Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) are the primary greenhouse gases (GHG) emitted from the soil due to agricultural activities. In the short-term, increases in CO₂ emissions indicate increased soil microbial activity. Soil microorganisms decompose crop residues and release CO₂ as a byproduct. The decomposition of crop residues releases nutrients for plant use. Abundance of microbes also improves soil structure quality by stabilizing aggregates. Thus, increased CO₂ emissions (microbial activity) may indicate improved soil health. A balance, however, is needed between CO₂ emissions and carbon (C) stored in soil to maintain soil productivity. This article discusses the impact on CO₂ emissions of baling and grazing corn residue. It is important to emphasize that CO₂ emissions explain only one phase of the carbon cycle. Measurements of soil C stocks along with non-CO₂ emissions are actual determinants of defining whether the system is a sink or source of greenhouse gases.

Does Residue Baling and Grazing Increase CO₂ Emissions? A Literature Review

Residue baling and grazing may impact soil CO₂ emissions by decreasing residue inputs and altering soil physical conditions, both of which could affect soil microbial activity. The net impact of residue removal may depend on site-specific factors such as local climate, soil type, interaction with other management operations (i.e., tillage), and the amount of residue removed. The amount of crop residue removed through baling depends on cutting height of the shredder and moisture conditions of windrows during baling. The amount of crop residue removed through grazing at recommended stocking rates is generally less than 25%. A recent multi-site study reported that grazing left 2.4 times more residue than baling (Ulmer et al., 2016). Thus, baling is expected to reduce carbon (C) input and C substrate for microbes, which would reduce CO₂ emissions as compared to residue grazing.

The higher rates of residue removal under baling can have a greater influence on soil microclimatic conditions that alter CO₂ emissions than would grazing. Residue baling, in general, can cause higher daytime soil temperatures than residue grazing. As with any biochemical reaction, the decomposition process is likely to accelerate with increased soil temperatures under residue baling, and greater CO₂ emissions are expected. However, at the same time, residue baling can reduce the soil water content due to greater evaporative loss or soil compaction issues related to increased machine traffic, thereby decreasing microbial activity. In contrast, grazing is likely to cause less evaporation and abrupt temperature fluctuations compared to baling as it removes less residue than baling. Residue grazing can, however, alter the microbial decomposition process by affecting the physical and chemical structure of residues. First, cattle trampling can crush residue into smaller parts. Smaller particles offer more surface area, which can accelerate the microbial activity to decompose residues and increase CO₂ emissions. Second, adding manure and urine can increase C substrate to enhance the microbial activity to emit CO₂.

A meta-analysis of 176 published studies indicated that residue removal decreases CO₂ emissions by 27.8% (Liu et al., 2014). The decreased CO₂ emissions were mainly due to reduced microbial biomass, soil water content, and soil porosity.  Research studies from the US Corn Belt have, however, reported a range of impacts from a 4% decrease to a 24% increase in CO₂ emissions with mechanical residue removal (Al-Kaisi and Yin, 2005; Jin et al., 2014). Such variation in CO₂ emissions could be due to site specific residue inputs and climatic conditions. Research data on residue grazing impact on CO₂ emissions are few. Two studies reported either no effect or increased CO₂ emissions due to residue grazing (Tracy and Zhang, 2008; Barsotti et al., 2013).

Recent Data from Nebraska

Soil CO₂ emissions were monitored under corn residue grazing and baling near Clay Center. The experimental treatments were 1) control (no residue removal), 2) corn residue baling (about 70% residue removal), and 3) corn residue grazing (about 18% residue removal) managed under irrigated strip tillage continuous corn (Figure 1).

₂ emissions for three corn residue removal treatments in a strip-till continuous corn experiment near Clay Center.

Soil CO₂ emissions were not affected by residue baling or grazing compared to the control on individual sampling dates (Figure 2). On average, CO₂ emissions were 1.69 lb per acre per hour with a range of -0.59 lb per acre per hour during December to 3.78 lb per acre per hour during June sampling. Due to high fluctuations in CO₂ emissions on a temporal basis, cumulative CO₂ emissions were calculated to evaluate the net impact of residue baling and grazing on an annual basis. Residue grazing increased cumulative CO₂ emissions by 1.4 times compared to control but baling had no effect (Figure 3) on an annual basis. The increased CO₂ emissions could be due to increased available C substrate from manure addition by grazing animals (Barsotti et al., 2013). This can enhance microbial activity to increase CO₂ emissions.

₂ emissions due to three crop residue management practices in strip-till continuous corn near Clay Center.

₂ emissions under three crop residue management practices in strip-till continuous corn near Clay Center. Bars with different letters indicate significant differences among treatments.

Conclusion

Soil CO₂ emissions are mainly governed by residue removal rate, soil microclimatic conditions, and related soil properties that impact biological activity and gaseous exchange reactions. A literature review suggests that residue baling may reduce soil C stocks and change soil conditions to alter CO₂ emissions whereas residue grazing may impact CO₂ emissions due to manure input.

Based on our one-year data, residue grazing and baling appear to have no impact on CO₂ emission on a daily basis. However, when the daily CO₂ emissions are summed to obtain annual CO₂ emissions, residue grazing appears to increase the cumulative CO₂ emissions in irrigated crop-livestock systems. Baling had no effect. Additional research is required to conclusively ascertain the extent to which baling and grazing affect CO₂ emissions under different management conditions and climatic zones. These CO₂ results explain only one phase of C cycle, examining the net change in soil organic C and emissions of non-CO₂ gases will be essential to discern whether systems under residue baling or grazing are a net source or sink for greenhouse gases.

References

Al-Kaisi, M. M., and X. Yin. 2005. Tillage and Crop Residue Effects on Soil Carbon and Carbon Dioxide Emission in Corn–Soybean Rotations. Journal of Environmental Quality 34:437-445. 

Barsotti, J.L., U.M. Sainju, A.W. Lenssen, C. Montagne, and P.G. Hatfield. 2013. Crop yields and soil organic matter responses to sheep grazing in US northern Great Plains. Soil and Tillage Research 134: 133-141.

Jin, V.L., J.M. Baker, J.M. Johnson, D.L., Karlen, R.M. Lehman, S.L. Osborne, T.J. Sauer, D.E. Stott, G.E. Varvel, R.T. Venterea, M.R. Schmer, and B.J. Wienhold. 2014. Soil greenhouse gas emissions in response to corn stover removal and tillage management across the US Corn Belt. BioEnergy Research 7: 517-527

Liu, C., M. Lu, J. Cui, B. Li, C. Fang. 2014. Effects of straw carbon input on carbon dynamics in agricultural soils: a meta-analysis. Global Change Biology 20: 1366-1381.

Tracy, B.F., and Y. Zhang. 2008. Soil compaction, corn yield response, and soil nutrient pool dynamics within an integrated crop-livestock system in Illinois. Crop Science 48:1211–1218.

Ulmer, K., 2016. Managing Corn Residue and Double Cropped Forages in Crop and Livestock Systems. Theses and Dissertations in Animal Science. University of Nebraska-Lincoln.