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ISU: Optimum N maintains soil, water quality
 
BY DOUG SCHMITZ
Iowa Correspondent
 
AMES, Iowa — The application of nitrogen (N) fertilizer at optimum levels to corn and soybeans maintains soil carbon, as well as contributes to soil health and water quality, which improves yields, according to new research released by Iowa State University agronomists.
 
“Our data show that nitrogen fertilizer, when applied at a level that maximizes crop production, is critical to maintain soil carbon for sustainable agricultural systems,” said Michael Castellano, an ISU associate professor of agronomy and co-author of the study. “In fact, I’d say it plays an essential role.”
 
Researchers collected soil samples from four ISU research farms in 1999 and 2000 and again 15 years later, after each site received regimented applications of N fertilizer. The sites included both continuous corn and rotated corn-soybean production systems.
 
Funded by the USDA’s National Institute for Food and Agriculture, the study’s results show N helped build soil carbon content for the acres with only corn, while it was essential to maintain soil carbon content in the acres devoted to a corn-soybean rotation. Insufficient N in both systems led to soil carbon losses.
 
Hanna Poffenbarger, a graduate assistant in agronomy and lead author of the paper, said N fertilization leads to increased crop yields, which leaves greater amounts of crop residue on the soil after harvest. She said the crop residue feeds soil microbes and builds carbon in the soil as it decomposes.
 
“Soil carbon content is positively associated with crop yield, and it’s an important indicator of soil health,” she explained.
Poffenbarger said soil carbon improves the ability of the soil to hold water, which reduces
runoff and improves downstream water quality; carbon also helps soil retain nutrients, including nitrates. But Castellano said the study’s findings do contradict some previous studies that examined the impact N has on soil carbon because the research team focused on finding the optimal rate of N to maximize crop production for each field without using excess fertilizer – a step that resulted in more precise results.
 
He said the optimal application rate can vary depending on weather, geography and other soil variables. “You really have to understand the optimum nitrogen rate to gauge the effect it has in a field,” he noted. “Applying fertilizer to corn that is already well-supplied will not have the same impact on soil carbon content as a nitrogen application that boosts crop production.”
 
Clarke McGrath, on-farm research and extension coordinator at ISU’s Iowa Soybean Research Center and ISU extension agronomist, said growers and agronomists have known for a long time “if we short the production system on nutrients,
we’ll degrade the system, over time.
 
“While we can use soil testing and crop removal to track some nutrients and target applications work towards maintaining ‘optimum levels,’ nitrogen is not so cooperative,” he said. “We know our corn needs N, but the optimal application rate can vary, depending on weather, geography and other soil variables.
 
“Unlike relatively soil stable phosphorus (P) and potassium (K), our N applications are much more vulnerable to losses, so we have to work hard – and smart – to dial in the right rates, timing, forms of N fertilizer and N rates to optimize and balance production, economics and environmental issues.” 
 
McGrath said the ISU study highlights how important it is for growers and agronomists to keep striving to strike these balances. “Too little N, and we can see short-term and longer-term consequences. If we apply too much N, we see consequences from that as well.
 
“Balancing all of this is important to all of us, from producers to consumers; a quick look at efforts like the (Iowa) Nutrient Reduction Strategy will illustrate the importance of continually improving our understanding of nutrients and managing them,” he added. “We’ve come a long way with N management over the years and we continue to gain a better understanding of the N cycle and the fate of N in the environment.”
 
On the other hand, he said work like this illustrates “we are still learning and have to continue to research ways to maximize our returns from N, while also looking for ways to reduce N losses.
 
“Expect to continue to see research – and results from our research – to work towards those goals.” 
3/29/2017