Chapter 2: The Impact of Tillage Methods on Soil Carbon Levels and Crop Yield Kevin G. Harrison1*, Michelle G. Segal2, Matthew H. Hoskins3, and Alan L. Kafka1 Keywords: Conservation tillage, soil carbon sequestration 1

Boston College, Department of Geology and Geophysics, 213 Devlin Hall,

140 Commonwealth Ave., Chestnut Hill, MA 02467 USA

2

PO Box 984, Far Hills, NJ 07931 USA

3

609 Bradley Street, Laramie, WY 82072 USA

*Corresponding author ([email protected]). ABSTRACT This research adds data to the growing body of literature examining how agricultural practices influence soil carbon levels and crop yield. We collected 20 soil samples each from three adjacent plots of land on a farm in Brighton, Iowa (60 samples total). These cultivated sites all shared the same silty loam soil characteristics, and included three different methods of soybean farming: conservation tillage (also known as no-till drilling), conventional tillage, and organic farming, where tillage is used to excess. The no-till drill method produced 15% more soybeans than conventional tilling and 110% more soybeans than the organic method. The mean levels of soil carbon were the highest in the no-till drill plots. The drill method plots had 41% more soil carbon than conventionally-tilled plots and 48% more carbon than the organic plots. The carbon difference is statistically significant (P=0.01). The drill method produced the highest

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crop yield and preserved more soil organic carbon than either the conventional or organic methods.

INTRODUCTION Catching and holding carbon in the soil, which is known as sequestration, is beneficial to the environment; it keeps carbon from being incorporated into carbon dioxide or other greenhouse gases that contribute to global warming. Our research suggests that farmers can make decisions about tillage that can help pull carbon out of the atmosphere and help mitigate the effects of global warming. Identifying methods of agriculture that preserve soil carbon levels while increasing crop yield will help feed the increasing world population, make more effective use of land under cultivation, and slow the release of carbon dioxide to the atmosphere from deforestation and cultivation. Researchers have compared the carbon losses and crop yields for different agricultural methods. In general, plowing the soil—used both in conventional and organic farming—releases carbon stored there, degrades the soil and leads to soil erosion (Stewart and Lal, 1995; Lal et al., 1997; Richter and Markewitz, 2001; Kimble et al., 2002). Trewavas (2001) found that conventional farming produced a 30 to 50% greater yield than organic farming for a given amount of land. Organic farming uses frequent, usually mechanical, weeding to replace herbicides. Mechanical weeding can damage nesting birds and worms, and consumes fossil fuels (Trewavas, 2001). Weeding aerates the soil, which accelerates the microbial oxidation of soil carbon to carbon dioxide. Dersch and Böhm (2001) found that Austrian cultivation techniques, which use a minimal amount of tillage, produced significantly higher amounts of organic carbon in the top ten centimeters of soil compared to conventional tilling. The minimally-tilled soil contained 4.68 tons per hectare more C than the conventionally-tilled soil. Lal et al. (1997) have

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estimated that restoration of degraded soil can sequester 3 Gt C/year. Some of this restoration potential can be achieved through conservation tillage (Kerr, 1992; Kern and Johnson, 1993).

STUDY AREA Our study compares soil carbon inventories of silty loam (Wilson, 1986) collected from three adjacent plots of land on a 142-hectare farm in Brighton, Iowa. Brighton is located in southeastern Iowa (latitude 41 09’ 45”, longitude 91 54’ 20”) (Figure 1). Farmers there practiced annual crop rotation: corn was replaced by soybeans which were then replaced by wheat. This study sampled the 1999 soybean fields and three different agricultural practices: conventional tillage, conservation tillage (or drill method), and organic farming, which requires extensive tillage (Figure 1). The drill method does not require tilling the soil before seeds are planted. Instead, a drill is used to plant seeds at a depth of 3.8 cm in rows that are 19.2 cm apart. No fertilizers were used at this site, although conservation tillage often uses fertilizers. 2.4 liters per hectare of Round-Up® herbicide were used. The soybeans have been engineered to resist the herbicide. This land use represented 81 hectares (or 57%) of the farm in 1999 (Hoskins, 2000). Conventional farming at this site involved preparing the soil with a disc cultivator before planting the soybeans at a depth of 3.8 cm in rows 96.8 cm apart. No fertilizers were used at this site, although conventional farming often uses fertilizers. The beans were sprayed with 2.4 liters per hectare of Round-Up® herbicide, which they have been engineered to resist. This technique was used on 26 hectares (or 18%) of the farm in 1999. The 24 hectares (17%) that were being farmed organically in 1999 had lain fallow from 1986 to 1996. From 1996 to 1999, the organic plot was cultivated using the protocols set forth by the Organic Growers and Buyers Association. The crops and land were not treated with any chemical fertilizers or herbicides. No organic fertilizers were used. The fields were tilled with a mouldboard plow, disk, and a harrow. The soybeans were planted at a depth of 3.8 cm and the 3

rows were spaced 96.8 cm apart with a conventional planter. After the beans have sprouted, a cultivator and a rotary hoe are used at least twice to till the soil betweeen the rows to control weeds. This is how all crop production was done on the farm before the introduction of herbicides and genetically-engineered seeds in the 1960’s.

SAMPLING METHODS We collected soil samples in 1999 from depths of 0-5, 5-10, 10-20 and 20-30 cm with a soil probe. Five cores were collected at each of the three land-use sites, yielding a total of 60 soil samples. Each sample was dried to a constant weight at 60 degrees Celsius to inhibit microbial oxidation of soil organic carbon and to stabilize the soil for storage. Then the soil was ground into a fine powder and sifted through 2.0 mm and 0.6 mm sieves. Soil organic material and rocks were removed from the soil by visual inspection. Any remaining litter and charcoal was removed by flotation to ensure that only soil-bound carbon was measured. After delittering, the soil samples were dried at 90 degrees Celsius to a constant weight before we measured carbon concentrations with a Carlo Erba NC2100 gas chromatograph. The Carlo Erba analyzes total carbon in the soil by flash combustion with a precision of ±0.3% for carbon (Verardo et al., 1990). We detected no carbonates at this site.

RESULTS The conservation tillage method produced the highest crop yield: 15% higher than conventional tilling and 110% more than the organic method (Table 1). Conservation tillage also yielded 41% more soil carbon than the conventionally-tilled land and 48% more carbon than the organic plot (Table 1, Figure 2).

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DISCUSSION Statistical analysis There are 20 data points for each site; we do not know if the data are normally distributed, so we applied a non-parametric test, the Wilcoxon Rank Sum test. The null hypothesis is that there is no difference between the carbon inventories in the soil cultivated by the drill method and soil from each of the other land use methods. The alternative hypothesis is that the carbon inventories of the no-till drilled plots will be higher than the others. The Wilcoxon Rank Sum test was applied to compare the conservation tillage data points to the data points from each of the two other land use areas (Table 2). The carbon differences were statistically significant (P=0.01). Crop yield Based on 1990 figures, 11% of the Earth’s land is being conventionally-cultivated to feed the world population (Schlesinger, 1997). Switching from a conventional till method to conservation tillage could, based on our results, produce 15% more food on the same amount of land, or produce the same amount of food on 13% less acreage. In our study, the drill method was found to be about twice as productive as organic farming; 52% less land would be needed to grow soybeans with the drill technique than with organic farming. This will be even more important if world population increases, as projected by 50% in the next 70 years (Lutz et al., 2001). Farming and carbon sequestration Cultivating land typically reduces the amount of carbon in the soil, because of the reduced annual input of plant residues and increased decomposition (Harrison et al., 1993). For example, Post and Mann (1990) examined 625 paired native and cultivated sites and found that cultivated land had an average of 22% less organic carbon than native land (0-30 cm). In our study, the drill method had higher carbon inventories than any of the other land uses analyzed. If 5

these results are representative of other drill, till, and organic farms, converting till and organic farms into drill farms could minimize the loss of carbon to the atmosphere from cultivation. If the improved crop yield allows farmers to feed the growing world population on the same amount of land that is under cultivation today, carbon that is currently stored in vegetation and in soil will be preserved. This would also preserve native land, which may be removing carbon dioxide from the atmosphere (Keeling et al., 1996).

Tradeoffs What are the benefits and drawbacks of each agricultural method? Organic farming is considered healthy for ecosystems and people because it does not require the application of chemical fertilizers or pesticides. Fertilizers can leach from agricultural land, contaminating groundwater and causing eutrophication of surface water (Donner and Kucharik, 2003; Nosengo, 2003). Consuming produce that contains pesticides may cause cancer. Organic produce costs more than conventionally-farmed produce (Trewavas, 2001). The benefits of conservation tillage, which uses pesticides and chemical fertilizers, include higher crop yield, greater soil carbon inventories, and inexpensive produce with greater nutrient levels. Because conservation tillage requires no tilling of the soil, even though chemicals are used, less chemical residue is stirred up and carried into groundwater. Using the no-till method of cultivation, in conjunction with fertilizers and pesticides that do not cause environmental problems, would be ideal. Research should be directed toward developing fertilizers and pesticides that do not cause environmental problems such as groundwater contamination and eutrophication.

CONCLUSION

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Ogle et al. (2003) estimate that US agricultural soil stored 1.3 Tg C yr-1 from 1982 to 1997 due to land use and management change. Global population continues to rise, and the need to feed more people using less land is becoming more urgent. The ideal cultivation technique maximizes crop yield while preserving soil fertility. Maximizing crop yield decreases the amount of land needed for agriculture and slows the release of carbon dioxide to the atmosphere due to deforestation and cultivation. In this study, the drill method of farming produced the largest crop yield and maintained the highest soil organic carbon levels of all of the agricultural methods we studied. ACKNOWLEDGMENTS This study was supported by the United States Department of Agriculture and Boston College Graduate School of Arts and Sciences.

References Dersch, G., and K. Böhm. 2001. Effects of agronomic practices on the soil carbon storage potential in arable farming in Austria. Nutrient Cycling in Agroecosystems, 60, 49-55. Donner, S.D and C. J. Kucharik. 2003. Evaluating the impact of land management and climate variability on crop production and nitrate export across the Upper Missippi Basin. Global Biogeochemical Cycles, 17, 3, article 11, 1-16. Harrison, K.G., W.S. Broecker, and G. Bonani, 1993, The effect of changing land use on soil radiocarbon: Science, 262, 725-726. Hoskins, S.G., 2000, Personal Communication. Keeling, R.F., S.C. Piper, and M. Heimann. 1996. Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature, 381, 218-221. Kern, J.S. and M.G. Johnson. 1993. Conservation tillage impacts on national soil and atmopsheric carbon levels. Soil Sci. Soc. Am. J., 57, 200-210. Kerr, R.A. 1992. Fugitive carbon dioxide: it’s not hiding in the ocean. Science, 256, 35. Kimble, J.M., R. Lal, and R.F. Follett. 2002. Agricultural Practices and Policies for Carbon Sequestration in Soil. New York: Lewis Publishers Inc. Lal, R., J. Kimble, and R. Follett. 1997. Soil quality management for carbon sequestration. In Soil properties and their management for carbon sequestration, ed. R. Lal, J. Kimble, and R. Follett. Lincoln NE: USDA. Lutz, W., W. Sanderson, and S. Scherbov. 2001. The end of world population growth. Nature, 412, 543-545. Nosengo, N. 2003. Fertilized to Death. Nature, 425, 894-895. Ogle, S.M., F.J. Breidt, M.D. Eve, and K. Paustian. 2003. Uncertainty in estimating land use and management impacts on soil organic carbon storage for US agricultural lands between 1982 and 1997. Global Change Biology, 9, 1521-1542. 7

Post, W.M., and L. K. Mann. 1990. Changes in soil organic carbon and nitrogen as a result of cultivation. In Soils and the Greenhouse Effect, ed. by A.F. Bouwman. New York: John Wiley and Sons, 401-406. Richter, D.D. and D. Markewitz. 2001. Understanding soil change: soil sustainablity over millennia, centuries, and decades. Cambridge University Press. Schlesinger, W.H. 1997. Biogeochemistry, An Analysis of Global Change. San Diego, CA: Academic Press. Stewart, B.A. and R. Lal, Eds. 1995. Soil Management: Experimental Basis for Sustainability and Environmental Quality. New York: Lewis Publishers Inc. Trewavas, A., 2001, Urban myths of organic farming; Organic agriculture began as an ideology, but can it meet today’s needs? Nature, 410, 409-410. Verardo, D.J., P.N. Froelich, and A. McIntyre. 1990. Determination of organic carbon and nitrogen in marine sediments using the Carl Erba NA-1500 Analyzer. Deep-Sea Research, 37, 157-165. Wilson, J.H. 1986. Soil survey of Washington County, Iowa, United States Department of Agriculture.

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Table 1. Carbon inventories and crop yield. Land Use

Carbon Inventory (g C/cm2 ± standard deviation)

Crop Yield (m3/ha)

Conservation tillage

0.65 ± 0.03

4.5

Conventional tillage

0.46 ± 0.05

3.9

Organic

0.44 ± 0.03

2.2

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Table 2. Wilcoxon Rank Sum Test: Conservation Tillage vs. other methods

Land Use Method

Carbon Inventory

Conventional Tillage

99%

Organic Farming

99%

10

Till

Till

Organic

Drill

Figure 1. Southeastern Iowa farmland sampled. The farm measures 142 hectares. Scale: 1 cm = 200 meters.

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Figure 2a Drill Carbon Histogram

Frequency

10 8 6 4 2 0

0.02

0.06

0.10

0.14

0.18

0.22

0.26

0.30

0.34

0.38

0.34

0.38

2

C (g/cm )

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Figure 2b Till Carbon Histogram

Frequency

10 8 6 4 2 0 0.02

0.06

0.10

0.14

0.18

0.22

0.26

0.30

2

C (g /cm )

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Figure 2c Organic Carbon Histogram

Frequency

10 8 6 4 2 0

0.02

0.06

0.10

0.14

0.18

0.22

0.26

0.30

0.34

0.38

2

C (g/cm )

Figure 2. Frequency of Carbon concentrations in soil samples.

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