Green Horizons

Three decades after the implementation of the Clean Water Act of the 1970s, nonpoint source pollution – pollutants such as sediments, nutrients or pesticides that originate upslope of a stream, lake or pond – is a major challenge for protecting and restoring water quality.

The U.S. Environmental Protection Agency noted the most common pollutants to rivers and streams from livestock grazing include pathogens, siltation, organic enrichment and nutrients. Poor grazing management practices not only increase contamination of surface and ground waters but also reduce farm income. Control of nonpoint source pollution from grazing is important to improve water quality.

Previous studies suggest nutrient loss can be reduced and production can be improved through proper management of grazing animals and pastures. Adoption of alternative practices that improve soil and water quality and farm income are essential for the sustainability of small family livestock operations. Agroforestry practices have been recognized as a measure to address many issues related to water quality. However, studies examining buffer effects on the quality of water from grazed pastures are limited.

Ranjith Udawatta, Harold E. Garrett and Robert Kallenbach, three researchers with The Center for Agroforestry at the University of Missouri, set out to study just that, beginning in 2001. Six treatment areas, two with agroforestry buffers, two with grass buffers, and two control treatments, were used to test the hypothesis that agroforestry and grass buffers can be used to effectively reduce nonpoint source pollution from pastured watersheds. Vegetation in grass buffer and pasture areas include red clover and lespedeza planted into fescue. Eastern cottonwood trees were planted into fescue in agroforestry buffers. Composite water samples were analyzed for sediment and total nitrogen after each runoff event to compare treatment differences.

The annual discharge of water per area differed greatly among treatments and years. On average, buffer treatments produced only 30 percent and 59 percent of the runoff of the control treatment in 2004 and 2008, respectively. In years with a very small number of runoff events, the difference between the buffer and control treatments was small and differences were not significant. The control treatment produced significantly more runoff during 2004 than during any other study year, a year with 115 percent precipitation of the long-term mean.

Soil loss was significantly affected by treatments. The grazed treatment with agroforestry buffers lost only 51 percent of the sediment compared to the control treatment. The unbuffered control treatment lost 36 percent more soil than the average for the agroforestry and grass buffer treatments. Results of this study demonstrate that buffers with trees may be more effective than grass alone, probably due to improved soil properties and greater resistance to surface flow.

Total nitrogen loss was significantly affected by treatments. The control treatment lost 4 and 3.2 times more total nitrogen than the agroforestry and grass buffer treatments, respectively.

In summary, grazed watersheds with agroforestry and grass buffers had significantly lower runoff volumes, sediment and total nitrogen losses than the control.

Releasing hard mast-producing trees will not only produce more nuts and acorns, but also more wood.

It is anticipated as trees grow and roots occupy more soil volume, the reduction of nitrogen in runoff will continue to increase on the agroforestry watershed. This study suggests that greater emphasis should be placed on management strategies that minimize runoff and nonpoint source pollution losses. Upland buffers, as a protective measure, can clearly help reduce soil erosion and nutrient losses from pastured land and thereby protect water quality.

Ranjith P. Udawatta is a Research Assistant Professor with the Department of Soil, Environmental and Atmospheric Sciences, and The Center for Agroforestry, School of Natural Resources, University of Missouri.

Harold E. Garrett is the former Director of The Center for Agroforestry, MU.

Robert L. Kallenbach is a Professor in the Department of Ag Experimental and Plant Sciences, MU.

Complete results of this study can be found in the journal Agroforestry Systems: DOI: 10.1007/s10457-010-9288-9