Central Minnesota Research Articles

Integrated management of edaphic and biotic factors limiting yield of irrigated soybean and dry bean in Minnesota

Root rot of dry bean ( Phaseolus vulgaris L.) caused by Fusarium solani f. sp. phaseoli in a complex with Rhizoctonia solani and Fusarium oxysporum is an increasingly important problem for dry bean and soybean producers in central Minnesota. Shortened rotation intervals, reduced tillage, use of susceptible cultivars, and soil compaction are all factors that may be contributing to a more favorable environment for the development of root rots caused by these pathogens. A Bt horizon, characteristic of sandy soils in central Minnesota is exacerbated by reduced tillage, impairs root development and contributes to plant stress. The objective of this research was to determine if reduced tillage or moldboard plowing and seed treatment with a peat formulation of Bacillus subtilis and either Rhizobium leguminosarum (dry bean) or Bradyrhizobium japonicum (soybean) would reduce root rot severity and allow increased yield of dry bean and soybean ( Glycine max L.). The study consisted of experiments planted at Staples in 2000, and Staples and Verndale in 2001. Dry bean and soybean were planted in adjacent experiments arranged in a split-plot design with tillage as the main plot and seed treatment as the subplot. The soil type at both sites is a Verndale sandy loam. The impedance layer was located at a shallower depth at Staples than at Verndale. Mean cone index values in reduced tillage plots exceeded 2500 Pa from 60 to 300 mm in the soil profile at Staples in 2000, and from 105–300 to 165–300 mm at Staples and at Verndale, respectively in 2001. Moldboard plowing reduced cone index to less than 2000 Pa in the upper 300 mm of the soil profile at Staples in 2000 and 2001 and the upper 285 mm of the soil profile at Verndale in 2001. Gravimetric soil moisture at 15–22 cm depth was 4 and 6% less in moldboard plowing than reduced tillage in 2000 and 2001, respectively. Dry bean yield was greater in moldboard plowing than reduced tillage at Staples in 2000 and 2001 (1751 vs. 1148 and 653 vs. 495 kg ha −1, respectively) and at Verndale in 2001 (2003 vs. 1395 kg ha −1, respectively). Seed inoculation with R. leguminosarum and B. subtilis increased dry bean yield by 11% at Staples in 2000 and 16% in 2001. Soybean yields were not increased significantly by moldboard plowing. At Verndale in 2001, seed inoculation with B. subtilis and B. japonicum increased soybean yields from 2203 to 3021 kg ha −1. Seed treatment did not produce a consistent effect on disease severity in dry bean at Staples in 2000 or at Verndale in 2001. However, seed treatment decreased disease severity in dry bean at Staples in 2001 and in soybean at Staples and Verndale in 2001. The effect of the treatment factors, inoculation with B. subtilis and Rhizobium spp. and tillage, differed among crops, sites, and environments. A combination of management practices such as deep tillage, and seed inoculation with B. subtilis and Rhizobium spp. will be necessary to restore and maintain optimum dry bean and soybean productivity.

Why do diners tip: rule-of-thumb or valuation of service?

When diners decide how much to tip, is the decision based on social convention or on conscientious appraisal of server productivity? Previous researchers in economics and social psychology are generally inconclusive on this question. A common finding in the literature is that tip size and service quality are unrelated, a result usually obtained from OLS regressions. OLS is only appropriate if service quality is exogenous. It is argued that service quality is very likely endogenous in any regression of tip size; good quality encourages good tips, but server expectations of good tips encourage good quality. This simultaneity is accounted for by jointly estimating percentage tips and customer rankings of service quality on a sample of 247 diners in a Central Minnesota restaurant. Included are explanatory variables consistent with both the social psychology and economic views of tipping. In contrast to previous studies, it is found that service quality significantly affects tip size and when servers expect higher tips, customers rank service quality higher. Also it is found that patronage frequency and coupon redemption have no effect on percentage tips, but server gender influences quality significantly. It is concluded that the results are generally supportive of an economic hypothesis of tipping.

Characterization of lake water and ground water movement in the littoral zone of Williams Lake, a closed‐basin lake in north central Minnesota

AbstractWilliams Lake, Minnesota is a closed‐basin lake that is a flow‐through system with respect to ground water. Ground‐water input represents half of the annual water input and most of the chemical input to the lake. Chemical budgets indicate that the lake is a sink for calcium, yet surficial sediments contain little calcium carbonate. Sediment pore‐water samplers (peepers) were used to characterize solute fluxes at the lake‐water–ground‐water interface in the littoral zone and resolve the apparent disparity between the chemical budget and sediment data. Pore‐water depth profiles of the stable isotopes δ18O and δ2H were non‐linear where ground water seeped into the lake, with a sharp transition from lake‐water values to ground‐water values in the top 10 cm of sediment. These data indicate that advective inflow to the lake is the primary mechanism for solute flux from ground water. Linear interstitial velocities determined from δ2H profiles (316 to 528 cm/yr) were consistent with velocities determined independently from water budget data and sediment porosity (366 cm/yr). Stable isotope profiles were generally linear where water flowed out of the lake into ground water. However, calcium profiles were not linear in the same area and varied in response to input of calcium carbonate from the littoral zone and subsequent dissolution. The comparison of pore‐water calcium profiles to pore‐water stable isotope profiles indicate calcium is not conservative. Based on the previous understanding that 40–50 % of the calcium in Williams Lake is retained, the pore‐water profiles indicate aquatic plants in the littoral zone are recycling the retained portion of calcium. The difference between the pore‐water depth profiles of calcium and δ18O and δ2H demonstrate the importance of using stable isotopes to evaluate flow direction and source through the lake‐water–ground‐water interface and evaluate mechanisms controlling the chemical balance of lakes. Published in 2003 by John Wiley & Sons, Ltd.

Growth and yield response of Cuphea to row spacing

Presently, in USA there is no domestic crop source of oil rich in medium chain fatty acids (MCFAs) (i.e., oils composed of triglycerides with fatty acid chains between 8 and 14 carbons long). Several species of the genus Cuphea (Lythraceae) accumulate saturated MCFAs in their seed-storage lipids, and some species grow well in short-season temperate climates. Recent efforts have been successful in semi-domesticating genotypes of Cuphea for crop production. However, little is known about best management practices for production. A 2-year study was conducted in west central Minnesota, USA, to determine optimum inter-row spacing for row culture production of Cuphea. Seed was sown in inter-row spacings of 0.125, 0.25, 0.375 and 0.50 m in 1999, and a fifth spacing of 0.75 m was added in 2000. In 2000, seed yield averaged nearly 1000 kg ha −1, which was about 40% greater than in 1999. Seed yields were not significantly affected by row spacing. Plants in wider rows compensated for yield by producing more branches and seed pods per plant. The number of filled pods per plant was as much as 70% greater for plants in the widest as compared to the narrowest row spacing. Data indicate that this was due in part to competition among plants for available light and nutrient resources. Row culture of Cuphea in the northern Corn Belt of the US appears favorable. However, due to its indeterminate habit, growth and yield of Cuphea in row culture may be more responsive to plant population density than inter-row spacing.

RED-NECKED GREBES BECOME SEMICOLONIAL WHEN PRIME NESTING SUBSTRATE IS AVAILABLE

Abstract During 1995–1998 we studied the dynamics of nesting dispersion in a population of 160–200 Red-necked Grebes (Podiceps grisegena) on Lake Osakis, a large, windblown lake in central Minnesota. Breeding grebe pairs arriving on the lake were highly attracted to several large floating cattail (Typha angustifolia) mats that had broken off and drifted away from the mainland to become “islands” snagged within rooted emergent beds. These mats provided prime nesting habitat protected from waves and predators that was chosen by nearly 40% of the breeding population. Most pairs establishing nests on the islands averaged 5–10 simultaneously active neighbors within 50 m, compared to pairs that established territories along the shoreline, most of which had no neighbors within 50 m. The overall success of the two strategies varied widely from year to year, depending on the water levels, nesting habitat availability, timing of windstorms, and predator activity. During most years, however, pairs that established n...

Interrelationships among Production, Density, Growth, and Mortality of Northern Pike in Seven North-Central Minnesota Lakes

Abstract Secondary production can be an important indicator of performance for a fish population. In this study we examined how density, growth, and mortality operated together to alter production of northern pike Esox lucius in seven north–central Minnesota lakes. Estimated densities of age-2 and older northern pike were 3–59 fish/ha and estimated production rates were 0.77–8.31 kg·ha−1·year−1 across the seven lakes. The relationship between density and production among the lakes was curvilinear, production being moderated by slow growth rates at high northern pike densities. The negative relationship between indices of individual fish growth and production was counterintuitive and demonstrated the overwhelming influence of density on northern pike population dynamics. Relationships between total mortality and either density or production were not apparent. Recreational fisheries in Minnesota appear to be harvesting much of the annual production of northern pike. Yet many northern pike populations are ma...

A regional survey of malformed frogs in Minnesota (USA) (Minnesota malformed frogs).

In late 1995, school children discovered malformed frogs in a south central Minnesota pond. Press coverage resulted in numerous citizen reports of frog malformation across Minnesota in 1996. After some initial site investigation, 3 affected frog sites and 4 nearby reference sites were selected for more detailed evaluation. Field biologists made 89 visits to study sites beginning spring 1997 through fall 1999 to examine the number and type of frog malformations. Over 5,100 Leopard frogs (Rana pipiens) were captured and examined at all study sites. Water elevations and associated littoral inundation were recorded from 1997-2000. Results indicate that malformation occurred at all study sites above historical background levels. Rana pipiens malformation across all sites over three seasons averaged 7.9% and ranged from 0 to 7% at reference sites and 4 to 23% at affected sites. At one northern Minnesota site, mink frog (Rana septentrionalis) malformation was 75% in 1998. A site characteristic common to the most affected sites was an elastic zone of littoral inundation. Climate driven hydrologic variation likely influenced water depth and associated breeding locations.

Development of a Fish-Based Index of Biotic Integrity for Small Inland Lakes in Central Minnesota

We developed a fish-based index of biotic integrity (IBI) for a set of Minnesota lakes having similar geophysical and chemical features. Fish data were collected by means of trap nets, gill nets, shoreline seines, and backpack electrofishing. Of 30 evaluated metrics, we identified 16 metrics of three types: species richness, community assemblage, and trophic composition. In contrast to lotic IBIs, where a single sampling gear is usually used, data from all four sampling gears were necessary for IBI development. We selected metrics based on responses to measures of human-induced stress based on watershed land use patterns and human population density. Species richness and community composition metrics describing intolerant or habitat specialist species were most sensitive to differences in human-induced stress. Because these species were found in the nearshore zone of lakes, effective sampling of the nearshore fish community was essential to the development and performance of the IBI. Trap-net- and gill-net-based metrics, however, were essential to the development of trophic composition metrics, the trap nets providing the best insectivore and omnivore metric and the gill nets providing the best top carnivore metrics. Lake IBI scores reflected differences in land use patterns, trophic state, and aquatic vegetation. Sensitivity analysis indicated that selected metrics contributed to IBI performance, but the relative contribution of individual metrics varied among human-induced stress categories. Additional work is needed to test the performance of this IBI on an independent group of lakes, to set the range of applicability, and to determine the effectiveness of the IBI as an assessment tool for lakes.

Estimating Aquifer Sensitivity to Nitrate Contamination Using Geochemical Information

AbstractMethods for predicting aquifer sensitivity to contamination typically ignore geochemical factors that affect the occurrence of contaminants such as nitrate. Use of geochemical information offers a simple and accurate method for estimating aquifer sensitivity to nitrate contamination. We developed a classification method in which nitrate‐sensitive aquifers have dissolved oxygen concentrations > 1.0 mg/L, Eh values >250 mV, and either reduced iron concentrations < 0.1 mg/L or total iron concentrations < 0.7 mg/L. We tested the method in four Minnesota aquifer systems having different geochemical and hydrologic conditions. A surficial sand aquifer in central Minnesota exhibited geochemical zonation, with a rapid shift from aerobic to anaerobic conditions 5 m below the water table. A fractured bedrock aquifer in east‐central Minnesota remained aerobic to depths of 50 m, except in areas where anaerobic ground water discharged upward from an underlying aquifer. A bedrock aquifer in southeast Minnesota exhibited aerobic conditions when overlain by surficial deposits lacking shale, whereas anaerobic conditions occurred under deposits that contained shale. Surficial sand aquifers in northwest Minnesota contained high concentrations of sulfate and were anaerobic throughout their extent. Nitrate‐nitrogen was detected at concentrations exceeding 1 mg/L in 135 of 149 samples classified as sensitive. Nitrate was not detected in any of the 109 samples classified as not sensitive. We observed differences between our estimates of sensitivity and existing sensitivity maps, which are based on methods that do not consider aquifer geochemistry. Because dissolved oxygen, reduced iron, and Eh are readily measured in the field, use of geochemistry provides a quick and accurate way of assessing aquifer sensitivity to nitrate contamination.

Yield and Growth Response of Cuphea to Sowing Date

ABSTRACTSelect germplasm of Cuphea, developed from an interspecific hybridization of C. viscosissima Jacq. and C. lanceolata f. Silenoides W.T. Aiton, shows good potential for commercial production in short‐season temperate climates. Cuphea seed oil could serve as a domestic source of medium‐chain fatty acids, which are in high demand by the chemical manufacturing industry. However, little is known about proper management practices for Cuphea agronomic production. A field study was conducted in west central Minnesota to determine optimum time of sowing in the northern Corn Belt region and describe influences of sowing date on growth and seed yield components of a semidomesticated germplasm line (PSR23). Seed was sown on 15 April, 1 May, 15 May, 1 June, and 15 June 1999 and 2000. Sowing in May resulted in greatest seed and seed‐oil yields, which were as high as 1.09 and 0.29 Mg ha−1, respectively. Seed yield declined as much as 31% when sown 15 April and 65% when sowing was delayed until 15 June. Plants developed from seed sown before June tended to form more branches and accumulated a greater amount of aboveground biomass. Typically, there was a distinct decrease in plant biomass accumulation and most seed yield components when sowing date was delayed until June. Cuphea PSR23 can be successfully grown in the northern Corn Belt, with early to mid May being the best time for sowing.

Using chemical, hydrologic, and age dating analysis to delineate redox processes and flow paths in the riparian zone of a glacial outwash aquifer‐stream system

A combination of chemical and dissolved gas analyses, chlorofluorocarbon age dating, and hydrologic measurements were used to determine the degree to which biogeochemical processes in a riparian wetland were responsible for removing NO3− from groundwaters discharging to the Otter Tail River in west central Minnesota. An analysis of river chemistry and flow data revealed that NO3− concentrations in the river increased in the lower half of the 8.3 km study reach as the result of groundwater discharge to the river. Groundwater head measurements along a study transect through the riparian wetland revealed a zone of groundwater discharge extending out under the river. On the basis of combined chemical, dissolved gas, age date, and hydrologic results, it was determined that water chemistry under the riparian wetland was controlled largely by upgradient groundwaters that followed flow paths up to 16 m deep and discharged under the wetland, creating a pattern of progressively older, more chemically reduced, low NO3− water the farther one progressed from the edge of the wetland toward the river. These findings pose challenges for researchers investigating biogeochemical processes in riparian buffer zones because the progressively older groundwaters entered the aquifer in earlier years when less NO3− fertilizer was being used. NO3− concentrations originally present in the groundwater had also decreased in the upgradient aquifer as a result of denitrification and progressively stronger reducing conditions there. The resulting pattern of decreasing NO3− concentrations across the riparian zone may be incorrectly interpreted as evidence of denitrification losses there instead of in the upgradient aquifer. Consequently, it is important to understand the hydrogeologic setting and age structure of the groundwaters being sampled in order to avoid misinterpreting biogeochemical processes in riparian zones.

Response of russet norkotah clonal selections to nitrogen fertilization

The low vine vigor and high N requirement of Russet Norkotah may lead to N loss and groundwater contamination on coarse-textured soils. Recent clonal selections from Texas have produced strains that have larger and stronger vines, which may alter N requirements. This twoyear study examined the N use efficiency (NUE), yield, and quality of Russet Norkotah strain selections fertilized with different N levels on a Hubbard loamy sand in central Minnesota. The selections, Texas Norkotah Strain (TXNS) 112, TXNS 223, and TXNS 278 were grown with standard Russet Norkotah under irrigated conditions and received total N levels of 28, 112, 224, or 336 kg ha-1. Total, marketable, and large (>340 g) tuber yields increased linearly (P>0.05) with rate of N application in 1998 but not in 1997. The genotype main effect was not significant for any of the tuber yield parameters measured based on fresh weight. Vine, tuber, and total dry biomass yields were 116%, 5.8%, and 13.2%, respectively, higher with the selections than Russet Norkotah. Harvest index (HI), or the proportion of total dry matter partitioned to tubers, was 7% greater for Russet Norkotah than the TXNS selections, reflecting the larger vine growth of the selections. The selections accumulated significantly higher N in the vines (0.113 kg kg-1 N) than the standard clone (0.053 kg kg-1 N) as N rate increased from 28 to 336 kg ha-1, and the difference between the selections and the standard clone was larger at higher N rates than at lower N rates. Russet Norkotah partitioned 10% more N to tubers than did the TXNS selections, reflecting the difference in HI between the standard cultivar and its clones. Nitrogen recovered from fertilizer N applied in addition to the 28 kg ha-1 starter N (NUE28) averaged 36% and varied little with genotype, N rate, or cropping year. Biomass accumulation from similar N additions (AUE), however, was significantly higher for the selections than Russet Norkotah at 112 kg N ha-1 in 1997 only. At low N rate (112 kg ha-1), the selections had higher physiological use efficiency (PUE28) (mean 45.9 g g1) than Russet Norkotah (25.9 g g1). Results from this study demonstrate that, although N recovery was similar for the four genotypes, the Texas Norkotah strains produced greater biomass than Russet Norkotah per kg N applied at low rate in 1997 and per kg of fertilizer N absorbed by the plant in both years. However, under the conditions of this study, higher biomass of the selections did not translate into a marketable yield advantage over the standard cultivar.

Harvest caused soil disturbance decreased suckering capacity of quaking aspen ( Populus tremuloides Michx.) following growing season harvests in Minnesota, USA

The effect of soil disturbance on quaking aspen ( Populus tremuloides Michx.) sucker density and height growth was measured in mixed aspen and hardwood stands in east central Minnesota, USA. Loggers harvested stands during the growing season resulting in a range of soil disturbance conditions. Soil disturbance and parent stand aspen density accounted for a significant amount of the variation in suckering two full growing seasons after the harvest. Mineral soil exposure did not significantly affect total sucker density, but decreased sucker height growth as measured by the cumulative height of all suckers in a plot (sucker length) and density of suckers more than 1.2 m tall. Both shallow (<5 cm) and deep (5–15 cm and >15 cm) soil disturbance caused by machine traffic reduced sucker density and sucker length. Results indicated an effect of machine traffic on suckers in trafficked areas, but not adjacent areas. The long-term effects of reduced height growth in disturbed areas and severely reduced density in trafficked areas are not well understood. Loss in early height growth is likely to have long-term effects on productivity, and it is unlikely that aspen will recolonize rutted areas in time to recover full aspen productivity in trafficked areas in one rotation.

Feeding Habitat Selection by Great Blue Herons and Great Egrets Nesting in East Central Minnesota

Abstract Great Blue Herons (Ardea herodias) and Great Egrets (Casmerodius albus) partitioned feeding habitat based on wetland size at Peltier Lake rookery in east central Minnesota. Great Blue Herons preferred large waterbodies (350 ha), whereas Great Egrets fed most often at small ponds (<25 ha). Forty-nine percent of Great Blue Herons used wetlands 301-400 hectares in size and 83% of Great Egrets fed in wetlands <100 ha in size. Great Blue Herons selected large wetlands more often than expected both at the regional (30-km radius) and local (4-km radius) scales. Habitat use by Great Egrets was in proportion to availability at the regional scale, but they selected smaller wetlands for feeding more often than expected at a local scale. The median flight distance of Great Blue Herons was 2.7 km, similar to distances reported elsewhere. Great Egrets flew farther to feeding sites than Great Blue Herons, and flew farther (median = 13.5 km) than reported in other geographic areas.

Intergenerational family relationships of older women in central Minnesota

This paper applies the family lifecourse perspective to the lives of rural older women in the Midwestern United States based on the findings of the Rural Older Women’s Project, an ethnographic study of the daily lives and systems of support of 30 women. The focus is on the relationships with children, grandchildren, and parents if they are still living, of rural older women in central Minnesota. Three case studies are used to demonstrate the full range of circumstances. From those who are still actively providing support and assistance to their own parents, children, and grandchildren, to those who are primarily the recipients of care and support, to those who are unable to receive the necessary assistance from their family system. These variations are significant in terms of the ways in which each of them face the increasing needs which often come with ageing.

The relationships among root and leaf traits of 76 grassland species and relative abundance along fertility and disturbance gradients

For 76 annual, biennial, and perennial species common in the grasslands of central Minnesota, USA, we determined the patterns of correlations among seven organ‐level traits (specific leaf area, leaf thickness, leaf tissue density, leaf angle, specific root length, average fine root diameter, and fine root tissue density) and their relationships with two traits relating to growth form (whether species existed for part of the growing season in basal, non‐caulescent form and whether species were rhizomatous or not). The first correlation of traits showed that grasses had thin, dense leaves and thin roots while forbs had thick, low‐density leaves and thick roots without any significant differences in growth form or life history. The second correlation of traits showed a gradient of species from those with high‐density roots and high‐density erect leaves to species with low‐density roots and low‐density leaves that were held parallel to the ground. High tissue density species were more likely to exist as a basal rosette for part of the season, were less likely to be rhizomatous, and less likely to be annuals. We examined the relationships between the two axes that represent the correlations of traits and previously collected data on the relative abundance of species across gradients of nitrogen addition and disturbance. Grasses were generally more abundant than forbs and the relative abundance of grasses and forbs did not change with increasing nitrogen addition or soil disturbance. High tissue density species became less common as fertility and disturbance increased.

Stream flow and ground water recharge from small forested watersheds in north central Minnesota

In hydrologic studies of forested watersheds, the component of the water balance most likely to be poorly defined or neglected is deep seepage. In the complex glaciated terrain of the northern Lake States, subsurface water movement can be substantial. On the Marcell experimental forest (MEF) in north-central Minnesota, ground water table elevations measured in observation wells in recharge areas were used to calculate rates of ground water recharge. In northern Minnesota winters, precipitation is stored on the surface as snow and ground water recharge ceases. Water table elevations in recharge areas decline over winter at calculable rates. Deviations from these rates during other times of the year are due to ground water recharge. On 10–50ha watersheds on the MEF, ground water recharge varies among watersheds but constitutes about 40% of the total water yield. Annual ground water recharge amounts were found to vary linearly with precipitation. Even in high precipitation years, the infiltration capacity of the watersheds was not exceeded. Regression equations were developed relating yearly ground water recharge, stream flow, and total water yield, to seasonal precipitation amounts, summer and autumn precipitation during the previous year, and non-winter air temperature.

THE CONCEPT OF HYDROLOGIC LANDSCAPES1

ABSTRACT: Hydrologic landscapes are multiples or variations of fundamental hydrologic landscape units. A fundamental hydrologic landscape unit is defined on the basis of land‐surface form, geology, and climate. The basic land‐surface form of a fundamental hydrologic landscape unit is an upland separated from a lowland by an intervening steeper slope. Fundamental hydrologic landscape units have a complete hydrologic system consisting of surface runoff, ground‐water flow, and interaction with atmospheric water. By describing actual landscapes in terms of land‐surface slope, hydraulic properties of soils and geologic framework, and the difference between precipitation and evapotranspiration, the hydrologic system of actual landscapes can be conceptualized in a uniform way. This conceptual framework can then be the foundation for design of studies and data networks, syntheses of information on local to national scales, and comparison of process research across small study units in a variety of settings. The Crow Wing River watershed in central Minnesota is used as an example of evaluating stream discharge in the context of hydrologic landscapes. Lake‐research watersheds in Wisconsin, Minnesota, North Dakota, and Nebraska are used as an example of using the hydrologic‐land‐scapes concept to evaluate the effect of ground water on the degree of mineralization and major‐ion chemistry of lakes that lie within ground‐water flow systems.

Genetic differentiation among populations of house flies (Diptera: Muscidae) breeding at a multiple-barn, egg-laying facility in central Minnesota.

Mitochondrial gene diversity was used in house fly, Musca domestica L., populations to examine gene flow within and among 16 sealed barns in a large egg-laying facility in Renville, MN. Haplotypes in poultry barns were compared with those in outdoor house fly populations nearby and in St. Paul, MN. Haplotype diversities were greater in the closed than in the open populations. There was significant gene flow among poultry barns, and export of flies from barns was observed. Nevertheless, of three haplotypes detected in the closed populations, one was undetected in the open populations. A significant change in haplotype frequencies within poultry barns between years is attributed to genetic drift. The geographical origin of one haplotype is obscure.

EFFECTS OF TOPOGRAPHY AND SOIL PROPERTIES ON RECHARGE AT TWO SITES IN AN AGRICULTURAL FIELD1

ABSTRACT: Field experiments were conducted from 1992 to 1995 to estimate ground water recharge rates at two sites located within a 2.7‐hectare agricultural field. The field lies in a sand plain setting in central Minnesota and is cropped continuously in field corn. The sites are located at a topographically high (upland) site and a topographically low (lowland) site in an effort to quantify the effects of depression focusing of recharge. Three site‐specific methods were used to estimate recharge rates: well hydrograph analysis, chlorofluorocarbon age dating, and an unsaturated zone water balance. All three recharge methods indicated that recharge rates at the lowland site (annual average of all methods of 29 cm) exceeded those at the upland site (annual average of 18 cm). On an annual basis, estimates by the individual methods ranged from 12 to 44 percent of precipitation at the upland site and from 21 to 83 percent at the lowland site. The difference in recharge rates between the sites is primarily attributed to depression focusing of surface water runon at the lowland site. However, two other factors were also important: the presence of thin lamellae at the upland site, and coarser textured soils below a depth of 1.5 m at the lowland site.