Excess nitrogen from agricultural runoff can enter surface waters with devastating effects, including algal blooms and fish kills. However, riparian buffer zones — areas of grasses, perennials, or trees between farmlands and streams or rivers — can help.
“Riparian buffer zones are nature's hydraulic shock absorbers,” says Deanna Osmond, a soil scientist at North Carolina State University.
The buffer zones can reduce pollution and provide habitat for wildlife. Trees can hold stream banks together and provide food for animals. These buffer zones can also dampen the flow of agricultural runoff, which can lead to lower amounts of nitrogen reaching streams and rivers. But, what type of vegetation makes buffer zones most efficient at removing nitrogen from runoff?
In a recent study, Osmond and her colleagues found that — at least for some areas — it doesn't matter what type of vegetation is used in buffer zones. There appeared to be no significant differences in how efficiently the vegetation removed nitrogen from agricultural runoff.
Irrespective of vegetation type, wider buffer zones were more effective than narrower ones. It is important to consider the width of buffers, says Osmond. “There is a trade-off between productive farmlands and buffer zones.” Farmers cannot grow crops in buffer zones.
Previous studies had typically involved buffer zones measuring at least 30 meters wide. The buffers in this study were either 15 or 8 meters wide, but even these narrower buffer zones lowered the amount of nitrogen reaching streams. The 15-meter-wide buffers were often more than twice as efficient at removing nitrogen from runoff compared to the 8-meter-wide buggers.
Buffer zones can reduce the amount of nitrogen reaching water sources as plants in buffer zones absorb nitrogen or soil microbes transform nitrates into the atmosphere as nitrogen gas.
Some previous studies had found that certain vegetation types were more effective at nitrogen removal. However, those differences could have been caused by soil conditions and stream flow.
“Location matters when studying riparian buffer zones,” says Osmond.
For example, most previous studies were in areas where streams are more connected to their floodplains. The increased connectivity leads to a higher groundwater table. In areas where the water table is high, nitrates are transformed more efficiently to nitrogen gas.
This study was conducted in the upper coastal plains of North Carolina where the streams were not connected to their floodplains, which might have affected how efficiently the different vegetation types removed nitrogen from runoff.
Also, microbes need carbon to survive. The process is most efficient when levels of dissolved organic carbon in the soil are high. At all the measurement sites in this study, the dissolved organic carbon levels were low, which might have limited the amount of nitrates being removed from runoff and equalized differences between vegetation types.
Finally, although many studies measure the effectiveness of preexisting buffer zones, Osmond says this study started from scratch. That's important as it more closely reflects farms where buffer zones are not preexisting. Osmond's study also tracked the buffer zones for 12 years, far longer than most other studies. The longer time span could also explain why these results are different from some previous studies.
“Many factors affect how efficiently riparian buffer zones remove nitrogen from runoff,” says Osmond.
Read more about Osmond’s study in the Journal of Environmental Quality.