Farmers should avoid excessive nitrogen inputs to maintain soil organic matter, according to new research by University of Illinois.
Nitrogen causes diminishing of soil organic carbon in corn production systems. University of Illinois researchers tested residues from corn grown with and without high nitrogen fertilisation. They found the presence of nitrogen – either exogenously applied to residue or already incorporated in growing corn tissue – accelerated residue decomposition and produced more carbon dioxide. The form of nitrogen applied, potassium nitrate or ammonium sulfate, made no difference.
“With intensive nitrogen fertilisation, you may get more corn biomass and yield, which means you end up putting more residue into the soil. But you cannot keep that carbon in the soil,” says Richard Mulvaney, professor in the Department of Natural Resources and Environmental Sciences (NRES) at the University of Illinois. “The nitrogen in the residues stimulates the microbes to burn carbon off through respiration. So you can put more in, but you can’t keep it.”
“The carbon in corn residue comes from the atmosphere, and it returns to the atmosphere during decomposition. That’s not an issue,” Mulvaney says. “The problem is that when microbes have a high nitrogen supply, they also have a high demand for carbon as an energy source. With high nitrogen rates their demand may exceed the carbon supply in residues, which may cause them to attack stable organic matter. And therein lies the long-term problem.”
During the first month of soil incubation, residue carbon decomposition was more rapid in the presence than absence of nitrogen fertiliser. However, carbon dioxide production in the second month was slower for fertilised than unfertilised soil. By the end of the study, the total amount of carbon dioxide produced was greater with than without added nitrogen.
The results explain why soil organic carbon fails to build in high-input cornfields and suggest farmers should avoid excessive nitrogen inputs to maintain soil organic matter.
The study, “Short-term effect of nitrogen fertilization on carbon mineralization during corn residue decomposition in soil,” is published in Nitrogen.