Atmospheric Pollutants and Trace Gases

Fluxes of Carbon Dioxide, Nitrous Oxide, and Methane in Grass Sodand Winter Wheat-Fallow Tillage Management

Anabayan Kessavalou, Arvin R. Mosier, John W. Doran,* Rhae A. Drijber, Drew J. Lyon, and O. Heinemeyer

ABSTRACT

Cropping and tillage management can increase atmospheric CO,,N:O, and CH4 concentrations, and contribute to global warming anddestruction of the ozone layer. Fluxes of these gases in vented surfacechambers, and water-filled pore space (WFPS) and temperature ofsurvace soil were measured weekly from a long-term winter wheat(Triticiim aestivum L.)-fallow rotation system under chemical (no-tillage) and mechanical tillage (noninversion subtillage at 7 to 10 cmor moldboard plowing to 15 cm) follow management and comparedwith those from "native" grass sod at Sidney, NE, from March 1993to July 1995. Cropping, tillage, within-field location, time of year, soiltemperature, and WFPS influenced net greenhouse gas fluxes. Meanannual interrow CO2 emissions from wheat-fallow ranged from 6.9to 20.1 kg C ha"1 d ' and generally increased with intensity and degreeof tillage (no-till least and plow greatest). Nitrous oxide flux averaged<1.2 g N ha"1 d"1 for sod and 1 to 2 g N ha"1 d"1 for wheat-fallow.Tillage during fallow increased N,O flux by almost 100%. Nitrousoxide emissions were 1.5 to 3.7 times greater from crop row thaninterrow locations with greatest differences occurring during periodsof highest N,O emission. Mean annual N2O flux over the 3 yr of studywere 1.54 and 0.76 g N ha ! d"1 for row and interrow locations.Methane uptake ranged from 5.9 to 9.9 g C ha"1 d ' and was notinfluenced by row location. Seasonal CO, and N,O flux, and CH4

uptake ranked as spring s summer > autumn > winter. Winterperiods accounted for 4 to 10% and 3 to 47% of the annual CO, andN,O flux, respectively, and 12 to 21% of the annual CH4 uptake.Fluxes of CO, and N,O, and CH4 uptake increased linearly with soiltemperature. No-till fallow exhibited the least threat to deteriorationof atmospheric or soil quality as reflected by greater CH4 uptake,decreased N,O and CO, emissions, and less loss of soil organic C thantilled soils. However, potential for increased C sequestration in thiswheat-fallow system is limited due to reduced C input from intermit-tent cropping.

A. Kessavalou and R.A. Drijber, 279 Plant Science, Dep. of Agron-omy, Univ. of Nebraska-Lincoln, Lincoln, NE 68583; A.R. Mosier,USDA-ARS, Post Box E, Fort Collins, CO 80522; J.W. Doran, USDA-ARS, 116 Keim Hall, East Campus, Univ. of Nebraska-Lincoln, Lin-coln, NE 68583; D.J. Lyon, Panhandle Research and Extension Cen-ter, Scottsbluff, NE 69361; and O. Heinemeyer, FAL, Braunschweig,Germany. Contribution of the University of Nebraska-Lincoln andUSDA-ARS, Fort Collins. Journal Ser. no. 11811. Received 13 Jan.1997. ""Corresponding author (jdoran@unlinfo.unl.edu).

Published in J. Environ. Qual. 27:1094-1104 (1998).

CROPPING PATTERN, tillage practice, and N and irriga-tion management can influence the exchange of

CO2, N2O, and CH4 between soil and the atmosphere(IPCC, 1996). More CO2 emissions can occur from atilled than from an undisturbed soil, as tillage producesa soil microenvironment favorable for accelerated mi-crobial decomposition of plant and animal residues(Doran, 1980; Doran and Linn, 1994).

Losses of N2O are often higher in undisturbed thanplowed soils (Aulakh et al., 1984; Linn and Doran,1984), though tillage may result in elevated N2O emis-sions from soil for a short time (Matthias et al., 1980).Emissions of N2O from soil also increase due to N fertil-ization. It has been estimated that about 1.25% of thefertilizer N applied to crops is lost through N2O emis-sions (IPCC, 1996). But N2O losses from agriculturalland vary depending on the type of fertilizer used andcrops grown. Ammonium containing N fertilizers andatmospheric N2-fixing legumes are known to increaseN2O emissions from soil (Eichner, 1990). Nitrous oxideis emitted under both aerobic and anaerobic soil condi-tions, since it is produced during nitrification and deni-trification processes. Therefore, soil water content andWFPS play an important role in regulating the produc-tion, consumption, and release of N2O from soil (Linnand Doran, 1984; Doran et al., 1990).

Cultivation reduces CH4 oxidation capacity of soils(Mosier et al., 1991; Lessard et al., 1994). In northeasternColorado, tilled and irrigated soils cropped to wheatand corn (Zea mays L.) exhibited 90% reduction inthe CH4 oxidation capacity when compared to nativegrassland soils (Bronson et al., 1992; Bronson and Mo-sier, 1993). Also, in the same region, grassland soil lowin available N exhibited a 41 % reduction in CH4 uptake8 yr after N fertilization as compared to an unfertilizedcontrol, while the CH4 oxidation capacity of a soil highin mineral N remained unaffected by fertilization (Mo-

Abbreviations: WFPS, water-filled pore space; SOC, soil organiccarbon.

KESSAVALOU ET AL.: FLUXES OF GREENHOUSE GASES FROM GRASS & WHEAT-FALLOW TILLAGE SYSTEMS 1095

sier et al., 1991, 1996). Methane oxidation capacity ofthe soil decreases with increasing soil water content.Mosier et al. (1996) found that CH4 uptake of sandygrassland soil increased with increasing WFPS, between15 and 24% WFPS, and decreased thereafter with in-creasing WFPS.

Although agronomic practices influence the fluxes ofgreenhouse gases between soil and the atmosphere, themagnitude of gas emissions at a given time from a soilis mostly determined by seasonal climatic conditions.In the temperate humid climate of Ohio, Lal et al. (1995)observed average CO2 flux (soil respiration) rates 192 and 115 kg C ha-1 d-1 from alfalfa (Medicago sativaL.) and corn fields, respectively, as compared to 89 kgC ha-1 d-1 from an adjacent undisturbed coniferousforest. However, regardless of cropping pattern, sea-sonal CO2 and N20 fluxes and CH4 uptake followed theranking of summer > autumn > spring > winter. Mosieret al. (1996) found that N20 flux and CH4 uptake weregreatest during the summer and autumn in the semiaridshortgrass steppe of northeastern Colorado, with winterseason N20 and CH4 fluxes representing 15 to 30% ofannual fluxes. However, during some years, winter NzOemissions were greater than summer emissions probablydue to denitrification events in thawed surface soilsoverlying frozen subsoils. Greenhouse gas exchangerates also vary spatially (Lessard et al., 1994). This maybe due, partly, to the natural heterogeneity of soil, al-tered microenvironments created by crop rows, andmechanized management operations that disturb orcompact the soil in zonal patterns.

Winter wheat is an important crop in Nebraska, withabout 0.69 million ha of land under winter wheat-fallowcultivation. Winter wheat-fallow is the predominantcropping system in the central High Plains grasslandecosystem. There is little natural grassland vegetationremaining in this region and most grasslands are domi-nated by introduced species such as crested wheatgrass[Agropyron cristatum (L.) Gaertn.]. Little informationexists on the dynamics of CO2, N20, and CH4 fluxes inthese grassland agroecosystems and their contributionsto the global greenhouse gas budget. A thorough under~standing of the long-term effects of cultivation on thesefluxes is essential to develop alternative farming prac-tices for mitigating greenhouse gas emissions. There-fore, objectives of this experiment were to (i) study theseasonal and annual CO~, N20, and CH4 flUX patternsin a long-term winter wheat-fallow rotation system, asinfluenced by no-tillage (no-till), age (subtill) and mold-board plow (plow) tillage management, in comparisonwith "native" grass sod and (ii) estimate the rate

depletion of soil organic C in no-till, subtill, and plowplots.

MATERIALS AND METHODSThis study was conducted at the High Plains Agricultural

Research Laboratory at Sidney, NE (41°14’ N, 103000’ W)from 18 Mar. 1993 through 6 July 1995. The experimental site(1313 m above sea level) had been in a winter wheat-fallowrotation since 1970. The soil was a Duroc loam (a fine-silty,mixed, mesic Pachic Haplustoll). Treatments included: (1)cropping and tillage: (i) "native" grass sod dominated crested wheatgrass and with some minor species namely, nee-dle and thread (Stipa comata Trin & Rupr.), blue grama (Bou-teloua gracillis Lag. Ex Steud.), sideoats grama [B. curtipen-dula (Michx.) Tort.], and sand dropseed [Sporoboluscryptandrus (Torr.) Gray], and (ii) winter wheat-summer low with no-till, subtill (V blade, noninversion) and plow(moldboard plow, soil inversion) fallow management practicesand (2) location: (i) row and (ii) interrow. The weed controlmanagement practices compared represented a wide rangein degree and intensity of soil cultivation and crop residuedisturbance ranging from chemical no-till involving minimaldisturbance, through subtill that represented subsurface dis-turbance to a depth of 7 to 10 cm, to the plow treatmentwhich resulted in complete inversion of soil and mixing ofcrop residues to a depth of 15 cm. Treatments were arrangedin a randomized complete block split plot design (croppingand tillage as main plot and locations as subplot) with threereplicates. Plot size was 45.5 by 8.5 m. Selected physical andchemical characteristics of the soil are given in Table 1. Aver-age (30-yr) annual precipitation of this region is 411 mm.Annual precipitation totaled 524 mm in 1993, 462 mm in 1994,and 515 mm in 1995. Seasonal distribution pattern of theprecipitation during the study period is shown in Fig. 1. Winterwheat ("Siouxland") was planted in 30 cm rows, during thesecond week of September in 1992 and 1994, and was har-vested during the third week of July in 1993 and 1995. No N,P, and K fertilizers were applied to these plots since theirestablishment. Details of tillage operations and important cropmanagement history are provided in Table 2.

Gas Flux Measurements

Representative row and interrow locations were selectedwithin each replicate of tillage/wheat-fallow and grass sodplots. A bare soil spot without grass was selected to representthe interrow location for the sod treatment. Within each plot,two gas chamber anchors (20.3 cm inner diam. polyvinyl chlo-ride pipe), one each in the row and interrow location, weredriven 8 cm into the soil to install surface gas chambers formeasuring gas flux (Mosier et al., 1991). Anchors in subtilland plow plots were removed at time of tillage and werereinstalled at the same location within 7 d for measuring flux.Flux determinations were made weekly, between 1000 and1200 h of the day and were assumed to represent the average

Table 1. Selected physical and chemical properties of the soil (0-15 cm).

Partide sizeSoil bulk

Treatment density Sand Silt Clay pHOrganic

C Total N CO~C

g cm-3 %Sod 1.05 35 42 23 7.1 33.1No-tillage 1.23 33 38 29 6.9 28.8Subfill 1.20 35 37 28 7.0 28.2Plow 1.27 41 32 26 7.0 24.7

-- Mg ha 1 --

3.02.82.82.5

0.050.080.020.12

Organic C = (total C - C03-C).

1096 J. ENVIRON. QUAL., VOL. 27, SEPTEMBER-OCTOBER 1998

24

E 16

N 12

6

22

20

18

L.

2(01

J F M A

199319941995

I ~O N D

MonthFig. 1. Distribution of precipitation during the study period at Sidney,

NE, 1993 to 1995.

flux for that day. Soil was first covered with vented surface gaschambers (7.5-cm high and 20.3-cm diam.; chamber volume 2433 cm3) for 30-min. Gas samples were then collected fromthe chambers at 0, 15, and 30 min of cover with a 30-mLpolypropylene syringe fitted with a nylon stopcock and trans-ported to the laboratory. Because a period of <60 min of soilcover is recommended to accurately estimate gas fluxes underaerobic conditions (Jury et al., 1982), a convenient 30 minperiod was used to determine flux. During the growing season,grass and wheat present inside the gas chamber in the croprow positions was clipped to 2.5 cm so that the chamber couldbe closed to make flux measurements. Gas samples collectedwere analyzed for CO2, N20, and CH~ within 12 h after sam-pling. Nitrous oxide and CH4 contents were determined bygas chromatography using a Porapak Q column and electroncapture detector, and a Poropak N column and flame ioniza-tion detector, respectively, as described by Mosier et al. (1.991).Carbon dioxide content was determined using a gas chromato-graph with a thermal conductivity detector maintained at110°C as described by Weier et al. (1993). Concentrations greenhouse gases inside the chamber usually increased linearlywith time; where the repeated measurements indicated non-linearity, the corresponding flux rates were corrected as de-scribed by Hutchinson and Mosier (1981). This approach al-lowed reliable measurement of CO2 fluxes, though the ventedsurface chambers were not specifically designed for measuringfluxes of this gas.

At time of gas sampling, two soil cores (1.7 cm diam.)were collected to a depth of 15 cm, close to the gas samplingchambers from the row and interrow zone to estimate gravi-metric soil water content (Gw). Bulk density (BD) of the 15 cm of soil was determined frequently (before and aftertillage) during the study period as described by Sarrantonioet al. (1996). Since the bulk density of the soil did not change

significantly during the study period, except following tillage,an average BD value for each treatment (Table 1) was usedin all calculations. Soil water-filled pore space was calculatedfrom soil bulk density and gravimetric water content usingthe relationship of volumetric water content (Vw) / total soilporosity (P), where Vw = Gw × BD, and P = [(1- (BD/2.65)](Sarrantonio et al., 1996). Temperature of the top 7.6 cm soil was measured during each sampling time. Initial and finalair temperatures were also measured at each gas samplingtime to compute flux rates. Occasionally, soil NO3-N in thetop 15 cm was determined for each cropping/tillage treatment.In the autumn of 1995, soil samples were collected between0 and 7.6, 7.6 and 15.2, and 15.2 and 30.5 cm from all theplots and were analyzed for soil microbial biomass C by theSubstrate Induced Respiration method (Anderson aridDomsch, 1978; Heinemeyer et al., 1989).

Carbon input into the soil during a 2-yr rotation period(1993 and 1994) was computed for three tillage systems estimate the rate of depletion of soil organic C (SOC). Sourcesof C input included wheat straw, root dry matter, root exu-dates, and weed dry matter. Wheat straw dry matter produc-tion was calculated from the 1993 straw yield. Root dry matteryield was estimated from the aboveground dry matter (grain straw dry matter yield) based on a straw to root dry matterratio of 1.13 as described by Buyanovsky and Wagner (1986).Carbon contributions from straw and root residues were esti-mated by assuming a plant C content of 40%. Estimates ofrhizodeposited C (root released C), insect derived C, and othermiscellaneous inputs during the growing season (1993) weremade by multiplying root C by 1.5 as proposed by Lucas andVitosh (1978). Carbon added through weed biomass (assuming40% C) was assumed to average 600 kg C ha-1 for no-till, 200kg C ha-1 for subtill, and 200 kg C ha-1 for plow during the2-yr rotation cycle (D.J. Lyon, 1996, personal communication).A soil C budget was then computed using measured CO2emissions and the estimates of plant C input.

Statistical analysis on data were performed using StatisticalAnalysis System (SAS Institute, 1992). Because flux rates weremeasured repeatedly over time from the same spot, analysisof variance for split-plot design over space and time was com-puted using general linear model procedures to test the signifi-cance of treatment effects and to detect differences amongtreatment means. A probability level of 5% or lower wasused to test the statistical significance of all treatment effects.Appropriate contrasts were also used to test the effects oftillage, time, and cropping on trace gas fluxes. Also, a TableCurve program (Jandel Scientific, 1994) was used to evaluatethe relationship between WFPS, soil temperature, and gasfluxes.

RESULTS AND DISCUSSIONGreenhouse Gas Fluxes

Carbon Dioxide Flux

Carbon dioxide flux under different treatments, mea-sured weekly, during the study period is shown in Fig.

Table 2. Field management history of the experimental site at Sidney, NE, 1993-1995.

OperationsTillage management

Field operations-~ Depth per year No-fill Sub-till Plow Sod

cm

Plow 15 1 - - April-MayV-blade (subtill) 7-10 2-4 - May-Sept. -Field cultivator 5-7.5 2-3 - - May-Aug.Rotary rod weeder 2.5 2 - July-Sept. July-Sept.Spray (herbicides) - As needed April-Sept. - -

"~ Performed during 1994 fallow. One V-blade operation was done in September during cropping years 1993 and 1995. No N, P, and K fertilizers wereapplied to these plots since their establishment.

KESSAVALOU ET AL.: FLUXES OF GREENHOUSE GASES FROM GRASS & WHEAT-FALLOW TILLAGE SYSTEMS 1097

160 ~(a) CO~ flux ~ Sod140 ~ ....

No-Till

120~- .........Sub-Till

0

(b) N20 flux

..

January 1994 I! i!! ~t:: January 1995

2421

z 12

9630

-3

~, ~o

x: 16

~ 12

ff 4

o0

(c) 4 uptake

Januar

If "~’:10 20 30 40 50

1994 January 1995

~-,,Y .... ,,~,,i .... , .... I60 70 80 90 100 110 120 130

Week (from 18 March 1993 through 6 July 1995)

Fig. 2. Weekly COz, N~O flux, and CH4 uptake under wheat-fallow rotation as influenced by tillage and "native" grass sod at Sidney, NE, 1993to 1995.

2 and 3. Season, cultivation, and location influencedCO2 flUX (P < 0.05).

Because CO2 emitted from rows during the growingseason included CO2 derived from shoot and root respi-ration of severely injured plants, less importance wasgiven to these flux rates. On the other hand, CO2 emittedfrom the interrow location (bare soil) was assumed reflect soil respiration even though some of it would

have been derived from root respiration, especially dur-ing the active growing season (April-August). Root-derived CO2 could not be separated from that of thesoil with the surface chamber method.

Monthly COz flux (value in parenthesis representmean annual flux) from bare soil locations (interrow)in sod ranged from 2.7 to 32.5 kg C ha-~ d-1 (18.1) 1993, 3.6 to 43.1 kg C ha-~ d-~ (16.9) in 1994, and 3

120

100

8O

6O

"q’, 40

s 20.t:: 0(O 120~ lOO

~ 80

O 6o4o20

-- Row Sod AInter Row [~t

...... Mean

~:\t

Sub-Till

Jan Jun Dec Jun Dec Jun1993 1994 1995

No-Till

Plow

Jan Jun Dec Jun Dec Jun

1993 1994 1995Fig. 3. Mean monthly COz flux pattern from row and interrow under wheat-fallow rotation as influenced by tillage, and "native" grass sod at

Sidney, NE, 1993 to 1995.

1098 J. ENVIRON. QUAL., VOL. 27, SEPTEMBER-OCTOBER 1998

Table 3. Mean annual COz and N20 flux and CIt4 uptake at row and interrow io¢alions under wheat-fallow rotation and native sod atSidney, NE, 1993-1995.

1993 (cropped) 1994 (fallow) 1995 (cropped)’~

Treatment Row Interrow Mean Row Interrow Mean Row Interrow Mean

C02 flux, kg C ha-1 d-1

Sod~: 27.7 18.1 22.9 21.6 16.9 19.2 44.7 32.9 38.8(5.0)§ (0.2) (2.6) (0.8) (0.8) (0.5) (2.3) (0.8) (1.2)

No-till 23.3 11.1 17.2 8.4 6.9 7.6 38.0 13.3 25.7(2.0) (1.5) (1.2) (0.7) (0.4) (0.4) (3.0) (2.2) (2.6)

Subtill 23.8 11.9 17.8 10.0 9.6 9.8 40.8 17.5 29.1(0.2) (1.5) (0.8) (0.8) (0.3) (0.6) (5.0) (2.0) (2.7)

Plow 22.1 8.6 15.4 10.9 I0.0 10.4 33.7 20.1 26.9(4.2) (2.4) (3.3) (2.4) (1.3) (0.6) (8.0) (6.3) (6.2)

NzO flux g ha-~ d-1

Sod:~ 0.83 0.46 0.64 0.88 0.78 0.83 1.80 0.59 1.19(0.56) (0.13) (0.34) (0.11) (0.24) (0.11) (1.1) (0.22) (0.57)

No-till 0.99 0.77 0.88 1.32 0.87 1.10 2.02 0.34 1.18(0.15) (0.29) (0.13) (0.16) (0.18) (0.14) (1.2) (0.10) (0.58)

Subtill 1.05 0.55 0.80 2.24 1.32 1.78 2.32 0.39 1.36(0.22) (0.09) (0.16) (0.96) (0.06) (0.46) (1.8) (0.11) (0.96)

Plow 0.93 0.72 0.82 2.24 1.46 1.85 1.91 0.85 1.38(0.34) (0.33) (0.33) (0.96) (0.34) (0.57) (2.3) (0.63) (1.05)

CI-I~ uptake, g C ha-~ d-~

Sod:[: 7.3 8.7 8.0 7.6 8.8 8.2 10.4 9.4 9.9(0.7) (0.8) (0.2) (0.5) (0.8) (0.4) (0.4) (1.0) (0.4)

No-till 8.1 6.8 7.5 7.8 7.0 7.4 9.2 7.1 8.1(0.8) (0.4) (0.5) (0.5) (1.0) (0.7) (1.1) (0.8) (0.9)

Subtill 7.3 6.9 7.1 6.7 6.4 6.5 8.0 8.1 8.1(0.7) (1.1) (0.9) (0.9) (1.1) (1.0) (2.2) (1.7) (1.8)

Plow 6.0 5.8 5.9 6.6 6.3 6.5 7.8 8.3 8.0(1.7) (1.7) (1.7) (3.0) (1.5) (1.9) (0.7) (2.6) (1.5)

Measurement made from January to July.For sod ’Row’ represents grassy spot and ’Interrow’Number in parentheseis is standard deviation.

represents non-grassy spot.

95.5 kg C ha-1 d-1 (32.9) in 1995 (Fig. 3 and Table Under wheat-fallow, interrow CO2 flux across tillagesystems ranged from 0.9 to 29.7 kg C ha-1 d-1 (8.6-11.1)in 1993, 1.1 to 27.5 kg C ha-1 d-~ (6.9-10.0) in 1994, and1.4 to 51.3 kg C ha 1 d 1 (13.3-20.1) in 1995. Emissionsof CO2 from rows in sod and wheat (crop years: 1993and 1995) was 1.3 to 1.5 and 1.7 to 2.9 times, respectively,greater than from bare soil locations (Table 3) possiblydue to more root and shoot respiration. Carbon dioxideemitted from interrow in the wheat-fallow system aver-aged 34 to 60% of the grass sod CO2 emissions duringboth cropped and fallow years. Since grass roots oftenare densely spread in the soil, it was possible that in-creased root respired CO2 had led to greater interrowCO2 emissions under sod than wheat-fallow. Higher CO2emissions during 1995 may be due, in part, to increasedsoil and root respiration and because it was an averageof weekly CO2 emissions measured mostly during theactive growing season (January through July).

Fallow tillage practices greatly influenced (P < 0.05)CO2 emissions from wheat fields during the fallow yearfrom March through July, as soil was tilled frequentlyduring this time period (Fig. 3). Annual CO2 flux (meanof row and interrow) for the fallow year averaged 10.4kg C ha-1 d-1 for plow, 9.8 kg C ha-1 d-1 for subtill,and 7.6 kg C ha-~ d-1 for no-till. Presumably, increasedsoil microbial activity due to soil disturbance coupledwith greater soil water content and temperatures at timeof tillage led to elevated CO~ flux. Because of the ab-sence of shoot and root-derived CO2 emissions, CO2 fluxrates measured during the fallow year perhaps represent

the CO2 loss inherent to this system under given soil con-ditions.

During crop years, CO2 emitted from interrow wasnot always affected by tillage treatments imposed duringfallow. In 1993, CO2 flux was not influenced by thefallow tillage practices and ranged from 8.6 to 11.1 kgC ha-~ d-1. But in 1995, CO2 emissions ranged from13.3 to 20.1 kg C ha-a d-1, with no-till fallow exhibitingthe least CO2 flux.

Regardless of cropping and tillage management, thelargest CO2 emissions occurred during spring and de-creased in the order of spring (April-June) > summer(July-September) > autumn (October to December) winter (January to March) (Fig. 3). However, time occurrence of the greatest CO2 flux differed betweengrass sod and the wheat-fallow rotation. Under grasssod, peak CO2 flux occurred in June and July. For wheat-fallow, greatest CO2 flux occurred in May and July dur-ing the crop and fallow year, respectively. Althoughhigh CO2 emissions occurred during spring and summer,about 7 to 18% of the total annual COz loss occurredduring autumn and 4 to 10% during winter.

Nitrous Oxide Flux

Weekly N20 flux for each treatment is shown in Fig.2. Season, cropping, and row location significantly (P 0.05) influenced N20 flUX. Also, large seasonal varia-tions were observed in N~O flux which varied with tillagemanagement practice (Fig. 4).

Cropping with fallow tended to increase N20 flux

KESSAVALOU ET AL.: FLUXES OF GREENHOUSE GASES FROM GRASS & WHEAT-FALLOW TILLAGE SYSTEMS 1099

11.5

9.5

7.5

5.5

3.5

1.5

-13.517.515.513.511.59.57.55.53.51.5

-0.5

-- RowInter RowMean

Sod

Jan Jun Dec

1993

No-Till

Jun Dec Jun Jan Jun Dec Jun Dec Jun

1994 1995 1993 1994 1995Fig. 4. Mean monthly N~O flux pattern from row and interrow under wheat-fallow rotation, as influenced by tillage, and "native" grass sod at

Sidney, NE, 1993 to 1995.

as compared to grass sod and annual N20 flux acrosstreatments averaged < 1 g N ha-~ d-1 for sod and be-tween 1 to 2 g N ha-1 d-1 for wheat-fallow rotations(Table 3). The increase in N20 flux was pronouncedduring spring and summer months (P < 0.05) (Fig. and for wheat-fallow rotations it coincided with in-creases in CO2 flux (Fig. 3). Several episodic emissionsof N20 occurred during the spring and summer monthsof cropping, and also during the fallow year for subtilland plow treatments. Some of these episodic N20 emis-sions occurred when WFPS in the top 15 cm was above50 to 60% (Fig. 5), suggesting that denitrification wasthe major source for some of these emissions. Becauselittle is known about the immediate effects of wettingand tillage on N20 emissions in this ecosystem, the dura-tion of these wetting and tillage-induced episodic N20pulses could not be ascertained in this study and is thesubject of another experiment.

Average N20 flux for wheat-fallow tended to increasewith degree of soil disturbance. During the fallow year,subtill exhibited an average N:O pulse of 6.3 g N ha-~

d-1 during May through July with pulses being lower

during the rest of the year (Fig. 4). Plowing resulted two major N20 pulses: an average of 3.6 g N ha-~ d-1

(accounting for 47% of the annual flux) during winter(January to March) and 2.8 g N -1 d- ~ (accountingfor 37% of the annual flux) during May to July of thefallow year. Significant N~O flux also occurred with no-till between January and March, accounting for 31% ofthe total annual flux. Presumably, the increase in N~Oflux because of cropping was due, in part, to greater Nturnover as a result of cropping, fallow, and frequenttillage. Greater levels of NO3 measured for wheat-fallowsoils (across tillage systems) than for soils under sodsuggested that increased N:O flux generally coincidedwith increased net N mineralization and nitrification. Inaddition, lower levels of microbial biomass C observedunder cropped soils (see soil microbial biomass C andsoil NO3 section) suggested that the food supply to mi-croorganisms was more limiting and soil organic C andN pools were undergoing rapid net mineralization, ascompared to grass sod. The grassland ecosystem resultsin a more closed N cycle and lower net mineralizationand nitrification due to more continuous supply of high

9o807o

¯ o 50~- 40

~ Sod.... No-Till......... Sub-Till

/-.. --- Plow

~,.\ _...~- //~,

.,

J FM AMJ J AS O NDJ FM AMJ d AS O NDJ FM AMJ d A1993 1994 1995

Month (indicated by the first letter)Fig. 5. Mean monthly water-filled pore space observed in the top 15 cm soil under wheat-fallow rotation with different tillage management and

"native" grass sod at Sidney, NE, 1993 to 1995.

1100 J. ENVIRON. QUAL., VOL. 27, SEPTEMBER-OCTOBER 1998

C/N substrates to microorganisms which results ingreater immobilization and microbial biomass.

Seasonal N20 flux ranked in the order of spring >summer > winter > autumn. But in 1994, more N20flux occurred during summer than spring. Winter andautumn N20 flux represented 3 to 47% and 2 to 24%of the total annual N20 flux, respectively (Fig. 4). Simi-lar large winter N20 fluxes were also observed by Mosieret al. (1996) under the shortgrass steppe ecosystem Colorado. Nitrification, rather than denitrification, waspresumably the major source of spring, summer, andautumn N20 flux, as soil WFPS during these seasonsindicated predominantly aerobic soil conditions (Fig.5). In thawed surface soils overlying frozen subsoils,conversely, denitrification was likely the source of win-ter season N20 flux, as impeded drainage maintainedsoil WFPS above 60% for a longer time (Linn andDoran, 1984; Doran et al., 1990). However for grasslandsoils, considerably less nitrification usually occurs thanin cropped soils, allowing less opportunity for denitrifi-cation to occur.

More N20 was produced from the row than interrowlocations, especially during periods of greatest N20emission (P < 0.05) (Fig. 4 and Table 3). Mean annualN20 flux, averaged across management systems, was 1.5to 3.7 times greater from the row than the inter rowzone. Over the 3 yr of study this corresponded to amean annual N~O flux from row and interrow locationsof 1.54 and 0.76 g N ha-1 d-1, respectively. This wasprobably due, in part, to more available C and subse-quently greater nitrification and denitrification activitiesin the vicinity of plant roots where inputs of residuewere greatest. Therefore, these results indicate that N~Oflux under any ecosystem must be measured near andaway from plant roots to obtain a representative flux.

Methane Uptake

Weekly and seasonal CH4 uptake measured duringthe study period are shown in Fig. 2 and 6. Season andcropping influenced CH4 uptake (P < 0.05). No majordifferences in CH~ uptake were observed between rowand interrow location (Table 3). Monthly CH~ uptakeby soil in this grassland ecosystem across treatmentsranged from 1.3 to 12.3 g C ha-~ d-1 during the study

~ 18 ~ Sod---- No-Till

~ 15.~ ...... Sub-Till

O~ 12~

Plow ,,~ . /

FMAMJJASONDJ FMAMJJASONOJ FMAMJ Ja1993 1994 1995

Month (first letter)Fig. 6. Mean monthly CH4 uptake pattern under wheat-fallow rota-

tion as influenced by tillage, and "native" grass sod at Sidney, NE,1993 to 1995.

period (Fig. 6). Average annual CH4 uptake was greatestunder sod (8.0-9.9 g C ha-I d-~) and tended to decreasewith increasing soil disturbance in the order of sod >no-till (7.4-8.1 g C ha-~ d-1) -> subtill (6.5-8.1 g C ha-1

d-~) - plow (5.9-8.0 g C ha-~ d-~) as shown in Table3. Overall, the decline in CH4 uptake due to croppingranged from 6 to 26% within tillage treatments. A simi-lar reduction in CH4 uptake under winter wheat in Colo-rado was reported by Mosier et al. (1991). Also, CH4uptake tended to decrease as NzO flux increased. Thisconforms with earlier findings that an inverse relation-ship exits between CH4 and N20 flux (Mosier et al.,1991 and 1996).

Methane uptake was greater during spring (March-June) and summer (July-September), as compared autumn (October-December) and winter (January-March). However, winter and autumn CH4 uptakeranged from 12 to 21% and 14 to 31% of total annualuptake, respectively. Possibly, greater soil water contentand low soil temperatures during autumn and winterled to increased CH4 production and or decreasedCH~ oxidation.

Soil Environment

Water-Filled Pore Space and Temperature

Cropping and season influenced soil WFPS (0-15 cm)during most of the study period (P < 0.05). Although66 to 88% of the precipitation received during the studyperiod occurred during summer and spring months,WFPS values during that time were relatively .low, ascompared to winter and autumn months, due to greaterevapotranspiration demand during summer and spring.Regardless of season, WFPS of no-till soil was greaterthan that for other treatments during most of the studyperiod (Fig. 5). In 1993 and 1995 (crop year), (0-15 cm) averaged 55 to 61% for no-till, 46 to 52%for sod, 47% for subtill, and 46% for plow (Fig. 5).Further, during 1993, WFPS for no-till was often above60%, indicating potential for more anaerobic activitiesunder no-till than for other treatments (Linn and Doran,1984). During fallow, annual WFPS averaged 59% forno-till, 50% for subtill, 52% for plow, and 38% for sod.

Regardless of cultivation, WFPS influenced green-house gas fluxes throughout the study period. A curve-fitting approach was used to evaluate the relationshipbetween gas flux (CO2 and N20 fluxes were adjustedto a common base temperature of 25°C using a Q~0 factorof 2) and WFPS. Nitrous oxide flux tended to increaselinearly with increasing WFPS (Fig. 7), while CO2 fluxshowed a similar, but weaker trend (not shown). Meth-ane uptake, however, decreased linearly with increasingWFPS over the range of about 20 to 80% (Fig. 7) andthere was some indication of reduced uptake at lowerWFPS values, in accordance with other published re-ports (Mosier et al., 1996).

Soil temperatures (0-7.6 cm) were greatest in summer(13.8-26.3°C), followed by spring (8-22.8°C), and lowest in autumn (-0.9-12.9°C) and winter (-1.4-7.4°C) (Fig. 8). Greenhouse gas fluxes were stronglyinfluenced by soil temperature. Carbon dioxide emis-

KESSAVALOU ET AL.: FLUXES OF GREENHOUSE GASES FROM GRASS & WHEAT-FALLOW TILLAGE SYSTEMS 1101

8O

V 60

:~ 40

~ eo

z 0

10

20

~ 15

N20 = -6.39+0.23*WFPS r~--0.13Flux adjusted to 25°C (P = 0.05)

¯ ,o ¯ - ¯ u-,~=0%-

20 30 40 50 60 70 80

CH4=14.58-0.14*WFPS (P = 0.05)r~=0.27

10 20 30 40 50 60 70 80

Water-filled pore space (WFPS),

Fig. 7. Nitrous oxide flux and CH4 uptake as influenced by water-filled pore space in the top 15 cm soil at Sidney, NE.

sions (interrow for sod and wheat, and the mean ofrow and interrow during fallow) increased linearly withtemperature about 2.3-fold between 5 and 15°C, and 1.6fold between 15 and 25°C (Fig. 9). In the Ontario regionof Canada, Lessard et al. (1994) observed a linear rela-tionship between CO2 emissions and soil temperaturein a poorly drained corn field, but reported a Q~0 valueof 3 for soil temperatures between 5 and 15°C. Nitrousoxide emissions and CH~ uptake also increased linearlywith soil temperature. Soil CH4 uptake exhibited 1.13-to 1.31-fold increase for each 10°C rise in temperaturebetween 5 and 25°C (Fig. 9). This value is comparableto that (1.53) observed by Lessard et al. (1994) for Cana-dian forest soils at soil temperatures between 5 and15°C. Similar trends between temperature and N20 fluxand CH4 uptake were also reported by Mosier et al.(1996) in a Colorado shortgrass steppe ecosystem.

Soil Microbial Biomass Carbon and Soil Nitrate

Cropping and tillage influenced the amount and dis-tribution of soil microbial biomass C in the top 30.5 cm

30

25

2O

120

100

80

6040

20

20

z

z

o 12

o 0

CO~=2.21+0.97*temperature~=0.36 (P= 0.05)

0 5 10 15 20 25 30

N20=1.00+0.11 *temperaturer~=O.09

0 5 10 15 20 25 30

CH4=5.62+0.15*temperaturer~=0.30 (P= 0.05)

-5 0 5 10 15 20 25 30

Temperature (t),

Fig. 9. Influence of soil temperature in the top 7.6 ¢m soil layer onCOz (interrow) and N~O flux and CH4 uptake averaged over rowand interrow at Sidney, NE.

soil layer (P < 0.05). Greatest microbial biomass C the top 30.5 cm soil was found under grass sod (1765kg C ha-~), followed by no-till (1048 kg C ha-~), subtill(988 kg C ha-~) and plow (946 kg C ha-~) (Fig. 10).Reduced and less continuous inputs of C to soil werelikely responsible for a smaller microbial biomass Cpool in the cultivated than in grass sod soil. Surface soil(0-7.6 cm) contained the greatest relative amounts microbial biomass C especially under grass sod, no-till,and subtill plots. On the other hand, relatively moremicrobial biomass C accumulated in the lower layers of

0

-5J FMAMJ

--- Sod

\ ~.~/ \~ ’;,~/ ’~ ......... Sub-Till / /\\ ,.~.~" ’~ f~ ’~ --- Plow

~’~""

)~ 1994 t/~ ,995 ":"

......... .......d ASONDJ FMAMJ J ASONDJ FMAMJ J A

Month (indicated by the first letter)Fig. 8. Average monthly soil temperature in the top 7.6 cm soil layer under "native" grass sod and wheat-fallow rotation with different tillage

management at Sidney, NE, 1993 to 1995.

1102 J. ENVIRON. QUAL., VOL. 27, SEPTEMBER-OCTOBER 1998

2000

1600

1200

800

400

~ 0-7.6 cm~ 7.6-15.2 cmr---I 15.2-30.5 cm

Sod No-till Sub-till PlowFig. 10. Distribution of microbial biomass C in the top 30.5 cm soil

under native grass sod and wheat-fallow with different tillage man-agement at Sidney, NE. Treatment means for depth intervals con-taining the same letter are not significantly different at P = 0.05.

the plow plots, due to intensive soil mixing and resi-due incorporation.

Cropping and tillage influenced soil NO3 levels duringthe study period (P < 0.05). Average soil NO3 levels the top 15 cm layer were highest for no-till, intermediatefor subtill and plow and lowest for sod (data not shown).During the study period, NO3-N levels were 1 to 12 kgha-1 for sod and 2 to 65 kg NO3-N ha-1 under wheat-fallow rotation. This suggests greater nitrification withcropping. The highest amount of NO3 was found in thesummer of the fallow year, averaging 64 kg N ha-1 forno-till and subtill plots and 49 kg N ha-1 for plowed soil.

RELEVANCE OF FINDINGS

Because the balance between C sequestration andCO2 loss in cultivated soils has a significant impact on

the global C cycling, an attempt was made to relatemeasured CO2 loss to the dynamics of soil organic C(SOC) in this ecosystem. The rate of depletion of SOCin different tillage systems were estimated by computinga rough soil C budget (Table 4) with measured CO2emissions and estimates of plant C input into the soil(after Doran et al., 1998). Total losses of SOC throughsoil respiration during a 2-yr wheat-fallow rotation pe-riod, 1993 and 1994 for three tillage treatments werecalculated by doubling the average (mean of row andinterrow) CO2 loss measured during fallow year (1994)(Table 3). We followed this approach because, CO2emitted from the interrow during crop years especiallyduring the growing season would also contain root re-spired CO2 and may therefore result in the overestima-tion of SOC loss.

Net SOC loss estimated using the above approachaveraged 396 kg ha-1 yr-1 for no-till, 1684 kg ha-I yr-1

for subtill and 1436 kg ha-1 yr-~ for plow (Table 4). Therate of SOC losses predicted by Methrell (1994) withthe CENTURY model for these treatments averaged400 for no-till, 500 for subtill, and 550 kg C ha-1 yr-1for plow. Rate of depletion of SOC determined by sam-pling the top 1.2 m soil profile, 25 yr after the establish-ment of these plots averaged 420 kg C ha-1 yr-~ for no-till, 500 kg C ha-~ yr-~ for subtill, and 760 kg C ha-1

yr-~ for plow (Doran et al., 1998). The trend in the rateof depletion of SOC as estimated by our approach forthe 1993 to 1994 rotation period was comparable to thatof the long-term CENTURY model predictions andthose for soil measured values for no-till but overesti-mated for subtilled and plowed soils. This may be attrib-uted to the yearly variations in the capacity of wheatplants to fix atmospheric C, and our inability to accu-rately quantify the amount of C added to the soil, andthe variability in mean CO2 flux rates determined duringthis short 2-yr rotation period. Moreover, differences inthe magnitude of the rate of depletion of SOC observed

Table 4. Estimated soil C budget and C losses for wheat-fallow rotation under different tillage management at Sidney, NE, during 1993and 1994.

Carbon input or loss No-fill Subtill Plow

Residue and grain yield (1993), kg ha-1Grain yieldStraw yieldEstimated root yield’~

C inputs during 2-yr rotation, kg ha-1Wheat straw C (assuming 40% C)Estimated root residue C (assuming 40% C)Estimated rhizo deposited C and miscellaneous inputs$Estimated C input from weed plantsTotal C inputs in 2 yr, kg ha 1

Total C outputsMeasured soil C loss as CO2 in 2 yr, kg C ha-~Estimated annual soil C loss§, kg C ha-1 yr-1

Simulated and measured soil C loss, kg C ha-t yr-~Simulated][ for surface soil (0-20 cm), 1972-1992Measured for surface soil (0-30 cm), 1970-1992Measured for soil profile (1.2 m)’~’, 1970-1995

2924 2359 30663235 2791 35182863 2470 3113

1294 1116 14071145 988 12451718 1482 1868600 200 200

4757 3786 4720

5548 7154 7592396 1684 1436

400 500 550390 430 490420 500 760

"~ Estimated using a straw to root dry matter ratio of 1.13 (Buyanovsky and Wagner, 1986).~ 1.5 times the root residue C at harvest (Lucas and Vitosh, 1978).§ Estimated annual soil organic C loss = (2-yr measured C loss - 2-yr C input)/2.][ Simulated by Metherell (1994) using the CENTURY model.~’~ Determined in 1995, 25 yr after establishing experimental plots; loss based on changes in profile (0-1.2 m) organic content relative tograss sod(120.5 Mg ha-~). Respective profile organic C contents for no-till, subtill, and plow were 110, 107.9, and 101.6 Mg ha-t. Values averaged for Tillage Cand D fields.

KESSAVALOU ET AL.: FLUXES OF GREENHOUSE GASES FROM GRASS & WHEAT-FALLOW TILLAGE SYSTEMS 1103

between short- and long-term estimates indicate thatrate of SOC depletion across tillage systems couldchange from year, to year as a function of net C inputand soil respiration. Our results indicate that, exceptfor no-till, caution should be taken in using measuredCO2 losses as an indicator of soil C depletion in wheat-fallow tillage systems. No-tillage appears a viable tillagemanagement option for reducing the loss of SOC in thiswheat-fallow rotation system. Moreover, the study alsodemonstrates that increasing C storage under winterwheat-fallow cropping to the level exhibited by sod isnot possible due to reduced C input.

The maximum average annual N2O emissions in thewheat-fallow system ranged from 321 g N2O-N ha yr"1

during the crop year (1993) to 675 g N2O-N ha'1 yr~'during the fallow year. On the 0.69 million ha of wheat-fallow land in Nebraska, this would amount to 0.22 XIO6 kg and 0.47 X IO6 kg N2O-N ha yr'1 during thecrop and fallow year, respectively, as compared to anestimated 0.18 X IO6 kg N2O-N ha yr"1 from an equiva-lent area of sod. Therefore, in a 2-yr rotation period,up to 0.69 X IO6 kg N2O-N may be emitted into theatmosphere due to wheat-fallow cultivation under zeroN input conditions, as compared to the background(sod) emission of 0.36 X IO6 kg N2O-N. Further, if weassume that N is applied to wheat at the rate of 56kg N ha'1 (mostly NH4 form) and 1.25% of that N isconverted to N2O (IPCC, 1996), the net emissions fromwheat-fallow rotation in 2-yr period would then be1.17 X IO6 kg N2O-N, as compared with 0.36 X IO6 kgN2O-N ha from sod. This is about three times the emis-sion from an equivalent area of sod.

This grassland ecosystem serves as a net sink for CH4

and accumulates about 3.2 kg ha"1 yr"1. Regardless oftillage management, cropping and inclusion of fallowled to an average 19% reduction in CH4 uptake. Studiesindicate that the CH4 oxidation capacity of the cultivatedsoil also declines following N fertilization (Mosier et al.,1991). Based on the findings of Mosier et al. (1991), weassumed that about a 41% (1.31 kg ha"1 yr"1) reductionin CH4 oxidation capacity of the grassland ecosystemmay occur due to cropping, and fallow, combined withN fertilization. With this assumption, we estimate thatthe atmosphere is likely to be enriched with about 0.90 XIO6 kg of CH4 each year, as a result of wheat-fallowrotation (0.69 X IO6 ha) in Nebraska.

SUMMARY AND CONCLUSIONSWhen compared to "native" grass sod, winter wheat-

fallow cropping significantly increased net soil CO2 andN2O emissions into the atmosphere, while reducing CH4

uptake by soil. Large seasonal variations occurred ingreenhouse gas fluxes, with greatest fluxes during springand summer, when changes in environmental conditionsand biological activity were most pronounced, and lowerfluxes occurring in autumn and winter. However, winterseason N2O flux and CH4 uptake accounted for 3 to47% and 12 to 21% of the annual fluxes, respectively,a surprisingly large relative proportion given the lowlevels of soil biological activity expected at low tempera-

tures. Mid-winter flushes of N2O were associated withperiodic thawing of frozen surface soil, apparently re-sulting in creation of optimal conditions for microbialdenitrification for short periods.

Average annual flux of CO2 from grass sod was gener-ally 10 to 20% greater than that from wheat-fallow sys-tems, likely due to a larger contribution from plant respi-ration. However, among wheat-fallow managementsystems annual CO2 flux from subtill and plow systems(9.8-10.4 kg C ha"1 d"1) during fallow was significantlyhigher than that from no-till (7.6 C ha"1 d"1). In wheat-fallow systems the greatest pulses in CO2 occurred inspring and summer and generally coincided with pulsesin N2O. However, mean annual flux of N2O over the3 yr of study was least for systems where soil disturbancewas minimal (sod and no-till, 0.9 and 1.05 g N ha"1 d"1)and increased by 30 to 50% with fallow tillage (subtilland plow, 1.31-1.35 g N ha*1 d"1). Over the 3 yr ofstudy, CH4 uptake from soil tended to be greatest ingrass sod (8.7 g C ha"1 d"1) and declined with wheat-fallow cropping and degree of soil disturbance by tillage(no-till, 7.7 g C ha"1 d"1; subtill, 7.2 g C ha"1 d"1; plow,6.8 g C ha"1 d"1). Other research at this site indicatesthat major pulses in greenhouse gas flux in response toepisodic tillage and precipitation events are of relativelyshort duration (8-24 h) but contribute significantly tototal annual fluxes.

Cropping and tillage led to a significant decline insoil organic C with the rate and pattern of its depletionvarying possibly from year to year as a function of totalC input into the system and soil respiration. No-till man-agement appears best for reducing the detrimental ef-fects of cropping and fallow on atmospheric quality andfor maintaining soil C storage. However, with wheat-fallow cropping systems, soil C sequestration to remedi-ate increases in atmospheric CO2 will likely only bepossible with increased intensity of cropping and re-duced frequency of fallow.

ACKNOWLEDGMENTSWe thank Mr. Larry A. Tisue and Robert A. Higgins for

their technical assistance in this research. Our appreciation isalso extended to Dr. James F. Power, Dr. James S. Schepers,and Dr. Shashi B. Verma for reviewing this manuscript.

1104 J. ENVIRON. QUAL., VOL. 27, SEPTEMBER-OCTOBER 1998