Soil organic carbon is a cornerstone of soil health. It improves soil structure while increasing the capacity to retain water and nutrients, key factors for any agricultural production system. To build it, farmers incorporate crop residues into the soil.
So why, despite decades of residue inputs, is soil organic carbon declining in corn production systems? Short answer: it’s nitrogen.
“With intensive nitrogen fertilization, you can get more corn biomass and yield, which means you’ll end up putting more residue in the soil. But you can’t keep that carbon in the ground, ”says Richard Mulvaney, a professor in the Department of Natural Resources and Environmental Sciences (NRES) at the University of Illinois. “The nitrogen in the tailings stimulates microbes to burn carbon through respiration. So you can put more, but you can’t keep it.
The concept that nitrogen fertilization affects the decomposition of residues – and therefore the incorporation of residues into soil organic matter reserves – is not new. But previous studies have shown conflicting results. This is why Mulvaney and Tanjila Jesmin, PhD student at NRES, set out to clarify how residue quality and nitrogen form affect the decomposition of corn residue in typical Corn Belt soil.
Using historic plots in Morrow, Illinois, the team was able to test residue from corn grown with and without high nitrogen fertilization.
“We designed an aerobic incubation study, adding these two residues to a typical cultivated soil with or without two forms of nitrogen. We then observed the decomposition process by continuously measuring the production of carbon dioxide, as well as periodic measurements of enzyme activities and microbial biomass, ”explains Jesmin.
The researchers found that the presence of nitrogen – applied exogenously to the residue or already incorporated into growing corn tissue – accelerated the decomposition of the residue and produced more carbon dioxide. The form of nitrogen applied, potassium nitrate or ammonium sulfate, made no difference.
“The carbon in corn residue comes from the atmosphere and returns to the atmosphere when it decomposes. It’s not a problem, ”says Mulvaney. “The problem is, when microbes have a high nitrogen supply, they also have a high demand for carbon for energy. With high nitrogen levels, their demand can exceed the supply of carbon in the tailings, which can cause them to attack stable organic matter. And this is where the long term problem lies.
During the first month of incubation in the soil, carbon decomposition in the tailings was faster in the presence than in the absence of nitrogen fertilizer. However, carbon dioxide production in the second month was slower for fertilized soils than for unfertilized soils. At the end of the study, the total amount of carbon dioxide produced was higher with than without added nitrogen.
“It’s like burning leaves in the fall. You put more leaves on the fire and you get more flames. And so, with this added nitrogen, the residue goes faster at the start of incubation. Then the fire goes out because you have already burned the easily decomposable substrate. We get there earlier with nitrogen, ”he says.
The results explain why soil organic carbon fails to accumulate in high-yield corn fields and suggest that farmers should avoid excessive nitrogen inputs to maintain soil organic matter.
According to the researchers, other studies are underway to assess the effect of mineral nitrogen on the decomposition of residues in soils with contrasting characteristics.
“Because our incubation used only one type of soil, the results might not be valid everywhere. With soils of low native fertility, intensive fertilization is often effective in increasing carbon inputs from residues. We want to see if these inputs help build soil organic carbon, ”says Jesmin.