Landscape position and age of reconstructed prairies effect on soil organic carbon sequestration rate and aggregate associated carbon

Transcription

1 oi:1.2489/jsw Lnspe position n ge of reonstrute priries effet on soil orgni ron sequestrtion rte n ggregte ssoite ron Jose G. Guzmn n Mhi Al-Kisi Astrt: Chnges in griulturl ln use suh s the estlishment of priries on previously ultivte ropln provie n opportunity for greter soil orgni ron (SOC) sequestrtion rtes. In this stuy, topo- n hrono-sequene pproh ws use to investigte reltionships etween SOC n soil ggregte formtion. Our hypothesis ws tht the gretest inreses in SOC sequestrtion rtes re ssoite with most reently estlishe priries on ultivte ln, where SOC ws the most eplete, ue to the estrution of soil ggregte. The stuy ws onute in Jsper n Wrren ounties in Iow, from 25 to 28. Soils in oth ounties were forme in loess uner ntive vegettion of tllgrss pririe. There were three reonstrute pririe sites vrying in estlishment yer 1993, 1998, 23; row rop proution site uner no-tillge; n pririe remnnt site. All soil n plnt smpling plots were lote on summit, mislope, n toe-slope positions. Results show tht time sine estlishment n slope position h signifint impt on SOC sequestrtion rtes in the top 15 m (6 in) soil epth only. In the summit position, the gretest SOC sequestrtion rtes were oserve in the youngest pririe t 2.15 Mg h 1 y 1 (.96 tn 1 yr 1 ), n erese to ner equilirium fter pproximtely eight yers. Aitionlly, SOC ws shown to inrese linerly t rte of.73 Mg h 1 y 1 (.33 tn 1 yr 1 ) sine pririe estlishment uring the 14-yer perio. In the toe-slope position, SOC sequestrtion rtes were sustntilly lower t.59 Mg h 1 y 1 (.26 tn 1 yr 1 ). Furthermore, inreses in SOC sequestrtion rtes oinie with inreses in wter stle ggregte ssoite ron (C) in.25 to 2 mm (.1 to.8 in) size frtions. This ourre in the youngest reonstrute pririe (23) in the summit n in the mislope positions on priries reonstrute in 1998 n This suggests tht the mehnisms of soil ggregte formtion i in stilizing SOC, whih is signifintly ffete y slope position n yers sine pririe estlishment. Key wors: hrono-sequene soil orgni ron tllgrss pririe topo-sequene Chnges in ln use n ltertion of the eosystem n signifintly ffet soil orgni ron (SOC) ynmis. Uner similr onitions, permnent grsslns ten to hve greter SOC ontent in the surfe thn ultivte ropln ue to slowly eomposing plnt resiues. Conversely, tillge tht isturs the soil surfe inreses eomposition of soil orgni mtter y reking own ggregtes n exposing silt-ly omplexes to miroil tivities (Oes 1984; Elliott 1986; Pustin et l. 1997). Nonetheless, this reution in SOC gives rise for the potentil of greter SOC sequestrtion rtes when reversing the effets of ultivtion (Post n Kwon 2). However, little is known on interreltionships etween soil ggregte formtion n SOC sequestrtion fter tillge hs ese (Jstrow 1987; Jstrow et l. 1998). Intertions etween plnt resiues n soil orgni mtter with the soil re importnt ftors in the formtion of stle soil ggregtes for SOC sequestrtion (Hynes n Bere 1996; Blno-Cnqui n Ll 24). These ftors inlue (1) ove- n elow-groun input of orgni mterils, (2) the eomposility of the orgni mterils, (3) soil epth t whih the orgni mteril is ple, n (4) physil protetion either y ggregte formtion or y sorption to silt-ly minerls (Jones n Donnelly 24). Newly e orgni mteril suh s root exutes, fungl n myorrhizl hyphe, n miroil eris re reite for formtion n stiliztion of mro-ggregtes (Tisll n Oes 1982). In return, soil ggregtes protet orgni ron (C) from miroil eomposition tivities. However, the C sequestere to stilize mroggregtes is onsiere only short-term ( few weeks) ue to reltive ese of eomposition y miroorgnisms n is gretly ffete y tillge prties (Cmrell n Elliot 1993). Highly eompose orgni mterils suh s humi ompouns n polymers tht resist further rekown re losely ssoite with miroggregtes (Six et l. 1999). This is ue to the physil protetion y miroggregtes reuing miroil effets, therefore enhning long-term SOC sequestrtion. Aitionlly, these ioti tivities re hevily influene y topogrphy (Hook n Burke 2; Brye n Kuhrik 23), limte (Brye et l. 24; Rih n Potter 1995) n soil texture (Feller n Bere 1997; Hssink 1997) over time (Rihter et l. 1999; Akl n Ll 21). Thus, topo- n hrono-sequene pprohes provie grient of topogrphy n time neee to give insights into the reltionships etween SOC sequestrtion n soil ggregte formtion. Chnges in ln use, suh s reonstrution of priries on previously ultivte ln, provie opportunities to evlute potentil improvement in SOC, plnt iomss, n physil soil properties n how these hve een influene y lnspe position within reestlishe pririe system (Ber et l. 22). In prtiulr, the gretest hnges in SOC sequestrtion rtes re expete in the erly yers of estlishment, n sequestrtion rtes my ese to hnge signifintly in long-estlishe systems (Jstrow 1987; MLuhln et l. 26; n Kuhrik 27). Informtion on lnspe position effets on SOC sequestrtion rtes is lso neee. Stuies hve shown signifint ereses in SOC ontent in high-risk erosion soils, suh s the mislope position, n onsequently, Jose G. Guzmn is grute reserh ssistnt n Mhi Al-Kisi is n ssoite professor in the Deprtment of Agronomy t Iow Stte University, Ames, Iow.

2 SOC is then eposite in the toe-slope position, leing to greter SOC ontent (Gregorih et l. 1998). Our hypothesis is tht hnges in soil C ynmis in newly estlishe pririe systems on previously ultivte row rop proution ln will e highly ffete y slope position n temporl vriility, where the gretest SOC sequestrtion rtes will e in the most reently estlishe pririe systems. This stuy ws onute to investigte the effets of lnspe position n pririe ge sine onversion from ultivte ropln on SOC n soil ggregte formtion ynmis. In prtiulr, this reserh n provie nswers to questions relte to hnges in SOC n ssoite soil properties, suh s soil ggregte stility in newly estlishe pririe systems. Mterils n Methos Site Desription n Experimentl Design. This stuy ws onute in Jsper n Wrren ounties in Iow from My 25 to My 28. During this perio, nnul preipittion verge 94 mm (35.6 in), n nnul men temperture ws 1.3 C (5.5 F). Soils in oth Jsper n Wrren ounties forme in loess uner ntive vegettion of tllgrss pririe (Brynt n Woster 1978; Nestru n Woster 1979). All of the reonstrute pririe sites were lote in the Nel Smith Ntionl Willife Refuge (41 35'N, 93 14'W). The remnnt pririe ws lote pproximtely 95 km (59 mi) southwest in Rolling Thuner Pririe (41 1'N, 93 43'W). Historil kgrouns on the onversion of row rop proution to reonstrute pririe in the Nel Smith Ntionl Willife Refuge sites s well s present vegettive onitions on ll sites re summrize in tles 1 to 3. The three reonstrute pririe sites vrie in estlishment yer of 1993, 1998, n 23, whih were tegorize s P-1993, P-1998, n P-23, respetively. These sites were then ompre to n jent ropln proution site tht ws tegorize s no-till site estlishe in 23 (NT-23). A pririe remnnt (P-Remnnt) site ws inlue to ientify the upper limits for SOC n totl nitrogen (TN) of pririe eosystem efore onversion to ultivte ropln. To mesure the effets of slope position, time sine estlishment, n soil epth of ll sites on SOC, TN, miroil iomss C, n wter stle ggregtes, eh plot ws trete s n experimentl unit for totl of 45 oservtions n tretment mens for omprisons (three replite plots three slope positions five sites). Due to restritions in mngele worklo (i.e., limite ville sites n time onstrints on smpling), tretment omprisons were unvoily se on pseuo-replition. Detile ssessments of soil texture, ulk ensity, vegettion growth, n other relte properties were etermine to hrterize site vriility etween sites n to inorporte ifferenes into the nlyses n interprettions. All smpling plots were lote on summit, mislope, n toe-slope positions n were hosen y yer of estlishment n the presene of reltively similr soils (tles 1 to 3). The experiment ws esigne so tht the yer of estlishment of the site ws the min tretment, replite three times long eh slope position in plots of pproximtely 4 m 2 (43 ft 2 ) n 3 m (98 ft) prt. Soil Smpling n Anlyses. From 25 to 27, soil smples were ollete nnully in erly My, to mesure ny hnges in SOC, TN, n ph. Ten to twelve 1.7 m (.67 in) soil ores were rnomly ollete from eh of the following epths of to 15, 15 to 3, 3 to 45, n 45 to 6 m ( to 6, 6 to 12, 12 to 18, n 18 to 24 in) in eh plot. Soil ores for eh epth were omine into single homogeneous smple. Soil smples were 2 mm (.8 in) sieve n then ir rie efore eing nlyze for ph (1:1; soil to wter) using n AR15 ph meter (Aumet Reserh, Fisher Sientifi Interntionl In.). SOC n TN were etermine y ry omustion using LECO CN nlyzer (LECO Corportion, St. Joseph, Mihign). At the sme time, three ulk ensity smples were ollete rnomly from eh plot, using 1.7 m (.67 in) imeter soil proe for eh soil epth. Soil ores were tken t the sme soil epths n were then oven rie t 15 C (221 F) for 24 hours n weighe. Bulk ensity (ρ [g m 3 ]) ws lulte s the rie soil mss ivie y the soil ore volume (Blke n Hrtge 1986). The SOC n TN onentrtions (mg g 1 ry soil) were multiplie y men ρ vlues n soil epth of 15 m (6 in) to onvert SOC onentrtion to mss per re sis (Mg h 1 ) for ll sites. Mesurments of SOC, miroil iomss ron (MBC) n TN mesurements were one in the first yer (25) n re shown in (tle 4). Initil SOC, miroil iomss ron (MBC) n TN in 25 re shown in tle 4. Miroil Biomss Cron. Smples were ollete using 7.6 m (3 in) imeter golf ourse hole utter to 15 m (6 in) epth in mi-august in 25 n 27. Miroil iomss ws etermine y performing the fumigtion extrtion metho (Horwth n Pul 1994). Soil smples were fumigte with ethnol-free hloroform (CHCl 3 ) for 24 hours in vuum esitor. The soil smples were extrte for 3 minutes with 1 ml (.2 pt) of.5m potssium sulfte (K 2 SO 4 ) n then were filtere through Whtmn No. 42 filter pper. A similr extrtion ws performe on the non-fumigte soil smples while the others were eing set up for fumigtion. The extrtnt (K 2 SO 4 ) lone ws lso filtere to etermine the kgroun level of C in the filter pper n extrtnt. Cron reovere in the extrt ws mesure with n Elementr liquitoc ron nlyzer (Ameris In., Mt. Lurel, New Jersey). The MBC ws lulte on n oven ry weight sis. Aggregte Stility n Assoite Cron n Nitrogen. Soil smples were tken using 7.6 m (3 in) imeter golf ourse hole-utter to soil epth of 15 m (6 in) in three replitions for eh plot in mi- My in 25 n 28. Soil smples were then gently sieve through n 8 mm (.3 in) sieve to remove ny unesirle plnt resiues n roks. Soil smples were then ir rie n store for nlysis. The wter stle ggregtes (WSA) size istriution ws etermine following the proeure y Kemper n Rosenu (1986), with some moifitions. One hunre grms (3.5 oz) of soil smple ws use for wet sieving for five minutes in eionize wter t 21 C (7 F), y lowering n then rising the sieves with stroke length of 2 mm (.8 in) n frequeny of 9 strokes per minute, using ustom-me sieving mhine where 2 m (7.9 in) imeter sieves oul fit. Seven ggregte-size frtions were ollete. Aggregtes tht psse through ll sieves, inluing the.53 mm (.2 in) sieve were tegorize s <.53 mm. The other six frtions were.53 to.25,.25 to.5,.5 to 1, 1 to 2, 2 to 4, n >4 mm (.2 to.1,.1 to.2,.2 to.4,.4 to.8,.8 to.16, n >.16 in). Eh soil smple ws first miste n then sumerge in wter in the top sieve for t lest five minutes efore wet sieving egn to 1

3 Tle 1 Summit prmeters: Estlishment yer, urrent plnt speies, physiogrphy, n selete soil properties for eh site t the top 15 m soil epth. Site esription Chrteristi NT-23 P-23 P-1998 P-1993 P-Remnnt Eosystem Row rop Pririe Pririe Pririe Pririe Estlishment yer 187s Ntive remnnt Tle 2 Mislope prmeters: Estlishment yer, urrent plnt speies, physiogrphy, n selete soil properties for eh site t the top 15 m soil epth. Site esription Chrteristi NT-23 P-23 P-1998 P-1993 P-Remnnt Eosystem Row rop Pririe Pririe Pririe Pririe Estlishment yer 187s Ntive remnnt No-till sine 23 Dominnt plnt speies 25 n 27 Shizhyrium Anropogon Anropogon gerrii Shizhyrium soprium Ze mys L. soprium gerrii Shizhyrium Soligo nensis 26 Glyine Sorghstrum nutns soprium Boutelou urtipenul mx L. Merr. Sorghstrum nutns Shrus Root iomss (Mg h 1 ) Aove-iomss (Mg h 1 ) Physiogrphy Slope (%) 1 to 16 5 to 8 6 to 9 4 to 1 1 to 14 Soil properties No-till sine 23 Dominnt plnt speies 25 n 27 Anropogon gerrii Anropogon Anropogon gerrii Anropogon gerrii Ze mys L. Shizhyrium gerrii Shizhyrium Shizhyrium soprium 26 Glyine soprium soprium Sorghstrum nutns mx L. Merr. Elymus nensis Soligo nensis Soligo nensis Trifolium prtense Root iomss (Mg h 1 ) Aove-iomss (Mg h 1 ) Physiogrphy Slope (%) to 1 to 2 to 6 3 to 4 2 to 4 Soil properties Soil ssoition Otley-Mhsk Otley-Mhsk Otley-Mhsk Otley-Mhsk Log-Gr-Armstrong Soil type Otley Otley Otley Otley Air-Shrpsurg Sn (%) Silt (%) Cly (%) ph Chmerist fsiult Soil ssoition Otley-Mhsk Otley-Mhsk Otley-Mhsk Otley-Mhsk Log-Gr-Armstrong Soil type Mhsk Mhsk Tm Mhsk Shrpsurg Sn (%) Silt (%) Cly (%) ph

4 Tle 3 Toe-slope prmeters: Estlishment yer, urrent plnt speies, physiogrphy, n selete soil properties for eh site t the top 15 m soil epth. Site esription Chrteristi NT-23 P-23 P-1998 P-1993 P-Remnnt Eosystem Row rop Pririe Pririe Pririe Pririe Estlishment yer 187s Ntive remnnt No-till sine 23 Dominnt plnt speies 25 n 27 Silphium linitum Sprtin Sprtin petint Helinthus grosserrtus Ze mys L. Vrious other fors petint Vrious other fors 26 Glyine mx L. Merr. Root iomss (Mg h 1 ) Aove-iomss (Mg h 1 ) Physiogrphy Slope (%) to 1 1 to 5 to 1 4 to 5 1 to 4 Soil properties Soil ssoition Otley-Mhsk Otley-Mhsk Otley-Mhsk Otley-Mhsk Log-Gr-Armstrong Soil type Shely Shely Shely Shely Colo-Ely Sn (%) Silt (%) Cly (%) ph Tle 4 Effets of slope position n time sine pririe estlishment on selete soil properties of ifferent soil epths sine 25. Summit soil epth (m) Mislope soil epth (m) Toe-slope soil epth (m) Site to to 3 3 to to 6 to to 3 3 to to 6 to to 3 3 to to 6 SOC (Mg h 1 ) NT P P P P-Remnnt TN (Mg h 1 ) NT P P P P-Remnnt C/N NT P P P P-Remnnt MBC (g m 2 ) NT P P P P-Remnnt Notes: SOC = soil orgni ron. TN = totl nitrogen. C/N = ron/nitrogen rtio. MBC = miroil iomss ron. Numers followe y ifferent lower se letter within sme olumn n slope position re signifintly ifferent t p.1. 12

5 slke off ir-rie soil. Following wet sieving, the soil smples were immeitely poure into tus n oven rie t 65 C (15 F) until ll wter ws ompletely evporte, n ry weight ws reore for eh size frtion. In ition, WSA ry weights were juste to soil moisture orretions from ir-rie susmples of WSA. The WSA ssoite ron (C) n TN onentrtions were etermine y ry omustion using LECO CN nlyzer (LECO Corportion, St. Joseph, Mihign) for eh size frtion fter ggregtes were groun to fine power using mortr n pestle. Orgni C n TN onentrtions were then juste for sn-free frtion WSA. Snfree frtion WSA ws etermine y wet sieving seon set of soil smples, using the sme WSA proeure mentione erlier. Soil ggregte ftions from eh sieve were ollete, rie, n the weights were reore. The soil ggregte frtions were then ple into 25 ml (.5 pt) Nlgene ottle, n soium hexmetphosphte solution equivlent to 5:1 (milliliter of solution to grm of soil) ws e n left overnight in mehnil shker t 35 rpm. The solution ws then psse through.53 mm (.2 in) sieve, where sn ws ollete, wshe with eionize wter, n oven rie t 65 C (15 F). Sttistil Anlysis. Dt were nlyze using Pro Mixe proeure (SAS Institute In. 22). Mens were seprte using ANOVA F-test when tretment effets were signifint. Sttistil signifine ws evlute t p.1. Results n Disussion Effets of Lnspe Position n Pririe Age on Soil Cron Sequestrtion Rtes. Soil epth showe no signifint effet on SOC sequestrtion rtes; thus, the ifferenes etween sites ross slope positions re presente for the top 6 m (24 in) soil epth (figure 1). Differenes mong sites were only oserve in the summit position where NT-23 n P-1998 sites h the gretest SOC sequestrtion rtes n the P-23, P- 1993, n P-Remnnt sites were t or ner zero rtes. Comprisons etween sites SOC sequestrtion rtes for eh slope position in the top 15 m (6 in) soil epth show the effets of pririe ge or esstion of tillge on SOC hnge (figure 1). In the summit position, the most reently estlishe sites (NT-23 n P-23 sites) h the gret- Figure 1 Chnge in soil orgni ron (SOC) stok for eh site from 25 to 27 in the () top 6 m n () 15 m soil epths s influene y slope position. Tretments within the sme slope position with the sme letter re not signifintly ifferent oring to the lest-squres mens test t p.1. Error rs inite stnr error. () () SOC sequestrition rte (Mg h 1 y 1 ) SOC sequestrition rte (Mg h 1 y 1 ) Summit Mislope Toe-slope Slope position NT-23 P-23 P-1998 Summit Mislope Toe-slope Slope position P-1993 P-Remnnt 13

6 Figure 2 Soil orgni ron ontent over 2- to 14-yer perio of reonstrute priries of soil epth of 15 m. A no-till site n remnnt pririe were inlue for omprison with reonstrute priries ut were not inlue in the liner est fit. Eh point represents one plot tht ws verge from 25 to 27. Soil orgni ron (Mg h 1 ) Soil orgni ron (Mg h 1 ) Soil orgni ron (Mg h 1 ) r 2 =.81, p =.22 y =.73x Summit Remnnt r 2 =.72, p =.151 y =.73x Mislope Remnnt r 2 =.57, p =.1362 y =.59x Toe-slope Remnnt Reonstrute pririe Yers sine estlishment P-Remnnt NT-23 est SOC sequestrtion rtes (2.59 n 2.15 Mg SOC h 1 y 1 [1.15 n.95 tn 1 yr 1 ], respetively) followe y the P-1998 site, the P-Remnnt, n P-1993 site, respetively, whih were t or ner zero rtes. Similr finings y Kuhrik (27) hve shown SOC sequestrtion rtes of 1.3 Mg C h 1 y 1 (.58 tn 1 yr 1 ) uring the first four to five yers, 2.1 Mg C h 1 y 1 (.94 tn C 1 yr 1 ) uring 6 to 1 yers, n ner zero hnge in SOC rte t 25 m (1 in ) soil epth in 11 to 16 yers sine estlishment of reonstrute priries on previously ultivte Mollisols in Wisonsin. In the mislope n toe-slope positions, vriility in SOC sequestrtion rtes inrese rmtilly ompre to the summit positions. In the mislope position, the lrgest SOC sequestrtion rte ourre in the P-1993 site t 2.9 Mg SOC h 1 y 1 (1.29 tn SOC 1 yr 1 ), lthough it ws not signifintly ifferent from the P-1998 n NT-23 sites,.7 n 1.77 Mg SOC h 1 y 1 (.3 n.79 tn SOC 1 yr 1 ), respetively. In the toe-slope position, there were signifint elines in SOC sequestrtion rtes oserve in the most reently estlishe pririe site (P-23) t 4.88 Mg SOC h 1 y 1 ( 2.17 tn SOC 1 yr 1 ) n the P-Remnnt site t 2.57 Mg SOC h 1 y 1 ( 1.14 tn SOC 1 yr 1 ). However, ifferenes in SOC sequestrtion rtes within eh site t the toe-slope position were not signifint ue to the high vriility within eh site. The rte of SOC inrese in the top 15 m (6 in) soil epth in the summit t.73 Mg SOC h 1 y 1 (.32 tn SOC 1 yr 1 ) n toe-slope position t.59 Mg SOC h 1 y 1 (.26 tn SOC 1 yr 1 ) positions showe liner reltionship over time uring the 2- to 14-yer perio (figure 2). Comprle rtes of inrese in SOC sequestrtion rtes hve een reporte of.62 Mg SOC h 1 y 1 (.27 tn SOC 1 yr 1 ) in Minnesot (MLuhln et l. 26) n.45 Mg SOC h 1 y 1 (.2 tn SOC 1 yr 1 ) in Texs (Potter et l. 1999). Lower SOC sequestrtion rtes in the toeslope position ompre to the summit position might e ttriute to less rsti ifferenes in SOC ontent etween reonstrute priries n the P-Remnnt site, whih is hypothesize to e ner sturtion. In the mislope position, SOC ontent generlly erese sine yer of estlishment t.73 Mg SOC h 1 y 1 (.32 tn SOC 1 yr 1 ). However, in the P-1993 n 14

7 P-1998 sites, SOC ontent i inrese from the 25 to 27 perio in the mislope position. This suggests tht the mislope position hs lower SOC sequestrtion rtes ompre to the summit n toe-slope positions, t lest in the first four yers of pririe estlishment efore n inrese of SOC ontent n e oserve. This ws likely ue to lower ontriutions of orgni mtter from root iomss in the P-23 mislope position ompre to the mislope positions on oler estlishe priries (tle 2). In ition, it is well known tht in the mislope position, there is greter potentil of C loss ue to soil erosion ompre to the summit n toe-slope positions (Hook n Burke 2). This might explin why there is n initil loss of SOC ontent in the erly yers of pririe estlishment in the mislope position. Generlly in this stuy, vegettion (oven elow-groun plnt iomss) n iogeohemil properties (SOC, TN) were gretly reue in the mislope position n were enhne in the toe-slope position s esrie in tles 1 to 3. The gretest potentil mehnism for soil erosion in these sites ws wter runoff ething soil prtiles n epositing them on the toe-slope position. In the NT-23 site, signifint inreses in SOC ontent were oserve over two yers in the summit n mislope positions ut not in the toe-slope position (figure 1). These results vrie from the P-23 site, where ereses in SOC ontent were oserve in the mislope position, even though tillge opertions h ese t oth sites in the sme yer. This might e ttriute to the onservtion prties in the NT-23 site, suh s ontour plnting n leving rop resiue on the surfe, reuing soil erosion. The P-Remnnt site on verge h greter SOC ontent in the summit n mislope positions thn i the other sites, lthough SOC in the toe-slope position ws similr to tht of the reonstrute priries n NT- 23 sites (figure 2). Rte of hnge in MBC vrie y site n slope position (figure 3). The rte of hnge or inrese in MBC of the P-Remnnt site ws signifintly lower thn the newly reonstrute priries t ll slope positions n the NT-23 t the toe-slope position (figure 3). Miroil iomss C is n initor of positive or negtive hnges in the soil C pool s influene y hnges in ln use, lnspe position, n mngement. The Figure 3 Chnge in miroil iomss ron s influene y slope position n site from 25 to 27. Tretments within the sme slope position with the sme letter re not signifintly ifferent oring to lest-squres mens test t p.1. Error rs inite stnr error. Miroil iomss ron (g m 2 y 1 ) Summit Mislope Toe-slope Slope position NT-23 P-1993 P-23 P-Remnnt P-1998 results suggest tht the higher rte of MBC hnge is ssoite with the newly estlishe pririe system. This supports the ie tht the P-Remnnt site is in stle onition, while the reonstrute priries n NT-23 site re unergoing postmngement effets. In generl, yers fter esstion of tillge n estlishment of pririe grsses, positive trens re oserve in MBC ontent (figure 4) ross ll slope positions of reonstrute priries n the NT-23 site. However, the slope of these lines is muh greter t the toe-slope position, whih is refletion of greter orgni C umultion t tht position, ue to eing wetter n oler ompre to summit n mislope positions. Other stuies reporte inreses in MBC ontent s ge of reonstrute priries n grsslns inrese s well (Knops n Tilmn 2; MLuhln et l. 26; Kuhrik 27). Site h signifint effet on soil TN umultion in the top 6 m (24 in) soil epth t ifferent slope positions (figure 5). Chnges in TN ontent sine pririe sites estlishment were not well esrie y liner regression funtion t ll slope positions s oserve with SOC ontent. Inreses or no hnge in soil TN ontent ourre in every site with the exeption of the P-23 site in the toe-slope position (figure 5). Knops n Tilmn (2) showe tht vegettion omposition n signifintly ffet TN n SOC pools. In their stuy, priries with high legume popultions inrese SOC n TN umultion rtes, C3 grsses n fors erese SOC, n C4 grsses inrese SOC only (ue to high ron to nitrogen rtio). This my explin why in the top 6 m soil epth (figure 5), inrese in TN sequestrtion rtes were oserve in sites where high popultion of legumes ourre in this stuy. These sites inlue P-1993 n P-Remnnt s esrie in site esription tles 1 to 3. In ition, signifint inreses in TN sequestrtion rtes were oserve in the P-23 n NT-23 sites in the top 15 m (6 in) soil epth (figure 5). This my e ttriute to the esstion of tillge prties uner row ropping systems suh s no-till n grss systems, whih hve een well oumente in inresing SOC n TN in the soil surfe (Al-Kisi et l. 25). In the top 15 m soil epth, similr results in TN ontent were oserve s in the top 6 m (figure 5). Other stuies reporte inreses in TN sequestrtion rtes in the top 5 to 3 m (2 to 12 in) soil epth only (Gehrt et l. 1994; Frnzlueers n Stueemnn 25; Kuhrik 27), ut hnges in SOC n TN ontent were highly vrile s soil epth inrese, leing to unetetle hnges. 15

8 Figure 4 Miroil iomss ron ontent over 2- to 14-yer perio of reonstrute priries of soil epth of 15 m. A no-till site n remnnt pririe were inlue for omprison with reonstrute priries ut were not inlue in the liner est fit. Eh point represents one plot tht ws verge from 25 to 27. Miroil iomss ron (g m 2 ) Miroil iomss ron (g m 2 ) Miroil iomss ron (g m 2 ) Summit r 2 =.261, p =.362 y = x Remnnt 16 Mislope 14 r 2 =.1522, p = y = x Remnnt 16 Toe-slope 14 r 2 =.8153, p = y = x Remnnt Yers sine estlishment Reonstrute pririe P-Remnnt NT-23 Effets of Lnspe Position n Pririe Age on Cron n Nitrogen Sequestrtion Assoite with Wter-Stle Aggregtes. Initil WSA istriutions y mss in 25 re shown in figure 6. Age of pririe site plye mjor role in the formtion of WSA. In generl, the two youngest (less thn three yers) estlishe sites (P-23 n NT-23 sites) h the lowest perentge of WSA greter thn 1 mm (.4 in) ompre to the oler (greter thn three yers) estlishe priries n P-Remnnt site (figure 6). Conversely, these two sites h greter perentge of WSA less thn 1 mm. This suggests these two sites hve the gretest potentil for inreses in soil ggregtion. It is likely the WSA > 1 mm istriution of these two sites n potentilly e similr to those of other reonstrute priries n P-Remnnt sites within ten yers. These results gree with the finings of Jstrow et l. (1996) tht soil ggregtes in reonstrute priries n reh ggregte size istriutions omprle to remnnt priries in pproximtely 1 yers fter esstion of tillge. Yers fter esstion of tillge n slope position s of 25 lso h n effet on WSA ssoite C istriution y size frtions (figure 7). The P-Remnnt site h its gretest WSA ssoite C onentrtions typilly in the 1 to 4 mm (.4 to.16 in) size frtions, while the lowest onentrtion ourre in the silt n ly frtion (<.53 mm [<.2]), followe y the >4 mm (.16 in) n <.5 mm (<.2 in) size frtions. In the summit n toe-slope positions, the two oler reonstrute pririe sites (P-23 n NT-23 sites) h similr WSA ssoite C istriution s the P-Remnnt site with lower C onentrtions. However, the P-23 n NT-23 sites n two oler reonstrute pririe sites in the mislope position h their gretest WSA ssoite C onentrtion in the.5 to 2 mm (.2 to.8 in) size frtion. Over time these reonstrute priries will eventully evelop similr WSA ssoite C istriutions s the remnnt pririe. Also, these progressive hnges in WSA n result in potentil inreses in ggregte-ssoite C onentrtion for WSA greter thn.53 mm (.2 in). This my suggest tht there is shift of WSA ssoite C from.5 to 2 mm ggregtes to lrger ggregtes (2 to 4 mm). Mny stuies hve shown tht when soil is left unisture n there is suffiient C input, mroggregte formtion is 16

9 inrese s well s miroggregtes within mroggregtes (Six et l. 1999), whih n e mjor eterminte in long-term SOC sequestrtion. Signifint inreses of WSA ssoite C onentrtion etween 25 n 28 were oserve only in the.25 to 2 mm (.1 to.8 in) size frtions n vrie with site n slope position (figure 8). In generl, the NT-23 n P-23 sites h the gretest inreses in WSA ssoite C onentrtion in the summit position. However, in the mislope position, the two oler estlishe priries, P-1998 n P-1993, h the gretest inreses in WSA ssoite C onentrtion. This ws expete, sine the mislope position h lower SOC onentrtion in 25 thn the summit n toe-slope positions, whih resulte in higher potentil for SOC sequestrtion rte. Lower C input n greter erosion uring the first few yers of pririe estlishment might explin the low WSA ssoite C onentrtion in the P-23 mislope position. In ition, the P-Remnnt site ws onsistently mong the lowest in WSA ssoite C sequestrtion rtes, suggesting this site might hve rehe equilirium (Guzmn n Al-Kisi 29). Only smll hnges in WSA ssoite C onentrtion were oserve in the toe-slope position, whih my e ttriute to greter SOC onentrtion ompre to the summit n mislope positions (tle 4). Sustntil inreses in SOC were not expete sine ifferenes etween remnnt pririe n reonstrute priries were not signifint. Inreses in WSA ssoite C onentrtion in.25 to 2 mm (.1 to.8 in) size frtions (figure 8) seeme to oinie with inreses in SOC sequestrtion (figure 1) proviing linkge etween the two. Although one oneivly might think tht lrge mroggregtes (>2 mm) shoul hve greter SOC sequestrtion rtes thn intermeite mroggregtes (.25 to 2 mm) n miroggregtes (.53 to.25 mm [.2 to.1 in]), in prt ue to lrge mroggregtes eing omprise of mny miroggregtes oun with freshly e orgni mterils (Oes 1984; Six et l. 22), the freshly e C tht ins miroggregtes into lrger mroggregtes is onsiere to e soil C unprotete from eomposition y miroorgnisms n more suseptile to isruptive proess suh s wter movement. This implies tht reently Figure 5 Soil totl nitrogen (TN) sequestrtion rtes for eh site from 25 to 27 in the () top 6 m n () 15 m epths s influene y slope position. Tretments within the slope position with the sme letter re not signifintly ifferent oring to the lest-squres mens test t p.1. Error rs inite stnr error. () () TN sequestrtion rte (Mg h 1 y 1 ) TN sequestrtion rte (Mg h 1 y 1 ) Summit Mislope Toe-slope Slope position NT-23 P-23 P-1998 Summit Mislope Toe-slope Slope position P-1993 P-Remnnt 17

10 Figure 6 Distriution of wter-stle ggregtes frtions s influene y site n slope position in 25 for eh site. Tretments within the sme size frtion with the sme letter re not signifintly ifferent oring to the lest-squres mens test t p.1. Error rs inite stnr error. Wter-stle ggregtes (%) Wter-stle ggregtes (%) Wter-stle ggregtes (%) Summit Mislope < to.25 to.5 to 1 1 to 2 2 to 4 > NT-23 P-23 Toe-slope Size frtion (mm) NT-23 P-23 < to.25 to.5 to 1 1 to 2 2 to 4 > P-Remnnt < to.25 to.5 to 1 1 to 2 2 to 4 > e C input from plnt mterils is less importnt for long-term SOC sequestrtion thn physilly protete C in miroggregtes (Monrel n Kom 1997) n intermeite mroggregtes (.25 to 2 mm) s suggeste in this stuy. However, inputs of plnt resiues n le to greter umultion of C ining gents tht further stilize miroggregtes n intermeite-mroggregtes within lrger mroggregtes over time, whih provies further protetion for freshly e C from miroil eomposition tivities (Six et l. 1999) n inreses WSA formtion (Six et l. 2; Mikh n Rie 24). Differenes in WSA ssoite TN onentrtion etween sites ross ll slope positions were not oserve (t not shown). One gin, lthough site h signifint effet on WSA ssoite TN ontent, type of vegettion (fors, legume, n C3 or C4 grsses) n its C/N rtio n lignin ontent seem to e the etermining ftors, in ition to ge of pririe site n slope position effets. The WSA ssoite TN sequestrtion rtes were gretest in sites n slope positions where high popultions of legumes ourre, suh s the P-1993 n P-Remnnt in the summit n P-23 in the toe-slope position s esrie in tles 1 to 3. In the NT-23 site, plnt vegettion vrie y yer, sine it ws in orn soyen rottion (25 n 27 in orn), n N fertilizer ws pplie when orn ws plnte, resulting in muh ifferent N ynmis when ompre to the priries. In ition, size of the frtion ws not signifint ftor in WSA ssoite TN sequestrtion rtes, suggesting tht soil TN ws not signifint ftor in ggregte formtion. Summry n Conlusions Slope position n ge sine estlishment h signifint impt on SOC sequestrtion rtes in the top 15 m (6 in) soil epth only. At the summit position, the youngest estlishe pririe (P-23) h the gretest SOC sequestrtion rte t 2.15 Mg SOC h 1 y 1 (.96 tn SOC 1 yr 1 ), lthough rtes shrply erese s ge inrese over 14-yer perio. Another interesting fining of this stuy ws tht SOC inrese linerly over the 14-yer perio t rte of.73 Mg SOC h 1 y 1 (.33 tn SOC 1 yr 1 ). However, there ws greter vriility in SOC sequestrtion rtes t the mislope n toe-slope positions ompre to the 18

11 summit position. For the most prt, this oul e expline y losses of SOC n proutivity of soil in the mislope position when soil erosion ws high n eposition of SOC umulte in the toe-slope position. As result, SOC sequestrtion rtes in reonstrute priries in the mislope position were negtive, espeilly uring the first five yers efore inreses were oserve. At the toe-slope position, n verge sequestrtion rte of.59 Mg SOC h 1 y 1 (.26 tn SOC 1 yr 1 ) ws etermine over the first 14 yers sine pririe estlishment. For soil TN sequestrtion rtes, slope position n ge sine estlishment were not signifint eterminnts. Inste, vegettion type for eh site n slope position ws etermine to e the min ftor in TN pools in these reonstrute priries. In the NT-23 ropln site, SOC ontent inrese similrly to tht of the P-23 site in the summit n toe-slope positions. However, in the mislope position of the NT-23 site, inreses in SOC ontent were similr to tht of the summit position, ue to resiue n ontour plnting prties reuing soil erosion ompre to the P-23 site. The P-Remnnt site, whih typilly h the gretest SOC ontent ross ll slope positions, h no signifint hnge in SOC rte. Slope position n ge of pririe or time sine esstion of tillge lso h signifint impt on WSA formtion. In generl, the two most reently estlishe sites (P-23 n NT-23) h the lowest perentges of WSA greter thn the 2 mm (.8 in) frtion ross ll slope positions. However, formtion of mroggregtes of newly estlishe sites ourre rpily n oul potentilly reh similr WSA perentge istriutions to tht of oler reonstrute n remnnt priries within pproximtely 1 yers. Furthermore, the gretest inreses in WSA ssoite C onentrtion were oserve in the 1 to 2 mm (.4 to.8 in) frtion, n the lest were in the <.53,.53 to.25, n >4 (<.2,.2 to.1, n >.16 in) frtions. In summry, this stuy provies eviene for the existene of feek etween the formtion n stiliztion of WSA ggregtes n SOC sequestrtion s ffete y time sine estlishment (or esstion of tillge) n slope position effets. The finings of this reserh showe tht the gretest potentil for SOC sequestrtion ws ssoite with the newly estlishe pririe sites, where SOC ontent ws most signifintly Figure 7 Wter-stle ggregtes ssoite ron istriution y size frtions s influene y site n slope position in 25. Tretments within the sme site with the sme letter re not signifintly ifferent oring to the lest-squres mens test t p.1. Error rs inite stnr error. Aggregte ssoite ron (g C per ggregte) Aggregte ssoite ron (g C per ggregte) Aggregte ssoite ron (g C per ggregte) NT-23 P-23 P-1998 P-1993 P-Remnnt e ef f <.53 mm.53 to.25 mm Summit Site e e.25 to.5 mm.5 to 1 mm e NT-23 P-23 P-1998 P-1993 P-Remnnt Mislope Toe-slope NT-23 P-23 P-1998 P-1993 P-Remnnt f e 1 to 2 mm 2 to 4 mm e >4 mm 19

12 Figure 8 Chnges in wter-stle ggregtes ssoite soil orgni ron (SOC) onentrtion y size frtion s influene y site n slope position etween 25 n 28. The otte lines represent the signifint level of hnge in SOC rte from zero hnge in SOC for eh site within eh frtion, oring to the lest-squres mens test t p.1. Error rs inite stnr error. Chnge in SOC (mg C g 1 y 1 ) Chnge in SOC (mg C g 1 y 1 ) Chnge in SOC (mg C g 1 y 1 ) Summit < to 1 1 to 2 2 to 4 >4 to.25 to.5 < to 1 1 to 2 2 to 4 >4 to.25 to.5 Size frtion (mm) NT-23 P-23 Mislope < to 1 1 to 2 2 to 4 >4 to.25 to.5 Toe-slope P-1998 P-1993 P-Remnnt eplete ompre to nery P-Remnnt site. This usully ourre in the most reently estlishe pririe sites, n the mislope position, where soil erosion h ourre. In ition, inreses in SOC sequestrtion rtes oinie with inreses in WSA ssoite C onentrtion in the intermeite mroggregtes n miroggregtes (.25 to 2 mm [.1 to.8 in] size frtions). This provies strong eviene of the reltionship etween SOC sequestrtion n the mehnism of soil ggregte formtion tht is in proteting miroggregtes from miroil eomposition. Referenes Akl, V.A., n R. Ll. 21. Soil orgni ron pools n sequestrtion rtes in relime mine soils in Ohio. Journl of Environment Qulity 3(6): Al-Kisi, M.M., X. Yin, n M.A. Liht. 25. Soil ron n nitrogen hnges s ffete y tillge system n rop iomss in orn-soyen rottion. Applie Soil Eology 3(3): Ber, S.G., D.J. Kithen, J.M. Blir, n C.W. Rie. 22. Chnges in eosystem struture n funtion long hronosequene of restore grsslns. Eologil Applitions 12(6): Blke, G.R., n K.H. Hrtge Bulk ensity. In Methos of Soil Anlysis Prt 1: Physil n Minerlogil Methos 2n e, e. A. Kluste, Mison WI: Agronomy Soiety of Ameri n Soil Siene Soiety of Ameri. Blno-Cnqui, H., n R. Ll. 24. Mehnisms of ron sequestrtion in soil ggregtes. Critil Reviews in Plnt Sienes 23(6): Brynt, A.A., n J.R. Woster Soil survey of Wrren ounty, Iow USDA-SCS. Wshington DC: US Government Printing Offie. Brye, K.R., n C.J. Kuhrik. 23. Cron sequestrtion in two pririe topo- hronosequenes on ontrsting soils in southern Wisonsin. Amerin Miln Nturlist 149(1):9-13. Brye, K.R., C.P. West, n E.E. Gur. 24. Soil qulity ifferenes uner ntive tllgrss pririe ross limosequene in Arknss. Amerin Miln Nturlist 152(2): Cmrell, C.A., n E.T. Elliot Cron n nitrogen istriution in ggregte from ultivte n ntive grssln soils. Soil Siene Soiety of Ameri Journl 57(4): Elliott, E.T Aggregte struture n ron, nitrogen, n phosphorus in ntive n ultivte soils. Soil Siene Soiety of Ameri Journl 5(3): Feller, C., n M.H. Bere Physil ontrol of soil orgni mtter ynmis in the tropis. Geoerm 79(1):

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