FAYETTEVILLE, Ark. — University of Arkansas researchers studying an intermediate zone in mantled limestone-karst terrain have found that seasonal differences in rainfall and water load in this zone, called the interflow zone, can make a difference in how excess nitrogen is processed. These findings could impact how farmers use organic fertilizers such as poultry litter and help create and maintain more sustainable practices in agriculture that would lead to better water quality.

Phillip Hays, an associate research professor of geosciences and scientist with the U.S. Geological Survey, graduate students Jozef Laincz and Bryon Winston, and Sue Ziegler, professor of earth sciences at the Memorial University of Newfoundland, were invited to give a presentation at the upcoming Geological Society of America meeting in Philadelphia.

Jozef Laincz, graduate student in environmental dynamics, measures water quality as it comes out of a seep in the Savoy Experimental Watershed.

The researchers use weirs, like the one shown here, to measure water discharge.

Researchers use the Savoy research plot to acquire water samples from rainfall, land surface, soil, epikarst and the conduit zones.

Limestone terrain covers about a quarter of the Earth’s landmass. It provides large quantities of the world’s drinkable water, but it also is vulnerable to contamination. Studies to date have focused attention on the processing of nutrients in the overlying soil zone and on the conduits that form in limestone terrain — caves and sinkholes where water flows rapidly.

Hays’ group studied a third zone, labeled the “epikarst,” the area between the soil and conduit zones that has a high rock to water ratio and slower water velocities than the conduits.

“We need to see what happens in this interflow zone,” Hays said. “By understanding these dynamic processes, we can learn how to manage our nutrient use to achieve sustainability with agricultural practices.”

The researchers conducted controlled experiments at the Savoy Experimental Watershed, a University of Arkansas facility in Northwest Arkansas. They were able to design and install a sampling system to acquire water samples from rainfall, land surface, soil, the epikarst and the conduit zones. They then applied organic fertilizer — poultry litter — at a rate of one ton per acre — which is a common application rate for poultry litter applied to farm land.

“A lot of the nitrogen is being used up by the grass,” Hays said. In addition, microbes in soils act as natural filters for nitrogen compounds, but cannot handle excessive amounts of nitrogen.

When they sampled the concentration from the epikarst to the conduits, they found decreases in the concentration of nitrogen from one zone to the other. They then needed to answer the question, what was causing the decrease — dilution or microbial processing? Dilution would not be enough in times of minimal rainfall to mitigate the effects of excessive nitrogen.

To answer this question, the researchers used chloride and nitrate tracers in the system. Chloride does not react much with soil microbes, so any loss of chloride in the system from the epikarst to the conduit could be attributed to dilution. They found that while chloride concentrations decreased by 68 percent moving through the epikarst, nitrate concentrations decreased by 93 percent.

“That last amount we would attribute to denitrification,” Hays said.

The researchers followed up on this study with an examination of changes in the stable isotope composition of nitrate as it goes through the system. Stable isotopes are atoms of the same type with slightly different atomic weights. Various nitrate sources contain a signature isotope ratio, and changes in the composition of that ratio can help researchers determine the nature of uptake and use of nutrients in a given ecosystem. The scientists used the University of Arkansas Stable Isotope Facility to conduct these experiments, and the results corresponded well with the dilution experiments.

In terms of processing nitrates in the epikarst, timing appears to be everything — at least the time that the water with the excess nitrates spends in that layer. The researchers found that the time that water remained in the epikarst layer varied seasonally. Tracers used to monitor the time water spent in different parts of the ecosystem showed that during the dry summer months, water can take months to move 100 meters. In the winter time or during heavy rains, water can move 100 meters in a couple of days.

“Things move through quickly in the winter season, and microbes and plant growth both slow,” making winter a less than ideal time to be putting organic matter on fields, Hays said.

The next step in the scientists’ research will be to work towards sustainable agricultural practices by weighing the costs and benefits of different management approaches to fertilizer application and grazing.

“By understanding this we can learn how to manage our nutrient use to achieve sustainability,” Hays said.

In addition to addressing the Geological Society of America this fall, Hays and Laincz were invited to the Slovak Republic to talk about the group’s research this summer.

The research is being conducted in the Savoy Experimental Watershed, which is designed specifically to examine water quality issues. This project and others conducted in the watershed are collaborative efforts between the University of Arkansas, the Natural Resources Conservation Service National Water Management Center, the U.S. Geological Survey, the U.S.D.A.-Agricultural Research Service and the Arkansas Department of Environmental Quality. Hays and his colleagues are in the geosciences department of the J. William Fulbright College of Arts and Sciences.