Corn Residue Removal and CO2 Emissions

 

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.

 

 

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).

 

 

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).

 

 

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.

 

 

 

 

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.