DESIGN AND PLACEMENT OF A MULTISPECIES RIPARIAN BUFFER STRIP SYSTEM

A multi-species riparian buffer strip (MSRBS) system was designed and placed along a Central Iowa stream in 1990. Bear Creek, is typical of many streams in Central Iowa where the primary land use along the stre am's length is row crop (corn and soybeans) production agriculture or intensive riparian zone grazing. The Bear Creek watershed is long (sim ilar to 35 km), narrow (3-6 km), and drains 7,661 ha of farmland. The MSRBS system is a 20 m wide filter strip consisting of four or five ro ws of fast-growing trees planted closest to the stream, then two shrub rows, and finally a 7 m wide strip of switchgrass established next to the agricultural fields. The 1.0 km long system is located on an oper ational farm and is laid out in a split block design on both sides of Bear Creek. An integral part of this system is a streambank stabilizat ion soil bioengineering component and a constructed wetland to interce pt NPS pollutants in field drainage tile water flow. It is hypothesize d that this system will function effectively as a nutrient, pesticide, and sediment sink for NPS pollutants coming from the upslope agricult ural fields. Prior to establishment of the MSRBS system, the riparian zone along Bear Creek was grazed and row cropped to the stream edge. S ince 1990 there has been dramatic alteration in the appearance and fun ctioning of this riparian zone. After four growing seasons, the fast-g rowing tree species (cottonwood, silver maple, willow, and green ash) range in height from 2.4 m to over 5.5 m. Mean (four-year) biomass pro duction of silver maple was 8.4 dry Mg ha(-1), more than twice to seve n times the yield from other silver maple research plots in Central Io wa. The shrub species, selected because of desired wildlife benefits, have done well in terms of survival and growth with ninebark, Nannyber ry viburnum and Nanking cherry doing the best. The switchgrass has dev eloped into a dense stand that effectively stops concentrated flow fro m the agriculture fields and allows for infiltration rates well above the field rate. Early root biomass data indicate significantly more ro ots below the MSRBS than agricultural fields. This suggests better soi l stabilization, absorption of infiltrated water, and soil-root-microb e-NPS pollutant interaction characteristics within the MSRBS system th an the cropped fields. Nitrate-nitrogen concentrations in the MSRBS ne ver exceed 2 mg l(-1) whereas the levels in the adjacent agricultural fields exceed 12 mg l(-1). The water quality data collected suggest th at the MSRBS is effective in reducing NPS pollutants in the vadose and saturated zone below the system. The soil bioengineering revetments h ave stabilized the streambank and minimized bank collapse. Initial res ults (from 4 months of operation) from the constructed wetland (built in summer 1994) indicate nitrate-nitrogen concentrations of the tile i nflow water > 15 mg l(-1) whereas the outflow water had a nitrate-nitr ogen concentration of < 3 mg l(-1). Over time this wetland should beco me more effective in removing excess nitrogen moving with the tile flo w from the agricultural fields because of the accumulation of organic matter from the cattails. Overall the MSRBS system seems to be functio ning as expected. This MSRBS system offers farmers a way to intercept eroding soil, trap and transform NPS pollution, stabilize streambanks, provide wildlife habitat, produce biomass for on-farm use, produce hi gh-quality hardwood in the future, and enhance the aesthetics of the a groecosystem. As a streamside best management practice (BMP), the MSRB S system complements upland BMPs and provides many valuable private an d public market and non-market benefits.