M.J. Lindstrom and D.C. Reicosky

Soil Scientists, USDA-Agricultural Research Service, North Central Soil Conservation Research Lab, 803 Iowa Avenue, Morris, MN 56267; (320)589-3411; (320)589-3787(FAX); Email: mlindstrommail.mrsars.usda.gov and dreicosky@mail .mrsars.usda. gov. * * * All programs and services of the U.S. Department of Agriculture are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap.

ABSTRACT

There are concerns about increased greenhouse gases in the atmosphere because of potential climate changes that could result in an increase in temperature and drought. Agricultural production systems and specifically tillage effects on C0(2) fluxes to the atmosphere are not well understood. Two studies were conducted in the Northern Corn Belt to 1) measure the effect of fail tillage methods after wheat harvest on C0(2) flux from the soil compared to no-tillage and 2) evaluate short-term C0(2) emission from the soil after various conservation tillage tools compared to moldboard plowing and no-tillage. Four fall tillage methods were used in the first study: moldboard plow, moldboard plow plus disk harrow twice, disk harrow, and chisel plow. In the second study, five conservation tillage tools common to the area and designed to leave at least 30% or greater surface residue cover after planting were evaluated. The C0(2) flux was measured with a portable chamber system commonly used to measure canopy gas exchange of field crops. Moldboard plowing had the highest initial C0(2) flux and maintained the highest flux throughout a 19-day study period. Total C0(2) flux for the study period was in the order: moldboard plow> moldboard plow plus disk harrow> disk harrow> chisel plow> no-tillage. The major factors contributing to C0(2) flux were depth of soil disturbance that resulted in a rougher surface and large voids. Little difference was observed with the conservation tillage tools which produced C0(2) emissions that were approximately 30% of the moldboard plow treatment. Tillage methods can effect short-term C0(2) emission from the soil and results from these studies suggest improved soil management can minimize agriculture's impact on global C0(2) increase.

INTRODUCTION

Management of crop residues and soil organic matter is important in maintaining soil fertility, productivity, and minimizing agricultural impact on environmental change. Soil is our most valuable resource for food and fiber production. Soil organic matter or soil carbon is a critically important component which is directly related to soil productivity and soil quality Carbon is the key element for the foundation of life. Soil organic carbon gives soil its dark color and is closely linked to soil physical, chemical, and biological properties associated with soil productivity.

The possibility of global greenhouse warming due to an increase in atmospheric carbon dioxide (C0(2)) has received much attention (Wood, 1990; Post et al., 1990). The burning of fossil fuels has the potential effect of elevating C0(2) concentrations above the current 0.03% atmospheric concentration. This concern is warranted because potential climatic changes from elevated C0(2) concentrations could result in increased temperature and drought over present agricultural production areas (Wood, 1990).

Agriculture plays an important role in the overall global carbon balance. Carbon dioxide from the atmosphere in combination with photosynthesis provides the organic raw material and energy for all synthetic reactions in plants. In effect, carbon is the raw material from which plants develop. However as plants die and decompose, C0(2) is released back to the atmosphere through microbial oxidation of readily decomposable plant material, but two groups of organic carbon compounds tend to remain in the soil. The first group is resistant compounds such as oils, fats, waxes, and lignin; the second group is synthesized by microorganism and held as part of their tissue as microbial biomass. These two groups, one modified from the original plant material and one newly synthesized by microorganisms, provide the basic framework for humus. Humus, the major proportion of soil organic carbon, is quite resistant to further microbial attack. Maintaining or increasing the humus content in soils is an important role that agriculture can play in reducing atmospheric C0(2) concentrations.

Although humus is generally resistant to microbial attack, it is not immune. For example, when grasslands are tilled, soil organic carbon rapidly declines in the first ten years and then gradually declines to an equilibrium value dependent on cropping system and climate (Bauer and Black, 1981; Ismail et al., 1994). The decline in soil organic carbon following tillage results from the rapid influx of oxygen, similar to stoking a slow-burning fire. The increases in soil temperature when surface residues are mixed into the soil further increases microbial activity and decomposition. Both process result in a stimulation of microbial activity under moist soil conditions and consequently an accelerated loss in soil organic carbon.

No-till systems have the advantage of avoiding rapid mineralization of soil organic carbon that occurs when a grass sod is tilled. Evaluation of soil organic carbon after ten years of no-till and conventional moldboard plow corn production following grass showed approximately twice the carbon in the surface layer (0-5 cm depth) of the no-till compared to the conventionally tilled system (Blevins et al., 1983). Starting from a conventionally tilled system, microbial biomass carbon in an introduced no-till corn system in Ohio came to equilibrium after 15 years to approximately 30% greater than the conventional system (Staley et al., 1988). However, pasture had 50% greater microbial biomass carbon than the no-till. A study in Kentucky found that soil organic carbon content was restored to that of a bluegrass sod after 20 years of no-till corn with a winter rye cover crop (Ismail et al., 1994).

Data from long-term field experiments leads to two key management factors affecting soil organic carbon; 1) the quantity and quality of residues added to the soil, and 2) the type and intensity of tillage Current changes in agricultural practices, e.g. , increased use of crop rotations, cover cropping, decreased fallow frequency, and increased use of conservation tillage and no-till, provide an opportunity to reverse the decrease in soil organic carbon loss from agricultural soils, changing them from sources of C0(2) gas emission to sinks for carbon. Present data supports increased adoption of no-till management practices and offers a significant potential to preserve or increase soil organic carbon levels. Reversing this trend will be beneficial to agriculture as well as to the global population through positive adjustments in the global carbon balance.

Information is needed on the impacts of various tillage methods on carbon dynamics within agricultural production systems. Studies were initiated at the North Central Soil Conservation Research Laboratory, Morris, Minnesota, U.S.A. to measure C0(2) release from the soil with tillage The objectives were to 1) measure the effect of different fall tillage methods on C0(2) flux from the soil after wheat harvest compared to no-tillage and 2) evaluate the effect of conservation tillage tools on short-term C0(2) emissions from soil compared to moldboard plowing and no-tillage

EXPERIMENTAL PROCEDURES

Study 1. - Tillage methods effect on C0(2 ) flux.

This experiment was conducted in 1991 on a clay loam and originally reported by Reicosky and Lindstrom (1993). Tillage treatments were designed to cover a range of tillage depths, estimated degree of soil disturbance, and estimated degree of residue incorporation. A no-tillage area was established as a check treatment. The four tillage treatments established included a moldboard plow (MP) with 0.46 m wide bottoms, to a depth of 0.25 m that resulted in complete inversion of the surface layer and nearly 100% incorporation of the residue. The second treatment was moldboard plow to 0.25 m depth followed by a disk harrow twice (MP+D2X). This resulted in the same depth and degree of soil disturbance and residue incorporation, but a surface soil with smaller aggregates and a less porous surface. The third treatment was a disk harrow (DH) that resulted in shallow soil disruption (0.075 m) and partial incorporation of residue. The fourth treatment was a chisel plow (CP) with shanks on 0.30 m centers to a depth of 0.15 m with 0.076 m twisted shovels staggered on three bars, resulting in partial soil disruption and residue incorporation. All tillage treatments were done on 4 September (DY 247) on harvested spring wheat ground.

The C0(2) flux from the soil surface was measured on all treatments using a large portable chamber described by Reicosky (1990) and Reicosky et al. (1990). Measurements for C0(2) flux were initiated within five minutes of the tillage pass. Briefly, the chamber (volume of 3.25 m(3) covering a horizontal land area of 2.67 m(2) was moved over the soil surface, lowered and data rapidly collected at 2-second intervals for a period of 80 seconds to determine the rate of C0(2) increase inside the chamber. After appropriate lag times, data for a 30-second period was used to convert the volume concentration of C0(2) to a mass basis and then linearly regressed as a function of time. The slopes of these regression lines which reflect the rate of C02 increase within the chamber are expressed on a unit horizontal land area basis.

The total time for a single measurement to collect required data and computation was about two minutes. Triplicate measurements were made at each plot for each sampling period before moving to the next plot. Initially, up to five repeated measurements per day were made on all treatments to provide limited data on the diurnal dynamics of C0(2) fluxes. When the fluxes on the tilled plots had decreased substantially, only two measurements per day were made. Measurements were conducted over a 19-day period (Reicosky and Lindstrom, 1993).

Study 2 - Evaluation of conservation tillage tools

This study was conducted in 1994 on a loam and originally reported by Reicosky et al. (1995). Short-term C0(2) flux was measured for five hours after tillage using the same procedure described in Experiment 1 following six tillage implements ( moldboard plow and five conservation tillage tools) and compared to a no-tillage check area on harvested spring wheat ground. Point measurements were taken again 5, 14, and 28 days after tillage on the same areas. Tillage treatments were completed on 24 August. Two cutting heights of 0.05 and 0.30 m on the wheat stubble were established prior to tillage Conservation tillage equipment was supplied and operated by local area dealers. Because of the wide variety of conservation tillage implements, the specific equipment used in this study will be briefly described.

The moldboard plow, used in this study as a reference, was a John Deere Model 2800 (Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the u~c of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.) that consisted of six 0.46 m wide bottoms pulled by a 142-kW tractor at 8 km/h The plow was set to a depth of 0.25 m and left between 7 to 9% surface residue cover.

The Howard Paraplow Model 410b had four plows set to a depth of 0.25 m spaced 0.51 m apart. This tool was pulled by a 104-kW tractor at 8 km h-' and left 76 to 84% surface residue cover.

The White Model 445 Conser-Till consisted of a gang of disk coulters in front to cut and partially incorporate residue to a depth of 0.10 m. The front gang was followed by two gangs of twisted chisel shovels set to a depth of 0.25 m on 0.30 m centers and then followed by deep till subsoil shanks set to a depth of 0.41 m on 0.76 m centers. This equipment was 5.2 m wide and pulled by a 224-kW tractor at a speed of 9.7 km/h and left 31 to 35% surface residue cover.

The DM1 Model 530 Ecolo-tiger consisted of an "X" frame with nine concave disks 0.56 m in diameter on 0.38 m centers. The disk gangs were followed by five subsoil shanks on 0.76 m centers set to a depth of 0.41 m. Five additional shanks were mounted ahead and between the subsoil shanks to work the middles and provide more fracturing. These shanks ran at a depth 0.10 m shallower than the subsoil shanks. This equipment was 3.8 m wide and pulled by a 1 86-kW tractor at a speed of 9.7 km/h and left 29 to 43% surface residue cover.

The Glencoe Model 55 7400 Soil Saver consisted of a gang of 0.51 m diameter coulters on 0.19 m centers. The gang of coulters was followed by three gangs of deep till 0.10 m twisted chisel shovels set to a depth of 0.30 m on 0.38 m centers. This equipment was 4.1 m wide and pulled by a 127-kW tractor at a speed of 9.7 km/h and left 32 to 36% surface residue cover.

The John Deere Model 510 Disk Ripper consisted of a disk gang with 0.61 m diameter disks spaced 0.28 m apart followed by a gang of ripper shanks on 0.76 m spacing set to a depth of 0.38 m. The subsoil shanks were then followed by an additional gang of disk set at the opposite angle to smooth the soil surface. This equipment was 3.8 m wide and pulled by a 1 86-kW tractor at a speed of 9.7 km h-' and left 24 to 34% surface residue cover.

RESULTS AND DISCUSSION

Study 1

The short-term (55 hours after tillage) effect of tillage on C0(2) flux is summarized in Fig. 1. Each data point is the mean of three replicates; error bars represent +1 standard deviation. When error bars are not visible, the error bars are contained within the data symbol. Positive values indicate C0(2) flux from the soil surface to the atmosphere. Relatively large initial fluxes (29 g C0(2) m(2 )W') from the MP treatment were observed. The large C0(2) fluxes observed with the MP, MP+D2X, and CP treatments immediately after tillage probably reflect a flux of C0(2) released from large voids generated by the tillage operation (Blevins et al., 1984; Hendrix et al., 1988). The initial short-term C0(2) flux (55 hours) was largest for MP followed by the CP, apparently reflecting the depth of soil disturbance and increased void fraction in both tillage treatments. The third highest flux was MP+D2X which had the same depth of tillage as the MP treatment, but the surface porosity was reduced by the two diskings. The DH treatment had a relatively small C0(2) flux and was only slightly larger than the NT treatment. The cumulative C0(2) flux for the 55-hour period in Fig 1. was estimated by calculating the area under the curves and resulted in 247, 88, 78, 37, and 22 g C0(2) m(2 )for the MP, CP, MP+D2X, DH, and NT respectively.

Measurement of the C0(2) flux from the tilled surfaces was continued after a major rainfall event (3 day total of 49 mm) for 19 days after the initial tillage (Fig. 2.). Throughout the remaining period, MP consistently had the highest C0(2) flux followed by MP+D2X. Both treatments completely incorporated surface residue. The DH and CP treatments had relatively low fluxes that were similar to the NT treatment during the remainder of the study. The results indicate that, for at least 19 days after tillage, moldboard plowing caused more C0(2) to reenter the atmosphere compared to other treatments. The cumulative C0(2) flux from each tillage treatment for the 19-day period after tillage was 913,475,391,366, and 183 g C0(2) m(2 ) for the MP, MP#D2X, DH, CP, and NT respectively.

The C0(2) amounts released during the 19-day period can be compared with the equivalent carbon contained in the residue of the previous wheat crop. Accepting the common approximation that 45% of the wheat residue is carbon, then the carbon equivalent of the 4.12 Mg ha' of wheat residue is 1.85 Mg C/ha. With moldboard plowing only, the C0(2) loss was greater than the equivalent carbon input from the previous wheat crop. The carbon released as C0(2) during the 19 days following tillage would account for 134, 70, 58, 54, and 27% for the MP, MP+D2X, DH, CP, and NT treatments, respectively, of the carbon contained in the current year's crop residue.

Considerably more carbon was lost as C0(2) from the tilled plots than from the area not tilled; the treatments that had total incorporation of the residue (MP and MPD2X) also show more loss than treatments with only partial incorporation of the residue (DH and CP). Differences in C0(2) flux between the DH and CP treatments probably reflects differences in void fraction and surface roughness. However, given the short duration of sampling, presumably, the wheat residue was only starting to decompose and did not contribute greatly to the intermediate-term CC0(2) flux. These results suggest that the depth of soil disturbance was more important than residue incorporation in determining the magnitude of short-term C0(2) flux and these results are in general agreement with the work of Grabert (1968), but contradict those of Richter (1974) and Hendrix et al. (1988). Changes in soil properties such as crust formation and soil consolidation will change the long-term dynamics of C0(2) fluxes and possibly explain the reason for these conflicting reports.

STUDY 2

Total C0(2) flux from the various conservation tillage treatments during the first five hours, the longest time period common to all tillages, is shown in Fig 3. The average initial flux for the moldboard plow was 49 g C0(2) m(2)/h and then decreased to about 7 g C0(2) m(2 ) The paraplow was the next highest with the four conservation tillage tools having fluxes that ranged from 14 to 21 g C0(2) m(2 )h and eventually decreased to about 3 to 4 g C0(2) m(2 )h in five hours. The flux from the non tilled area was about 1 g C0(2) m2/h. The flux decreased rapidly with time after tillage primarily due to soil drying and continued gas loss.

The cumulative C0(2) loss for five hours after tillage was 81 g CC0(2) m(2 )for the moldboard plow and the smallest on the treatment area not tilled (6 g C0(2) m(2)). The paraplow was the second highest with 79 g C0(2) m(2 )while the remaining conservation tillage tools ranged from 23 to 28 g C0 m(2). All conservation tillage tools produced more C0(2) than the non tilled area, but significantly less than the moldboard plow.

The C0(2) flux measured on the paraplow treatment five days after tillage became similar in magnitude to the other conservation tillage systems. This change resulted from soil reconsolidation and settling in the paraplow treatment from 27 mm rainfall the evening after tillage The moldboard plow treatment maintained a greater C0(2) flux than the conservation tillage treatments on all days of measurement. The flux rates for the conservation tillage treatments measured 5, 14, and 28 days after tillage showed some variation between treatments; however, the flux rates were generally greater than the non tilled area, but not always. This variation resulted from the interaction between soil temperature and soil moisture contents at the time of measurements.

CONCLUSION

Soil carbon levels are affected by agricultural management practices through a complex interaction of processes determined by carbon inputs and decomposition rates. Carbon input is controlled by the level of residue inputs and is largely determined by crop choice, fertilization, and climate. The amount of carbon leaving the system is controlled by crop grain and residue removal, biological oxidation, and soil erosion. These latter two mechanisms of soil organic carbon loss are substantially reduced when tillage is reduced or eliminated. Intense tillage, particularly moldboard plowing, results in large gaseous losses of carbon The large losses of C0(2) following moldboard plowing compared to the relatively small losses of C0(2) with no-till or conservation tillage illustrates why crop production systems involving moldboard plowing have decreased soil organic carbon levels and why no-till crop production systems are reversing this trend. The data suggests the potential for using the soil as a sink for carbon through improved soil and tillage management

Keeping crop residues on the soil surface and reducing tillage intensity not only reduces soil erosion, but also reduces physical release of C0(2) and possibly the biological oxidation of soil organic carbon. The moldboard plow not only fractures, inverts, and introduces large void fractions which allow rapid C0(2) nd oxygen exchange, but also incorporates residue into the soil which generates a microbial explosion. With conservation tillage or no-till, most or all of the crop residues are left on the soil surface and only a small portion is in intimate contact with the soil moisture and available to microorganisms. As a result, residues decompose more slowly. Improved management practices need to be developed to control the rate of biological oxidation necessary to maintain the optimum soil processes and properties for sustained crop production.

LITERATURE CITED

Bauer, A., and A.L. Black. 1981. Soil carbon, nitrogen, and bulk density comparisons in two cropland tillage Systems after 25 years and in virgin grassland. Soil Sci. Soc. Am. J. 45:1166-1170.

Blevins, R.L., M.S. Smith, and G.W. Thomas. 1984. Changes in soil properties under no-tillage. Pp.190-230. In RE. Phillips and S.H. Phillips (eds.) No-tillage Agriculture: Principles and Practices. Van Nostrand Reinhold, New York.

Blevins, R.L., G.W. Thomas, M.S. Smith, W.W. Frye, and P.L. Cornelius. 1983. Changes in soil properties after 10 yearscontinuous non tilled and conventionally tilled corn. Soil Till. Res. 3:135-146.

Grabert, D. 1968. Measurements of soil respiration in a model experiment on the deepening of the arable layer. (In German; Engl. Summary.) Albrecht-thaer-Arch. 12:681-689.

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

Ismail, I., R.L. Blevins, and W.W. Frye. 1994. Long-term no-tillage effects on soil properties and continuous corn yields. Soil Sci. Soc. Am. J. 58:193-198.

Post, W.M., T.H. Peng, W.R. Emanuel, A.W. King, V.H. Dale, and D.L. DeAngelis. 1990. The global carbon cycle. Am. Scientist 78:310-326.

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Reicosky, D.C., M.J. Lindstrom, S. Musielewicz. 1995. Conservation tillage tools and carbon dioxide loss. Soil and Water Conserv. Soc. Abstract. p.21.

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Figure 1. Short-term effect of fall tillage method on carbon dioxide flux versus time:a) moldboard plow, moldboard plow plus disk twice, and no-tillage; b) disk harrow, chisel plow, and no-tillage. Tillage was done on September 4, 1991. From Reicosky and Lindstrom, 1993).

Figure 2. Intermediate-term effect of fall tillage method on carbon dioxide flux versus time following a major rainfall event: a) moldboard plow, moldboard plow plus disk harrow, and no-tillage; b) disk harrow, chisel plow and no-tillage. (From Reicosky and Lindstrom ,1993)

Figure 3. Total carbon dioxide flux from five conservation tillage tools during the first five hours after tillage compared to moldboard plowing and no-tillage. (From Reicosky et al., 1995)