Contrary to what many farmers feel about riparian buffer zones, they do not need to take a lot of acres out of production.
Prairie wildlife and conservation agencies have often touted riparian buffer zones as wide as 90 metres, which is maybe why we don’t see more of them.
However, a 12-year study in North Carolina suggests that an eight metre-wide buffer zone can do a pretty good job of keeping nitrates and sediment out of waterways.
Deanna Osmond, a soil science professor at North Carolina State University who calls buffer zones nature’s shock absorbers, is comparing how effectively different types remove nitrogen from runoff water. She is monitoring two widths of buffers and two types of vegetation.
Many factors affect how efficiently a buffer zone functions. Field size, type of stream, local species, temperature, amount of moisture and rainfall infiltration into the buffer zone are just a few of the parameters that differ from region to region.
With this in mind, specific buffer designs will be different, but the principles stay the same. Osmond said her ideal buffer is 15 metres wide with both a grass and a tree species.
“In the first half of the buffer, it’s important to have a grass because the grass can really slow down the water and dump out any sediment that might be leaving the field, and also the attached phosphorus,” she said.
“The next half of the buffer I’d put into trees because the trees really help hold the stream together. Depending on the kind of buffer you use, you can manipulate the kind of species.
“You can manage your buffer for hunting, or if you value having certain species around, you can manage your buffers for that, too. Wildlife can be an important part of buffers.”
Osmond found that any type of vegetation would do the trick as far as nitrogen removal was concerned. Statistically, she didn’t see any real differences in nitrogen reduction between the grass and tree species.
“(Trees) …might end up being better over time,” she said.
“There are a lot of studies in the U.S. that suggest that 75 percent or more of the sediment from agricultural watersheds are coming from the stream banks and the stream sediments.
Tree buffers do a much better job of holding the stream banks together than the grass buffers or the other kinds of vegetation.”
Buffer width made the biggest difference in nitrogen removal. The study tested widths of eight and 15 metres wide.
“The buffer width that was two times greater had two and a half times better processing of the nitrogen,” she said.
“The additional width gave us more than the additive effect of having a buffer two times as wide. We saw pretty good denitrification … even from the narrower width buffer.
She said it was encouraging that even the smaller buffers were successful because many recommendations for buffer zones in the U.S. are based on widths of 15 to 45 metres.