Although approximately 20% of Illinois cropping systems are planted with corn continuously, it is nearly impossible to find fields planted this way for decades at a time. Yet long-term experiments like the one at the University of Illinois, including more than 40 years of continuous corn under varying nitrogen fertilizer rates, offer incredible learning opportunities and soil management lessons for farmers. researchers and farmers.
This is especially true for the soil microbiome studies, two of which were led by Nakian Kim, who holds a Ph.D. from the Department of Crop Sciences at U of I. For Kim, the long-term experience provided a unique and biologically stable framework to explore baselines and observe microbial responses to the inclusion of cover crops, a management change introduced in 2018 and supported by a USDA-NIFA award to María Villamil’s team.
Importantly, Kim found that short-term use of cover crops cannot undo decades of soil microbial dynamics in response to continued use of corn and heavy nitrogen fertilizers.
“In the Midwest, our soils are healthy and resilient, but we shouldn’t overestimate them. Soil subjected to unsustainable practices for too long could reach an irreversible threshold,” Kim says.
In her first study, published in Agronomy, Kim characterized changes in microbial communities at the genus level, a taxonomic resolution far higher than previous studies. The more detailed view of the microbial community revealed kinds of indicators representing critical aspects of soil health and function.
“Most of the studies have looked at microbes very broadly, at the phylum level. But even a single phylum can have incredibly vast microbial diversity. Analyzing at such levels could not provide sufficiently detailed information, so I dug deeper,” says Kim. “Genus-level responses or stasis can tell us how a soil and its microbial community responds to soil management practices.”
For example, he found that long-term fertilization and cover crops favored microbes that could increase the risk of nitrous oxide emissions. At the same time, cover crops also improved soil biodiversity, as microbes with more diverse niches and functions were associated with the practice. Details like these – particularly that cover crops can have both positive and negative effects on soil microbes – may have been missed with the broader microbial analyzes of the past.
In a second study, published in Frontiers in Microbiology, Kim focused on the nitrogen cycle by identifying functional microbial genes in soil, rather than characterizing the microbes themselves.
“The application of lots of nitrogen fertilizer definitely disrupted the nitrogen cycle communities,” Kim says. “Ammonia-oxidizing archaea declined significantly with fertilization, but bacteria were not so responsive. Within denitrifying communities, those harboring the nitrite-reducing nirK gene were not as responsive, while that others with the nirS gene were negatively affected.
Kim found that two years of cover crops had no impact on potential microbe nitrification and denitrification rates, proxy indicators of nitrate leaching, and nitrous oxide emissions.
“If a system is exposed long enough to disruption of microbial nitrogen cycle communities, it can develop resistance to conservation practices,” Kim says.
In other words, two years of cover cropping may not have been enough to reverse the damage caused by 36 years of continuous application of corn and nitrogen fertilizer. But Kim is keen to test the effects of longer-term cover crop management.
Villamil, a professor in the Department of Crop Sciences and co-author of both papers, says, “Our Midwestern soils are resilient in ways we might not have anticipated. Closer examination reveals that microbial communities respond dynamically to unsustainable practices and adapt to the resulting soil conditions. Reversing these changes can take enormous effort and time, and cover cropping should be one of the many strategies we deploy to increase the spatial and temporal diversity of our agricultural systems to protect our soils and our future on this planet.
References: Kim N et al. Limited impacts of cover crops on soil nitrogen cycling microbial communities of long-term maize monocultures. Front. Microphone. doi:10.3389/fmicb.2022.926592/abstract
Kim N, Riggins CW, Zabaloy MC, Allegrini M, Rodriguez-Zas SL, Villamil MB. High-resolution indicators of soil microbial responses to nitrogen fertilization and cover cropping in maize monocultures. Agronomy. 2022;12(4):954. doi:10.3390/agronomy12040954
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