll 2005) nd the formtion of soil struture is gretly relted to soil ggregtion proesses. Reently, the physil frtiontion hs been widely pplied to evlute the effet of different griulturl mngement prties on soil orgni rbon (SOC) storge nd stbility. Physil frtiontion tehniques, espeilly the less destrutive nd more seletive ggregte frtiontion one, re diretly relted to the struture nd funtion of SOM in situ (Christensen 1992; Cmbrdell nd Elliott 1994). Indeed, the inrese in SOM nd the formtion of soil ggregte re the two mutully relted proesses: SOM filittes the ggregtion ting s binding gents, nd on the other hnd, ggregtes n protet SOM from minerliztion physilly (Tisdll nd Odes 1982; Elliott 1986; Bere et l. 1994). Thereby, it is importnt to investigte the ggregte pools of SOC nd totl nitrogen (TN) s influened by long-term fertiliztion. Reserh hs shown tht ggregte stbility nd distribution re sensitive to mngement prties (Pinheiro et l. 2004). Tillge n derese ggregte stbility prlleled by loss of SOM (Elliott 1986; Bere et l. 1994; Six et l. 2000b; Pinheiro et l. 2004; Mdri et l. 2005), while notillge led to greter proportion of mroggregtes nd inresed C nd N sequestrtion (Wright nd Hons 2005). Fertiliztion, s nother most ommon mngement prtie, n lso lter the ggregte distribution, nd, in turn, ffet the qulity nd quntity of SOM signifintly (Aoym et l. 1999; Mnn et l. 2006). Rudrpp et l. (2006) found tht blned fertiliztion (NPK) is more effetive on enhning C umultion in soil thn unblned fertiliztion (N or NP). The inrese in SOM indued by orgni mnure pplition is observed primrily in mroggregte (Aoym et l. 1999; Su et l. 2006), wheres, Kong et l. (2005) found tht mjority of the umultion of SOC derived from dditionl orgni C inputs is preferentilly sequestered in the miroggregteswithin-smll-mroggregtes. The ombined pplition of NPK nd frmyrd mnure (FYM) is the most effiient in sequestering orgni C due to intertive effets (Mnn et l. 2006; Rudrpp et l. 2006) nd enhning the formtion nd stbility of mroggregtes (Su et l. 2006; Hti et l. 2007). However, beuse of the different limte, unique ropping, nd mngement system in our Gongzhuling site, the umultion pttern of SOC nd TN in ggregte pools would be different from those of others; hene, it is still interesting to ondut study of this kind. Nevertheless, SOC nd TN umultion nd dynmis re long proess; ny onvinible evidene n surely be obtined by long-term experiments. Our objetives were therefore to identify how the long-term fertiliztion tretments would ffet the ggregte distribution pttern in the Chinese Mollisol, how SOC nd TN ould be sequestered or depleted in different lss ggregtes, nd wht would be the role of soil ggregtion on the storge nd stbiliztion proess of SOC nd TN. This study hs gret implitions for the sustinble use of the Chinese Mollisols.

3 2 Mterils nd methods 2.1 Site nd soil Our study ws onduted on the bse of long-term fieldloted experiment of fertiliztion estblished in 1979 in Gongzhuling, Jilin Provine, Chin ( E, N) nd the experiment hs been mnged by the Institute of Soil nd Fertilizer, Jilin Ademy of Agriulturl Sienes. There, the men nnul preipittion rnged from 500 to 650 mm, with bout 70% flling in June, July, nd August. It experienes four distint sesons, with old winter nd wrm summer. The hottest months re June, July, nd August (20 23 C) nd the oldest is Jnury ( 15 C). The ly lom soil ws lssified s Mollisol (Typi Hpludoll; Soil Survey Stff 2003). The long-term experiment is omposed of three sets of fertilizer tretments, inluding minerl fertilizers, orgni mnure, nd different ombintions of minerl fertilizers nd orgni mnure. The experiment onsists of 24 fertiliztion tretments nd eh tretment plot overs 400 m 2. The plots were rrnged rndomly without replition on the sme lnd (similr to the design of the Brodblk in the Rothmsted; Yng et l. 2003), nd the design ws modified by dividing eh plot in three subplots in 1998 in order to run sttisti nlysis. Before the setup of the long-term experiment, the lnd hd been intensively ultivted for t lest 50 yers (ontinuous orn for bout 20 yers), but the experimentl field hd been homogenized by orn ropping for 3 yers without fertiliztion. Therefter, min soil hemil nd physil properties, espeilly SOC nd TN, were not signifintly different in ll plots. Mize (Ze mys L.) monoulture is pplied ll yers seeding in lte April nd hrvesting in lte September. Above-ground residues re removed fter hrvesting. We seleted the following six tretments: () CK, (b) NPK, () M1, (d) M1NPK, (e) M2, nd (f) M2NPK, respetively. CK indites no fertilizer; M1 nd M2 men pig mnure pplied t the rte of 4.5 nd 9.0 t orgni mtter h 1 yr 1, respetively; NPK re hemil N (ure), P 2 O 5 (multiple superphosphte), nd K 2 O (potssium sulfte) fertilizers dded t the rte of 165, 82.5, nd 82.5 kg h 1 yer 1, respetively. Three omposite surfe soil smples (0 20 m) from sub-plots of eh tretment plot were olleted in April Soil properties re shown in Tble 1. Afterwrds, the field-moist soil smples were gently broken prt nd sieved to pss through 5-mm sieve nd ir-dried for further use. 2.2 Aggregte size seprtion Four lsses of ggregtes were frtionted by using the method reported by Cmbrdell nd Elliott (1994), nd eh frtion ws nmed s lrge mroggregte (>2,000 μm), smll mroggregte (250 2,000 μm), miroggregte ( μm), nd silt + ly frtion (<53 μm), respetively. In this method, the pre-wetted soil subsmples were submerged in wter t room temperture on the top of the 2,000 µm sieve for 5 min before sieving. Aggregte seprtion ws omplished by mnully moving the sieve 3 m vertilly 50 times over period of 2 min. Wter, plus soil tht pssed through the sieve, ws poured onto the next finer-size sieve nd the sme proess ws repeted. Mterils remining on the eh sieve were oven-dried t 50 C. The silt + ly frtion ws entrifuged 10 min t 2,500 g, nd then the pellet ws oven-dried t 50 C. The dried ggregte frtions were weighed nd stored t room temperture. Before nlysis, the subsmples were ground in mortr with pestle for further use. 2.3 SOC nd TN determintion SOC nd TN in bulk soils nd ggregtes were nlyzed (in three replites) using C/N/H/S-nlyzer (Vrio ElementrIII, Tble 1 Some soil properties of different fertiliztion tretments Tretment Avilble N (mg kg 1 ) Avilble P (mg kg 1 ) Avilble K (mg kg 1 ) Mirobil biomss-c (mg kg 1 ) ph Mehnil omposition (%) Corse snd 250 2,000µm Fine snd µm Silt 2 53 µm Cly <2µm CK NPK M M1NPK M M2NPK CK zero fertilizer, NPK minerl fertilizers, M1 nd M2 orgni mnure t lower nd higher level, respetively, M1NPK ombintion of M1 nd NPK, M2NPK ombintion of M2 nd NPK

4 Germny). Sine the smples were free of inorgni C (rbontes), the totl C ontent mesured diretly ws tken to be equivlent to the quntity of orgni C in the smples. The reoveries of smple mss, SOC nd TN fter the ggregte frtiontion verged 98.3, 96.6, nd 98.8%, respetively. 2.4 Sttistil nlysis The nlysis of vrine ws used to ompre the effets of different tretments on the mesured properties by the SPSS The LSD test (P<0.05) ws pplied to evlute the signifine of min effets from one-wy nlysis of vrine. 3 Results 3.1 The effet of tretments on ggregte distribution The distribution pttern of the four ggregtes ws different. The smll mroggregtes omprised the lrgest proportion, whih ounted for % of the totl weight, nd the frtion of miroggregtes ws the seond lrgest, being %. The lrge mroggregtes nd silt + ly frtions were the lest omponents, ounting for only % nd %, respetively (Fig. 1). Both minerl fertilizers nd orgni mnure exerted ler influene on soil ggregte distribution. Compred with the ontrol (CK), the orgni mnure pplition lone deresed the proportion of smll mroggregte remrkbly, but inresed tht of lrge mroggregte nd there Aggregte perentge in soil (%) b b b d d b CK M1 M2 b b b b b b > <53 Aggregte size lss (µm) NPK M1NPK M2NPK Fig. 1 Aggregte size distribution in different fertiliztion tretments. See Tble 1 for the bbrevition of the six tretments. Error brs represent stndrd errors of the mens (n=3). Different smll letters indite signifint differene within the sme lss ggregte t 0.05 probbility level ws little differene between the two orgni mnure level tretments. The influene of orgni mnure pplition on the proportion of miroggregte nd silt + ly frtion ws not signifint. In ontrst, the ddition of minerl fertilizers enlrged the perentge of the two smll-size ggregtes (miroggregte nd silt + ly frtion) in the soil. Menwhile, the mount of lrge mroggregte ws not influened by minerl fertilizers ddition, while the proportion of smll mroggregte deresed gretly. When minerl fertilizers were ombined with orgni mnure, the minerl fertilizers did not reverse the ting diretion of orgni mnure on the distribution of the four ggregtes, but minerl fertilizers plyed n intertion role to some extent, espeilly on miroggregte nd silt + ly frtion. 3.2 SOC nd TN onentrtion of bulk soil with different tretments SOC ontent in bulk soil inresed signifintly with the inresing mounts of orgni mnure pplition (P< 0.05). Compred with CK, SOC ontent ws inresed by 33.0% in M1, nd 49.3% in M2, respetively. For M1NPK nd M2NPK, SOC ontent ws 34.0% nd 74.8% more thn tht in CK (Tble 2). Even SOC ontent in M2NPK ws still 17.2% higher thn tht in M2. However, there ws no signifint differene in SOC ontent between M1 nd M1NPK. The distribution pttern of TN in the different fertiliztion tretments ws similr with tht of SOC. The highest ontent (3.36 g kg 1 ) ws found in M2NPK tretment, nd the lowest one ws still in the two tretments without orgni mnure pplition (1.63 g kg 1 for CK nd 1.60 g kg 1 for NPK). 3.3 SOC nd TN onentrtion of ggregtes with different tretments SOC nd TN onentrtion (grms per kilogrm frtion) in the four ggregtes followed trend similr to tht in bulk soil on the whole, but the mgnitude mong the four frtions ws different. Although orgni mnure pplition resulted in the inrese in the onentrtions of SOC nd TN in ll frtions, more signifint inrese ws found espeilly in the mroggregtes nd miroggregtes (Tble 2). In generl, the influene of minerl fertilizers ddition lone on the ggregte-ssoited rbon nd nitrogen onentrtion ws not signifint, but SOC ontent in the lrge mroggregte deresed signifintly with the NPK fertilizers pplition. In the tretment of the ombined pplition of inorgni nd orgni fertilizers, M2NPK ws more effetive on inresing eh ggregtessoited rbon nd nitrogen onentrtion thn M1NPK.

5 Tble 2 SOC nd TN onentrtion in bulk soil nd ggregtes (g kg 1 frtion) Tretment SOC TN >2,000µm 250 2,000µm µm <53µm Bulk soil >2,000µm 250 2,000µm µm <53µm Bulk soil CK 21.0±0.4b 19.7± ± ± ± ± ± ± ± ±0.05 NPK 19.4± ± ± ± ± ± ± ± ± ±0.01 M1 28.5±0.1d 29.6±0.0b 25.6±0.5b 17.8±0.0b 27.0±0.0b 2.44±0.00b 2.64±0.03b 2.43±0.06b 1.92±0.00b 2.51±0.05b M1NPK 26.1± ±0.3b 27.1±0.6b 18. 5± ±0.5b 2.52±0.01b 2.75± ±0.02b 2.12± ±0.06b M2 31.2±0.5e 31.4± ±0.3b 18. 9± ± ± ±0.04b 2.55±0.01b 2.06± ±0.12 M2NPK 34.3±0.2f 36.6±0.8d 32.8± ±0.3d 35.5±1.0d 3.25±0.02d 3.57±0.07d 3.40± ±0.03d 3.36±0.02d See Tble 1 for the bbrevition of the six tretments. Different smll letters indite signifint different t 0.05 probbility level within the sme lss ggregte 3.4 SOC nd TN storge of ggregtes with different tretments The smll mroggregtes omprised the mjor SOC nd TN pools (stoks) mong the four ggregte frtions in the soil studied, regrdless of different fertiliztion ptterns. They ontined gc kg 1 soil nd gn kg 1 soil, ounting for % of SOC nd % of soil totl nitrogen, respetively; nd gc kg 1 soil were stored in the miroggregtes nd lrge mroggregtes, respetively; nd the silt + ly frtions were the lest SOC pool, only bout gc kg 1 soil being in these frtions (Tble 3). The TN stok in ggregte frtions ws similr to tht of SOC. As muh s nd gn kg 1 soil were present in the miroggregtes nd lrge mroggregtes, respetively, but gn kg 1 soil ws in the silt + ly frtions. As to the fertiliztion effets, our results showed tht SOC nd TN stoks in ll the four ggregtes in the mnure tretments were inresed ompred with the CK, regrdless with or without minerl fertilizers, but the minerl fertilizers ould enhne the stoks of SOC nd TN in miroggregte nd silt + ly frtion. 4 Disussion 4.1 Effet of long-term fertiliztions on soil ggregtion The soils with orgni mnure pplition ontined more lrge mroggregte (>2,000 µm) while the CK tretment ontined more ggregtes in the 250 2,000 µm size lss. This observtion is similr to the results reported by Gregorih et l. (2003). Inrese in lrger ggregtes onomitnt with the inresed levels of SOM is ommon feture when soils reeiving orgni mnure (Angers nd N'Dyegmiye 1991; Aoym et l. 1999). The dt(see Fig. 1) lso indite tht orgni mnure filitted soil ggregtion proesses beuse the orgni mendments n ontribute binding gents for soil ggregtion (Tisdll nd Odes 1982). However, Su et l. (2006) hve found tht long-term pplition of FYM substntilly inreses the proportion of both lrge mroggregte (>2 mm) nd smll mroggregte (250 2,000 µm) ompred with the soils without FYM pplition. The lower SOC level in the investigted soils might be responsible for the phenomenon. Hene, we onlude tht when SOM is reltively high, the formtion of lrge mroggregte from smll mroggregte is very neessry in order to store more SOM in the soil reeived orgni mnure. However, when SOM is originlly low, the re-ggregtion of miroggregte nd silt + ly frtion into lrger ggregte frtions is importnt for SOC sequestrtion in the soil mended with orgni mnure. Aording to the hierrhil model of ggregte formtion reported by Tisdll nd Odes (1982), miro-

6 Tble 3 SOC nd TN stok in ggregtes (g kg 1 soil) Tretment SOC TN >2,000µm 250 2,000µm µm <53µm >2,000µm 250 2,000µm µm <53µm CK b NPK b 2.13d b 0.20 M b 1.59b d b M1NPK 3.48b 13.9b 6.30d 2.70e 0.34b e 0.31e M d b e 0.55d 0.18b M2NPK 5.39d 18.3e 7.96e d 1.78f 0.83f 0.22d Stok lulted s SOC nd TN onentrtion in n ggregte multiplied by the mount of the ggregte in one kilogrm soil. See Tble 1 for the bbrevition of the six tretments. Different smll letters indite signifint different t 0.05 probbility level within the sme lss ggregte ggregtes re held together by reltively persistent binding gents, e.g., romti humi mteril in ssoition with morphous Fe nd Al ompounds nd polyvlent metl tions. In ddition, those persistent binding gents re hrdly hnged by mngement mesures nd thus, miroggregtes should be firly stble over long periods of time. However, we found tht there ws n inrese in the proportions of miroggregte nd silt + ly frtion long with long-term minerl fertilizers ddition, nd menwhile, the proportion of smll mroggregte deresed signifintly. It is suggested in our se tht the smll mroggregte is likely to be broken down into miroggregte nd, in turn, to silt nd ly fter the long-term pplition of minerl fertilizers. This finding therefore indites tht the minerl fertilizers pplition n led to deggregtion effet, espeilly of smll mroggregte. 4.2 Effet of long-term fertiliztions on the SOC nd TN umultion in bulk soil The results showed tht the minerl fertilizer pplition lone hd no signifint effet on SOC sequestrtion ompred with CK. This ws onsistent with results from Yng et l. (2003), who reported tht inorgni N nd NPK fertiliztion were indequte for mintining SOC levels under onditions of onventionl mngement ssoited with no bove-ground rop residues returning to soil. However, we found n intertive effet between minerl fertilizers nd higher mount of orgni mnure on SOC umultion in bulk soil. The pttern of TN ontent in bulk soil ws similr to tht of SOC s influened by the orgni nd inorgni fertilizers pplition, onfirming tht there is lose reltionship in the dynmis of these two omponents, but, the C/N rtio showed different pttern (Fig. 2). The nrrower C/N rtio in the mnure tretments is evident ompred with the other tretments nd our finding is in line with the results from nother long-term fertiliztion experiment in the Northwest of Chin (Su et l. 2006). Generlly, during orgni mnure deomposition, C losses re fster thn N losses nd then this n result in the dereses in C/N rtios (Antil et l. 2005). Consequently, TN inrement ws muh higher thn tht of SOC fter the orgni mnure pplition (see Tble 2). These results Fig. 2 C/N rtios of different lss ggregte frtions nd bulk soil. See Tble 1 for the bbrevition of the six tretments. Different smll letters indite signifint differene within the sme lss ggregte t 0.05 probbility level C/N d db CK NPK M1 M1NPK M2 M2NPK b b b b b b b b b b b b 4 0 > <53 Bulk soil Aggregte size lss (µm)

7 suggest tht orgni mnure pplition is more effetive on improving TN umultion thn SOC umultion in soil. This finding lso implies tht one SOC is lost, its restortion needs long-term effort. 4.3 Effet of long-term fertiliztions on the SOC nd TN umultion nd distribution in ggregtes In ll the tested soils, the lowest SOC onentrtions (grms per kilogrm frtion) were found in the silt + ly frtion. A similr observtion hs been reported by Mnn et l. (2006) through ontinuous fertiliztion on Ineptisol in Indi. The silt + ly frtion in our experiment did not ontin snd prtiles, so the lower SOC onentrtion in this frtion must be influened by the presene of free silt prtiles (e.g., smll qurtz grins) with low or no binding pity of SOC. Also, the protetion pity of silt nd ly prtiles is limited sometimes by sturtion phenomenon (Hssink 1997; Six et l. 2002). In our experiment, the SOC onentrtion ws reltively uniform ross the ggregtes greter thn 53 µm, but it hs been found in nother investigtion tht the SOC ontent in the miroggregtes of whet field is less thn tht in mroggregtes (Elliott 1986). However, it is still unler wht used the differene, different ropping systems or different soil or site properties? Evidene shows tht plnt roots nd hyphl enmeshment re involved in soil struture stbiliztion nd this n be different mongst different plnts (Chn nd Heenn 1996); hene, our monoropping system should be responsible for the disrepny. Although the SOC ontent of ll the four ggregte frtions inresed with orgni mnure pplition, the inresing degree vried mong the different ggregtes. The inrement of SOC used by orgni mnure pplition minly hppened in the ggregtes lrger thn 53 µm, onfirming tht those lrge ggregtes ply n importnt role in SOC sequestrtion. We found tht orgni mnure pplition hd positive effet on SOC stok in ggregtes, nd this ws in greement with the results reported by Su et l. (2006). However, in their investigtions, pproximtely 54 60% of totl SOC ws stored in miroggregtes nd silt + ly frtions without FYM pplition, wheres 57 64% of totl SOC ws stored in mroggregtes fter tretment with FYM. The different types of soil nd soil properties mybe ontribute to this disrepny. To ompre the distribution of SOC in vrious ggregte frtions isolted from different tretments, element enrihment ftors (E) re usully lulted for eh ggregte lss. The enrihment ftor (Christensen 1992) reltes the ontent of ertin omponent in prtiulte size frtion to its ontent in the bulk soil nd, for instne, the E of SOC n be lulted by the eqution: E C Frtion ¼ ðgc kg 1 frtionþ= ðgc kg 1 soilþ. E>1 indites enrihment of omponent in frtion, wheres, E<1 indites depletion (Amelung et l. 1998). The enrihment ftor is useful to study the hrteristi of rbon pool (Amelung et l. 1998; Zhng et l. 1999), sine it n eliminte the effet of different SOC ontent in different soil smples on evluting different frtions ontribution to totl SOC. Our results indite tht SOC in the silt + ly frtion (Tble 4) is reltively depleted ompred with tht in the other three lrger frtions, regrdless of tretments, but there is no signifint differene in enrihment ftors in the three lrger frtions. This finding suggests tht the hemil protetion for the umultion of SOC is limited in lrge ggregtes, but physil protetion plys mjor role in the Chinese Mollisol. We lso found tht when SOC is sequestered long with orgni mnure pplition, there is shift of SOC from the silt + ly frtion to lrger frtions. However, when SOC is depleted, it hppens preferentilly in the physilly proteted SOC in lrge ggregtes. Differenes in the reltive effet of fertiliztion mngement on the mount of SOC present in the Tble 4 Enrihment ftor for soil orgni rbon (E C ) nd totl nitrogen (E N ) in ggregte frtions of different tretments Tretment E C E N >2,000µm 250 2,000µm µm <53µm >2,000µm 250 2,000µm µm <53µm CK 1.03b b 0.78d b 0.91d NPK 1.00b 1.06b e 0.99b e M d 0.95b 0.66b 0.98b b 0.76b M1NPK d 1.00b b b 0.81 M2 1.03b 1.03b b M2NPK d b b 0.70 Enrihment ftor lulted s SOC nd TN onentrtion in n ggregte frtion divided by tht in the orresponding bulk soil. See Tble 1 for the bbrevition of the six tretments. Different smll letters indite signifint different t 0.05 probbility level within the sme lss ggregte

8 ggregte frtions mybe ontribute to the shift of the SOC present in these ggregte pools (Sleutel et l. 2006). The verge C/N rtio deresed to some extent with the deresing ggregte size nd this is onsistent with the observtions of Lrré-Lrrouy et l. (2004). The nrrowest C/N rtio in silt + ly frtion indites tht N is preferred to umulte in the fine lss ggregtes. This is onfirmed by the E C nd E N ftors in the ggregte frtions. Higher C/N rtios in the ggregtes lrger thn 53 µm suggest tht SOC in those frtions is reltive fresh or little ltered by miroorgnisms, wheres, the SOC ssoited the ggregtes smller thn 53 µm is reltive ged nd more mirobilly derived. However, it is more interesting to ompre the enrihment ftors of SOC nd TN. For instne, the muh higher E N reltive to E C in the silt + ly frtion ws evident, onfirming tht the preferentil depletion of lbile SOC results in reltive umultion of stble SOC pools in the finest frtion. 5 Conlusions Twenty-five-yer ontinuous fertiliztion of orgni mnure filittes ggregtion proesses nd the sequestrtion of SOC in the Chinese Mollisol, inditing tht the formtion of lrge mroggregte from smll mroggregte is essentil for the storge of SOC. However, the effet of deggregtion due to long-term minerl fertilizers pplition ws evident nd it ould inrese the risk of soil nd nutrient loss through erosion. The ombintion use of orgni mnure nd minerl fertilizers not only enhne the sequestrtion of SOC to some further extent, but lso improve the qulity of SOM by ggregtion effet. Therefore, good soil mngement prties should preferbly inlude orgni mnure pplition to mintin better soil struture nd SOM levels in the Mollisol. The hemil protetion over SOC is limited in lrge lss ggregtes, but physil protetion plys mjor role for the sequestrtion of the inresed SOC in the Chinese Mollisol. Therefore, when SOC is sequestered long with long-term orgni mnure pplition, there is shift of SOC from the silt + ly frtion to lrge ggregtes. However, when SOC is depleted, it hppens preferentilly in the physil proteted SOC in lrge ggregtes. This study hs gret implitions for better understnding SOC nd N dynmis s ffeted by long-term fertiliztions. Aknowledgements This work ws jointly funded by the Nturl Siene Foundtion of Chin (Projet No ) nd the Mjor Stte Bsi Reserh Development Progrm of Chin (973 Progrm) (Projet No. 2009CB118607). Referenes Amelung W, Zeh W, Zhng XD, Follett RF, Tiessen H, Knox E, Flh K-W (1998) Crbon, nitrogen nd sulfur pools in prtilesize frtions s influened by limte. Soil Si So Am J 62: Angers DA, N'Dyegmiye A (1991) Effets of mnure pplition on rbon, nitrogen, nd rbohydrte ontents of silt lom nd its prtile-size frtions. Biol Fertil Soils 11:79 82 Antil RS, Gerzbek MH, Hberhuer G, Eder G (2005) Long-term effets of ropped vs. fllow nd fertilizer mendments on soil orgni mtter II. Nitrogen. J Plnt Nutr Soil Si 168: Aoym M, Angers DA, N'Dyegmiye A (1999) Prtiulte nd minerl-ssoited orgni mtter in wter-stble ggregtes s ffeted by minerl fertilizer nd mnure pplition. Cn J Soil Si 79: Bere MH, Cbrer ML, Hendrix PF, Colemn DC (1994) Aggregteproteted nd unproteted orgni mtter pools in onventionl nd no-tillge soils. Soil Si So Am J 58: Blir N, Fulkner RD, Till AR, Poulton PR (2006) Long-term mngement impts on soil C, N nd physil fertility Prt I: brodblk experiment. Soil Till Res 91:30 38 Bronik CJ, Ll R (2005) Soil struture nd mngement: review. Geoderm 124:3 22 Cmbrdell CA, Elliott ET (1994) Crbon nd nitrogen dynmis of soil orgni mtter frtions from ultivted grsslnd soils. Soil Si So Am J 58: Chn KY, Heenn DP (1996) The influene of rop rottion on soil struture nd soil physil properties under onventionl tillge. Soil Till Res 37: Christensen BT (1992) Physil frtiontion of soil nd orgni mtter in primry prtile size nd density seprtes. Adv Soil Si 20:1 90 Elliott ET (1986) Aggregte struture nd rbon, nitrogen nd phosphorus in ntive nd ultivted soils. Soil Si So Am J 50: Gregorih EG, Bere MH, Stokls U, St-Georges P (2003) Biodegrdbility of soluble orgni mtter in mize-ropped soils. Geoderm 113: Hssink J (1997) The pity of soils to preserve orgni C nd N by their ssoition with ly nd silt prtiles. Plnt Soil 191:77 87 Hti KM, Swrup A, Dwivedi AK, Misr AK, Bndyopdhyy KK (2007) Chnges in soil physil properties nd orgni rbon sttus t the topsoil horizon of vertisol of entrl Indi fter 28 yers of ontinuous ropping, fertiliztion nd mnuring. Agri Eosyst Environ 119: Jones SK, Rees RM, Kosms D, Bll BC, Skib UM (2006) Crbon sequestrtion in temperte grsslnd; mngement nd limti ontrols. Soil Use Mnge 22: Kong AYY, Six J, Brynt DC, Denison RF, vn Kessel C (2005) The reltionship between rbon input, ggregtion, nd soil orgni rbon stbiliztion in sustinble ropping systems. Soil Si So Am J 69: Lrré-Lrrouy MC, Blnhrt E, Albreht A, Feller C (2004) Crbon nd monoshrides of tropil vertisol under psture nd mrket-grdening: distribution in seondry orgnominerl seprtes. Geoderm 119: Liu AB, Hn XZ, Song CY, Herbert SJ, Xing BS (2003) Soil orgni rbon dynmis in blk soils of Chin under different griulturl mngement systems. Comm Soil Si Plnt Anl 34: Mdri B, Mhdo PLOA, Torres E, de Andrde AG, Vleni LIO (2005) No tillge nd rop rottion effets on soil ggregtion

9 nd orgni rbon in Rhodi Ferrlsol from southern Brzil. Soil Till Res 80: Mnn MC, Swrup A, Wnjri RH, Singh YV, Ghosh PK, Singh KN, Tripthi AK, Sh MN (2006) Soil orgni mtter in west Bengl ineptisol fter 30 yers of multiple ropping nd fertiliztion. Soil Si So Am J 70: Pinheiro EFM, Pereir MG, Anjos LHC (2004) Aggregte distribution nd soil orgni mtter under different tillge systems for vegetble rops in Red Ltosol from Brzil. Soil Till Res 77:79 84 Rudrpp L, Purkysth TJ, Singh D, Bhdrry S (2006) Longterm mnuring nd fertiliztion effets on soil orgni rbon pools in Typi Hplustept of semi-rid sub-tropil Indi. Soil Till Res 88: Six J, Elliott ET, Pustin K (2000) Soil struture nd soil orgni mtter: II. A normlized stbility index nd the effet of minerlogy. Soil Si So Am J 64: Six J, Elliott ET, Pustin K (2000b) Soil mroggregte turnover nd miroggregte formtion: mehnism for C sequestrtion under no-tillge griulture. Soil Biol Biohem 32: Six J, Connt RT, Pul EA, Pustin K (2002) Stbiliztion mehnisms of soil orgni mtter: implitions for C-sturtion of soils. Plnt Soil 241: Sleutel S, Neve SD, Németh T, Tóth T, Hofmn G (2006) Effet of mnure nd fertilizer pplition on the distribution of orgni rbon in different soil frtions in long-term field experiments. Eur J Agron 25: Soil Survey Stff (2003) Keys to soil txonomy, 9th edn. United Sttes Deprtment of Agriulture nd Nturl Resoures Conservtion Servie, Wshington Su YZ, Wng F, Suo DR, Zhng ZH, Du MW (2006) Long-term effet of fertilizer nd mnure pplition on soil-rbon sequestrtion nd soil fertility under the whet whet mize ropping system in northwest Chin. Nutr Cyl Agroeosys 75: Tisdll JM, Odes JM (1982) Orgni mtter nd wter-stble ggregtes in soils. J Soil Si 33: Wright AL, Hons FM (2005) Soil rbon nd nitrogen storge in ggregtes from different tillge nd rop regimes. Soil Si So Am J 69: Xing BS, Liu XB, Liu JD, Hn XZ (2004) Physil nd hemil hrteristis of typil Mollisol in Chin. Comm Soil Si Plnt Anl 35: Yng XM, Zhng XP, Fng HJ, Zhu P, Ren J, Wng LC (2003) Longterm effets of fertiliztion on soil orgni rbon hnges in ontinuous orn of northest hin: rothc model simultions. Environ Mnge 32: Zhng XD, Amelung W, Yun Y, Smson-Liebig S, Brown L, Zeh W (1999) Lnd-use effets on mino sugrs in prtile size frtions of n Argiudoll. Appl Soil Eol 11: