Thomas M. Isenhart

Design and placement of a multi-species 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 streams length is row crop (corn and soybeans) production agriculture or intensive riparian zone grazing. The Bear Creek watershed is long (∼ 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 rows 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 operational 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 stabilization soil bioengineering component and a constructed wetland to intercept NPS pollutants in field drainage tile water flow. It is hypothesized that this system will function effectively as a nutrient, pesticide, and sediment sink for NPS pollutants coming from the upslope agricultural fields. Prior to establishment of the MSRBS system, the riparian zone along Bear Creek was grazed and row cropped to the stream edge. Since 1990 there has been dramatic alteration in the appearance and functioning of this riparian zone. After four growing seasons, the fast-growing 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 production of silver maple was 8.4 dry Mg ha−1, more than twice to seven times the yield from other silver maple research plots in Central Iowa. The shrub species, selected because of desired wildlife benefits, have done well in terms of survival and growth with ninebark, Nannyberry viburnum and Nanking cherry doing the best. The switchgrass grass has developed into a dense stand that effectively stops concentrated flow from the agriculture fields and allows for infiltration rates well above the field rate. Early root biomass data indicate significantly more roots below the MSRBS than agricultural fields. This suggests better soil stabilization, absorption of infiltrated water, and soil-root-microbe-NPS pollutant interaction characteristics within the MSRBS system than the cropped fields. Nitrate-nitrogen concentrations in the MSRBS never exceed 2 mg l−1 whereas the levels in the adjacent agricultural fields exceed 12 mg l−1. The water quality data collected suggest that the MSRBS is effective in reducing NPS pollutants in the vadose and saturated zone below the system. The soil bioengineering revetments have stabilized the streambank and minimized bank collapse. Initial results (from 4 months of operation) from the constructed wetland (built in summer 1994) indicate nitrate-nitrogen concentrations of the tile inflow water >15 mg l−1 whereas, the outflow water had a nitrate-nitrogen concentration of <3 mg l−1. Over time this wetland should become more effective in removing excess nitrogen moving with the tile flow from the agricultural fields because of the accumulation of organic matter from the cattails. Overall the MSRBS system seems to be functioning 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 high-quality hardwood in the future, and enhance the aesthetics of the agroecosystem. As a streamside best management practice (BMP), the MSRBS system complements upland BMPs and provides many valuable private and public market and non-market benefits.

Fine root dynamics, coarse root biomass, root distribution, and soil respiration in a multispecies riparian buffer in Central Iowa, USA

By influencing belowground processes, streamside vegetation affects soil processes important to surface water quality. We conducted this study to compare root distributions and dynamics, and total soil respiration among six sites comprising an agricultural buffer system: poplar (Populus × euroamericana‘ Eugenei), switchgrass, cool-season pasture grasses, corn (Zea mays L.), and soybean (Glycine max (L.) Merr.). The dynamics of fine (0--2 mm) and small roots (2--5 mm) were assessed by sequentially collecting 35 cm deep, 5.4 cm diameter cores from April through November. Coarse roots were described by excavating 1 × 1 × 2 m pits and collecting all roots in 20 cm depth increments. Root distributions within the soil profile were determined by counting roots that intersected the walls of the excavated pits. Soil respiration was measured monthly from July to October using the soda-lime technique. Over the sampling period, live fine-root biomass in the top 35 cm of soil averaged over 6 Mg ha-1 for the cool-season grass, poplar, and switchgrass sites while root biomass in the crop fields was < 2.3 Mg ha-1 at its maximum. Roots of trees, cool-season grasses, and switchgrass extended to more than 1.5 m in depth, with switchgrass roots being more widely distributed in deeper horizons. Root density was significantly greater under switchgrass and cool-season grasses than under corn or soybean. Soil respiration rates, which ranged from 1.4--7.2 g C m-2 day-1, were up to twice as high under the poplar, switchgrass and cool-season grasses as in the cropped fields. Abundant fine roots, deep rooting depths, and high soil respiration rates in the multispecies riparian buffer zones suggest that these buffer systems added more organic matter to the soil profile, and therefore provided better conditions for nutrient sequestration within the riparian buffers.

Soil-water infiltration under crops, pasture, and established riparian buffer in Midwestern USA

The production-oriented agricultural system of Midwestern United States has caused environmental problems such as soil degradation and nonpoint source (NPS) pollution of water. Riparian buffers have been shown to reduce the impacts of NPS pollutants on stream water quality through the enhancement of riparian zone soil quality. The objective of this study was to compare soil-water infiltration in a Coland soil (fine-loamy, mixed, superactive, mesic Cumulic Endoaquoll) under multi-species riparian buffer vegetation with that of cultivated fields and a grazed pasture. Eight infiltration measurements were made, in each of six treatments. Bulk density, antecedent soil moisture, and particle size were also examined. The average 60-min cumulative infiltration was five times greater under the buffers than under the cultivated field and pasture. Cumulative infiltration in the multi-species riparian buffer was in the order of silver maple > grass filter > switchgrass. Cumulative infiltration did not differ significantly (P < 0.05) among corn and soybean crop fields and the pasture. Soil bulk densities under the multi-species buffer vegetation were significantly (P < 0.05) smaller than in the crop fields and the pasture. Other measured parameters did not show consistent trends. Thus, when using infiltration as an index, the established multi-species buffer vegetation seemed to improve soil quality after six years.

Riparian forest buffers in agroecosystems – lessons learned from the Bear Creek Watershed, central Iowa, USA

Intensive agriculture can result in increased runoff of sediment and agricultural chemicals that pollute streams. Consensus is emerging that, despite our best efforts, it is unlikely that significant reductions in nutrient loading to surface waters will be achieved through traditional, in-field management alone. Riparian forest buffers can play an important role in the movement of water and NPS (non-point source) pollutants to surface water bodies and ground water. Riparian buffers are linear in nature and because of their position in the landscape provide effective connections between the upland and aquatic ecosystems. Present designs tend to use one model with a zone of unmanaged trees nearest the stream followed by a zone of managed trees with a zone of grasses adjacent to the crop field. Numerous variations of that design using trees, shrubs, native grasses and forbs or nonnative cool-season grasses may provide better function for riparian forest buffers in specific settings. Properly designed riparian buffers have been shown to effectively reduce surface NPS pollutant movement to streams and under the right geological riparian setting can also remove them from the groundwater. Flexibility in design can also be used to produce various market and nonmarket goods. Design flexibility should become more widely practiced in the application of this agroforestry practice.

Biomass, carbon and nitrogen dynamics of multi-species riparian buffers within an agricultural watershed in Iowa, USA

This study was conducted to determine biomass dynamics, carbon sequestration and plant nitrogen immobilization in multispecies riparian buffers, cool-season grass buffers and adjacent crop fields in central Iowa. The seven-year-old multispecies buffers were composed of poplar (Populus×euroamericana ‘Eugenei’) and switchgrass (Panicum virgatum L.). The cool-season grass buffers were dominated by non-native forage grasses (Bromus inermis Leysser., Phleum pratense L. and Poa pratensis L). Crop fields were under an annual corn-soybean rotation. Aboveground non-woody live and dead biomass were determined by direct harvests throughout two growing seasons. The dynamics of fine (0–2 mm) and small roots (2–5 mm) were assessed by sequentially collecting 35 cm deep, 5.4 cm diameter cores (125 cm deep cores in the second year) from April through November. Biomass of poplar trees was estimated using allometric equations developed by destructive sampling of trees. Poplar had the greatest aboveground live biomass, N and C pools, while switchgrass had the highest mean aboveground dead biomass, C and N pools. Over the two-year sampling period, live fine root biomass and root C and N in the riparian buffers were significantly greater than in crop fields. Growing-season mean biomass, C and N pools were greater in the multispecies buffer than in either of the crop fields or cool-season grass buffers. Rates of C accumulation in plant and litter biomass in the planted poplar and switchgrass stands averaged 2960 and 820 kg C ha−1 y−1, respectively. Nitrogen immobilization rates in the poplar stands and switchgrass sites averaged 37 and 16 kg N ha−1 y−1, respectively. Planted riparian buffers containing native perennial species therefore have the potential to sequester C from the atmosphere, and to immobilize N in biomass, therefore slowing or preventing N losses to the atmosphere and to ground and surface waters.

Nutrient and sediment removal by switchgrass and cool-season grass filter strips in Central Iowa, USA

AbstractSimulated rainfall and runoff were used to compare the effectiveness of 6 m and 3 m wide filter strips of switchgrass (Panicum virgatum) and cool-season filter strips consisting of bromegrass (Bromus inermis), timothy (Phleum pratense) and fescue (Festuca spp.) in reducing sediment, nitrogen and phosphorus in surface runoff from adjacent crop fields. The 6 m and 3 m wide strips represented 20:1 and 40:1 area ratios, respectively. Twelve plots, six each, in the switchgrass and cool-season grass strips, were laid out on Coland soil, having an average slope of 3%. Plots received simulated rainfall of 5.1 cm hr1 intensity and simulated runoff containing known quantities of sediment and nutrients. Three runon samples, each integrated over 15 minutes, and nine runoff samples, each integrated over five minutes, were collected from each plot and analyzed for sediment, total-N,

QUANTIFYING FINE‐ROOT DECOMPOSITION: AN ALTERNATIVE TO BURIED LITTERBAGS

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Assessing soil quality in a riparian buffer by testing organic matter fractions in central Iowa, USA

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