Soil-water infiltration under crops, pasture, and established riparian ...
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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:firstname.lastname@example.org;)
Received 25 July 2002; accepted in revised form 8 August 2002
Key words: Conservation buffer, Filter strip, Riparian forest buffer, Soil quality, Switchgrass
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.
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: 249257, 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 h1) 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 (4211 N, 9330 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,
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 June30 June, 7 August16 August, and 20 Oc-tober10 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 07 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 105C 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 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-
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 onmacropo...