Investigating the impact of rainfall on nutrient movement

These researchers are working together to bolster the findings of their predecessors at the Nutrient Management Spear Program (NMSP). “The phosphorus risk assessment tool recognizes that crop residue may reduce soil and nutrient loss from farm fields,” said Quirine Ketterings, professor of on-farm nutrient management and leader of the Nutrient Management Spear Program. “However, the interactions between manure and crop residue are less well understood, so this is an important dynamic for us to clarify.”

The NMSP researchers are conducting these studies to see if crop residue can reduce the risk of soil, nitrogen and phosphorus loss through runoff and infiltration, Irias explained. “Understanding these interactions will guide us with the development of improved risk assessment tools,” he said. “Ultimately, our goal is to refine recommendations that support both environmental stewardship and agricultural productivity.”

So, how does the study work?

The team is evaluating two different levels of wheat straw surface residue (zero residue versus ~45% or 90% coverage) at four different manure application rates. These field conditions are replicated in perforated trays which they position on a table under a specialized rainfall spigot. 

They then simulate consistent rainfall events on the first, second and third week after the manure is applied. The rainfall occurs over a slightly graded surface, and the researchers collect (water that slides down the graded surface and off the soil trays) and infiltrate (water that penetrates the soil trays completely) in jars attached to the simulation system. 

After a rainfall event, the team subsamples the water collected in these jars for analysis. The team analyzes sediment loss, nitrate (NO₃-N), ammonium (NH₄-N), total Kjeldahl nitrogen (TKN), dissolved reactive phosphorus (DRP) and total phosphorus (Total-P) in runoff and infiltrate samples, Irias explained.

What have they observed so far?

"We typically observe the greatest runoff volumes for bare soil,” said Irias. “When manure is surface-applied, the manure itself prevents soil erosion, seemingly creating a protective crust on the soil’s surface.

Earlier studies showed an increase in nitrogen and phosphorus loss at higher manure application rates and a benefit of incorporating manure versus surface application,” he continued. “But this is something that still needs to be investigated further.”

Future studies will focus on the impact of cover crop biomass as a way to reduce risk of soil, nitrogen and phosphorus loss, as well as the influences of initial soil test phosphorus level. 

The project is sponsored by the United States Department of Agriculture Natural Resources Conservation Service (USDA-NRCS) and involves researchers at Auburn University, Oregon State University and Cornell University. “These studies are conducted at all three locations but with different manure sources, relevant to the different states,” said Ketterings, the principal investigator for the Cornell University study. “For us, the evaluation of various dairy manure sources is very relevant and important.”

This research is meant to better our understanding of various beneficial management practices and assist farmers in their journey towards becoming more efficient by mitigating soil and nutrient loss, Irias explained. The team is looking forward to seeing the results of these trials and to applying these findings to future on-farm research and extension. They are planning to use them to further inform and refine regional nutrient management guidelines to help farmers make management decisions that allow them to maximize economic profit and minimize environmental impact.

Madeline Hanscom ‘22 is a writer for the Nutrient Management Spear Program.