Editor’s note: The following was written by Mahdi Al-Kaisi, Iowa State University professor of agronomy and Extension soil and water specialist, for the university’s Integrated Crop Management News website Nov. 5.
Crop residue serves an important role in physically protecting soil from erosion during rain events or high winds, as well as enhancing soil biological activity by providing sources of organic carbon and nitrogen for its energy needs.
In order to understand how residue decomposes, we need to understand how the degradation processes are influenced by environmental and soil conditions — namely, air and soil temperatures, soil moisture availability, soil pH, oxygen and type of microbial community.
The composition of crop residue includes lignin, cellulose, hemicellulose and macro and micronutrients. Certain biological and enzymatic processes, controlled by a wide range of microorganisms and influenced by other factors, must occur in order to release most of these organic forms.
In agriculture, annual cropping systems and other ecosystems management can influence these factors that are critical to the process of residue breakdown. There is a common belief among many farmers and agronomists that tillage can accelerate residue breakdown by the cutting of crop residue into small pieces or burying residue in the soil profile.
Also, there is the belief that the application of nitrogen fertilizer on crop residue (i.e., corn residue) after harvest can speed up the process of residue breakdown.
Neither assertion is correct.
A study was conducted to examine the effect of three different tillage systems that include deep tillage (DT), strip-tillage (ST) and no-till (NT) on residue breakdown of Bt and non-Bt corn residues.
The results of this three-year field and laboratory incubation studies show no signiﬁcant differences in the breakdown or percent of residue that remained among the three tillage systems with either Bt and non-Bt corn hybrid residues.
Also, after 12 months, there was no difference between tillage systems or Bt and non-Bt hybrid residue breakdown in the ﬁeld, where 34-49% of the corn residue still remained on the soil surface. The results show no significant difference in the breakdown or decomposition due to tillage or type of residue (Bt or non-Bt).
In a study on corn residue decomposition with different N rates in the no-tillage system, corn residue decomposition was evaluated by applying 32% UAN at three N rates (0, 30 and 60 lbs. N/acre) to corn residue immediately after harvest, where specific amounts of corn residue were weighed and placed in nylon mesh bags and left in the field immediately after harvest for decomposition evaluation.
The rate of residue decomposition was evaluated every three months for the entire year (12 months).
The results showed corn residue decomposition increased with time, with lesser amounts of residue remaining after each evaluation period, but no differences existed in the rate of residue decomposition as a result of N application or different N rates.
These results show that applying N fertilizer to facilitate residue decomposition is not effective.
The timing of N application for corn residue decomposition immediately after harvest, as practiced, is not an effective strategy, as the soil and air temperatures decrease over time after fall harvest.
Soil moisture and temperature are essential factors for microbial activity for the residue decomposition (moisture at field capacity and warm temperature above 50 degrees). Therefore, fall N application does not achieve the intended result of facilitating residue decomposition.
The same results were observed with laboratory evaluation of corn residue decomposition that was conducted with the same residue treated with different N rates in the field study. Corn residue samples from the field were incubated in the laboratory under constant temperatures of 32 degrees and 90 degrees for approximately 30 days each.
Again, no differences in residue decomposition/breakdown with different N rates were found. The laboratory study results confirmed the field results and demonstrated the role of temperature in controlling corn residue decomposition rather than N rate, where a slower rate of residue decomposition was observed at the low temperature and increased at the higher temperature without any effect of N application on residue breakdown.
The use of tillage or N application to increase residue decomposition can be counterproductive from economic and environmental perspectives. From an economic perspective, both options of management add additional costs of materials, time, labor, fuel and equipment.
Environmentally, both tillage and fall N application are not very sustainable practices.
Tillage can contribute to soil health and water quality deterioration by increasing soil erosion potential, sediment loss and water quality degradation, and fall N applications result in water quality risks.
Since residue decomposition is controlled by biological processes that are influenced by environmental and soil conditions, our research and many other studies do not support these practices regardless of the justification or claims that propose tillage equipment can manage residue. Disturbing the soil does not constitute an improvement in soil health nor increase in residue decomposition.