Soil management impacts on soil carbon sequestration by switchgrass

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  • 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 CO2buildup 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 influenced byuse of switchgrass as an energy crop have been conducted. Our objective was to determine the eect of dierent

    cultural practices on soil C sequestration under switchgrass. Field experiments were designed to provide dierencesin 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 (23 yr) after switchgrass establishment. However, after 10 yr under switchgrass soil organic C was45 and 28% higher at depths of 015 and 1530 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 benefit. 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 CO2concentration has been caused by the combinedimpact of population growth, industrial expan-sion, and deforestation [14]. 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 oers one possi-bility for reducing atmospheric CO2 while creat-ing a renewable and safe bioenergy source.

    Biomass and Bioenergy 18 (2000) 469477

    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.

  • 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 GJ1 basis, considerably lower thanemissions from fossil fuels, which on a kg CGJ1 basis are 13.8, 22.3, and 24.6 for naturalgas, petroleum, and coal, respectively [6].Switchgrass may improve soil quality by

    sequestering C in the switchgrasssoil 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 eect 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 of

    switchgrass 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]. Specific 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 humification [10]. Deep-rootedcrops with the capacity to produce high biomass[1113], 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

    able1

    Experiments,locationcharacterization,andadjacentsoils

    Experiment

    Timea

    Soilseries

    Location

    Classification

    Adjacentsoilsb

    11992

    Norfolk

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

    Fine-loamy,siliceous,thermicTypicPaleudult

    BermudagrassSoil

    21992

    Malbis

    GulfCoastSubstation,Fairhope,AL(G

    CS)

    Fine-loamy,siliceous,thermicPlinthicPaleudult

    Soybeansoil

    1992

    Norfolk

    PlantBreedingUnit,Tallassee,AL(PBU)

    Fine-loamy,mixed,thermicTypicHepludult

    Fallowsoil

    1992

    Pacolet

    PiedmontSubstation,CampHill,ASL(PSS)

    Clayey,kaolinitic,thermicTypicPaleudult

    Fallowsoil

    1992

    Decatur

    TennesseeValley

    Substation,BelleMiwa,AL(TVS)

    Clayey,kaolinitic,thermicRhodicPaleudult

    Soybeansoil

    1992

    Hartsells

    SandMountain

    Substation,Crossille,AL(SMS)

    Fine-loamy,siliceous,thermicTypicHapludult

    Bermudagrasssoil

    31988

    Norfolk

    PlantBreedingUnit,Tallassee,AL(PBU)

    Fine-loamy,mixed,thermicTypicHapludult

    Fallowsoil

    41994

    Norfolk

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

    Fine-loamy,siliceous,thermicTypicPaleudult

    Fallowsoil

    1994

    Pacolet

    PiedmontSubstation,CampHill,AL(PSS)

    Clayey,kaolinitic,thermicTypicPaleudult

    Fallowsoil

    aTimeofplantingswitchgrass.

    bAdjacentsoilsweresoilscollectedin

    falloworannuallycropped

    areasadjacentto

    switchgrassplots.

    Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469477470

  • With regard to eects 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 potentialbenefit 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 eects 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: Eects ofrow spacing and N rate. In this experimentAlamo 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 ha1 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; fine-loamy, siliceous, thermic TypicPaleudult). Experiment 2: Eect of harvest fre-quency. In this experiment Alamo switchgrasswas planted in 1992 at five 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; fine-loamy, siliceous, thermic PlinthicPaleudult); the Plant Breeding Unit (PBU), Tal-lassee, AL (Norfolk sandy loam; fine-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 fine sandy loam; fine-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 ha1 of fertilizerannually. Experiment 3: Eects of switchgrasscultivars in mature stands of switchgrass. Threeswitchgrass cultivars, Alamo, Kanlow, andCave-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 ha1

    each year. Areas that were cropped annually withcotton or left fallow were located immediatelyadjacent to this experiment for comparison. Ex-periment 4: Eects 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 ha1

    yr1.

    2.2. Sample collection and analysis

    Soil samples were taken primarily for assess-ment of total organic C. Soil C sequestrationwas determined via dierences 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 profilesamples 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 015-, 1530- 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 profileshad only one sample at lower depths due to

    Z. Ma et al. / Biomass and Bioenergy 18 (2000) 469477 471

  • 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 profile samples

    were 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, soilprofile 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 Alamoswitchgrass, which had the highest biomass ofthe switchgrass cultivars, were selected for ad-ditional sampling. Soil samples from the zoneswith the highest density of roots (015 and1530 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 soil

    samples. Approximatly 20 g of each soilsample were dried at 608C for 48 h andground to pass a 0.15 mm sieve. This finelyground 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 was

    collected 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 eects andtheir interactions on soil C and C/N. Significancewas set at P R 0.05. In order to obtain increasedhomogeneity of variances, a requirement foranalysis of variance procedures [18], depths ofsoil profiles 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 015, 1530 and 3060 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 nodierences in organic C among N rates at anydepth, and also no dierences in depth distri-bution of organic C throughout the soil profileamong N rates. Eects of N on organic C wouldmost likely occur near the soil surface [9], butthere were no eects in this study, which is incontrast to findings of other workers [15]. Lackof an N eect 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. 060 cm[Fig. 1(a)], again likely due to the relatively shorttime since switchgrass establishment.Organic C, regardless of depth, was not

    aected by row spacing [Fig. 1(b); Table 2]. Noeect of row spacing on organic C implied nodierences 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) 469477472

  • Dierences in yield among switchgrass culti-

    vars [7] could result in dierences in organic C.

    However, there were no dierences 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 profile distri-

    bution associated with root activity. However,

    Fig. 1. Depth distribution of soil organic C as influenced 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 aected 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...

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