Transcript
Page 1: Soil management impacts on soil carbon sequestration by switchgrass

Soil management impacts on soil carbon sequestration byswitchgrass

Z. Ma, C.W. Wood*, D.I. Bransby

Department of Agronomy and Soils, Auburn University, AL 36849, USA

Received 27 April 1999; accepted 30 November 1999

Abstract

Increased atmospheric carbon dioxide (CO2) could have negative impacts on the environment. Producing andcreating bioenergy in the form of biofuels and electricity from crops is a practical approach to reducing CO2

buildup by displacing fossil fuels and sequestering carbon (C). The use of switchgrass (Panicum virgatum L.) as anenergy crop can contribute to clean burning fuels, but no studies addressing soil C sequestration as in¯uenced byuse of switchgrass as an energy crop have been conducted. Our objective was to determine the e�ect of di�erent

cultural practices on soil C sequestration under switchgrass. Field experiments were designed to provide di�erencesin row spacing, nitrogen (N) rate, switchgrass cultivar, and harvest frequency on a variety of soils. Our resultsshowed that N application, row spacing, harvest frequency, and switchgrass cultivar did not change soil organic C

in the short-term (2±3 yr) after switchgrass establishment. However, after 10 yr under switchgrass soil organic C was45 and 28% higher at depths of 0±15 and 15±30 cm, respectively, compared with fallowed soil in an adjacent area.It appears that several years of switchgrass culture will be required to realize a soil C sequestration bene®t. 7 2000Elsevier Science Ltd. All rights reserved.

Keywords: Carbon sequestration; Switchgrass (Panicum virgatum L.); Energy crops; Cultural practices

1. Introduction

Growing international concern over the threatof adverse global climate change is a drivingforce behind the search for ways to avoid thispotentially devastating phenomenon. Carbondioxide (CO2) is primarily responsible for global

warming, and the increase in atmospheric CO2

concentration has been caused by the combinedimpact of population growth, industrial expan-sion, and deforestation [1±4]. There is no doubtthat rise in atmospheric CO2 is partly due to fos-sil fuel combustion, and fossil fuels are a nonre-newable and diminishing energy source.Conversion of crops to biofuel o�ers one possi-bility for reducing atmospheric CO2 while creat-ing a renewable and safe bioenergy source.

Biomass and Bioenergy 18 (2000) 469±477

0961-9534/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0961-9534(00 )00013-1

www.elsevier.com/locate/biombioe

* Corresponding author.

Page 2: Soil management impacts on soil carbon sequestration by switchgrass

Switchgrass (Panicum virgatum L.) has beenchosen by the US Department of Energy as themodel herbaceous energy crop to be used for bio-fuel related research in the Southeast [5]. Produ-cing and using biofuels can help reduceatmospheric CO2 buildup by replacing fossil fuelsas an energy source and recycling CO2 releasedwhen it is consumed. Carbon dioxide emissionfrom use of switchgrass as an energy crop is 1.9on a kg C GJÿ1 basis, considerably lower thanemissions from fossil fuels, which on a kg CGJÿ1 basis are 13.8, 22.3, and 24.6 for naturalgas, petroleum, and coal, respectively [6].

Switchgrass may improve soil quality bysequestering C in the switchgrass±soil agroecosys-tem owing to its high biomass [7], deep rootingsystem [8], and perenniality. Forages such as tallfescue (Festuca arundinacea Schreb.) and smoothbromegrass (Bromus inermis Leyss.) have beenshown to have some e�ect on environmentalquality through soil C sequestration [9]. As anexample, tall fescue and smooth bromegrassincreased the soil C pool by 17.2% comparedwith a corn (Zea mays L.)-soybean (Glycine maxL.) rotation [9].

At present, little is known about the impact ofswitchgrass on soil C pools. Increasing the stableproportion of aggregates and placement of or-ganic C deeper in soil by incorporation of bio-mass are two strategies or mechanisms for Csequestration [10]. Speci®c techniques forenhancement of C sequestration include the useof cover and perennial crops for increases inmicro-aggregates, deep-rooted crops for deep in-corporation of organic C in soil, balanced fertili-zer application for maximum biomassproduction, and returning crop residues to thesoil for increased humi®cation [10]. Deep-rootedcrops with the capacity to produce high biomass[11±13], and with perenniality [14], may enhancethe organic C content of deeper soil horizonswhere it is not easily mineralized and decom-posed. Application of some fertilizers couldincrease biomass both above-below-ground,increasing crop residues returned to soil and sub-sequently promoting soil organic C accumu-lation. Organic C has been shown to increase asN application increases [15]. T

able

1

Experim

ents,locationcharacterization,andadjacentsoils

Experim

ent

Tim

eaSoilseries

Location

Classi®cation

Adjacentsoilsb

11992

Norfolk

E.V.SmithResearchCenter,Shorter,AL(EVS)

Fine-loamy,siliceous,thermic

Typic

Paleudult

Bermudagrass

Soil

21992

Malbis

GulfCoast

Substation,Fairhope,

AL(G

CS)

Fine-loamy,siliceous,thermic

Plinthic

Paleudult

Soybeansoil

1992

Norfolk

PlantBreedingUnit,Tallassee,AL(PBU)

Fine-loamy,mixed,thermic

Typic

Hepludult

Fallow

soil

1992

Pacolet

PiedmontSubstation,CampHill,ASL(PSS)

Clayey,kaolinitic,thermic

Typic

Paleudult

Fallow

soil

1992

Decatur

TennesseeValley

Substation,Belle

Miwa,AL(TVS)

Clayey,kaolinitic,thermic

Rhodic

Paleudult

Soybeansoil

1992

Hartsells

SandMountain

Substation,Crossille,AL(SMS)

Fine-loamy,siliceous,thermic

Typic

Hapludult

Bermudagrass

soil

31988

Norfolk

PlantBreedingUnit,Tallassee,AL(PBU)

Fine-loamy,mixed,thermic

Typic

Hapludult

Fallow

soil

41994

Norfolk

E.V.SmithResearchCenter,Shorter,AL(EVS)

Fine-loamy,siliceous,thermic

Typic

Paleudult

Fallow

soil

1994

Pacolet

PiedmontSubstation,CampHill,AL(PSS)

Clayey,kaolinitic,thermic

Typic

Paleudult

Fallow

soil

aTim

eofplantingsw

itchgrass.

bAdjacentsoilsweresoilscollectedin

fallow

orannuallycropped

areasadjacentto

switchgrass

plots.

Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469±477470

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With regard to e�ects of cultural practices andsoil types on soil C sequestration where switch-grass is grown, no studies have been published.Without data on C sequestration, the potentialbene®t of increased soil C storage under switch-grass grown as an energy crop will remainunknown, which presents a gap in a high priorityresearch area [16]. The objective of the presentstudy was to evaluate the e�ects of cultural prac-tices (row spacing, N fertilizer, switchgrass culti-var, and harvest frequency) on soil Csequestration under switchgrass.

2. Materials and methods

2.1. Field setup

Selected characteristics of the four experimentswe used to determine the impact of cultural prac-tices on soil C sequestration under switchgrassare shown in Table 1. Experiment 1: E�ects ofrow spacing and N rate. In this experiment`Alamo' switchgrass was planted in 3 m � 9 mplots at row spacings of 20 and 80 cm, and ferti-lized with 0, 112, and 224 kg N haÿ1 within eachrow spacing in a randomized complete block de-sign with four replicates. The plots were plantedin 1992 on a Coastal plain soil at the E.V. Smith(EVS) Research Center, Shorter, AL (Norfolksandy loam; ®ne-loamy, siliceous, thermic TypicPaleudult). Experiment 2: E�ect of harvest fre-quency. In this experiment `Alamo' switchgrasswas planted in 1992 at ®ve locations (Table 1) in3 m � 9 m plots with 30 cm between rows. Lo-cations for this experiment were the Gulf CoastSubstation (GCS), Fairhope, AL (Malbis sandyloam; ®ne-loamy, siliceous, thermic PlinthicPaleudult); the Plant Breeding Unit (PBU), Tal-lassee, AL (Norfolk sandy loam; ®ne-loam,mixed, thermic Typic Hapludult); the PiedmontSubstation (PSS), Camp Hill, AL (Pacolet clayloam; clayey, kaolinitic, thermic Typic Haplu-dult); the Tennessee Valley Substation (TVS),Belle Miwa, AL (Decatur silty loam; clayey, kao-linitic, thermic Rhodic Paleudult); and the SandMountain Substation (SMS), Crossville, AL(Hartsells ®ne sandy loam; ®ne-loamy, siliceous,

thermic Typic Hapludult). Harvest frequency foreach location was either once only in early Octo-ber or in late July and early October for thosetwice harvested. There were four plots at each lo-cation resulting in two replications per location.All plots received 112 kg N haÿ1 of fertilizerannually. Experiment 3: E�ects of switchgrasscultivars in mature stands of switchgrass. Threeswitchgrass cultivars, `Alamo', `Kanlow', and`Cave-in-Rock', were established in 1988 at EVS.Plots were planted with 15 cm between rows in arandomized complete block design with fourreplications. All plots received 112 kg N haÿ1

each year. Areas that were cropped annually withcotton or left fallow were located immediatelyadjacent to this experiment for comparison. Ex-periment 4: E�ects of time, row spacing and har-vest frequency. This experiment, established inthe spring of 1994, was located at the E.V. SmithResearch Center, Shorter, AL (Norfolk sandyloam), and the Piedmont Substation, Camp Hill,AL (Pacolet clay loam). The experiment was arow spacing (3) x cutting frequency (2) factorialarranged in a randomized complete block withfour replications. Row spacings were 20, 60, and120 cm. Harvest frequency was once (mid-Octo-ber) or twice annually (late June and mid-Octo-ber). All plots were fertilized with 112 kg N haÿ1

yrÿ1.

2.2. Sample collection and analysis

Soil samples were taken primarily for assess-ment of total organic C. Soil C sequestrationwas determined via di�erences in soil C in theswitchgrass plots compared with that in fallow,other forages, or annually cropped areas adja-cent to switchgrass plots (Table 1). Soil pro®lesamples were collected in each plot during lateSeptember and early October 1995. Four ran-dom cores per plot were sampled to 3-m orlithic contact in 0±15-, 15±30- and each sub-sequent 30-cm depth increment using a tractor-mounted soil probe (Giddings Machine Co., FtCollins, CO) and 5-cm diameter by 122-cmlength soil tubes. Individual depth incrementswere composited for each plot. Some pro®leshad only one sample at lower depths due to

Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469±477 471

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variability of soil depth to bedrock. Plant resi-dues, primarily switchgrass roots, wereexcluded from the samples via sieving (0.5 cmopenings) before compositing. Soil samples col-lected in areas adjacent to switchgrass plotswere used to determine if switchgrass hadresulted in soil C accumulation. Samples wereplaced in soil boxes and transported to thelaboratory for drying, grinding, and analysis.

In Experiment 4, initial soil pro®le sampleswere collected on 19 May 1994 (EVS) and 12May 1994 (PSS), and consisted of four coresper replication with the same sampling pro-cedures as previously mentioned. In order todetermine the C sequestration compared withthe initial soil samples and fallow soil, soilpro®le samples from switchgrass plots and fal-low were collected again during November1996.

Increased time after switchgrass establish-ment could result in organic C accumulation.In order to verify whether switchgrass resultedin a greater C accumulation than in fallowsoil, Experiment 3, which ran for the longesttime (10 years) of the four, and `Alamo'switchgrass, which had the highest biomass ofthe switchgrass cultivars, were selected for ad-ditional sampling. Soil samples from the zoneswith the highest density of roots (0±15 and15±30 cm depth) were sampled on 2 April1998 in Experiment 3. These samples weresampled with a hand probe (2 cm diameter;20 cores per plot) for each `Alamo' switchgrassplot and the fallow, and composited by depthwithin plots.

Total C and N were determined for all soilsamples. Approximatly 20 g of each soilsample were dried at 608C for 48 h andground to pass a 0.15 mm sieve. This ®nelyground soil was analyzed for total organic Cand total N with a LECO CHN-600 analyzer(LECO Corp., St Joseph, MI).

In Experiment 2, aboveground biomass wascollected by harvesting plots with a Carter forageplot harvester during September 1995. Sub-samples were taken, transported to the labora-tory, and dried at 608C for 48 h for conversionof fresh biomass yields to dry matter yields.

2.3. Statistical analysis

Analysis of variance was performed using theSAS package [17], testing for all main e�ects andtheir interactions on soil C and C/N. Signi®cancewas set at P R 0.05. In order to obtain increasedhomogeneity of variances, a requirement foranalysis of variance procedures [18], depths ofsoil pro®les sampled were grouped into layers,based on the depth distribution of organic Ccaused primarily by tillage operations [19], plantroots, and soil formations. Layer I, layer II, andlayer III consisted of the 0±15, 15±30 and 30±60 cm depth, respectively. Layer IV consisted ofall remaining depths below 60 cm.

3. Results and discussion

Depth distribution of organic C in the Norfolksandy loam is shown in Fig. 1 (a). There were nodi�erences in organic C among N rates at anydepth, and also no di�erences in depth distri-bution of organic C throughout the soil pro®leamong N rates. E�ects of N on organic C wouldmost likely occur near the soil surface [9], butthere were no e�ects in this study, which is incontrast to ®ndings of other workers [15]. Lackof an N e�ect on soil C accumulation may beowing to the short duration of the study prior tosoil sampling (3 yr), because long time N fertili-zation of switchgrass may contribute to anincrease in soil C reserves due to increases inboth switchgrass aboveground biomass [20] androot biomass [21]. Switchgrass had lower organicC concentrations than bermudagrass (Cynodondactylon L.) soil to a depth of approx. 0±60 cm[Fig. 1(a)], again likely due to the relatively shorttime since switchgrass establishment.

Organic C, regardless of depth, was nota�ected by row spacing [Fig. 1(b); Table 2]. Noe�ect of row spacing on organic C implied nodi�erences in root biomass between row spacings.In the upper 60 cm of soil, organic C was lowerunder switchgrass than bermudagrass [Fig. 1(b)].The lower organic C with switchgrass than ber-mudagrass is probably related to lower root bio-mass with switchgrass than bermudagrass.

Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469±477472

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Di�erences in yield among switchgrass culti-

vars [7] could result in di�erences in organic C.

However, there were no di�erences in this study

[Fig. 1(c)].

Cutting or harvest shifts the allocation of C

from active root biomass to regrowth of leaves,

and consequently alters the soil C pro®le distri-

bution associated with root activity. However,

Fig. 1. Depth distribution of soil organic C as in¯uenced by (a) nitrogen rate (NR) (Experiment 1), (b) row spacing (RS) (Exper-

iment 1), and (c) switchgrass cultivars (Experiment 3). The LSD0.05 for NR, RS, and cultivars=NS (all layers).

Table 2

Soil organic C concentration as a�ected by row spacing (RS) (Experiment 4)

Norfolk sandy loam Pacolet clay loam

Row spacing (cm) Layer I Layer II Layer III Layer IV Layer I Layer II Layer III Layer IV

20 4.60a 2.85 1.23 0.73 6.90 3.55 1.28 0.69

60 4.95 3.18 1.40 0.84 6.60 3.58 1.33 0.77

120 4.60 3.00 1.45 0.92 7.65 3.35 1.60 0.79

Variance source Analysis of variance (P>F)

RS 0.953 0.904 0.228 0.695 0.511 0.790 0.401 0.675

Depth ± ± ± 0.056 ± ± ± 0.193

RS� depth ± ± ± 0.048 ± ± ± 0.194

a Statistical analyses for only samples taken in Fall 1995; Unit=g kgÿ1.

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harvest frequencies generally showed no di�er-ences in organic C in this study (Fig. 2). HigherC storage was found in the 0±15 cm depth ofMalbis sandy loam soil, but not in any of theother soils. Higher organic C with one ratherthan two harvests occurred in the upper depth. Itis not known why higher soil C with one harvestoccurred in the Malbis sandy loam and not inother soils in this study. When compared withvarious adjacent fallow soils, switchgrass did notalter soil organic C (Fig. 2).

In Experiment 4, there were no di�erences inorganic C between switchgrass soil and fallow inall layers of both the sandy and the clay soil(Fig. 3). Also no di�erences in organic C existedin switchgrass soil 2 yr after switchgrass estab-lishment as compared to initial levels. Apparentlytwo years is not an adequate amount of time toe�ect a change in organic C levels.

Soil C sequestration with switchgrass can betested by comparison with adjacent soils(cropped and fallowed). The above results didnot demonstrate that switchgrass produced soilorganic C accumulation, but this was probablylimited by the short time (2±3 yr) since switch-grass establishment. However, switchgrass couldincrease soil organic C levels over longer periodsof time due to perennality and a large amount ofunharvested roots. Bransby et al. [22] statedswitchgrass compared to row crops such as cot-ton and corn promotes a net system C gain, sosoil organic C gains under switchgrass are morelikely than under row crops over the long-term.In order to con®rm whether switchgrass resultsin net soil C storage, one needs to evaluate soilorganic C levels under long-term switchgrass ex-periments. In Experiment 3, soil samples fromupper soil layers (0±15 and 15±30 cm depth)

Fig. 2. Depth distribution of soil organic C as in¯uenced by harvest frequency (HF) in Decatur silty loam (TVS), Hartsells ®ne

sandy loam (SMS), Malbis sandy loam (GCS), Norfolk sandy loam (PBU), and Pacolet clay loam (Experiment 2). LSD0.05 is the

test for C di�erences between harvest frequencies within each layer.

Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469±477474

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Fig. 2 (continued)

Fig. 3. Depth distribution of soil organic C as in¯uenced by switchgrass (Experiment 4) compared with fallow and initial soil treat-

ments. The LSD0.05 for mean separation treatments=NS (all layers).

Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469±477 475

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were sampled 10 yr after switchgrass establish-ment. The results indicated that soil organic Cconcentration under switchgrass was signi®cantlyhigher than for fallow soil, with an increase of44.8 and 28.2% for 0±15 and 15±30 cm depths,respectively. However, the C/N did not changefor the same layers (Fig. 4). Our results suggestthat switchgrass culture will result in soil C ac-cumulation, but it may take several years beforeany increases are detectable.

4. Conclusions

Switchgrass management practices such as Napplication, row spacing, and harvest frequencydid not alter soil C concentration in this study.Lack of cultural practice e�ects on soil organic Cin our experiments was likely owing to the shorttime (2±3 yr) since switchgrass establishment. Inthe long-term (10 yr) experiment, however,switchgrass resulted in more soil C than an adja-

cent fallow soil. It appears that several years ofswitchgrass culture will be required to realize asoil C sequestration bene®t.

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