Although continuous corn is planted in about 20% of Illinois cropping systems, it's nearly impossible to find fields that have been planted this way for decades. Long-term experiments, such as one at the University of Illinois that included over 40 years of continuous corn under various nitrogen fertilizer rates, provide incredible learning opportunities and soil management lessons for both researchers and farmers.
This is especially true for studies of the soil microbiome, such as two led by Nakian Kim, a doctoral graduate of the University of Illinois' Department of Crop Sciences. For Kim, the long-term experiment provided a unique, biologically stable setting in which to investigate baselines and monitor microbial responses to the addition of cover crops, a management change implemented in 2018 and supported by a USDA-NIFA grant to Mara Villamil's team.
Importantly, Kim discovered that short-term cover crop use cannot reverse decades of soil microbial dynamics caused by continuous corn and nitrogen fertilizer use.
"Our soils in the Midwest are healthy and resilient, but we shouldn't take them for granted. A soil that has been subjected to unsustainable practices for an extended period of time may reach an irreversible tipping point "Kim explains.
Kim characterized shifts in microbial communities at the genus level in his first study, which was published in Agronomy, with a far higher taxonomic resolution than previous studies. The microbial community was examined in greater detail, revealing indicator genera that represent critical aspects of soil health and function.
"The majority of research has focused on microbes in broad terms, at the phylum level. However, even a single phylum can have a staggering amount of microbial diversity. Such levels of analysis couldn't provide enough information, so I dug a little deeper "Kim explains. "Genus-level responses, also known as stasis, can reveal how a soil and its microbial community react to soil management practices."
Long-term fertilization and cover crops, for example, favoured microbes that could increase the risk of nitrous oxide emissions, he discovered. Cover crops, on the other hand, increased soil biodiversity by attracting microbes with a wider range of niches and functions. Details like these—particularly the fact that cover crops can have both positive and negative effects on soil microbes—could have been overlooked in previous microbial analyses.
Kim focused on nitrogen cycling in a second study, which was published in Frontiers in Microbiology, by identifying microbial functional genes in soil rather than characterizing microbes.
"It was clear that using a lot of nitrogen fertilizers disrupted nitrogen cycling communities," Kim says. "Ammonia-oxidizing archaea decreased significantly as a result of fertilization, but bacteria did not. Those with the nitrite-reducing nirK gene were less sensitive in denitrification communities, while those with the nirS gene were negatively affected."
Kim discovered that two years of cover crops had no effect on the rates of potential nitrification and denitrification in microbes, both of which are indirect indicators of nitrate leaching and nitrous oxide emission.
"A system may develop resistance to conservation practices if it is exposed to disruption of nitrogen-cycling microbial communities for a long time," Kim says.
To put it another way, two years of cover cropping might not have been enough to repair the damage caused by 36 years of continuous corn and nitrogen fertilizer application. Kim, on the other hand, is eager to investigate the effects of longer-term cover crop management.
Villamil, a co-author on both papers and a professor in the Department of Crop Sciences, says "Our soils in the Midwest are resilient in ways we might not have expected. Microbial communities react dynamically to unsustainable practises and adapt to the resulting soil conditions, according to a closer look. Cover cropping should be one of many strategies we use to increase the spatial and temporal diversity of our agricultural systems in order to protect our soils and our future on this planet."
(Source: Phys.org)
