Over the last few weeks, I have written and posted several articles about the value of/need for long-term research to properly assess the agronomic benefits that can be realized from particular farming practices (click here and here for recent examples). Regrettably, such long-term research projects often are not viable in today’s research environment because of short-term funding from grants, the short-term tenure of graduate students who often are in charge of such research, and/or the need for research scientists to publish results on an annual basis.
One of the mainstays of U.S. crop production is the corn-soybean rotation system. The yield benefits of this system are accepted by all involved parties, and are outlined in an article here. The antithesis of this rotation system is a system of continuous corn or continuous soybean production–i.e. a monocrop system of either crop. The above-linked article supports the aforementioned rotation system of the two crops from a yield perspective, but there is a dearth of available information that can be used to discern and assess the long-term value of this rotation as it pertains to soil health that may contribute to the long-term sustainability of this dominant U.S. cropping system.
A recent article titled “Acidification in corn monocultures favor fungi, ammonia oxidizing bacteria, and nirK-denitrifier groups” by Behnke et al. appears in the journal Science of the Total Environment 720 (2020) 137514 [https://doi.org/10.1016/j.scitotenv.2020.137514], and presents results from a 20-year field experiment that demonstrated the detrimental effects of a continuous corn production system to soil and environmental health. A brief summary of the conduct of and findings from that research follow. (Access the above-cited article to read the details of the treatments and methodology used in the study, plus the terminology that relates to the soil health parameters measured in the study. Also, click here for a reader-friendly summary of the research and its results provided by Lauren Quinn with quotes from co-authors Drs. Behnke and Villamil.)
• The study site was established in 1996 in northern Illinois. This study is one of few to comprehensively evaluate the effects of crop rotation and tillage on soil health as it relates to soil properties and microbial abundance.
• Research plots were located on silt loam soils to study the effects of rotation and tillage practices on corn and soybean yields when both were monocropped and grown in a biennial rotation with each other every year during the 20-year span of the study.
• Nitrogen (N) fertilizer as urea ammonium nitrate liquid was applied in the spring before corn planting. No N fertilizer was applied to soybean.
• Soil samples were collected following harvest of each crop at the end of the 20-year experimental period, and selected soil parameters were measured.
• Quantification of abundance for nine microbial marker genes to determine fungal and bacterial activity was obtained.
• Long-term crop rotation–in this case, a biennial rotation of corn and soybeans–and tillage decisions affected soil health as related to soil properties and microbial paramaters. Specifically, 1) no-till management increased soil organic matter (SOM), decreased soil pH, and increased numbers of ammonia-oxidizing bacteria (AOB); 2) crop rotations with more corn (including continuous corn) increased SOM, reduced soil pH, reduced ammonia-oxidizing archaea (AOA), and increased AOB and fungal abundance compared to soybean monoculture (AOB and AOA play a key role in conversion of ammonia to nitrite, an essential step in the complete mineralization of organic matter that leads to the accumulation of nitrate in soil); 3) a continuous corn monocrop increased denitrifiers carrying nirK genes which code for nitrite reductases that are key enzymes in the denitrification process, which is a dissimilatory process that ultimately leads to gaseous N products that are lost to the environment; and 4) continuous soybean vs. continuous corn had the opposite effect on soil parameters listed above.
• Overall, including more corn years in the rotation increased the amount of N fertilizer needed to sustain yield levels, thereby intensifying the N cycle in this system. This leads to acidification and enhanced bacterial nitrification, and creates an environment primed for soil N losses and increased N2O emissions.
• The authors composed a graphical abstract that pictorially shows the effects of the monocropped and rotated systems on soil health parameters.
The above results lead to two major conclusions. 1) A biennial rotation of corn and soybeans is not only advantageous for yield enhancement–it also leads to healthier soil and potentially fewer gaseous emissions to the atmosphere. 2) The long-term nature of the above study likely led to its significant findings as related to soil health. In today’s world of perceived climate change and its likely causes, this is a very important result that shows how cropping system diversity can be used to mitigate possible environmental contaminants. This finding will undoubtedly be forthcoming from other long-term research that investigates less common cropping system diversity.
Composed by Larry G. Heatherly, Aug. 2020, email@example.com