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

L. Bharati1, K.-H. Lee2, T.M. Isenhart3 and R.C. Schultz3,*1Center for Development Research, Ecology and Resource Management, Walter-Flex-Str. 3, 53113 Bonn,Germany; 2School of Forest Resources and Conservation, University of Florida, 5988 Highway 90, Building4900, Milton, Florida, USA; 3Department of Natural Resource Ecology and Management, Iowa StateUniversity, 253 Bessey Hall, Iowa 50011 Ames, USA; *Author for correspondence (e-mail:rschultz@iastate.edu;)

Received 25 July 2002; accepted in revised form 8 August 2002

Key words: Conservation buffer, Filter strip, Riparian forest buffer, Soil quality, Switchgrass

Abstract

The production-oriented agricultural system of Midwestern United States has caused environmental problemssuch as soil degradation and nonpoint source (NPS) pollution of water. Riparian buffers have been shown toreduce the impacts of NPS pollutants on stream water quality through the enhancement of riparian zone soilquality. 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 cultivatedfields 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 wasfive times greater under the buffers than under the cultivated field and pasture. Cumulative infiltration in themulti-species riparian buffer was in the order of silver maple > grass filter > switchgrass. Cumulative infiltrationdid not differ significantly (P < 0.05) among corn and soybean crop fields and the pasture. Soil bulk densitiesunder the multi-species buffer vegetation were significantly (P < 0.05) smaller than in the crop fields and thepasture. Other measured parameters did not show consistent trends. Thus, when using infiltration as an index, theestablished multi-species buffer vegetation seemed to improve soil quality after six years.

Introduction

Highly efficient agriculture systems have producedmany nonpoint source (NPS) pollution problems. Ri-parian buffers can reduce NPS pollution from agricul-tural areas through the enhancement of riparian zonesoil quality (Schultz et al. 1995). The Soil ScienceSociety of America defines soil quality as: ‘The ca-pacity of a specific kind of soil to function, withinnatural or managed ecosystem boundaries, to sustainplant and animal productivity, maintain or enhancewater and air quality, and support human health andhabitation’ (Soil Science Society of America 1995).

Infiltration characteristics are closely related to soilstructure and may be a good indicator of changes insoil physical and biological properties (Radke and

Berry 1993). Water infiltration affects crop productionand the volume, transport route, and quality of agri-cultural drainage (Mukhtar et al. 1985). Several fac-tors such as slope of the landscape, soil texture andstructure, vegetation cover, management systems, an-tecedent water content and soil organic matter havean effect on infiltration (Radke and Berry 1993). In-creased infiltration usually delays the time during astorm, when surface runoff begins and during this ex-tra time, the infiltrating water can leach more of thechemicals out of the thin mixing zone of soil that in-teracts with rainfall and runoff (Mukhtar et al. 1985).Rapid infiltration rates would increase the contacttime between water transported NPS pollutants andthe associated ‘living plant-soil filter’ of a multi-spe-cies riparian buffer (Schultz et al. 1995). Also, as

249Agroforestry Systems 56: 249–257, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

more water infiltrates, surface runoff velocities willdecrease which in turn reduce soil erosion (Lee et al.2000).

Radke and Berry (1993) found ponded and simu-lated rainfall infiltration to be a good integrator ofstructurally related soil properties and used them tolearn whether changes in soil properties had occurredbefore running other tests to determine the causes.During rainfall events of limited amounts and dura-tions (e.g., � 5 cm h−1) less than 30 cm of surfacesoil is immediately involved with infiltrated water(Mukhtar et al. 1985). Surface soil conditions includ-ing the presence of macropores determine the amountof water entering the soil (Mukhtar et al. 1985).Deeper soils will play a role only under surface con-ditions of large transfer rates and low storage volume.These conditions can occur when the soil contains ahigh proportion of connected macropores.Macropores can greatly increase infiltration and arecreated by soil fauna and old root channels, fractureplanes caused by tillage, and soil cracks caused bydrying and freezing (Mukhtar et al. 1985; Radke andBerry 1993).

Different cropping, tillage, and management sys-tems, often change structurally related soil physicaland biological properties (Meek et al. 1992; Radkeand Berry 1993). Compaction caused by farm imple-ments and grazing animals increases soil bulk densityand reduces infiltration (Radke and Berry 1993). Asurface cover of live plants or crop residue can helpmaintain larger infiltration rates by reducing compac-tion from the impact of rainfall, crusting and decreas-ing soil water evaporation (Mukhtar et al. 1985;Radke and Berry 1993).

Potentially confounding the ability to detect man-agement effects on soil-water infiltration is the inher-ent variability of depositional soils. For example,Hammer et al. (1987) found that bottomland soilswere the most variable of three landscape units stud-ied. Stolt et al. (2001) recommended that a stratifiedsampling design be used to quantify soil variabilitywithin these zones and to separate systematic fromrandom soil components. This was the strategy em-ployed in this study, as sampling units were restrictedto a single soil mapping unit.

The multispecies riparian buffer (MRB) was de-signed to test the hypothesis that the establishment ofriparian buffers with tree, shrub and prairie vegetationon previously cultivated fields or pastures would de-crease the impacts of NPS pollutants on stream water

quality through the enhancement of riparian zone soilquality (Schultz et al. 1995).

The objective of this study was to compare therates of infiltration, bulk density, antecedent soilmoisture, and particle size in the different vegetativecommunities of an established MRB with similarsoils in which the traditional practice of cultivationright up to the edge of the stream or grazing in theriparian zone were being practiced.

Materials and methods

In 1990, a MRB was established along nearly 1,000m of Bear Creek on a farm in Story County, Iowa,USA (42°11� N, 93°30� W). About 87% of the water-shed is devoted to corn (Zea mays L.) and soybean[Glycine max (L.) Merr.] production with row cropsextending to the edge of the streambank along asmuch as 50% of the stream length (Schultz et al.1995).

Vegetation consisting of three combinations ofplanted trees, shrubs, native prairie grass, and twocontrols, are randomly distributed in 90 m long plotsalong the 1,000 m of Bear Creek. The tree zone con-sists of five rows of trees planted closest to and par-allel to the stream. The tree species used were hybridpoplar (Populus X euramericana ‘Eugenei’), greenash (Fraxinus pennsylvanica Marsh.), silver maple(Acer saccharinum L.), and black walnut (Juglans ni-gra L.). The shrub zone consists of two rows ofplanted shrubs. Upslope from the trees, a row of red-osier dogwood (Cornus stolonifera Michx.) and a rowof ninebark (Physocarpus opulifolius L.) wereplanted. Finally, a 7.1 m wide switchgrass (Panicumvirgatum L.) buffer is established upslope from theshrubs. Details of the MRB design, placement, andplant species are given in Schultz et al. (1995). Con-trols for the buffer site consists of cool season grasspasture strips that had been grazed until 1989 andwere allowed to grow following the removal of cattle(Schultz et al. 1995). The strip acts as a grass filter.Dominant grass species in the cool season grass siteswere smooth brome (Bromus inermis Leysser), timo-thy (Phleum pratense L.), and Kentucky bluegrass(Poa pratensis L.).

To compare the infiltration rate among the MRB,cultivated fields and continuously grazed pasture, sixtreatment sites were identified along the riparian zoneof Bear Creek. The six treatments were: 1) silver ma-ple, 2) switchgrass, 3) grass filter, 4) corn, 5) soybean,

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and 6) continuously grazed pasture. All the treatmentsites were located on Coland soil (fine-loamy, mixed,superactive, mesic Cumulic Endoaquoll). The silvermaple, switchgrass and control cool season pasturegrass sites were located in the established MRB site.The cultivation and grazing sites were located on ad-jacent farms. Once the treatment sites were located, a44 m long plot was marked in each of the treatmentsites. These plots were then divided into 24 1.8-mlong subplots.

Infiltration measurements were made three timesduring 1995 to establish changes over the growingseason. Eight measurements were made on each ofthe treatment sites during each of three time periods(20 June–30 June, 7 August–16 August, and 20 Oc-tober–10 November). Thus, for each measurement pe-riod, eight replicated infiltration measurements pereach site were made. These measurements wereblocked by day so that on each day, one measurementwas made at each of the six treatment sites. Subplotswere randomly selected for each day of testing fromthe 24 subplots at each site.

One ponded, falling head infiltration measurementwas conducted in each subplot using a double-ringinfiltrometer (702 cm2 area inside ring) following themethod outlined by Bouwere (1986). Rectangularshaped infiltrometers with the same area as the ringinfiltrometers were used to measure the infiltrationrate on the cultivated field sites. The infiltrometerswere pushed into the ground by placing a flat metal-lic lid or a wooden board on top and pounding with ahammer. The measurements were recorded by usingfloat-actuated stage recorders. Each measurement pe-riod lasted for at least 60 minutes. For many of thesites, the time was longer. Swartzendruber and Hog-arth (1991) stated that the pressure head of water pon-ded on the soil surface can increase the infiltration ofwater into soil. Therefore, the level of the water inthe infiltrometer was not allowed to fluctuate morethan approximately 5 cm by manually adding smallamounts of water. Sixty minutes cumulative infiltra-tion values were calculated by reading the change inwater level (cm) over time (min) from the stage re-corder.

Three surface soil samples from 0–7 cm depthwere collected from an undisturbed area immediatelyoutside of the infiltrometer to determine the bulk den-sity and soil water content preceding infiltration mea-surements. The volume of the probe used to collectthe samples was 244 cm3. The collected soil sampleswere placed in plastic lined soil bags, and transported

to the laboratory. Fifty gram sub-samples were takenfrom each of the soil samples and oven dried at 105°C for gravimetric water content and bulk density de-terminations (Blake and Hartge 1986). Because of theheterogeneous nature of soil, the 44-m plot at eachtreatment site was divided into thirds, and compositesamples were made from each area, for each of thesix treatment sites. Thus, there were a total of 18composite samples that were analyzed for particlesize by using the hydrometer method (Gee andBauder 1986).

The data were analyzed using analysis of variance(ANOVA), and differences were determined byT-tests (LSD), contrast analysis and correlation anal-ysis.

Results and discussion

60-min cumulative infiltration

There were significant treatment differences (P <0.05) between each of the three sampling periods forthe 60-min cumulative infiltration measurements.There were, however, no significant site treatment dif-ferences between individual testing days within anyof the three months. Fifty six percent of the variancefor the 60-min cumulative infiltration was due to thetreatment.

Because cumulative infiltration did not vary withinmonths across measurement days, for a given treat-ment, values were averaged across days (Figure 1).Between months, cumulative infiltration rate was sig-nificantly different for only the switchgrass treatment(P < 0.05). For all of the other treatments, mean cu-mulative infiltration did not significantly vary bymonth. The MRB (Silver maple, grass filter, andswitchgrass) had larger infiltration values than thecultivated fields and the grazed pasture (Figure 2).When statistical contrast analysis was used to test thedifferences between mean cumulative infiltration val-ues averaged across sampling periods from each ofthe six treatments, 60-min cumulative infiltration wasdifferent among silver maple, cool season grass, andswitchgrass treatments in the MRB with the order ofsilver maple > grass filter > switchgrass. 60-min cu-mulative infiltration was greater in the silver mapleand grass filter than in any of the crop fields or thegrazed pasture. Cumulative infiltration was greater inthe switchgrass than the crop fields or the pasture ex-cept for the soybean crop field. Cumulative infiltra-

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tion rates did not vary among any of the crop fieldsand the grazed pasture.

Since infiltration values are related to soil quality,these results show that this established MRB had apositive effect on soil quality by increasing infiltra-tion capacity by as much as five times after six grow-ing seasons. Cultivation of row crops and continuousgrazing reduced cumulative infiltration. The greatestdifferences occurred under the silver maple followedby the grass filter.

Soil organic matter is an important soil quality in-dicator because it has a strong relationship to criticalsoil functions like infiltration, productivity, erodibil-ity, and the capacity of the soil to act as an environ-mental buffer by absorbing or transforming potentialpollutants (Sikora and Stott 1996). The perennialgrass cover established under Conservation ReserveProgram (U.S. Department of Agriculture) at selectedlocations within the Great Plains has resulted in sig-nificant increases in soil organic carbon (Gebhar et al.1994). Bruce et al. (1992) reported that increasedphytomass input to a loamy sand increased aggregatestability and water infiltration. Soil structure improveswhen cultivated land is put into grass. Soil aggregatedistribution, stability, air permeability, and hydraulicconductivity improve with time in a grass culture(Lindstrom et al. 1998). In riparian buffers establishedon previously cultivated or heavily grazed soil, Mar-quez et al. (1999) found that soil organic matter in-

creased 8.5% over the original organic matter levelunder poplar (Populus X euramericana ‘Eugenei’)grown in association with cool season grass, and8.6% over the original organic matter level underswitchgrass (Panicum virgatum L.) over six growingseasons. Results obtained from this study are consis-tent with previous studies that show the influence ofvegetation and management systems on infiltration.Wood (1977) found that infiltration rates were greateron 14 of 15 undisturbed forest sites than in adjacentsites used for pastures, pineapple, or sugarcane pro-duction in Hawaii. In a study conducted by Meek etal. (1992), alfalfa roots were important in increasingthe infiltration rate by reforming macropores de-stroyed by tillage. Other studies also have demon-strated that perennial vegetation can increase infiltra-tion (Broersma et al. 1995).

Infiltration values under switchgrass were signifi-cantly (P < 0.05) smaller in August than in June andOctober/November measurements (Figure 2). Thelower cumulative infiltration under switchgrass maybe due to the higher density of living roots. In theMRB, root density of switchgrass reaches a peak inAugust and then declines in autumn (Tufekcioglu etal. 1999). Lesser root densities in spring (June) andfall (Oct/Nov) may be due to root death that leads toopening macropores. The effect of living roots onmacropore flow is not clear. Warner and Young (1991)using dye tracers showed that macropore flow was

Figure 1. Average 60-min cumulative soil water infiltration under multispecies riparian buffer (MRB) vegetation communities (silver maple,grass filter, and switchgrass), two cropping treatments (soybean and corn), and a continuously grazed pasture site in Story County, IA. Errorbars indicate standard error. (n = 8)

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occurring along living roots of corn plants. Gish andJury (1981), however, hypothesized that root growthmay initially decrease infiltration by compacting soiland obstructing existing macropores. Meek et al.(1992) measured increases in infiltration rate thatwere related to decreases in stand density under al-falfa.

Small infiltration found in the cultivated field treat-ments and the continuously grazed pasture could be aresult of the break-up of soil structure by cultivationand compaction from farm implements and grazinganimals. The Coland soil, on which all treatmentswere located is a floodplain soil. Taboada and Lavado

(1993) found that trampling often decreases porosityin grassland top soils because of the stress caused byanimal hooves. However, knowledge of how differ-ent pore sizes are affected by trampling is limited.Macroporosity which influences infiltration seems tobe more affected by trampling than total porosity (Ta-boada and Lavado 1993)).

According to Mukhtar et al. (1985), infiltration ofwater into the soil can vary during the crop growthperiod and one of the factors that can substantiallydecrease soil water intake is surface sealing. Soilswithout residue cover or with little crop canopy aremore susceptible to surface sealing due to raindrop

Figure 2. Average cumulative infiltration in June, August, and October/November under multispecies riparian buffer (MRB) vegetation com-munities (silver maple, grass filter, and switchgrass), two cropping treatments (soybean and corn), and a continuously grazed pasture site inStory County, IA. (n = 8)

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impact than those with a crop canopy (Mukhtar et al.1985). In this study, at the time of the first measure-ment period (June), the corn and soybean had alreadybeen planted in the fields and in the last measurementperiod (October/November) the crops had just beenharvested. Rearrangement of soil particles or filtrationof finer particles into soil pores can decline the infil-tration, as water moves into the soil profile (Pikul andAase 1995). Such soil sealing may have been a factorthroughout this period because no cultivation wasconducted during the measurement periods. As a re-sult, there were no significant differences among thethree measurement periods (June, August, Oct/Nov)for the cultivated fields.

Bulk density, moisture content, and particle size

As the bulk density increased, infiltration decreased,except under silver maple (Figure 3). 82% of the var-iance in bulk density was due to treatment. The bulkdensity and infiltration values were averaged over thethree sampling periods because 60-min cumulativeinfiltration curves were similar among sampling peri-ods. Results from previous studies also have demon-strated the inverse relationship. Goldhammer andPeterson (1984) found that the major effect of soilcompaction in an irrigated sandy loam soil was thereduction in infiltration rate. Meek et al. (1992), us-ing a course textured soil, measured four fold de-creases in infiltration rate when traffic compacted thesoil from a bulk density of 1.7 to 1.89 g cm−3. Paststudies also have found that bulk density of cultivatedfields and grazed pastures are generally greater thanthose of native grassland or forest soils (Jaiyeoba1995; Meek et al. 1992; Taboada and Lavado 1993).The results from this study show that removing live-stock from pastures and establishing perennial plantcommunities on previously cultivated fields can de-crease bulk density significantly after six growingseasons. The bulk density under the grass filter andswitchgrass was not significantly different (P < 0.05).However, the bulk density under the silver maple wassignificantly greater than both under the grass filterand switchgrass (P < 0.05). Jaiyeoba (1995) found asimilar increase in soil bulk density under Eucapyp-tus camaladulensisand Mangifera indica(mango)plantations and also under arable and fallow field con-ditions when compared with areas under natural sa-vanna vegetation in Nigeria.

It is interesting to note that soils under silver maplehave not only the largest bulk density of the MRB but

also the highest cumulative infiltration. The silver ma-ple treatment may have a higher bulk density than theswitchgrass and the grass filter because of a six per-cent higher sand content (Table 1). Coarse-texturedsoils are generally more dense than fine-textured soils(Hille 1982). Bulk density and soil texture greatly af-fect piston-like flow of water through soil. Soil bulkdensity however, has little or no relationship to waterflow through macropores (Meek et al. 1992). In thisstudy, the large infiltration found under the silver ma-ple might be due to the large number of macroporesassociated with earthworm activity (personal observa-tion). Large numbers of earthworm pores can be seenespecially in fall under silver maple because theworms collect and pull leaves into the pores leavingpetioles extending above the ground (personal obser-vation).

Infiltration rates under silver maple might also begreater because of soil moisture conditions. Gravi-metric moisture contents were significantly different(P < 0.05) among treatments as well as among mea-surement times.

Gravimetric moisture contents of the grass filterand the switchgrass were consistently larger than un-der the crop fields and the pasture. Gravimetric mois-ture contents in the silver maple treatment were sig-nificantly smaller than in the grass filter andswitchgrass treatments during all measurements (Ta-ble 2). Wood (1977) found that undisturbed forestsoils had greater detention storage associated withmacropores at all soil depths than abandoned farmsoil, but the latter had greater retention storage asso-

Figure 3. Sixty minute cumulative infiltration rates vs. bulk den-sities under multispecies riparian buffer (MRB) vegetation commu-nities (silver maple, grass filter, and switchgrass), two croppingtreatments (soybean and corn), and a continuously grazed pasturesite in Story County, IA. (n = 24)

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ciated with micropores at the 15–45 cm depths. Largeevapotranspiration rates in forests might also accountfor the relatively small moisture values. Investigatorshave noted a rise in the soil water table following theremoval of the forest canopy, due to reduction inevapotranspiration (Trousdell and Hoover 1955).

Summarized results from particle size analysis arepresented in Table 1. Even though all the sites weremapped as the same soil mapping unit, results fromparticle size analysis showed that the treatment siteshad significantly different textures (P < 0.05). Thus,even though particle size distribution probably had aninfluence on these parameters, treatment influenceshad a greater effect. The high percentage of sand un-der pasture might account for the low moisture con-tent (Table 1). These results emphasize the heteroge-neity of depositional soils and that this heterogeneityhas to be taken into account when comparisons forinfiltration, bulk density, and moisture are made. Such

findings reaffirm the conclusions of Hammer et al.(1987) and others, that a stratified sampling designmay be very important in quantifying soil quality pa-rameters and the effects of land management. As con-cluded by Stolt et al. (2001), such spatial variabilityis important to consider when assessing the environ-mental and ecological functions of depositional soils.

Conclusions

Results showed that soil infiltration capacities aregenerally greater under the established MRB than un-der cultivated fields and grazed pasture. The average60-min cumulative infiltration rates from the MRBwere five times greater than from the cultivated fieldsand pasture. Cumulative infiltration was not signifi-cantly different between the crop fields and the grazedpastures. However, cumulative infiltration was signif-icantly different among each of the three MRB veg-etation with an order of silver maple > grass filter >switchgrass. While the grass filter had higher infiltra-tion rates during this study, Corre et al. (1999) sug-gested that switchgrass may need up to 15 years be-fore it has modified soil quality to its full potential.

The bulk densities of the cultivated field and thegrazed pasture were significantly larger than that ofthe MRB vegetation. Among the MRB vegetation, thegrass filter and the switchgrass had smaller bulk den-sity values than the silver maple site. Except in thesilver maple site, 60-min cumulative infiltrationshowed an inversely proportional relationship withbulk density. The large infiltration found in the silvermaple site may be due to the presence of a greaterpercentage of sand and a large number of macroporesformed through decayed roots and soil fauna activity.

Gravimetric soil moisture content varied by treat-ments and measurement time. However, results didnot show a strong correlation between infiltration andsoil moisture.

The soil moisture contents of the grass filter andthe switchgrass treatments were consistently higherthan those of the crop fields and the pasture. The sil-ver maple treatment had a smaller soil moisture con-tent than all but the pasture and soybean field. Thelower soil moisture content under the silver maplecould be due to the greater absorption of soil waterthrough the profile by the well-developed roots underthe silver maples.

This study was conducted with the assumption thatthe impact of different vegetative and management

Table 1. Mean particle size distribution of duplicate samples takenfrom composite soil samples within multispecies riparian buffervegetation communities (silver maple, grass filter, and switch-grass), two cropping treatments (soybean and corn), and a continu-ously grazed pasture site in Story County, IA.

Treatment Particle size distribution (%)

Sand Silt Clay

(2–0.05 mm) (0.05–0.002 mm) (< 0.002 mm)

Silver maple 38.0 44.2 17.8

Grass filter 32.5 46.3 21.2

Switchgrass 32.7 48.8 18.5

Soybean 38.3 35.2 26.5

Corn 42.6 41.7 15.7

Pasture 58.0 25.7 16.3

Each value represents the average of 72 individual samples.

Table 2. Mean gravimetric moisture content (�m) of top 7-cm soilfor the three time periods in 1995 within multispecies riparianbuffer vegetation communities (silver maple, grass filter, andswitchgrass), two cropping treatments (soybean and corn), and acontinuously grazed pasture site in Story County, IA.

Treatment/Time June August Oct./Nov.

�m

Silver maple 0.24 (0.06)@ 0.30 (0.04) 0.21 (0.06)

Grass filter 0.33 (0.07) 0.39 (0.03) 0.34 (0.03)

Switchgrass 0.37 (0.12) 0.40 (0.02) 0.39 (0.02)

Soybean 0.27 (0.05) 0.33 (0.04) 0.29 (0.02)

Corn 0.24 (0.11) 0.30 (0.05) 0.25 (0.02)

Pasture 0.18 (0.07) 0.24 (0.06) 0.19 (0.06)

@ Values in parentheses are standard deviations.

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system treatments is reflected in the infiltration pro-cess, and that infiltration is a good indicator of ‘soilquality’. The results show that the 6-year old MRB,through its influence on infiltration, has a positive in-fluence on riparian zone soil quality that may be ef-fective in the reduction of NPS pollution in agroeco-systems.

Acknowledgements

Journal Paper No. J-18963 of the Iowa Agricultureand Home Economics Experiment Station, Ames,Iowa, Project No. 3605, and supported by Hatch Actand State of Iowa funds. This material was preparedwith the support of a grant from Agriculture in Con-cert with the Environment (ACE) program, which isjointly funded by the USDA, Cooperative State Re-search, Education and Extension Service, and theUSEPA under Cooperative Agreement No. 94-COOP-1-0809. The research also has been funded in part bythe Iowa DNR through a grant from the USEPA un-der the Federal Non-point Sources Management Pro-gram (Section 319 of the Clean Water Act), by theLeopold Center for Sustainable Agriculture, a State ofIowa Institution located at Iowa State University, andthe USDA National Research Initiative CompetitiveGrants Program award No. 95-37192-2213.

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