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Subsurface lithology plays an important role in many riparian zone processes, but few studies have examined how sediment nutrient concentrations vary with depth. In this study, we evaluated concentrations of nutrients (N, C and P) with depth in a riparian zone of the glaciated Midwest. A total of 146 sediment samples were collected from 24 cores that extended to a maximum depth of 3.6 m at eight sites in the riparian zone of Walnut Creek. Subsurface deposits were predominantly silt loam, becoming coarser and more variable with depth. Nitrogen and carbon content ranged from

Reducing non-point source phosphorus (P) pollution is often necessary to improve water quality in agricultural streams. Soil total P (TP) concentrations and compaction are soil characteristics that can influence P losses to streams. The objective of this study was to compare these two soil characteristics among riparian forest buffers, grass filters, pastures with cattle fenced out of the stream, intensive rotational, rotational and continuously grazed pastures and row-cropped fields in three physiographic regions of Iowa. Soil TP and compaction for the seven riparian land-uses were determined in stream bank and surface riparian soils. Total P concentrations in stream bed material along the seven riparian land-uses were also measured. Total P concentrations in riparian stream bank soils among land-uses ranged from 303 to 398 mg kg−1 in the central region, to 432–518 mg kg−1 in the northeast, to 360–555 mg kg−1 in the southeast. In the surface riparian soils TP among land-uses ranged from 434 to 649 mg kg−1 in the central region, to 493–764 mg kg−1 in the northeast region, to 428–716 mg kg−1 in the southeast region. Finally, the TP concentrations in the stream bed sediments among land-uses ranged 194–307 mg kg−1 in the central region, to 169–461 mg kg−1 in the northeast, to 389–964 mg kg−1 in the southeast. Few soil TP significant differences among riparian land-uses within regions were found. Soil compaction under some of the grazing practices was significantly higher than under the conservation practices. Limited differences in soil TP concentrations and compaction were partially due to the young age of the conservation practices and the rotational and intensive rotational pastures. Past land-use on these sites had been either continuous grazing or row-cropping and those uses were still influencing these soil parameters. Within riparian land-uses, surface riparian soils generally had higher TP than the stream bank riparian soils. Stream bed samples had lower TP concentrations than the riparian soils in the northeast and central regions but were higher than the riparian soils in some of the land-uses in the southeast region. Overall TP concentrations in the stream bank, surface riparian soils and stream bed material were high indicating that these areas can potentially be important source areas of P and the focus should be on reducing the transport of P.

Phosphorus and sediment are major nonpoint source pollutants that degrade water quality. Streambank erosion can contribute a significant percentage of the phosphorus and sediment load in streams. Riparian land-uses can heavily influence streambank erosion. The objective of this study was to compare streambank erosion along reaches of row-cropped fields, continuous, rotational and intensive rotational grazed pastures, pastures where cattle were fenced out of the stream, grass filters and riparian forest buffers, in three physiographic regions of Iowa. Streambank erosion was measured by surveying the extent of severely eroding banks within each riparian land-use reach and randomly establishing pin plots on subsets of those eroding banks. Based on these measurements, streambank erosion rate, erosion activity, maximum pin plot erosion rate, percentage of streambank length with severely eroding banks, and soil and phosphorus losses per unit length of stream reach were compared among the riparian land-uses. Riparian forest buffers had the lowest streambank erosion rate (15-46 mm/year) and contributed the least soil (5-18 tonne/km/year) and phosphorus (2-6 kg/km/year) to stream channels. Riparian forest buffers were followed by grass filters (erosion rates 41-106 mm/year, soil losses 22-47 tonne/km/year, phosphorus losses 9-14 kg/km/year) and pastures where cattle were fenced out of the stream (erosion rates 22-58 mm/year, soil losses 6-61 tonne/km/year, phosphorus losses 3-34 kg/km/year). The streambank erosion rates for the continuous, rotational, and intensive rotational pastures were 101-171, 104-122, and 94-170 mm/year, respectively. The soil losses for the continuous, rotational, and intensive rotational pastures were 197-264, 94-266, and 124-153 tonne/km/year, respectively, while the phosphorus losses were 71-123, 37-122, and 66 kg/km/year, respectively. The only significant differences for these pasture practices were found among the percentage of severely eroding bank lengths with intensive rotational grazed pastures having the least compared to the continuous and rotational grazed pastures. Row-cropped fields had the highest streambank erosion rates (239 mm/year) and soil losses (304 tonne/km/year) and very high phosphorus losses (108 kg/km/year).

Spatial data on soils, land use, and topography, combined with knowledge of conservation effectiveness, can be used to identify alternatives to reduce nutrient discharge from small (hydrologic unit code [HUC]12) watersheds. Databases comprising soil attributes, agricultural land use, and light detection and ranging-derived elevation models were developed for two glaciated midwestern HUC12 watersheds: Iowa's Beaver Creek watershed has an older dissected landscape, and Lime Creek in Illinois is young and less dissected. Subsurface drainage is common in both watersheds. We identified locations for conservation practices, including in-field practices (grassed waterways), edge-of-field practices (nutrient-removal wetlands, saturated buffers), and drainage-water management, by applying terrain analyses, geographic criteria, and cross-classifications to field- and watershed-scale geographic data. Cover crops were randomly distributed to fields without geographic prioritization. A set of alternative planning scenarios was developed to represent a variety of extents of implementation among these practices. The scenarios were assessed for nutrient reduction potential using a spreadsheet approach to calculate the average nutrient-removal efficiency required among the practices included in each scenario to achieve a 40% NO-N reduction. Results were evaluated in the context of the Iowa Nutrient Reduction Strategy, which reviewed nutrient-removal efficiencies of practices and established the 40% NO-N reduction as Iowa's target for Gulf of Mexico hypoxia mitigation by agriculture. In both test watersheds, planning scenarios that could potentially achieve the targeted NO-N reduction but remove

Human alterations to the Iowa landscape, such as elimination of native vegetation for row crop agriculture and grazing, channelization of streams, and tile and ditch drainage, have led to deeply incised channels with accelerated streambank erosion. The magnitude of streambank erosion and soil loss were compared along Bear Creek in central Iowa. The subreaches are bordered by differing land uses, including reestablished riparian forest buffers, row ciop fields, and continuously grazed riparian pastures. Erosion pins were measured from June 1998 to July 2002 to estimate the magnitude of streambank erosion. Total streambank soil loss was estimated by using magnitude of bank erosion, soil bulk density, and severely eroded bank area. Significant seasonal and yearly differences in magnitude of bank erosion and total soil loss were partially attributed to differences in precipitation and associated discharges. Riparian forest buffers had significantly lower magnitude of streambank erosion and total soil loss than the other two riparian land uses. Establishment of riparian forest buffers along all of the nonbuffered subreaches would have reduced streambank soil loss by an estimated 77 to 97 percent, significantly decreasing sediment in the stream, a major water quality problem in Iowa.

An observational study was conducted at the watershed scale using land cover (vegetation) data to assess the absence or presence of riparian buffers in three northeastern Missouri watersheds. Forests and grasslands lying within a 61 m (200 ft) parallel band directly adjacent to streams were considered "buffers" for improving or protecting water quality and were characterized according to their length, width, and vegetation type. Results indicated that riparian buffers were abundant throughout the watersheds but were typically narrow along first-order and second-order streams; in many cases they may not have been wide enough to provide adequate stream protection. At least 90 percent of all streams had buffer vegetation immediately adjacent to the streambanks, but as few as 31 percent of first-order streams had buffers extending to 61 m from the stream on at least one side. On-site evaluations are needed to determine the condition of these forests and grasslands and their ability to process nonpoint source pollutants. The results will be useful for providing natural resource managers with knowledge of current watershed conditions as well as in identifying specific locations for future conservation efforts within each watershed.

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.

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.

Our understanding of fine-root decay processes is derived almost exclusively from litterbag studies. However, preparation of roots for litterbag studies and their sub- sequent decay within litterbags represent major departures from in situ conditions. We hypothesized that litterbag studies misrepresent fine-root decay and nutrient release rates during decomposition. To test these hypotheses we developed a new intact-core technique that requires no a priori root processing, retains natural rhizosphere associations, and main- tains in situ decay conditions. Using both litterbags and intact cores, we measured annual decay rates and nitrogen release from newly senesced fine roots of silver maple, maize, and winter wheat. After one year, mass loss was 10-23% greater, and nitrogen release was 21-29% higher within intact cores. Differences appeared to result from litterbag-induced alterations to decomposer dynamics and from unavoidable changes to fine-root size-class composition within litterbags. Our results suggest that fine-root decay and nutrient turnover occur significantly faster than estimated from litterbag studies. By minimizing disturbances to roots, soil, and rhizosphere associates prior to root decay, the intact-core technique provides an improved alternative for measuring fine-root decomposition.

Riparian Management Systems (RiMS) have been proposed to minimize the impacts of agricultural production and improve water quality in Iowa in the Midwestern USA. As part of RiMS, multi-species riparian buffers have been shown to decrease nutrient, pesticide, and sediment concentrations in runoff from adjacent crop fields. However, their effect on nutrients and pesticides moving in groundwater beneath buffers has been discussed only in limited and idealized hydrogeologic settings. Studies in the Bear Creek watershed of central Iowa show the variability inherent in hydrogeologic systems at the watershed scale, some of which may be favorable or unfavorable to future implementation of buffers. Buffers may be optimized by choosing hydrogeologic systems where a shallow groundwater flow system channels water directly through the riparian buffer at velocities that allow for processes such as denitrification to occur.

We quantified rates of soil respiration among sites within an agricultural landscape in central Iowa, USA. The study was conducted in riparian cool-season grass buffers, in re-established multispecies (switchgrass + poplar) riparian buffers and in adjacent crop (maize and soybean) fields. The objectives were to determine the variability in soil respiration among buffer types and crop fields within a riparian landscape, and to identify those factors correlating with the observed differences. Soil respiration was measured approximately monthly over a two-year period using the soda-lime technique. Mean daily soil respiration across all treatments ranged from 0.14 to 8.3 g C m−2 d−1. There were no significant differences between cool-season grass buffers and re-established forest buffers, but respiration rates beneath switchgrass were significantly lower than those beneath cool-season grass. Soil respiration was significantly greater in both buffer systems than in the cropped fields. Seasonal changes in soil respiration were strongly related to temperature changes. Over all sites, soil temperature and soil moisture together accounted for 69% of the seasonal variability in soil respiration. Annual soil respiration rates correlated strongly with soil organic carbon (R = 0.75, P < 0.001) and fine root (

A study was conducted to evaluate the ability of a multispecies riparian buffer (MRB) to remove sediment, nitrogen, and phosphorus from cropland runoff. Simulated rainfall was applied to 4.1- by 22.1-m bare cropland source areas paired with either no buffer, a 7.1-m-wide switchgrass (Panicum virgatum L. cv. Cave-n-Rock) buffer, or a 16.3-m-wide switchgrass-woody plant buffer. Each treatment plot combination had three replicates. The switchgrass buffer trapped 70% of the incoming sediment, while the switchgrass-woody buffer trapped more than 92%. In general, these buffers retained 93% of sand and silt particles and 52% of clay particles. During a 2-h rainfall simulation at 25 mm h -1 , the switchgrass buffer removed 64, 61, 72, and 44% of the incoming total N, NO 3 -N, total P, and PO 4 -P, respectively. The switchgrass-woody buffer removed 80, 92, 93, and 85% of the incoming total N, NO 3 -N, total P, and PO 4 -P, respectively. During a 1-h rainfall simulation at 69 mm h -1 , the switchgrass buffer removed 50, 41, 46, and 28% of the incoming total N, NO 3 -N, total P, and PO 4 -P, respectively. The switchgrass-woody plant buffer removed 73, 68, 81, and 35% of the incoming total N, NO 3 -N, total P, and PO 4 -P, respectively. The switchgrass buffer was effective in trapping coarse sediment and sediment-bound nutrients. But the additional buffer width with high infiltration capacity provided by the deep-rooted woody plant zone was effective in trapping the clay and soluble nutrients.

Simulated 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, \({\text{NO}}_{\text{3}}^{\text{ - }} \)-N, total-P and \({\text{PO}}_{\text{4}}^{\text{ - }} \)P. The 6 m wide filter strips removed 77% while the 3 m removed 66% of the incoming sediment from surface runoff. The 6 m filter strips removed 46% of total-N, 42% of \({\text{NO}}_{\text{3}}^{\text{ - }} \)-N, 52% of total-P, and 43% of \({\text{PO}}_{\text{4}}^{\text{ - }} \)-P; and the 3 m filter strips removed 28% of total-N, 25% of \({\text{NO}}_{\text{3}}^{\text{ - }} \)-N, 37% of total-P and 34% of \({\text{PO}}_{\text{4}}^{\text{ - }} \)-P. Differences between 6 m and 3 m filter strips were significant (P < 0.05) for sediment and nutrient removal. Switchgrass filter strips removed significantly more total-N, \({\text{NO}}_{\text{3}}^{\text{ - }} \)-N, total-P and \({\text{PO}}_{\text{4}}^{\text{ - }} \)-P than cool-season grass filter strips (P < 0.05).

A multispecies riparian buffer strip (MRB) was established along Bear Creek in central Iowa by the Agroecology Issues Team at Iowa State University (ISU) in order to assess the ability of the MRB to positively impact soil erosion and process non-point source pollutants to improve water quality. Soil organic matter (SOM), and especially biologically-active soil organic matter, is considered to be an important soil quality indicator variable because of it has relationship to critical soil functions like erodibility and the capacity of the soil to act as an environmental buffer. The objectives of this study were to examine trends in SOM C accrual and to quantify intra-seasonal changes in SOM C and particulate organic matter (POM) C for each vegetation zone of a MRBS seven years after establishment on previously cultivated or heavily grazed soil. Total SOM C and POM C in soil under perennial vegetation (poplar, switch- grass and cool season grass) were significantly higher than under cropped soil. Total POM C changed within vegetation type over the four month study period, whereas total SOM C did not. After six growing seasons, SOM C increased 8.5% under poplar grown in association with cool season grass, and 8.6% under switchgrass. The results are very promising and suggest that changes in SOM C can occur in a relatively short time after the establishment of perennial vegetation in a MRB. These changes should increase the ability of MRB soil to process non- point source pollutants.

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.

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 stream's 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

A watershed's water quality is influenced by contaminant-transport pathways unique to each landscape. Accurate information on contaminant-pathways could provide a basis for mitigation through well-targeted approaches. This study determined dynamics of nitrate-N, total P, Escherichia coli, and sediment during a runoff event in Tipton Creek, Iowa. The watershed, under crop and livestock production, has extensive tile drainage discharging through an alluvial valley. A September 2006 storm yielded 5.9 mm of discharge during the ensuing 7 d, which was monitored at the outlet (19,850 ha), two tile-drainage outfalls (total 1856 ha), and a runoff flume (11 ha) within the sloped valley. Hydrograph separations indicated 13% of tile discharge was from surface intakes. Tile and outlet nitrate-N loads were similar, verifying subsurface tiles dominate nitrate delivery. On a unit-area basis, tile total P and E. coli loads, respectively, were about half and 30% of the outlet's; their rapid, synchronous timing showed surface intakes are an important pathway for both contaminants. Flume results indicated field runoff was a significant source of total P and E. coli loads, but not the dominant one. At the outlet, sediment, P, and E. coli were reasonably synchronous. Radionuclide activities of (7)Be and (210)Pb in suspended sediments showed sheet-and-rill erosion sourced only 22% of sediment contributions; therefore, channel sources dominated and were an important source of P and E. coli. The contaminants followed unique pathways, necessitating separate mitigation strategies. To comprehensively address water quality, erosion-control and nitrogen-management practices currently encouraged could be complemented by buffering surface intakes and stabilizing stream banks.

While water quality functions of conservatian buffers established adjacent to cropped fields have been widely documented, the relative contribution of these re-established perennial plant systems to greenhouse gases has not been completely documented. In the case of methane (CH 4 ), these systems have the potential to serve as sinks of CH 4 or may provide favorable conditions for CH 4 production. This study quantifies CH 4 flux from soils of riparian buffer systems comprised of three vegetation types and compares these fluxes with those of adjacent crop fields. We measured soil properties and diel and seasonal variations of CH 4 flux in 7 to 17 yr-oLd re-established riparian forest buffers, warm-season and cool-season grass filters, and an adjacent crop field located in the Bear Creek watershed in central Iowa. Forest buffer and grass filter soils had significantly lower bulk density (P < 0.01); and higher pH (P < 0.01), total carbon (TC) (P < 0.01), and total nitrogen (TN) (P < 0.01) than crop field soils. There was no significant relationship between CH 4 flux and soil moisture or soil temperature among sites within the range of conditions observed. Cumulative CH 4 flux was -0.80 kg CH 4 -C ha -1 yr -1 in the cropped field, -0.46 kg CH 4 -C ha -1 yr -1 within the forest buffers, and 0.04 kg CH 4 -C ha -1 yr -1 within grass filters, but difference among vegetation covers was not significant. Results suggest that CH 4 flux was not changed after establishment of perennial vegetation on cropped soils, despite significant changes in soil properties.

Simple and reliable procedures have been developed for analyses of N03-, total N, and organic N in freshwaters. NO,is determined by second-derivative UV spectroscopy. Total N and organic N are determined based on secondderivative analyses of NO,following persulfate digestion. Resolution of organic N determinations was increased by using ion-exchange resins to remove NO,from samples with high concentrations of N03prior to persulfate oxidation of the organic N. Although numerous methods have been developed for the determination of N in aqueous samples, there is continued need for simple and reliable procedures for both inorganic and organic N analyses. The most widely accepted methods for N03analysis include Cd reduction and ion chromatography (Am. Public Health Assoc. 1989). Both methods have been adapted for automated analysis and, with proper consideration of potential interferences, both are accurate and precise. Ion chromatography has the advantage of also measuring several additional anions in a single aliquot of samAcknowledgments This work was supported in part by the Iowa State Water Resour. Res. Inst., the Iowa DNR, the U.S. EPA, and Wetlands Res. Inc., Chicago, Illinois. We thank Iowa State University staff and researchers for their contributions to this research. Automated Cd reduction analyses were performed by the Analytical Services Laboratory (Iowa State Univ. Eng. Res. Inst.) and by the U.S. EPA ERL, Duluth, Minnesota. ple. Ion chromatography and Cd reduction, however, are relatively complicated and expensive. Samples must normally be filtered to remove suspended matter before analysis, and in the case of Cd reduction, sample color, metal ions, and phosphate are potential interferences (Olson 1980). The principal methods for organic N determination are based on conversion of organic N to NH,+ or N03and subsequent analysis of these inorganic forms. Separate analysis of ambient inorganic N allows organic N levels to be calculated by difference. For freshwater samples, conversion of organic N to inorganic forms is most commonly based on either Kjeldahl digestion (U.S. EPA 1979; Am. Public Health Assoc. 1989), photo-oxidation (Armstrong et al. 1966), or persulfate digestion (Koroleff 1976; D’Elia et al. 1977; Solorzano and Sharp 1980). Photo-oxidation requires more specialized equipment and may be ineffective for some compounds (Henriksen 1970). Kjeldahl digestion is a complicated and expensive procedure and may be unsuitable for systems receiving high inorganic N loads. High concentrations of N03severely interfere with standard Kjeldahl N determinations (Schlueter 1977; U.S. EPA 1979; Am. Public Health Assoc. 1989). N03--N concentrations 10-20 times greater than those of organic N result in >90% inhibition of Kjeldahl N determinations (Schlue

Denitrification within riparian buffers may trade reduced nonpoint source pollution of surface waters for increased greenhouse gas emissions resulting from denitrification-produced nitrous oxide (N2O). However, little is known about the N2O emission within conservation buffers established for water quality improvement or of the importance of short-term N2O peak emission following rewetting dry soils and thawing frozen soils. Such estimates are important in reducing uncertainties in current Intergovernmental Panel on Climate Change (IPCC) methodologies estimating soil N2O emission which are based on N inputs. This study contrasts N2O emission from riparian buffer systems of three perennial vegetation types and an adjacent crop field, and compares measured N2O emission with estimates based on the IPCC methodology. We measured soil properties, N inputs, weather conditions and N2O fluxes from soils in forested riparian buffers, warm-season and cool-season grass filters, and a crop field located in the Bear Creek watershed in central Iowa, USA. Cumulative N2O emissions from soils in all riparian buffers (5.8 kg N2O-N ha−1 in 2006–2007) were significantly less than those from crop field soils (24.0 kg N2O-N ha−1 in 2006–2007), with no difference among the buffer vegetation types. While N2O peak emissions (up to 70-fold increase) following the rewetting of dry soils and thawing of frozen soils comprised 46–70% of the annual N2O emissions from soils in the crop field, soils in the riparian buffers were less sensitive to such events (3 to 10-fold increase). The ratio of N2O emission to N inputs within riparian buffers (0.02) was smaller than those of crop field (0.07). These results indicate that N2O emission from soils within the riparian buffers established for water quality improvement should not be considered a major source of N2O emission compared to crop field emission. The observed large difference between measured N2O emissions and those estimated using the IPCC's recommended methodology (i.e., 87% underestimation) in the crop field suggests that the IPCC methodology may underestimate N2O emission in the regions where soil rewetting and thawing are common, and that conditions predicted by future climate-change scenarios may increase N2O emissions.

First paragraph: This chapter describes the use of buffers and vegetative filter strips relative to water quality. In particular, we primarily discuss the herbaceous components of the following NRCS Conservation Practice Standards

Improving soil quality and suppressing weeds are two challenges facing strawberry growers. Cover crops, such as perennial ryegrass (Lolium perenne) and sorghum-sudangrass (Sorghum sudanense), have been used in rotation with strawberry in the Midwest. The objective of the field study was to investigate the effects of various cover crops on soil quality and weed populations for strawberry production. The experiment was established in 1996 at the Iowa State Univ. Horticulture Station, Ames, in plots that previously were planted continuously in strawberry for 10 years. Nine treatments were arranged in a randomized complete-block design with three replications. Treatments included cover crops of Indian grass (Sorghastrum avenaceum), switchgrass (Panicum virgatum), big bluestem (Andropogon gerardii), black-eyed susan (Rudbeckia hirta), marigold (Tagetes erecta `Crackerjack'), sorghum-sudangrass, perennial ryegrass, strawberry (Fragaria ×ananassa `Honeoye'), and bare soil (control). Data from 1998 showed that both annual and perennial cover crops were established more readily (higher treatment-plant populations and less weed populations) than in 1997. Water infiltration rates were highest in bare soil plots and lowest in P. virgatum plots. Bare soil plots and S. sudanense plots had the lowest percent soil moisture.

Traditional cover crops, such as perennial ryegrass (Lolium perenne), and sorghum-sudan grass (Sorghum sudanense), commonly are used in rotation with strawberry in the midwestern United States to improve soil quality and suppress weeds. The objective of the field study was to investigate the effects of various cover crops on soil quality and weed populations. The experiment was established in 1996 at the Iowa State Univ. Horticulture Station, Ames, in plots that previously were planted continuously in strawberry for 10 years. Nine treatments were arranged in a randomized complete-block design with three replications. Treatments included cover crops of Indian grass (Sorghastrum avenaceum), switch grass (Panicum virgatum), big bluestem (Andropogon gerardii), black-eyed susan (Rudbeckia hirta), marigold (Tagetes erecta `Crackerjack'), sorghum-sudan grass, perennial ryegrass, strawberry (Fragaria xananassa `Honeoye'), and bare soil (control). Of the prairie plants, Andropogon gerardii was the most difficult to establish and thus had the highest percentage weed cover. Macroaggregate mass of the soil at 0- to 15-cm depth was least in the bare-soil treatment and in the other annual-plant treatment, Tagetes erecta `Crackerjack'.

Mass balances of nitrate nitrogen were determined for an agricultural stream receiving non-point inputs of inorganic nitrogen. Primary production and respiration of stream reaches were estimated from analyses of diurnal changes in dissolved oxygen and temperature. Substantial in-stream losses of nitrate were observed, averaging 0.66 g N m−2 day−1. The estimated nitrogen requirements to support observed rates of primary production ranged between 0.15 and 0.27 g N m−2 day−1. Laboratory investigations measuring nitrate nitrogen loss rates from stream water overlying intact sediment cores suggest that algal assimilation of inorganic nitrogen contributes to the overall nitrate decline in these systems.