Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study

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1 Journl of Environmentl Protetion, 21, 1, doi:1.4236/jep Pulished Online June 21 ( Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study Qingren Wng 1,2*, Yunong Li 1,2, Ashok Alv 3 1 Tropil Reserh nd Edution Center, Homested, USA; 2 Deprtment of Soil nd Wter Siene, University of Florid, Ginesville, USA; 3 USDA, ARS, Vegetle nd Forge Crops Reserh, Prosser, USA. Emil: qrwng@ufl.edu Reeived Mrh 1 th, 21; revised April 13 th, 21; epted April 14 th, 21. ABSTRACT Cover rop system hs shown potentil pproh to improving ron sequestrtion nd environmentl qulity. Six of eh winter nd summer over rops were susequently grown in two soils, Krome grvelly lom soil (KGL), nd Quiny fine sndy soil (QFS), in phytotrons t 3 tempertures (1/2, 15/25, 25/3 o C for winter/summer over rops) to investigte their ontriutions for ron (C) sequestrtion. Among winter over rops, the highest nd the lowest mounts of C umulted were y ellen (Vii f L.), 597 g/m 2 nd white lover (Trifolium repens), 149 g/m 2, respetively, in the QFS soil. Among summer over rops, sunn hemp (Crotlri june L.) umulted the lrgest quntity of C (481 g/m 2 ), while tht y storen (Riinus ommunis) ws 12 g/m 2 t 3 o C in the KGL soil. The men net C remined in the residues following the 127 d deomposition were 187 g/m 2 of C (73% of the totl) nd 91 g/m 2 (52% of the totl) for the winter nd summer over rops, respetively. Following whole yle of winter nd summer over rops grown, the men soil orgni C (SOC) inresed y 13.8 nd 39.1% in the KGL nd QFS soil, respetively, ompred to the respetive soils efore. The results suggest tht tritile, ryegrss, nd ellen re the promising winter over rops in the QFS soil, while sunn hemp, velveten (Muun pruriens), nd sorghum sudngrss (Sorghum iolor S. iolor) re reommended summer over rops for oth soils under fvorle tempertures. Keywords: Cron to Nitrogen Rtio (C:N), Greenhouse Gs (GHG), Krome Grvelly Lom (KGL), Quiny Fine Snd (QFS), Soil Orgni Cron (SOC) 1. Introdution Soil ron (C) sequestrtion y terrestril vegettion, s one of the min pprohes for greenhouse gs (GHG) mitigtion, hs long een identified y the Intergovernmentl Pnel on Climte Chnge [1]. Terrestril eosystems ssoited with lnd use nd soil mngement ply n importnt role in the glol C udget [2]. For exmple, the urrent glol terrestril sink for C is estimted to hold 55-7 Pg of C in vegettion nd Pg in soil orgni mtter [3]. Soil is the lrgest terrestril C pool, onstitutes t 25 Pg of totl C (orgni nd inorgni) within one meter depth [4]. This soil C pool is pproximtely two-thirds of the totl C in eosystems [5]. The former is out 3.3-fold greter thn the tmospheri C pool (76 Pg), nd 4.5-fold greter thn the ioti C pool (56 Pg) [6]. In ddition, soil orgni ron (SOC) pools hve the slowest turnover rtes in generl terrestril eosystems [7], therefore C sequestrted in soils hs gret potentil to mitigte CO 2 emission to the tmosphere [3]. Inresed fixtion of tmospheri CO 2 with terrestril vegettion, nd in turn, ontriuting to enhned SOC leds to redution in GHS emissions nd relted negtive effets on the environment. However, the effiieny of C sequestrtion y vrious vegettions differs lrgely s influened y differenes in their physiologil hrteristis, growth rtes, iomss umultion rtes, et., nd y mny environmentl ftors, suh s the soil type, temperture, et. Mximizing iomss prodution y optimizing input use is mjor gol in gro-eosystems. Conversion of plnt sequestered C to SOC is importnt sine the ltter is very stle with long residene time, i.e., hundreds nd even thousnds of yers [8]. Agriulturl soils under p-

2 74 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study proprite mngement n ontin sustntil mounts of soil C in the forms of soil orgni mtter (SOM). Exluding ronted roks, soils onstitute the lrgest surfe C pool, pproximtely 15 Gt, whih is equivlent to lmost three fold greter thn the quntity stored in the terrestril iomss nd twie the mount stored in the tmosphere [9]. Cover rops provide n effetive prtie to enhne SOC [1-13] in ddition to their role in improving soil nd wter onservtion [14]. Cover rops n lso enhne soil fertility nd produtivity for sustinle griulturl prodution [15,16]. A liner reltionship hs een reported etween the mount of C sequestered in the soil nd C input s plnt iomss or residues y numer of reserhers [17-19]. Removl of over rop tops from the soil gretly deresed soil C [2]. The growth rtes nd mounts of over rop iomss s well s sequestered CO 2 C differed mong different over rop speies nd environmentl ftors [3]. In wide rnge of grolimti regions, winter over rops re generlly grown during fll through spring. Summer over rops filitte onserving soil nd wter during the riny summer seson, improving soil fertility, leding to inresed yields nd qulity of the susequent sh rops [14-16,21]. In Cndin priries, growing summer over rops insted of fllow sequestered pproximtely 1.5 Tg CO 2 per yer from the tmosphere [22]. To improve C sequestrtion effiieny, plnt sequestered C in orgni forms need to e trnsferred to stle forms, suh s relitrnt SOC vi humifition or roniztion proesses. The stility of orgni C in plnt residues or in soil depends on the over rop speies used in the prodution system in ddition to effets of environmentl ftors, inluding soil type, temperture, nd moisture. Orgni C in plnts omprises tive nd intive omponents, whih lso refer to s lile nd relitrnt pools [23]. The tive orgni C onsists of four frtions; i.e., deomposle orgni C, resistnt orgni C, miroil iomss orgni C nd humified orgni C [24]. The reltive distriution of orgni C in the ove frtions depends on the physiologil nd hemil hrteristis of plnt residues, i.e., C:N rtio nd lignin ontent. Most of the previous studies on the role of over rops on C sequestrtion or SOC umultion re exlusively relted to griulturl prties: suh s tillge, ropping systems, rop rottion, lnd use or shifting ultivtion, fertiliztion, et. [9,12,13,19,2,25,26]. Informtion is lking on the influene of different over rop speies on the effiy of soil C sequestrtion. In ddition, it is hrd if it is not impossile to evlute oth winter nd summer over rops under filed onditions. Therefore, the ojetive of the urrent reserh ws, under the ontrolled environment, to eluidte effets of soil nd temperture onditions on the quntities of C umultion, iomss prodution, minerliztion rtes nd effiieny of C sequestrtion mong different winter nd summer over rops typilly grown in the temperte nd sutropil regions. 2. Mterils nd Methods 2.1 Soils nd Cover Crops A Krome grvelly lom (KGL) soil (lomy-skeletl, ronti, hyperthermi Lithi Udorthents) from Mimi Dde County, FL, nd Quiny fine sndy soil (mixed, mesi Xeri Torripsmments) from Benton County, WA, were used in this study. Some hrteristis of the ove soils re shown in Tle 1. The ove soils smpled t -15 m depth were sieved to remove lrge roks nd plnt residues. Plsti pots, 25 m dimeter, 23 m high, 11.3 liters in volume with pity of 8 kg soil per pot, were used. Six eh winter nd summer over rops, inluding three eh legumes nd nonlegumes were evluted. The winter over rops used in the experiment were in the order of: tritile (Tritile hexploide Lrt.), ryegrss (Lolium perenne ssp. Multiflorum), mustrd (Brssi june, ssp. Indin gold), ell ens (Vii f L.), purple veth (Vii enghlensis L.), nd white lover (Trifolium repens). The summer over rops inluded in the order of: sorghum sudngrss [Sorghum iolor S. iolor vr. sudnense (Piper) Stpf.], okr (Aelmoshus esulentus L.), stor en (Riinus ommunis), sunn hemp (Crotlri june L. v. Tropi Sun), velveten [Muun pruriens vr. utilis (Wll. ex Wright) Bker ex Burk], nd owpe (Vign unguiult L., v. Iron Cly). ph (wter) Tle 1. Seleted hrteristis of the two soils used in this study EC (µs/m) Orgni C (g/kg) CCO 3 equivlent (g/kg) Totl N (g/kg) Totl P (g/kg) KCl extrtle NH 4 -N (mg/kg) KGL soil ± QFS soil ND Soil ph ws mesured in rtio of 1:2.5 (soil: wter). Soil eletril ondutivity (EC) ws mesured in rtio of 1:2 (soil: wter). ± KGL = Krome grvelly lom, nd QFS = Quiny fine snd. ND: not deteted. NO 3 -N (mg/kg)

3 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study Experimentl Design, Phytotron Setup nd Mngement A nested ftoril design ws dopted with temperture s min ftor, soil types nd 6 over rop speies (for either winter or summer over rops) were nested s suftors. The experiment ws onduted in three individul phytotrons (Conviron 861, Conviron Produts Compny, Winnipeg, Cnd) to simulte different tempertures ut kept other prmeters, e.g., light intensity, reltive humidity nd dy length, et. the sme to reprodue the growth onditions for the winter nd summer over rops with respetive fllow. For winter over rops (Novemer 2 th, 27 through Mrh 7 th, 28), tempertures were: 1/8, 15/1, nd 2/15 o C (dy/night, d/n), following the seed germintion t 2 o C ross ll 3 tretments ove. Light intensity ws grdully inresed or deresed % per hour with rmp proedure with 1 h dy length. The light intensity ws µmol s -1 m -2, lirted y light intensity sensor (LI-COR, Quntum with LI-1 dtlogger), nd reltive humidity of 5/75% (d/n). The tempertures for summer over rops (April 9 th through June 25 th, 28) were 2/15, 25/2, nd 3/25 o C (d/n), the dy length of 14 h. Light intensity ws sme s tht used for the winter over rops, with reltive humidity of 75/85% (d/n). Plnt density ws 1 per pot for okr, ellen, purple veth, sorghum sudngrss nd sunn hemp; 3 for white lover, ryegrss nd tritile; while 3-5 per pot for stor en, velveten, mustrd nd owpe. Three grm of fertilizer (1 N-4.3 P-8.3 K) ws pplied per pot. No inoultion ws pplied to legumes. All plnts were irrigted through drip line to djust flow rte of 2 L/h, the frequeny nd durtion were determined sed on plnt growth stges for quntities of wter required. With seprte experiment, over rop deomposition rtes were preliminrily evluted in n extr phytotron with the single (KGL) soil individully for oth winter nd summer over rops. In this experiment, the sme over rop speies s in the previous study were grown nd hrvested t the sme temperture in the sme (KGL) soil, the oveground iomss ws ut into pproximtely 1-m piees, the fresh weight nd moisture ontent were determined with susmples nd those susmples were lso used for hemil nlysis. The ertin mount of fresh over rop residues ws evenly distriuted on the soil surfe of the sme pot used for respetive over rop growth. Residue surfe pplition insted of soil inorportion ws to extend the residue residene time in similr field pproh of no tillge prtie. Wter ontent of the soil ws mintined t 75% of the field pity for the respetive soil y weighing the pots one week. Tempertures mintined t 15 o C for winter nd 25 o C for summer over rops, nd the sme lighting onditions tht desried ove for the winter nd summer over rop growth studies were dpted. At the end of 127 d minerliztion study, the remining plnt residues were refully removed from eh pot, dried nd weighed, nd susmples were tken gin for hemil nlysis. The mss weight loss of plnt residues due to deomposition ws lulted sed on the differene etween the mount of residues pplied nd the mount remined fter deomposition. The totl mount of C deomposed ws lulted sed on the residue weight loss nd the residue C onentrtions determined in the susmples. The onentrtions of SOC nd totl N were determined efore nd fter the deomposition experiment. 2.3 Smpling nd Chemil Anlysis Soil smples were olleted from the enter of eh pot t -1 m depth prior to nd one month fter over rop growth. Soil smples were ir dried nd ground to pss through < 1 mm mesh sieve for hemil nlysis. The experiment ws terminted t the time when ny one of these over rops ws flowering, whih ws out 9 dys for oth winter nd summer over rops under the experimentl onditions. The oveground plnt iomss ws hrvested, nd fresh nd dry weights (t 75 o C for 7 d) of iomss were reorded. A susmple of the oveground iomss ws ground to pss through <.5 mm mesh sieve for hemil nlysis. Totl C nd nitrogen (N) ontents in the soil nd over rops were nlyzed using CNS Auto-nlyzer (Vrio Mx Elementr, Hnu, Germny). Soil inorgni C ws determined vi pressure limeter method nd the orgni C ws lulted y sutrting the inorgni C from the totl C [27]. 2.4 Sttistil Anlysis of the Dt The dt were sujeted to nlysis of vrine (ANOVA) using SAS [28] with nested design nd generl liner model. For the ANOVA with the nested design in the experiment, three different error strutures were onduted to ompre men squres of different soures of vrition. The min ftor ws tested ginst the min ftor error, where the su-tretment ws tested ginst the intertion of replites (or lok) su-tretment nd the intertion etween the min ftor nd the su-ftor were tested ginst the su-ftor error [29]. Further nlysis ws onduted for eh individul ftor to seprte mens with the single ftor or omintion of the ftors for the intertion effets using Dunn test t p =.5, s needed. 3. Results nd Disussion 3.1 Cover Crop Biomss, nd C nd N Contents Totl C nd N s well s iomss prodution of oth winter nd summer over rops were signifintly influ-

4 76 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study ened y over rop speies nd growth tempertures (Tle 2). The soil type hd no signifint influene on the ove prmeters for the summer over rops, while in the se of winter over rops, it signifintly influened the iomss prodution nd the totl N. Also, signifint intertion effets were found for temp rop for oth winter nd summer over rops on totl C nd N, nd soil rop for winter over rops for ll three evlution prmeters. Result implies tht soil types hve strong influene on the iomss prodution nd totl quntity of N umultion for the winter over rops rther thn the summer over rops. The growth tempertures rther thn soil types influene the iomss prodution nd the C:N rtio for the summer over rops rther thn the winter over rops nd signifint intertion effet (temp rop) ourred. 3.2 Totl C in Aoveground Biomss Winter over rops: Totl C in the oveground iomss vried mong the winter over rops (Tle 3). In the QFS soil, totl iomss C (men ross ll tempertures) ws greter in ellen s ompred to tht in the remining speies, exept tritile. In the KGL soil, totl iomss C deresed in the order: tritile > ryegrss > ellen = mustrd = purple veth = white lover. Overll iomss totl C ws signifintly greter in the QFS soil s ompred to tht in the KGL soil for ll over Tle 2. Anlysis of vrine (ANOVA) for iomss prodution nd totl iomss C nd N for winter nd summer over rops df F-vlue Cron Nitrogen Biomss C:N rtio Winter over rop Temperture (Temp) * 8.83 * ** ** Soil NS ** 5.94 * 5.94 * Cover rop (Crop) *** *** *** *** Temp Soil NS 3.31 NS 4.72 NS 4.72 NS Temp Crop *** 5.34 *** 2.7 NS 2.7 NS Soil Crop ** *** ** *** Temp Soil Crop NS 6.11 *** 4.46 ** 4.46 ** Summer over rop Temperture (Temp) * 1.9 * 1.45 *.89 NS Soil NS.72 NS 2.1 NS ** Cover rop (Crop) *** *** *** 9.94 *** Temp Soil NS.9 NS 2.6 NS 1.3 NS Temp Crop *** 3. ** 5.27 *** 4.6 ** Soil Crop 5.75 NS 2.21 NS.79 NS 6.75 ** Temp Soil Crop NS 2.89 * 2.2 NS 5.41 ** *Signifint t p.5; ** signifint t p.1; *** signifint t p.1; nd NS: no signifint differene t p.5. Tle 3. Totl C (g/m 2 ) in oveground iomss of winter over rops Soil Temp Tritile Ryegrss Bellen Mustrd Purple veth White lover QFS soil 25 o C 369 * o C o C d KGL soil 25 o C o C d 132 d 139 d 1 o C Mens QFS soil 388 A 344 A 435 A 299 A 285 A 122 Ad KGL soil 295 B 252 B 188 B 169 B 153 B 15 A 25 o C 317 A 285 A 379 A 24 A 261 A 191 A 15 o C 364 A 326 A 326 AB 284 A 27 A 139 B 1 o C 344 A 282 A 23 B 214 A 189 A 8 Cd KGL= Krome grvelly lom; nd QFS = Quiny fine snd. * Vlues followed y different letter (s), lower se within the sme row, nd upper se within the sme olumn of suset (either the soil or temperture), represent signifint differene t p.5.

5 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study 77 rops exept white lover. Temperture effet ws signifint only in ell en nd white lover speies. The gretest mount of iomss C (597 g/m 2 ) ws otined in ellen grown in the QFS soil t 25 o C. At 1 o C in the QFS soil, iomss C ws greter y tritile thn tht y the other over rops. In the KGL soil t 25 o C, the iomss C ws greter in tritile thn tht in either purple veth, mustrd or ellen. At 15 nd 1 o C, the iomss C in tritile nd ryegrss were greter thn tht in the remining over rops. Summer over rops: In the QFS soil t 3 o C, iomss C in sunn hemp ws signifintly greter tht y storen, okr, nd owpe (Tle 4). At 25 o C, sunn hemp iomss C ws signifintly greter thn tht y the remining five over rops. At 2 o C, owpe iomss C ws signifintly lower thn tht y the other over rops. Similrly, in the KGL soil, the reltive rnking of iomss C response mong the six over rop speies ws different t different tempertures. Soil type influene (men ross ll tempertures) ws signifint on iomss C only in sorghum sudngrss. Biomss totl C ws signifintly greter t 3 o C s ompred to tht t either 2 or 25 o C for sunn hemp, velveten, nd sorghum sudngrss. In generl, iomss C umultion ws greter in the winter thn in the summer over rops under the experiment onditions (Tles 3 nd 4). 3.3 Aoveground Biomss nd Totl N in Vrious Cover Crops The oveground iomss nd totl N response followed somewht similr pttern s totl C umultion mong vrious over rops (Tle 5). The men iomss prodution nd totl N of winter over rops were generlly greter in the QFS thn those in the KGL soil. No signifint differene ws oserved etween these two soils for summer over rop iomss nd/or totl N ut these summer over rops umulted more iomss nd N t high temperture thn t low temperture. This result hs onfirmed tht the soil type hd greter influene on the growth nd umultion y the winter over rops. Temperture ppered to hve dominnt influene on the ove response vriles of the summer over rops. Among the summer over rops, sunn hemp produed the gretest mount of iomss (Tle 5), equivlent to 11 Mg h -1 t 3 o C. This grees with results of our prllel field studies [21,3], whih showed 15-2 Mg h -1 of oveground iomss y sunn hemp. Nitrogen under some growth onditions is dominnt ftor to limit the iomss prodution nd C umultion. Therefore, legume over rops, y virtue of their ility to fix tmospheri N, n overome this limittion, thus re le to produe greter mount of iomss s ompred to tht y nonlegume over rops [12,3]. Some winter over rops, suh s ellen nd purple veth, umulted 3-35 g/m 2 of N under the optiml onditions (25 o C) in the QFS soil (Tle 5). All winter over rops, exept white lover, umulted greter mount of N while grown on the QFS soil thn tht on the KGL soil. A similr trend ws lso oserved with respet to umultion of orgni C (Tle 3). Tle 4. Totl C (g/m 2 ) in oveground iomss of summer over rops Soil Temp Sunn hemp Velveten Cstoren Sorghum sudngrss Cowpe Okr QFS soil 3 o C 48 * o C o C KGL soil 3 o C o C o C Mens QFS soil 338 A 183 A 134 A 222 A 89 A 136 A KGL soil 274 A 228 A 63 A 136 B 78 A 138 A 3 o C 481 A 32 A 133 A 255 A 134 A 142 A 25 o C 257 B 159 B 72 A 125 B 78 AB 112 A 2 o C 181 B 137 B 91 A 157 B 39 B 157 A KGL= Krome grvelly lom; nd QFS = Quiny fine snd. * Vlues followed y different letter (s), lower se within the sme row, nd upper se within the sme olumn of suset (either the soil or temperture), represent signifint differene t p.5.

6 78 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study Tle 5. Totl oveground iomss nd N (g/m 2 ) in winter nd summer over rops 25 o C/3 o C 15 o C/25 o C 1 o C/2 o C QFS KGL QFS KGL QFS KGL Winter over rops Biomss TC ± (g/m 2 ) RG BB MT PV d d 495 d 428 WC d 22 N (g/m 2 ) TC RG BB MT PV WC Summer over rops Biomss SH (g/m 2 ) VB CB SS CP OK N (g/m 2 ) SH VB CB SS CP OK The first temperture ws for winter nd the seond for summer over rops. ± TC: tritile, RG: ryegrss, BB: ellen, MT: mustrd, PV: purple veth, WC: white lover, SH: sunn hemp, VB: velveten, CB: storen, SS: sorghum sudngrss, CP: owpe, nd OK: okr. Vlues followed y different letters within the sme olumn of suset (y winter or summer over s nd y eh response prmeter) represent signifint differene t P Conentrtions of C nd N in Cover Crop Biomss Among the winter over rops, C onentrtion ws signifintly lower in white lover (36%) s ompred to tht in the remining five winter over rops (-43%) (Figure 1). Biomss N onentrtions were in the rnge of % for legumes, whih ws signifintly greter thn the rnge of.9-1.1% for the nonlegumes. The C:N rtio vried from 13.9 (white lover) to 52.4 (tritile) mong these six winter over rops. The men C onentrtion s well s C:N rtio ross ll winter over rops were greter for the KGL thn those for the QFS soil, while the onverse ws oserved for N onentrtions. The temperture effet ws not signifint on onentrtions of C nd N ross ll winter over rops nd soil types ut the C:N rtio ws signifintly lower t 1 o C thn tht t other tempertures (Figure 1). The C onentrtions in the summer over rop iomss were signifintly greter for sunn hemp, velveten, sorghum sudngrss nd owpe followed y storen nd okr (Figure 2). The N onentrtion ws the gretest for velveten, while the lowest for sorghum sudngrss. As result, the C:N rtio followed the pttern: sorghum sudngrss > storen = okr > sunn hemp = velveten = owpe. Overll, there ws no signifint differene found etween soil types for the C onentrtion ut plnts grown in KGL soil hd greter N onentrtion thn those in the QFS soil, whih resulted in greter C:N rtio of plnt iomss in the ltter thn tht in the former soil. The temperture effet ws non-signifint on the iomss C nd N ontents s well s C:N rtio. This result grees well with the previous studies. For instne, the C onentrtions in vrious winter over rops remined onstnt ut the N onentrtions differed gretly etween legume nd nolegume over orps, whih resulted in lrge vrition in C:N rtio in different over rop iomss [11,32]. 3.5 Residue Deomposition nd C Sequestered The rte of deomposition of rop residues over 127-d under similr onditions s those dpted during the respetive over rop growth period vried mong oth winter nd summer over rops. The totl C remined in the soil following 127 d of deomposition vried from 53 to 79% nd 18-58% of totl C umulted in the respetive winter nd summer over orps (Tle 6). This grees with the reports of Doss et l. [33], i.e % of the C dded s shru residue ws minerlized in 118 d.

7 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study 79 C % C % C % Tritile Ryegrss Bellen MustrdPurple Purple vethwhite White lover Veth lover Sndy soil Grvelly soil N % N % N % Tritile Ryegrss Bellen Mustrd Purple veth White lover 1.3 Sndy soil Grvelly soil C:N rtio Tritile Ryegrss Bellen Mustrd Purple veth Winter over rop d White lover C:N rtio Sndy soil Soil Grvelly soil C:N rtio Temperture ( ) (oc) Figure 1. Conentrtions of C, N nd C:N rtio in winter over rop iomss C % C % C % Sunn hemp Velveten Cstoren Sorghum sudngrss Cowpe Okr 41 Sndy soil Grvelly soil N % d d N % N % Sunn hemp Velveten Cstoren Sorghum sudngrss Cowpe Okr.2 Sndy soil Grvelly soil C : N rtio C : N rtio C : N rtio Sunn hemp Velveten Cstoren Sorghum sudngrss Cover rop Cowpe Okr 3 Sndy soil Soil Grvelly soil Figure 2. Conentrtions of C, N nd C:N rtio in summer over rop iomss Temperture (oc) ( )

8 8 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study Tle 6. Cron umultion in over rop oveground iomss nd tht retined in the soil following 127-d deomposition Cover rop Totl C input (g/m 2 ) Totl C deomposed (g/m 2 ) Amount of C left in residues (g/m 2 ) % of C retined in residues Winter over rops Tritile * Ryegrss Bellen Mustrd Purple veth White lover Men Summer over rops Sunn hemp Velveten Cstoren Sorghum sudngrss Cowpe d 18 Okr Men * Vlues followed y different letter (s) within the sme olumn of suset (winter or summer over rops) represent signifint differene t p.5. Among the winter over rops, totl C input s well s the mount of C left in the residues fter deomposition were greter for tritile, ryegrss nd ellen thn tht y the remining three over rop speies (Tle 6). White lover rnked the lowest for oth prmeters lthough its mount of leftover ws not signifintly different from purple veth. Among the summer over rops, sunn hemp rnked the highest nd owpe the lowest for the totl C left in the residues. The perent of C retined in the residues ws signifintly greter y storen, sunn hemp nd sorghum sudngrss thn tht y owpe. Perent of C retined in the soil ws greter for storen (72%) s ompred to tht for velveten, okr nd owpe (Tle 6). The N onentrtion ssoited with C:N rtio is often n importnt ftor to determine the iomss qulity [34-36] nd iomss deomposition [11,13,33,37-39], whih is losely relted to the C sequestrtion effiieny. The urrent study showed tht the deomposition rte (Tle 6) is relted to the N onentrtion or the C:N rtio, s in the ses of sunn hemp nd velveten ompred to storen or of ellen nd purple veth ompred to mustrd with vrious C:N rtios (Figure 1 nd 2). The quntities of C deomposed following 127 d were quite high for oth tritile nd sorghum sudngrss (Tle 6), whih hd high C:N rtios, 52 nd 67, respetively. Therefore, the iomss C:N rtio is n importnt trit of iomss qulity tht influenes deomposition rte. In ddition, some iohemil properties, suh s lignin, polyphenoli nd tnnin ontents lso influene residue deomposition rte [38]. 3.6 Soil Orgni Cron Chnges with Cover Crops nd Tempertures After winter over rops grown, onentrtions of SOC in either the QFS or the KGL soil showed no signifint differene regrdless of different over rops grown in omprison with fllow (dt not shown). After summer over rops grown, ompred to fllow, no ny signifint hnge in SOC ws oserved in the QFS soil t ll tempertures, nd it seemed hrdly to oserve suh hnge in the KGL soil due to onsiderle flutution even the onentrtion of SOC in one tretment ws signifintly greter thn tht in the other (Tle 7). Short durtion of over rop growth pproved to hve very little influene on the SOC hnges in the soil. However, sine the winter over rop residues were returned to the soil (soil surfe pplied) nd susequently the summer over rops were grown, the SOC t the termintion of summer over rop growth showed some hnges mong the over rop speies though suh hnges were flututed nd not signifint. Therefore, long term tril is needed to monitor SOC hnges with over rops in the griulturl system. The flutution hnges in SOC hve een oserved y other reserhers [13]. Our results gree with tht of Ll [2] who onluded tht the use of over rops s short-term green mnure my not neessrily enhne the SOC pool. However, inreses of SOC ourred fter over rops grown s ompred to the respetive soils efore the experiment. For instne, the SOC ontent (men ross ll winter over rops nd tempertures) inresed y.9 nd 4.8% in the KGL nd QFS soils, respetively, s ompred to the respetive soils prior to the experiment. The inreses in the SOC ontent following the growth of the summer over rops vs. winter over rops were 13.7 nd 25.9% for the respetive soils. The orresponding inreses fter the summer over rops (inluding winter over rop residues returned to the soil) ompred to the SOC prior to the experiment were 13.8 nd 31.9% (Tle 8). The SOC ontent of the QFS soil ws signifi-

9 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study 81 Tle 7. Conentrtions of SOC (g/kg) fter summer over rops grown in different soils t vrious tempertures Soil Temp Sunn hemp Velveten Cstoren Sorghum sudngrss Cowpe Okr Fllow QFS soil 3 o C 3.1 * o C o C KGL soil 3 o C o C o C 14.4 d 11.1 d Mens QFS soil 2.8 B 2.8 B 2.7 B 3.2 B 3. B 2.5 B 2.3 B KGL soil 16.6 A 12.1 A 14. A 17.4 A 18.9 A 14.4 A 19.6 A 3 o C 12.5 A 7.9 A 8.1 A 8.9 B 13.2 A 7.9 AB 11.7 A 25 o C 7.9 A 7.5 A 7.5 A 9.9 AB 8.7 A 6.5 B 12. A 2 o C 8.7 A 7.1 A 9.5 A 12.1 A 11. A 11.1 A 9.1 B KGL = Krome grvelly lom; nd QFS = Quiny fine snd. *Vlues followed y different letter (s), lower se within the sme row, nd upper se within the sme olumn of suset (either the soil or temperture), represent signifint differene t p.5. Soil Tle 8. The overll hnges of SOC (g/kg) efore nd fter over rops grown in different soils Prior to growing over rops After winter over rops grown After summer over rops grown Net hnge (winter over rops vs. prior) Net hnge (summer vs. winter over rops) Net hnge (winter + summer over rops vs. prior) QFS soil % 25.9% 31.9% KGL soil % 13.7% 13.8% KGL = Krome grvelly lom; nd QFS = Quiny fine snd. ntly lower thn tht of the KGL soil regrdless of over rop speies or fllow soil, nd the former soil rther thn the ltter soil hd more SOC inrese fter over rops grown s ompred to the respetive soils efore the over rop growth. This result grees with previous report tht soil with low SOC usully hs more potentil to regin orgni C thn soil with high SOC [2]. Aumultion of SOC is slow nd long-term proess. Sinju [19] reported tht to oserve differenes in SOC under field onditions even with sustntil C inputs y over rops requires more thn two yers. Our previous study [16] with two yers of over rops grown in the field (KGL soil), showed no SOC inrese s ompred to tht of the fllow soil. Inresed ropping intensity in rop rottions y reduing the frequeny of re fllow n inrese rop prodution nd C inputs to the soil [24]. Kuo nd Jellum [11] lso indited tht onentrtions of C nd N in the surfe soil (-15 m) inresed with inresing totl C input from over rops euse sol umultion of C nd N is funtion of the totl input of orgni C []. Bordovsky et l. [41] found tht the surfe (-5 m) SOC onentrtion inresed with time in their 11-yer field experiment following ontinuous ultivtion of grin sorghum [Sorghum iolor (L.) Moenh] nd whet (Tritium estivum L.) in the Miles fine sndy lom soil in Texs. In Georgi otton (Gossypium hirsutum L.) prodution region with winter over ropping system, SOC in the Dothn sndy lom t -1 m inresed y 6-8% over period of 3 yers with winter over rops, suh s rye (Sele erel L.) nd hiry veth (Vii villos Roth). The rte of SOC sequestrtion ws kg C h -1 yr -1 while the rte of loss ws 167 kg C h -1 yr -1 in the soil without these over orps [13]. Conversely, hnging rop-fllow to ontinuous monoulture or rottion ropping, or inresing the numer of rops in rottion system ws less effetive in sequestering SOC s ompred to shifting to no till prtie [26]. Integrted griulturl prties, inluding over rops with no tillge or t lest with onservtion tillge, re needed to improve soil C sequestrtion. Tillge or inorportion of plnt residues into the soil inreses SOC minerliztion [2,42]. In strip- nd hisel-tilled plots, the SOC deresed y 3-17% nd 4-17% in -1 m nd 1-3 m depth, respetively, ut in no till tretments, SOC inresed y 6-8% with winter over rops t -1 m nd y.4% with rye nd 3% with iulture of veth nd rye t -3 m [13]. The enefiil effets of growing over rops in enhning SOC pool hve een reported from round the world [43-45]. Furthermore, the enhnement of C sequestrtion y growing over rops ssoited with onservtion tillge hs een reported y numer of reserhers [12,16,43,46,47]. For exmple, Sinju et l. [32]

10 82 Growing Cover Crops to Improve Biomss Aumultion nd Cron Sequestrtion: A Phytotron Study oserved tht hiry veth under no till n improve SOC, nd over ropping ssoited with N fertiliztion n hve effets in storing SOC in no tilled soils due to the redution in minerliztion rtes of rop residues nd soil orgni mtter. Mety et l. [46] found tht no till with over rop (Crotlri) inresed the storge of C in the topsoil lyer (-1 m) ompred to dis tillge, with the ltter only less thn 1% of over rop residues returned to the soil. No tillge or onservtion tillge n onserve rop residues ut nnot inrese the soil C or iomss input. The SOC umultion or C sequestrtion requires n inrese in orgni mtter or rop residue inputs long with derese in deomposition rte of soil orgni mtter [2,3,48]. Pustin et l. [49] oserved tht SOC inreses linerly with inresed ddition of rop residues. Cover rops or over ropping systems not only serve lrge sink to remove the tmospheri CO 2 ut lso inreses the iomss input into the soil. Therefore, over ropping system omined with the onservtion tillge hs shown gret dvntge in improving C sequestrtion nd sustinle development in griulture. The ontriution of over rops to SOC or ron sequestrtion vi ssimilting tmospheri CO 2 into SOC hs gret potentil in reduing the CO 2 onentrtion from the tmosphere. This potentil my lst t lest few dedes euse the SOC hs een depleted over the world in rle lnd nd my reh new equilirium in 5-1 yers [3,26]. 4. Conlusions The totl mounts of C umulted y oveground iomss vried gretly mong oth winter nd summer over rops. Therefore, hoie of over rops speies is importnt for inresed effiieny in iomss prodution nd ron sequestrtion. Soil nd temperture influened the iomss prodution nd C umultion under some irumstnes. Biomss prodution nd C umultion of most winter over rops, exept white lover, were greter in the QFS soil thn those in the KGL soil. Suh differene ws not evident with respet to the summer over rops. When the oveground over rop iomss ws returned to the soil for deomposition with over 127 dys, out 73 nd 52% of the oveground iomss C ws retined from the winter nd summer over rop residues, respetively. After yer rottion summer over rops following winter over rops, SOC inresed y 13.8 nd 31.9% in the KGL nd QFS soil, respetively, ompred to the respetive soils prior to the experiment. This study hs demonstrted improved SOC umultion y sequestrtion of tmospheri C following the growth of over rops. 5. Aknowledgements The reserh ws founded y USDA-ARS with ollortion reserh progrm. We sinerely thnk Dr. Thoms Dvenport, Mr. Roert Stulefield, Jorge Vergel, Jo Hll, Ms. Guiqin Yu, nd Lur Rosdo t TREC, University of Florid for their supports with phytotron filities nd/or help in smple olletions nd hemil nlyses. REFERENCES [1] R. T. Wtson, I. R. Nole, B. Bolin, N. H. Rvindrnth, D. J. Verrdo nd D. J. Dokken, Lnd Use, Lnd Use Chnge nd Forestry, Intergovernmentl Pnel on Climte Chnge, Cmridge University Press, Cmridge, 2. [2] R. Ll, Soil Cron Sequestrtion to Mitigte Climte Chnge, Geoderm, Vol. 123, No. 1-2, 24, pp [3] K. Pustin, O. Andrén, H. H. Jnzen, R. Ll, P. Smith, G. Tin, H. Tiessen, M. Vn Noordwijk nd P. L. Woomer, Agriulturl Sols s Sink to Mitigte CO 2 Emissions, Soil Use nd Mngement, Vol. 13, No. 4, 1997, pp [4] H. Eswrn, P. R. Reih, J. M. Kimle, F. H. Beinroth, E. Pdmmhn nd P. Monhroen, Glol Cron Stoks, In: R. Ll, J. M. Kimle, H. Eswrn nd B. A. Stewrd, Eds., Glol Climte Chnge nd Pedogeni Crontes, Lewis Pulishers, Bo Rton, 2, pp [5] D. S. Shimel, B. H. Brswell, E. A. Hollnd, R. MKeown, D. S. Ojim, T. H. Pinter, W. J. Prton nd A. R. Townsend, Climti, Edphi, nd Bioti Controls over Storge nd Turnover of Cron in Soils, Glol Biogeohemil Cyles, Vol. 8, No. 3, 1994, pp [6] R. Ll, Cron Sequestrtion in Soils of Centrl Asi, Lnd Degrdtion & Development, Vol. 15, No. 6, 24, pp [7] S. E. Trumore, Potentil Responses of Soli Orgni Cron to Glol Environmentl Chnge, Proeedings of the Ntionl Ademy of Sienes, Vol. 94, No. 16, 1997, pp [8] R. Ll, J. Kimle, R. F. Follet nd B. A. Stewrt, (Eds.) Mngement of Cron Sequestrtion in Soil, CRC Press, Bo Rton, [9] M. Bernoux, C. C. Cerri, C. E. P. Cerri, M. S. Neto, A. Mety, A. S. Perrin, E. Sopel, T. Rzfimelo, D. Blvet, M. de C. Piolo, M. Pvei nd E. Milne, Cropping Systems, Cron Sequestrtion nd Erosion in Brzil, Review, Agronomy nd Sustinle Development, Vol. 26, No. 1, 26, pp [1] K. A. MVy, D. E. Rdliff nd W. L. Hrgrove, Winter Legume Effets on Soil Properties nd Nitrogen Fertilizer Requirements, Soil Siene Soiety of Amerin Journl, Vol. 53, No. 6, 1989, pp [11] S. Kuo, U. M. Sinju nd E. J. Jellum, Winter Cover Crop Effets on Soil Orgni Cron nd Crohydrte, Soil Siene Soiety of Amerin Journl, Vol. 61, No. 1, 1997, pp

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