Study: Intensive nitrogen stymies organic carbon accumulation

By Brian O'Connor

As carbon sequestration markets heat up and farmers eye adding a program to their roster of conservation tools, a longtime agricultural ally may be an obstacle.

Researchers at the University of Illinois’s College of Agriculture, Consumer and Environmental Sciences wanted to know why soil organic carbon matter is disappearing from agriculture fields despite years of intensive residue inputs. Previous studies into nitrogen as a potential culprit showed mixed results, according to a press release issued Dec. 15 by the college.

So natural resources and environmental sciences professor Richard Mulvaney and doctoral researcher Tanjila Jesmin set out to determine whether nitrogen fertilization might change the composition of corn residue, and whether that impacted carbon sequestration as a result. They used the Morrow Plots to test residue from both corn grown with intensive nitrogen fertilization and corn grown without.

SYMBIOTIC ENVIRONMENT. A humus-rich soil that is high in organic matter is the perfect habitat for a congregation of earthworms, which in turn gives nutrients back to the soil for the benefit of the accompanying crops.

“We designed an aerobic incubation study, adding these two residues to a typical cropped soil with or without two forms of nitrogen,” Jesmin says. “We then observed the decomposition process by continuously measuring carbon dioxide production, as well as periodic measurements of enzyme activities and microbial biomass.”

Researchers created nine separate soil samples representing a variety of combinations of soil and residue, and incubated them in Mason Jars for 60 days, measuring carbon dioxide production that resulted.

Ultimately, carbon dioxide production was greater in soil samples that contained added nitrogen than in samples that did not contain added nitrogen.

“The problem is that when microbes have a high nitrogen supply, they also have a high demand for carbon as an energy source,” Mulvaney says. “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.”

The soil used to make the samples was all taken from a single plot near Farmer City, Ill., which had been cropped on a corn-soybean rotation for 40 years, with nitrogen application of 180 kg per ha (160.6 lbs per acre) during corn years.

Researchers caution that because only one type of soil was used as the basis for study, their results may not be applicable, given that innate fertility rates and soil biology can vary widely.

The corn residue used in the study — stover consisting of leaves, stalks, husks and cobs — was from the University’s Morrow plots, and included residue from continuous corn and corn-soybean rotations.

The situation is analogous to a bonfire, Mulvaney says.

“It’s like burning leaves in the fall,” he says. “You put more leaves on the fire, and you get more flames. And so, with that added nitrogen, the residue goes more quickly early in the incubation. Then the fire dies down because you had already burned up the readily decomposable substrate. We get there sooner with nitrogen.”