Soil & Tillage Research, 12 (1988) 135-148 135 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Soil Respiration in Conventional and No-til lage Agroecosystems under Different Winter Cover Crop Rotations

P.F. HENDRIX, CHUN-RU HAN 1 and P.M. GROFFMAN ~

Institute of Ecology, University of Georgia, Athens, GA 30602 (U.S.A.)

(Accepted for publication 16 November 1987)

ABSTRACT

Hendrix, P.F., Han, Chun-Ru and Groffman, P.M., 1988. Soil respiration in conventional and no- tillage agroecosystems under different winter cover crop rotations. Soil Tillage Res., 12: 135- 148.

Carbon-dioxide efflux from conventional (CT) and no-tillage (NT) soils under winter rye- grain sorghum and crimson clover-grain sorghum rotations was measured for two consecutive cropping seasons using a static-absorption technique. Overall, C02 output was significantly higher from NT than from CT soils, and from soils cropped to clover than from those cropped to rye. During the cool season, soil respiration was similar in NT and CT soils, and in an adjacent forest and old field. Carbon dioxide production ranged from 5 to 50 g C02 m -~ day- ~ over the 17 months of observation. Pulses of CO~ production were observed, following mowing of the winter crops, in both CT and NT. Plowing did not stimulate CO2 production in CT as was expected, but annual CO~ production in these systems may have been underestimated. Tillage appeared to affect the timing rather than the total amount of C02 production. Linear regressions showed strong rela- tionships between temperature and respiration in both CT ( r = 0.88) and NT ( r = 0.80 ) soils but not between soil moisture and respiration. However, soil moisture was significantly related to the contribution of surface residue to total respiration in NT ( r = 0.80), suggesting that wetting and drying cycles may be more important to the decomposition of residues in NT than in CT.

INTRODUCTION

Cultivation is known to diminish levels of organic matter in soils (Campbell, 1978; Coleman et al., 1984). Tillage increases the rates of organic-matter de- composition and mineralization by aerating the soil, burying surface residues and breaking soil aggregates, thereby increasing the exposure of soil organic

Present addresses: IBeijing Agricultural University, Beijing, China ~Department of Natural Resources Science, University of Rhode Island, Kingston, R102881, U.S.A.

0167-1987/88/$03.50 © 1988 Elsevier Science Publishers B.V.

136

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TIME

Fig. 1. Hypothetical COx efflux curves from conventional (CT) and no-tillage (NT) soils, show- ing expected pulsed output after plowing in CT.

matter to microbial activity ( Phillips and Phillips, 1984). Conservation tillage practices leave crop residues on the soil surface and reduce soil disturbance, but the extent to which decomposition and mineralization of organic matter are reduced, relative to conventional practices, is not clear. In fact, it has been reported that the abundance and often the activity of decomposer organisms are greater in NT than in CT systems, especially in the upper layers of the soil profile (Blumberg and Crossley, 1983; Blevins et al., 1984; Warburton and Klimstra, 1984; House and Parmelee, 1985).

To explain this apparent contradiction, Blevins et al. (1984) suggested that immediately after plowing there is a rapid flush of microbial activity which is often missed in studies of agricultural soils ( Fig. 1 ). After this pulse of activity in CT soils, the more abundant substrates and higher moisture content of NT soils should provide a more favorable habitat and promote greater biological activity over a longer period of time. A similar pattern has been proposed for nitrogen mineralization in CT and NT soils (Fox and Bundel, 1986). It can be further reasoned that because the organic-matter content of CT soil de- creases over time, the total annual CO2 output should be higher from CT than from NT soil, until carbon equilibrium occurs (i.e. carbon input equals carbon output). Therefore, prior to equilibrium, the plowing-induced pulse(s) in CT (Area A in Fig. 1) should be somewhat greater than the annual amount by which NT exceeds CT (Area B in Fig. 1 ).

In this paper we address these hypotheses using the results of a 17-month study of soil respiration in a set of experimental CT and NT agroecosystems. Our objectives were to determine the effects of CT and NT management prac- tices on the amounts and timing of carbon flux from soil and plant residues, and to make preliminary comparisons of soil respiration in agroecosystems and unmanaged native ecosystems at the same site.

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MATERIALS AND METHODS

The study was conducted at the University of Georgia's Horseshoe Bend experimental area on the Georgia piedmont near Athens, Georgia, U.S.A. (de- scribed by Stinner et al., 1984). The 14-ha floodplain site consists of a 0.8-ha agricultural area surrounded by old fields and second-growth forests. The soil is a well-drained Hiwassee series (clayey, kaolinitic, thermic Typic Rhodu- dults) with a sandy clay loam Ap horizon (66~c sand, 13% silt, 21% clay) and located on 0-2% slopes. Rainfall and air temperature during the study period are shown in Fig. 2.

At the time of this study, eight 28 × 28-m experimental plots had been under CT and NT treatments (4 each t since 1978. Conventional tillage consisted of moldboard plowing to a depth of 15 cm, followed by disking or rotary tilling before planting summer and winter crops. Grain sorghum (Sorghum bicolor ( L. ) Moench) and soybeans (Glycine max L. ) had been used in 2- or :/-year rotations as summer crops, with winter rye { Secale cereale L. ) as a winter cover crop. In 1981, the 8 plots were split into separate winter cover crop treatments superimposed on the tillage treatments. Winter rye was grown on one halt' of each plot and received inorganic nitrogen fertilizer in spring before the plant- ing ot 'sumnler crops. Crimson clover ( Trifolium incarnatum L. ) was grown on the other half of each plot and no nitrogen fertilizer was applied.

At the end of each cropping season, crop residues were mowed and then plowed under in CT or left on the surface in NT. In spring, fertilizer (95 kg N ha ~ , 4 5 k g P h a ~ , 1 3 5 k g K h a - ~ ) was applied, except as noted above, and

13S

the herbicide glyphosate (Roundup) was sprayed (6.5 1 ha -1) on all plots. Summer crops were seeded in all plots with a no-till planter. In fall, CT plots were plowed after grain harvest. Winter crops were broadcast-seeded by hand and covered by mowing crop stubble over them in NT or with a drag-bar in CT.

Soil respiration was studied during 1983 and 1984 in conjunction with on- going studies of organic-matter decomposition and nutrient cycling (Groff- man et al., 1986, 1987). Grain sorghum was the summer crop in both years. Measurements were made in the rye and clover subplots within two of the CT plots and two of the NT plots for a total of 8 experimental units (2 replicates per t reatment combinat ion) . Treatments are hereafter termed NTR, NTC, CTR and CTC fbr no-till and conventional-till rye and clover.

Carbon-dioxide evolution from soil and litter was measured with the alkali- absorption method, as descrtibed by Coleman (1973). Aluminum cylinders 10 cm in diameter by 15 cm long and open at both ends were used as in situ res- piration chambers. Three replicate cylinders were inserted 7.5 cm into the soil in each CT plot and 6 replicates were placed similarly in each N T plot. Surface litter was removed from 3 of the cylinders in each N T plot to allow separate estimation of litter respiration (calculated as the difference between respira- tion rates in cylinders with and without litter). Between sampling dates, plas- tic mesh screen (1 mm) was placed in the cylinders without litter to minimize temperature and moisture effects, caused by litter removal. Cylinders were placed in the field at the beginning of each crop season and removed at the end to allow fbr field operations. The cylinders were placed in the crop inter-row during summer, and randomly in the broadcast-seeded cover crops during winter.

Sampling was conducted approximately monthly, except during spring 1984, when more intensive sampling was done to investigate the effects of field op- erations on soil respiration. On each sampling date, a 6-cm diameterX7-cm tall glass jar ( baby food jar) containing 20 ml 1.0 N NaOH was placed in each cylinder. Cylinders were then sealed with tight-fitting plastic tops and left to incubate for 24 h. In the laboratory, jars were t i trated to the thymolphthalein endpoint with 1.0 N HC1 in the presence of excess BaCI,~.2H20. Carbon dioxide was calculated from the difference in normality between samples and NaOH blanks.

Separate field incubations of freshly mowed and 5-day-old rye and clover residue were made in early May 1984. Replicate samples of material were placed in aluminum cylinders sealed at one end with a plastic cap. Jars of 1.0 N NaOH were placed in the cylinders, which were then sealed and incubated overnight /br 12 h in the field. Jars were t i trated in the laboratory as described above.

Climatic and soil physical conditions were measured routinely throughout the study. Soil moisture was measured as percent weight loss after 24 h at 1 0 5 C , and with gypsum blocks buried 10 cm in the soil. Soil temperature was

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measured using maximum - - minimum thermometers with probes buried 10 cm in the soil. Precipitation was measured with rainfall collectors located at the site.

Data were analyzed by analysis of variance after log-transfbrmation using a 2 factor model with tillage and winter cover as crossed main effects. A plot term was nested in the tillage effect and crossed with the cover effect /'or a repeated measures design.

R E S U L T S

Respiration rates over the study period are shown in Fig. 3. Because the study began in late spring 1983, after plowing and spring planting, the imme- diate response of soil respiration to field operations was not studied in that year. In 1984, measurements were made at closer intervals throughout May to capture short-term dynamics accompanying field operations. A large pulse of respiratory activity was observed in all treatments over a 3-week period fbllow- ing mowing of the winter cover crops. Apart from this pulse in 1984, respiratory patterns were similar during the summers of both years. In NT, activity de- creased over the summer to between 5 and 10 g CO., m-~ day- ~ by harvest time in September. Activity decreased more rapidly in N T R than in NTC in 1984. In CT, activity increased during early summer (June and July 1983, June 1984 )

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Fig. 3. Carbon dioxide ef i lux f r o m c o n v e n t i o n a l ( C T ) and no-tillage ( N T ) a g r o e c o s y s t e m s u n d e r rye ( R ) and clover ( C ) w i n t e r - c o v e r c rops , a n d f r o m a n adjacent fo res t ( F O ) and old field ( O F I. Forest and old field curves are inset above but have the same time axis. I,east significant differen( :e ( (t' -- 0 .05) = 1.25 g C O : m d a y ] for tillage and c o v e r - c r o p e f fec t s , excluding December t h r o u g h April.

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Fig. 4. Carbon dioxide efflux from conventional (CT) and no-tillage (NT) agroecosystems under rye (R) and clover (C) winter-cover crops on designated days after mowing of cover crops in May 1984. Times of field operations are shown with arrows, and rainfall events during the period are shown across the top. Least significant difference (~=0.05) = 1.97 g CO2 m -2 day -1 for tillage and cover-crop effects.

and then declined through September of both years. Analysis of variance over the two summer growing seasons (i.e. excluding winter 1983-1984) showed that respiration rates were significantly higher in NT than in CT (P<0.05=0.022) and in the clover residue plots than in rye residue plots ( P < 0.05 =0.024).

After fall plowing and planting of winter cover crops in 1983, respiration chambers were replaced and all surface litter removed to allow study of soil respiration, in the absence of litter, during winter and early spring. Similar chambers were established in an old field and a hardwood forest, adjacent to the plots, to allow comparisons between agricultural and native ecosystems. Temporal patterns of respiration were similar across the systems during the cool season 1983-1984. (Fig. 3). Rates declined to lowest values in February and increased to between 8 and 10 g CO2 m-2day - 1 by April. Although differ- ences were not significant (P < 0.05 ) among systems, NT showed highest res- piration throughout the period. During June 1984, the old field and forest reached peak activity of approximately 10 g CO2 m -2 day-1, declining to 9 and 7 g CO2 m -2 day -1, respectively, in July.

To examine more closely the effects of field operations on respiration, the time scale from spring and early summer 1984 has been expanded in Fig. 4 and expressed as days after mowing of the winter crops. Half of the plots were mowed on 2 May, and the other half on 7 May. Respiration chambers were re- installed on the latter date. The second and third time points in Fig. 4 were

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TABLE I

Calculated respiration rates and percent contr ibut ion to total respiration of rye (R) and clover ( C ) surface residues in no-tillage ( N T ) agroecosystems (s tandard error in parentheses)

Date g CO~ m ~ day- ~ Residue respiration from surface residue percent of total

N T R NTC NTR NTC

17 ,June 83 7.4 (2.8) 8.0 (1.9) 40 39 23 ,July 83 0 2.6 (1.4) 0 16 22 August 83 0.1 (0.5) 1.9 (0.2) 2 19 29 Sept. 83 1.6 (1.6) 2.4 (0.1) 17 25 8 May 84 ~ 9.5 27.6 29 58 16 May 84 10.5 (3.6) 13.7 (0) 35 38 23 May 84 24.5 (5.6) 26.8 (8.1) 61 63 6 June 84 6.7 (1.2) 9.8 (3.8) 32 39 21 June 84 1.4 (1.5) 6.7 (1.6) 10 29 18 July 84 5.7 (0.6) 10.7 (1.2) 39 47 20 Sept. 84 0.3 (0.3) 0 4 0

Walues calculated from only one replicate plot.

thus actually measured on the same date but represent a 5-day difference in time since mowing. Only a trace of precipitation occurred during the interval. Chambers were removed on Days 10 and 26 to facilitate plowing and planting. After each operation all chambers were re-installed in the usual fashion.

Respiration rates were at their peak values on the first day after mowing in all systems except NTC, which peaked on Day 5. Absorption of CO., depleted the absorbent in one of the three replicate chambers in CTC on Day 1 and in NTC on Day 5; mean peak respiration is thus shown as greater than 50 g CO, m ~ day -1 in these plots. Rates were from 30 to 100% higher in clover than in rye plots after mowing in both NT and CT plots. Weight-specific respiration rates of the two residue types were estimated in separate field incubations. For clover, values were 51 and 43 mg CO._, g - 1 day- 1 on Days 1 and 5, respectively, and for rye, 13 and 0.7 mg CO._, g 1 day 1 on the same days. Considering the biomass of rye and clover mowed down, freshly mowed clover accounted fbr 50-60% of total respiration, while fresh rye contributed 20-30% one day after mowing. Activity may have been partly caused by autotrophic respiration of' plant tissues on Day 1, but, after 5 days, plant material appeared mostly dead. Buyanovsky et al. (1986) at tr ibuted peak CO., output from recently-harvested wheat systems to decomposition of root tissues, although their data indicate a lag of I or 2 weeks between harvest and peak respiration.

Ten days after mowing, the CT plots were plowed and rotary-tilled. No res- piration response was detected 1 day later, except perhaps a slight increase in CTR. Eight days later, a sharp increase in CO2 flux was observed in CTC and

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in both NT plots; CTR showed no increase. These responses may have been influenced by rainfall (13 ram) the day before measurement and/or by nitro- gen-fertilizer application to the rye plots 4 days earlier. Negative responses of soil respiration to mineral-nitrogen fertilizer were observed by de Jong et al. (1974) and by Kowalenko et al. (1978). Over the next 12 days, respiration rates declined by about 50% in the NT plots and by 50-60% in CT, the latter converging at about 15 g CO2 m -2 day -1 by Day 30. Between 30 and 45 days after mowing, respiration in the CT treatments increased to around 20 g CO., m ~ day 1 but continued to decrease in NT, particularly under rye residues. These trends may have been affected by application of herbicides or P and K fertilizers or by planting of sorghum seeds, all of which were applied equally across the plots. Over the 45 days depicted in Fig. 4, the effects of cover crop on respiration were significant ( P < 0.01 ).

Surface-residue respiration and proportional contributions of surface resi- due to total respiration in NT during the spring/summer growing seasons of 1983 and 1984 are shown in Table I. One day after mowing in 1984 (8 May) , clover residue contributed 58% of total respiration while rye contributed 29%. After 8 days, the value for clover decreased to 38% but increased slightly for rye. Seven days later, contributions of both rye and clover residues to total respiration were greater than 60%, following N-fertilization (rye plots only) and rainfall. Throughout the summer months of 1983 and 1984, litter contri- butions to total respiration fluctuated, apparently in response to rainfall. Con- tributions were generally higher from clover than from rye residues during both years.

DISCUSSION

Soil respiration rates during the growing season in a wide variety of agricul- tural systems range from around one to as high as 50 g CO2 m -2 day - 1, aver- aging less than 10 g CO,~ m -'~ day-1 in the temperate zone (Singh and Gupta, 1977; Kowalenko et al., 1978). Rates in our systems fall within this range (Fig. 3) showing seasonal variation and pulses of activity accompanying field op- erations, as observed by de Jong et al. (1974), Buyanovsky et al. (1986) and Singh and Shekhar (1986).

In natural ecosystems, soil respiration rates of over 100 (Wisconsin pine forest in summer) and as low as zero (tallgrass prairie in winter) have been reported, with values around 2-10 g CO`) m -2 day -1 being more common dur- ing the warm season in tropical, temperate and subarctic systems (Singh and Gupta, 1977; Ceulemans et al., 1987; Ewel et al., 1987; Gordon et al., 1987; Moore, 1986). In some studies, fallow fields or native systems have shown lower respiration rates than adjacent agroecosystems (de Jong et al., 1974; Kaszubiak et al., 1977 ). This also occurred in our study and is consistent with the observation that cultivation accelerates the loss of organic matter from

14:/

agricultural soils ( Coleman et al., 1984 ). In contrast, Schimel (1986) reported higher rates of C02 evolution from native grassland soils than from nearby croplands. Similarly, Seto and Yui (1983a,b) found 2-7-fold higher rates of CO,_, evolution from forest soil than from adjacent cultivated systems in .Japan.

Causes for these differences are not clear, but a number of factors influence soil respiration and are probably affected by agricultural practices. As results from Seto and Yui (1983a,b) suggest, availability of carbon substrates to soil biota is of primary importance. Tillage affects the availability of fresh plant materials by incorporating them into the soil where they may be more rapidly colonized and utilized by soil organisms than materials which remain on the soil surface in undisturbed ecosystems. Tillage also affects carbon availability by disturbing soil structure and exposing protected organic materials. After extended periods of cultivation, soil microbial activity may become carbon- limited ( Schimel, 1986). Thus, while newly-plowed soils may show higher (CO~ efflux than undisturbed soils, the pat tern may reverse as available substrates become depleted.

Ultimately, primary productivity of a system determines the potential amount of organic-matter input to soil as plant residue and root exudates. Energy and nutrient subsidies can increase agroecosystem productivity above that of na- tive ecosystems (Mitchell, 1984), and therefore possibly increase fluxes of CO,, as well. Interestingly, there was no clear relationship in the present study be- tween soil respiration and primary production. Since the initiation of the ex- perimental cropping systems in 1978, there have been no consistent differences in aboveground net production between NT and CT ( Stinner at al., 1984; House et al., 1984 ). However, during the period of respiration measurements, biomass of winter cover crops that were mowed down in the spring was highest in NT rye (10620 and 6415 kg ha 1 in 1983 and 1984 respectively) and lowest in NT clover (3200 and 4002 kg h a - ' , respectively); values for CT rye (7135 and 4679 kg ha -1) and CT clover (7125 and 4352 kg ha -~) were intermediate between the NT values during both years (Groffman et al., 1987). Nonethe- less, CO._, tlux rates were significantly higher (P<0 .05) from clover plots in both NT and CT over the two summer seasons. Thus, high resource quality of the clover residue was apparently more important than total carbon input in regulating carbon availability to soil biota.

Although tillage was expected to stimulate respiration in our study, results indicate greater activity in NT soil over most of the period of observation (Fig. 3 ). The predicted pulse alter spring plowing in CT either did not occur or was missed by our measurements at 1, 7 and 21 days after plowing (Fig. 4). Inte- grated areas under the curves in Fig. 3 were estimated to be about 4300 g CO., m -~ yea r - ' in NT and about 3700 g CO2 m --2 year 1 in CT. Excluding a pos- sible fall plowing pulse, which was not studied here, the annual mean difference between NT and CT from October 1983 through September 1984 was therefore about 600 g CO,_, m -~. This is equivalent to about 1600 kg C h a -1 year '. As-

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suming that root respiration is approximately equal in NT and CT, this amount of evolved carbon would represent decomposition of about 4000 kg h a - 1 dry matter, an amount unlikely to have decomposed within 7 or 21 days. In fact, using decomposition constants from previous studies at our site ( - k = 0.028 day 1 from single exponential decay curves) and approximate amounts of plant residue incorporated into the CT soil in May 1984 (4500 kg ha 1 dry matter; Groffman et al., 1987), the amount of material decomposed would be approx- imately 800 kg ha -1 (117 g CO.) m -2) in 7 days and approximately 2000 kg ha J (293 g CO,_, m -2) in 21 days. Thus, it seems unlikely that spring plowing resulted in a tlush of CO2 the size of the N T - C T annual difference. It is plau- sible, however, that spring and fall plowing together could generate pulsed out- puts approaching 600 g COw m -~ year -1. Groffman (1985) observed a large pulse of denitrification activity and large increases in denitrifier and nitrifier biolnass on these plots after fall plowing in 1982. Since these pulses were driven by inputs of organic substrates from crop residue, concurrent increases in soil respiration would be expected.

It is interesting to note the apparent increase in soil respiration that oc- curred in CT during the early summer of both years (Fig. 3). Although the magnitude is not great, it suggests a possible delay between plowing and in- creased soil biotic activity. Data from Buyanovsky et al., (1986) also indicate a lag of a few weeks between plowing and peak CO~ output from wheat systems. Thus, two phases of activity might be hypothesized to occur alter plowing: (1) an initial and rapid pulse in which newly exposed, labile compounds are min- eralized (possibly missed in this study); (2) a later but perhaps longer phase in which incorporated plant residues are colonized and decomposed. In our study, CO._, production during the latter phase could have been greater than indicated in Fig. 3, since we have only one measurement over the 1.5-2-month period.

Because we do not know the magnitude of all possible tillage-induced pulses in CT, we are unable to evaluate fully the hypothesized relationships in Fig. 1. More frequent sampling is needed to capture short-term dynamics of soil res- piration, especially during spring and fall field operations. Also, our measure- ment techniques are subject to errors from (1) underest imation of CO,, tlux, particularly at high rates of output (Singh and Gupta, 1977), (2) extrapola- tion of rates from small (80 cm ~) to large (ha) areas assuming a degree of homogeneity that does not exist in the field, and (3) inclusion of root respi- ration which was not quantified separately from soil respiration in this study. These sources of error should be approximately the same across tillage treat- ments, however, and would not be expected greatly to alter comparative effects.

Also with respect, to Fig. 1, we do not know if' our systems have achieved a soil carbon equilibrium, in which case total annual CO2 output from CT and NT would by expected to be approximately equal, given equal inputs of carbon. Interestingly, Rice et al. (1986 ~ reported steady-state conditions for soil or-

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ganic nitrogen after approximately 10 years of cropping in Kentucky, U.S.A., but they did not indicate relative amounts of carbon flux in CT versus NT treatments. Data from Dalal and Mayer (1986) suggest equilibrium times of from < 5 to 40 years for various cultivated soils in Australia.

Despite reports of greater abundance of organisms in NT, it is not clear that overall biotic activity is actually greater than in CT. Soil meso-and macrofauna show the most obvious increases in NT relative to CT (Warburton and Klim- stra, 1984; House and Parmelee 1985) but their contribution to total soil res- piration is probably small (Golebiowska and Ryszkowski, 1977; Peterson and Luxton, 1982; Hendrix et al., 1987). Soil microbes are responsible for most heterotrophic soil respiration. In a comparison of CT and NT soils at several geographic locations in the U.S.A., Doran (1980) found that microbial abun- dance and activity were higher in surface layers of NT soils but not at depth or over the soil profile as a whole. He noted that NT systems resemble natural ecosystems in that the "status of higher microbial populations seems to be less oxidative and perhaps slower than those under conventional tillage." It is sug- gested that biotic activity may be more nearly equal in CT and NT than is apparent. Major differences may be in the timing rather than in the magnitude of activity.

A number of studies have shown that soil temperature and moisture are important regulators of soil respiration (Wildung et al., 1975; Bunnell et al., 1977; Kowalenko et al., 1978; Buyanovsky et al., 1986). Temperature affects metabolic activity directly, while water serves both as a medium for microbial activity and as an agent for solubilizing and increasing the availability of or- ganic substrates. Effects of tillage and residue placement on soil biotic activity may be exerted through influence on moisture and temperature. For example. the mulch layer on the surface of NT soil tends to insulate the soil from tem- perature extremes and from rapid desiccation, creating a potentially more sta- ble environment for biological activity (Blevins et al., 1984).

Linear regressions on data from our study showed significant relationships (P < 0.05) between temperature and respiration in both CT (r = 0.88) and NT (r= 0.80) soils. In contrast, soil moisture showed no relationship to respira- tion rates in CT or NT soil, but was significantly related ( P < 0.05) to percent contribution of surface litter to total respiration in NT ( r=0 .80) . These re- sults suggest that moisture may be only rarely limiting to activity in the soil profile in our systems but that wetting and drying cycles are important in the litter layer (i.e. especially in NT) .

Finally, data from our study also show that residue type can have a strong effect on soil respiration. Clover was a more labile residue than rye, and sys- tems with legume-N inputs had higher respiratory activity throughout the year than the rye systems which received fertilizer-N. Long-term effects of residue quality on nutr ient cycling and soil organic matter may be significant.

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CONCLUSIONS

Dur ing two consecut ive growing seasons, CO~ efflux was higher f rom N T t han f rom CT soils, and f rom soils t r ea t ed with clover t h an with rye residue. All systems, as well as an adjacent old field and forest, showed similar COe efflux dur ing win ter and early spring.

In the agroecosystems, pulses of CO2 ou tpu t were observed af ter mowing of win ter cover crops in bo th N T and CT but not, as expected, af ter plowing of CT soil. A gradual increase in CO2 ou tpu t f rom CT soil appeared to occur approx ima te ly I m o n t h af ter plowing, suggesting a lag phase be tween plowing and m a x i m u m soil biotic activity.

Soil resp i ra t ion was s t rongly re la ted to soil t e m p e r a t u r e in bo th CT and NT. Soil moisture, on the o ther hand, showed no relat ionship to respirat ion in e i ther system, but was s ignif icant ly re la ted to pe rcen t con t r ibu t ion of surface residue to total respirat ion. Mois ture may be only rarely l imit ing to biotic act ivi ty in the soil profi le in these systems, bu t wet t ing and drying cycles appear to be especially impor t an t to decompos i t ion of surface residue in NT.

ACNOWLEDGEMENTS

R.W. Parmelee , M.H. Beare, W.X. Cheng, D.A. Crossley Jr., E.P. Odum and K. Ma t son provided useful ideas in the in t e rp re t a t ion of da ta f rom this study. C.L. Langner provided able technica l assis tance. Th i s work was suppor ted by grant DEB 8207206 f rom the Na t iona l Science Founda t ion to the Univers i ty of Georgia Research Founda t ion ( E.P. Odum and D.A. Crossley Jr. ).

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Blumberg, A.Y. and Crossley, D.A., Jr., 1983. Comparison of soil surface arthropod populations in conventional tillage, no-tillage and old field systems. Agro-Ecosystems, 8: 247-253.

Bunnell, F.L., Tait, D.E.N., Flanagan, P.W. and Van Cleve, K., 1977. Microbial respiration and substrate weight loss--1. A general model of the influences of abiotic variables. Soil Biol. Biochem., 9: 33-40.

Buyanovsky, G.A., Wagner, G.H. and Gantzer, C.J., 1986. Soil respiration in a winter wheat eco- system. Soil Sci. Soc. Am. J., 50: 338-344.

Campbell, C.A., 1978. Soil organic carbon, nitrogen and fertility, in: M. Schnitzer and S.U. Kahn (Editors), Soil Organic Matter. Elsevier Scientific Publishing Co., New York, pp. 173-272.

Ceulemans, R., Impens, I. and Gabriels, R., 1987. CO,_, evolution from different types of soil cover in the tropics. Trop. Agric., 64: 68-69.

Coleman, D.C., 1973. Soil carbon balance in a successional grassland. Oikos, 24: 195-199. Coleman, D.C., Cole, C.V. and Elliott, E.T., 1984. Decomposition, organic matter turnover, and

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