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
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
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 . 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
* 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 . 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 .Switchgrass may improve soil quality by
sequestering C in the switchgrasssoil agroecosys-tem owing to its high biomass , deep rootingsystem , 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 . As anexample, tall fescue and smooth bromegrassincreased the soil C pool by 17.2% comparedwith a corn (Zea mays L.)-soybean (Glycine maxL.) rotation .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 . 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 . Deep-rootedcrops with the capacity to produce high biomass, and with perenniality , 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 . T
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 . 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
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 a