Mesic Typic Haplaquoll Research Articles

Exchangeable Cation Hydration Properties Strongly Influence Soil Sorption of Nitroaromatic Compounds

Nitroaromatic compounds (NACs) are commonly found as soil contaminants in military training sites and manufacturing facilities, and may adversely affect human and ecosystem health. Exchangeable cation effects on p‐nitrocyanobenzene (p‐NCB) and 1,4‐dinitrobenzene (1,4‐DNB) sorption by the Webster (fine loamy, mixed, mesic Typic Haplaquoll) soil series A (WA) and B (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant mineral in the horizons of this soil. Soil organic matter (SOM) removal increased p‐NCB sorption to the WA and WB horizons by ∼1.5 times, and increased 1,4‐DNB sorption to the A and B horizons by ∼1.2 to 2.2 times, respectively. Greater sorption of NACs by whole soils after SOM removal suggests that SOM suppresses p‐NCB and 1,4‐DNB sorption by soil mineral components. Clay (<2‐μm fraction) removal decreased p‐NCB sorption to the WA and WB horizons by ∼8 and 11 times, respectively; clay removal decreased 1,4‐DNB sorption to the WA and WB horizons by ∼4.8 and 6.7 times, respectively. Nitroaromatic compound sorption to different soil fractions was measured to identify the independent effects of soil components and exchangeable cations on sorption. For this purpose, 1,2,4‐trichlorobenzene (1,2,4‐TCB) and 1,4‐DNB sorption to two organic soils and a soil devoid of smectites, and p‐NCB sorption to whole soil and the soil clay‐sized fraction were determined. Exchangeable cation type was found to minimally affect sorption of 1,2,4‐TCB, p‐NCB, and 1,4‐DNB by SOM. Sorption of p‐NCB to homoionic soil clay‐sized fractions was generally greater for clays saturated with monovalent cations than clays saturated with divalent cations. Greater p‐NCB sorption followed decreases in cation hydration energy in the order Na+ < Li+ < < K+ < Cs+ and Ca2+ < Ba2+ < Mg2+ Similar trends were observed for whole‐soil samples exchanged with these cations. This indicates that differences in nitroaromatic sorption are due to exchangeable cation effects on the clay mineral fraction.

Nitrogen Fertilizer and Herbicide Transport from Tile Drained Fields

AbstractOffsite transport of N fertilizers and pesticides through subterranean drainage pipes (tiles) has been linked to surface water contamination in the U.S. Corn Belt. This study was conducted from water years 1995 to 1997 to evaluate N export from two tile systems (Tiles A and B) in adjacent fields [in seed corn‐soybean rotation (Zea mays L.‐Glycine max (L.) Merr.)] in response to timing and form of N application. In addition, during the 1997 water year, concentrations of two herbicides were determined on grab samples to relate tile herbicide losses with field application rates. During the 1995 and 1996 water years, Tile A exported approximately 20% more N per unit area than Tile B; however, during the 1997 water year, Tile A exported nearly 70% more N. This result was partly caused by the leaching of 142 kg of NH+4‐N following a winter application of (NH4)2SO4 fertilizer in the drainage area of Tile A. The fertilizer was applied on top of 10 cm of snow, and within 4 d, a series of afternoon melting events began. We hypothesize that increased tile flow rates were caused by rapid infiltration of melt water through partially frozen soil (Drummer silty clay loam, fine‐silty, mixed mesic Typic Haplaquolls) that allowed the NH+4 ion to bypass the soil matrix (conc. reached 278 mg NH+4‐N L−1). The incidence of elevated concentrations of N fertilizer and herbicides in tiles during high flow events following agrichemical application indicated rapid water transport via preferential flow paths, thereby limiting the contact of these solutes with the soil.

EVAPORATIVE EFFECTS ON MOBILITY OF 14C-LABELED TRIASULFURON AND CHLORSULFURON IN SOILS.

A laboratory soil column experiment was conducted to determine the mobility of 14 C-triasulfuron in A horizon material of Farnum loam, fine-loamy, mixed, thermic Pachic Argiustoll from Kansas, Norfolk loamy sand, fine-loamy, siliceous, thermic Typic Hapludult, and Rion sandy loam, fine-loamy, mixed, thermic Typic Hapludult from North Carolina, and Webster silt loam, fine-loamy, mixed, mesic Typic Haplaquoll from Iowa as well as 14 C-chlorsulfuron in Rion sandy loam and to measure the effects of evaporation on capillary transport of the leached herbicides upward when soil columns were in contact with free water and not over free water. Triasulfuron mobility was in the order Norfolk (R f = 0.52) = Rion (R f = 0.48) > Farnum (R f = 0.40) > Webster (R f = 0.26), and R f was inversely related to the organic and humic matter contents of the soils. Evaporation of water from the soil surface of leached triasulfuron-treated Rion and Norfolk soils over a 2-week period had pronounced effects on herbicide transported upward, particularly when soil columns were in contact with free water simulating a shallow water table. Evaporation had no effects on capillary transport upward in Farnum and Webster soils, probably because of their higher contents of organic and humic matter. Chlorsulfuron was 18% more mobile than triasulfuron in Rion sandy loam, and both herbicides were distributed in a similar pattern in the soil when water was applied to the surface and leached or applied to the base of the columns and transported upward by capillary action. Redistribution of the herbicides upward in capillary water will likely influence persistence of the chemicals, particularly in soils over shallow water tables.

SOIL WATER AND SOLUTE MOVEMENT AND BULK DENSITY CHANGES IN REPACKED SOIL COLUMNS AS A RESULT OF FREEZING AND THAWING UNDER FIELD CONDITIONS

Freezing and thawing affect water and nutrient movement within soil profiles. Understanding the freezing and thawing processes and their effects on water movement and soil structure is necessary to develop improved management strategies. Experiments were conducted to measure the effects of freeze/thaw on soil water and solute movement in a Webster silty clay loam (fine-loamy, mixed mesic Typic Haplaquolls). Polyvinyl chloride (PVC) cylinders (0.13 m inside diameter, 1.2 m long) were packed with topsoil, and potassium bromide tracer was placed in the top 0.05- to 0.15-m soil layers in some of the columns. The soil columns were buried vertically in the field and exposed to the winter freeze/thaw conditions at Ankeny, Iowa. Soil columns were removed from the field throughout the winter and sectioned into 0.05-m layers. Each layer was analyzed for water content, bulk density, and electrical conductivity. Water moved upward to the freezing zone, carrying some solutes along. Electrical conductivity values verified the movement of solutes during the freeze/thaw periods. Bulk density changed abruptly as a result of expansion and compression of the soil matrix during freeze/thaw periods. Thawed soil retained some of the physical property changes caused by freezing and remained more variable than unfrozen soil. Freezing action increases the heterogeneity of soil properties in the frozen zone. This additional variability increases the complexity of predicting overwinter changes in the soil profile and the difficulty of developing management practices for maximizing nutrient efficiency while minimizing pollution.

Nitrogen cycling and tile drainage nitrate loss in a corn/soybean watershed

Nitrogen (N) in surface waters has been linked to agricultural crop production, and more specifically, to NO 3 − exported by tile drainage. The objective of this study was to evaluate agricultural N pools and fluxes in a seed corn/soybean ( Zea maize L./ Glycine max L.) watershed (40 ha) to relate soil inorganic N pools with annual losses of NO 3 − in drainage tiles. During a 2-year period beginning in October 1993, soil samples in the top 50 cm located near the tile systems (predominantly Drummer silty clay loam, fine–silty, mixed mesic Typic Haplaquolls) were analyzed for microbial biomass C and N, inorganic N, and N mineralization rates. Water flow and NO 3 − concentrations were continuously measured in the three drainage tiles. Soil microbial biomass N ranged from 83 to 156 kg N ha −1, and appeared more closely related to soil moisture than soil inorganic N pools. Soil inorganic N ranged from a low of 13 kg N ha −1 during the soybean growing season to a high of 115 kg N ha −1 after N fertilization. Following good growing seasons in 1993 and 1994, high crop uptake of N resulted in relatively small soil inorganic N pools of 40 and 24 kg N ha −1, respectively, after crop harvest. In 1995, however, when poor growing conditions decreased crop N accumulation, 98 kg N ha −1 remained in the watershed after harvest. Based on an average effective drainage area of 30 ha, 38 and 64 kg N ha −1 leached out of the watershed through tile drainage for a total of 3.1 Mg N for the 1995 and 1996 water years, respectively. Tile N export from the watershed was greatest during high flow events when there concurrently existed large pools of soil inorganic N in the form of NO 3 −. Differences in annual N export for each tile were the result of a combination of factors including; timing and area of N fertilization, amount and distribution of precipitation, crop uptake of soil derived N, and inorganic N pools remaining after harvest.

Carbon Dynamics in Corn‐Soybean Sequences as Estimated from Natural Carbon‐13 Abundance

AbstractCarbon flow in terrestrial ecosystems regulates partitioning between soil organic C (SOC) and atmospheric CO2. Our objectives were to assess SOC dynamics using natural 13C abundance in corn (Zea mays L., a C4 species)‐soybean [Glycine max (L.) Merr., a C3 species] sequences. Fifteen treatments of continuous corn, continuous soybean, various sequences of corn and soybean, and fallow were initiated in 1981 at Lamberton, MN, on a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll). In 1991, soil and aboveground shoot samples from all treatments were analyzed for total organic C and δ13C. Carbon inputs, δ13C, and SOC were integrated into a two‐pool model to evaluate C dynamics of corn and soybean. Total SOC was similar across all treatments after 10 yr; however, differences in soil δ13C occurred between continuous corn (δ13C = −17.2‰) and continuous soybean (δ13C = −18.2‰). Modeled C dynamics showed SOC decay rates of 0.011 yr−1 for C4‐derived C and 0.007 yr−1 for C3‐derived C, and humification rates of 0.16 yr−1 for corn and 0.11 yr−1 for soybean. Decay and humification rates were slightly lower than those found in other Corn Belt studies. Levels of SOC were predicted to decline an additional 7 to 18% with current C inputs from either corn or soybean, respectively. Annual C additions required for SOC maintenance averaged 5.6 Mg C ha−1, 1.4 to 2.1 times greater than previously reported estimates. Controlled variation in natural 13C abundance in corn‐soybean rotations during a 10‐yr period adequately traced C dynamics.

Fate of Fertilizer Nitrogen as Affected by Time and Rate of Application on Corn

AbstractManagement of fertilizer N on corn (Zea mays L.) can greatly affect the efficiency of N use and the potential for adverse environmental effects. Field studies were conducted on two nonirrigated southern Minnesota soils — a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) and a Mt. Carroll silt loam (fine‐silty, mixed, mesic Mollic Hapludalf) — to determine the effect of time and rate of N application on recovery of fertilizer‐derived N (FDN) in corn grain and stover and in the soil. Nitrogen rates of 75 and 150 kg ha‐1 on the Mt. Carroll soil and 100 and 200 kg ha‐1 on the Webster were applied at planting or at the eight‐leaf stage as (NH4)2SO4 to the same plots from 1982 to 1984. Enriched 15N was applied to separate microplots each of the first 2 yr to allow measurement of FDN. Grain yield responded to applied N in 5 of 6 site‐yr, but not to time of application. Uptake of FDN in grain was increased by higher N rate in all cases, but by delayed application in only one site‐year. Total plant FDN recovery ranged from 31 to 60% at the low N rate and from 24 to 45% at the high rate. Both yields and FDN recovery were affected by unusually dry midseason conditions. Fertilizer‐derived N recovery from the soil after harvest ranged from 25 to 56%, with a large proportion at the high N rate in inorganic forms, especially with the late application, which increased the potential for leaching losses. Residual uptake of FDN by grain ranged from 1 to 10% of the initial N rate. The difference method for estimating FDN recovery gave different results from the 15N method, emphasizing the importance of examining both labeled and nonlabeled N pools for complete inter‐pretation of 15N studies.

Nitrogen Balance in and Export from an Agricultural Watershed

Surface water nitrate (NO−3) pollution from agricultural production is well established, although few studies have linked field N budgets, NO−3 loss in tile drained watersheds, and surface water NO−3 loads. This study was conducted to determine field sources, transport, and river export of NO−3 from an agricultural watershed. The Embarras River watershed at Camargo (48 173 ha) in east-central Illinois was investigated. The watershed is a tile-drained area of fertile Mollisols (typical soil is Drummer silty clay loam, a fine-silty, mixed mesic Typic Haplaquoll) with primary cropping of maize (Zea mays L.) and soybean (Glycine max L.). Agricultural field N sources and sinks, tile drainage NO−3 concentrations and fluxes, and river NO−3 export were estimated for the entire watershed. Large pools of inorganic N were present following each harvest of maize and soybean (average of 3670 Mg N yr−1 over a 6-yr period). The source of most of the inorganic N was divided between N fertilizer and soil mineralized N. High concentrations of NO−3 were found in four monitored drainage tiles (5–49 mg N L−1), and tile concentrations of NO−3 were synchronous with Embarras River NO−3 concentrations. High flow events contributed most of the yearly NO−3 loss (24.7 kg N ha−1 yr−1) from tile drained fields in the 1995 water year (1 Oct. 1994 through 30 Sept. 1995) where high rainfall events occurred in a low overall precipitation year (in one tile 21% of the annual load was exported in 1 d). During the 1996 water year, NO−3 export in tiles was much higher (44.2 kg N ha−1 yr−1) due to greater precipitation, and individual days were less important. On average, about 49% (average of 1688 Mg N yr−1 over a 6-yr period) of the field inorganic N pool was estimated to be leached through drain tiles and seepage and was exported by the Embarras River, although depending on weather and field N balances this ranged from 25 to 85% of the field N balance over the 6-yr period. It seems likely that agricultural disturbance (high mineralization inputs of N) and N fertilization combined with tile drainage contributed significantly to NO−3 export in the Embarras River.

Nitrogen Application Methods and Timing for Corn after Soybean in a Ridge-Tillage System

Nitrogen placement options for improved N efficiency are limited in ridge-tillage systems where primary and secondary tillage for fertilizer incorporation is absent. Studies were conducted for 3 yr on a Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquoll) in southern Minnesota to determine the effect of placement and time of N application on corn (Zea mays L.) production and postharvest residual soil nitrate (RSN) and to evaluate the spoke- wheel injector for precise placement of urea-ammonium nitrate solution (UAN, 28-0-0) for corn after soybean [Glycine max (L.) Merr.] in a ridge-tillage system. Nitrogen as UAN was either band-applied on the ridge, broadcast, or point injected into the ridge (PINJ-R) or valley (PINJ-V) and was compared with anhydrous ammonia (AA) injected into the valley (INJ-V). Single preplant and preemergence applications were compared with split applications where 30 to 40% was applied preemergence and 60 to 70% was applied at the V7 and V16 growth stages. Leaf N concentration at the RI stage and whole plant dry matter and N concentration at the R2 stage were generally enhanced by single preemergence applications compared witb split applications and by PINJ-R compared with the PINJ-V treatment. Grain yield, total N uptake, and net economic return were generally optimized by point injecting UAN and the injection of preplant AA compared with band and broadcast applications of UAN. Split applications did not produce higher yields, N uptake, or net economic return than single preemergence injected applications of N. However, RSN was highest with the V16 application of UAN under these nonirrigated conditions. Three-year average grain yield and net economic return were highest with the preemergence PINJ-R treatment. These results suggest that a single preemergence spoke-wheel application of UAN into the ridge or preplant AA in the valleys can be successfully used to optimize N management and corn production following soybean in a ridge-tillage system. Nitrogen efficiency will be enhanced and ammonia volatilization will be reduced with these application methods.

Nitrogen fertilizers promote denitrification

A laboratory study was conducted to compare the effects of different N fertilizers on emission of N2 and N2O during denitrification of NO3 – in waterlogged soil. Field-moist samples of Drummer silty clay loam soil (fine-silty, mixed, mesic Typic Haplaquoll) were incubated under aerobic conditions for 0, 2, 4, 7, 14, 21, or 42 days with or without addition of unlabelled (NH4)2SO4, urea, NH4H2PO4, (NH4)2HPO4, NH4NO3 (200 or 1000 mg N kg–1 soil), or liquid anhydrous NH3 (1000 mg N kg–1 soil). The incubated soil samples were then treated with 15N-labelled KNO3 (250 mg N kg–1 soil, 73.7 atom% 15N), and incubation was carried out under waterlogged conditions for 5 days, followed by collection of atmospheric samples for 15N analyses to determine labelled N2 and N2O. Compared to samples incubated without addition of unlabelled N, all of the fertilizers promoted denitrification of 15NO3 –. Emission of labelled N2 and N2O decreased in the order: Anhydrous NH3>urea \gg (NH4)2HPO4>(NH4)2SO4≃NH4NO3≃NH4H2PO4. The highest emissions observed with anhydrous NH3 or urea coincided with the presence of NO2 –, and 15N analyses indicated that these emissions originated from NO2 – rather than NO3 –. Emissions of labelled N2 and N2O were significantly correlated with fertilizer effects on soil pH and water-soluble organic C.

Yield and Nitrogen Uptake of Spring Wheat Planted in a Ridge-Till System

Conservation tillage and inclusion of a cover crop that provides surface residue cover are two management options for reducing soil erosion losses commonly found in a corn (Zea mays L.)-soybean [Glycine max (L.) Merr] rotation. Information is lacking, however, on the performance of a small grain planted in a ridge-tillage system in the Northern Corn Belt. A 3-yr (1992-1994) study was conducted to determine the effect of row position (ridge, shoulder, furrow, and wheel track furrows) on yield and N uptake of hard red spring wheat (Triticum aestivum L.) planted in a ridge-tillage system on poorly drained Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquoll) soil at two locations in southern Minnesota. Spring wheat was planted after soybean in 15 ft wide strips. Wheat performance was affected by the position of the row across the ridged seedbed. Averaged across the 3 yr, grain yield of the wheat rows planted on ridge top and shoulder positions was 31% greater than those rows planted in the furrows at both locations. Grain yield of wheat in the wheel track furrows was reduced by 38% at Freeborn and by 33% at Waseca compared with those planted on ridge tops and shoulders when averaged across years. Moreover, grain yield in the wheel track furrows was reduced by 18% at Freeborn and by 12% at Waseca compared with non-wheel track furrows across years. Similar to grain yield, straw yield and total N uptake were greater in the ridge top and shoulder positions than in either non-wheel track or wheel track furrow positions. Lesser grain yield in the furrows and wheel track furrows correlated well with reduced head numbers in these row positions compared with ridge top and shoulder positions. The results from this study indicate that wheat can be successfully planted in a ridge-tillage system. Yield losses, however, occur in the furrows and controlled wheel track furrows, especially in wet years in this ridge-till system.

Degradation of Atrazine in Two Soils as a Function of Concentration

AbstractDissipation of atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) in a Webster clay loam soil (fine loamy, mixed, mesic Typic Haplaquoll), and Estherville sandy loam (sandy, mixed, mesic typic Hapludoll) was determined over a concentration range of 5 to 5000 mg kg−1 in field and laboratory experiments. Over the first 6 mo in the clay loam soil, the persistence of atrazine (based on percent of applied) was greater for the high‐rate treatments than the low‐rate treatments. However, in the laboratory, there was no effect of concentration on dissipation; the amount of atrazine degraded increased proportionally with the increase of concentration. In the sandy loam, persistence was greater at high concentration in both field and laboratory studies. Mineralization was the most important pathway for the dissipation of atrazine at all concentrations in the clay loam soil and from 5 to 500 mg kg−1 in the sandy loam. It was postulated that soil microorganisms were able to use the N or C from the s‐triazine ring. Atrazine at 500 and 5000 mg kg−1 may have increased soil microbial growth and activity and thus increased the degradation of atrazine based on the increase in soil respiration in the clay loam soil. Degradation pathways in both soils apparently were not influenced by concentration. Ring cleavage and hydrolysis were the major metabolic pathways in both soils, with dealkylation of less importance. Addition of a dairy manure amendment increased the rate of atrazine mineralization, while corn meal decreased and (NH4)2HPO4 amendments prevented mineralization.

Soil Temperature and Planting Date Effects on Corn Yield, Leaf Area, and Plant Development

AbstractEarly‐season soil temperature has been reported to affect leaf appearance and expansion rates, and consequently corn (Zea mays L.) ontogeny. A 2‐yr field experiment was conducted on a Drummer silty clay loam (fine silty, mixed, mesic Typic Haplaquoll) at the Agronomy and Plant Pathology South Farm of the University of Illinois at Urbana‐Champaign. Treatments consisted of planting date (early May and early June) and soil temperature surrounding the corn growing point (5°C below ambient, ambient, and 5°C above ambient). Soil temperature was controlled using warm and cool water circulating through copper pipes buried next to the corn rows. Air temperatures were not controlled. Irrigation was used. The experimental design was a split plot in a randomized complete block with four replications. Ten plants were randomly selected within each replication and nondestructive measures of plant stage, individual leaf area, and leaf senescence were taken three times per week from emergence to V5, and once per week afterwards until complete leaf senescence. Growing degree days (GDD) were calculated using the modified growing degree day formula (MGDD). From planting to V5, MGDD were computed using the soil maximum and minimum daily temperatures from each treatment. After the soil temperature treatments were terminated, the MGDD calculations were based on maximum and minimum daily air temperatures. Lower early‐season soil temperature delayed corn development and modified individual leaf area. Plants under these cooler conditions went through vegetative developmental stages faster per unit of accumulated MGDD, showing the effect of temperature on corn ontogeny. The results also show that the current method of calculating MGDD (with base temperature of 10°C) does not adequately predict corn development under exceptionally warm or exceptionally cool soils. Warmer early‐season soil temperature linearly increased corn yield (β1 = 0.14 Mg ha−1 °C−1). Leaves in the lower half of the canopy were larger under cooler soil temperature treatments (β1 = −48.9 cm2 °C−1). There was a linear increase in the size of the leaves in the upper half of the canopy of the plants under warmer temperatures (β1 = 28.8 cm2 °C−1).

Effect of Tillage and Rotation on Agronomic Performance of Corn and Soybean: Twenty-Year Study on Dark Silty Clay Loam Soil

In the U.S. Corn Belt, tillage without plowing was used on more than 50% of corn (Zea mays L.) and soybean (Glycine max (L.) Merr.] acreage in 1994, while no-till planting was used on about 30% of the acreage. Few research studies have evaluated these reduced tillage systems for 20 yr or more. This study includes plow, chisel, ridge, and no-till systems in continuous corn, corn after soybean, soybean after corn, and continuous soybean rotations. The experiment was conducted on Chalmers (fine-silty, mixed, mesic Typic Haplaquolls) silty clay loam soil in north-central Indiana for 20 yr. Objectives were to determine the effect of tillage and rotation on stands, growth, maturity, and yield of corn and soybean and to determine trends that develop with time for these variables. Major stand reductions occurred only in no-till continuous corn, 8% less than plowing. Compared with plowing, chisel and ridge tillage systems reduced growth and yield and increased harvest moisture by less than 3% in continuous corn. No-till reduced 4-wk height of continuous corn by 17%, yield by 14%, and increased harvest moisture by 2.1 percentage points compared with plowing. For corn following soybean, only no-till corn showed a yield reduction, 2.8%, compared with plowing. Twenty-year mean soybean yield reductions averaged 4 to 7% in both rotation and monoculture for chisel, ridge, and no-till systems compared with plowing. Relative yields for no-till continuous corn tended to be less than other tillage-rotation systems over time, while no-till soybean yields tended to improve with time, especially during the last 5 yr of the study. Both corn and soybean yields were better in rotation than in continuous cropping for all tillage systems. Of the tillage systems-rotation combinations in this study, only no-till continuous corn is likely to suffer major yield loss on a long-term basis on dark prairie soils of the Central and Northern Corn Belt.

Effect of Concentration on Persistence of Alachlor in Soil

AbstractTo determine the behavior of alachlor [2‐chloro‐N‐(methoxymethyl)‐N‐(2,6‐diethylphenyl)‐acetamide] at concentrations common with spill and waste disposal sites, alachlor degradation was determined over a concentration range of 10 to 10 000 mg kg−1 in a Webster clay loam (fine loamy, mixed, mesic Typic Haplaquoll) and an Estherville sandy loam (sandy, mixed, mesic Typic Hapludoll) in laboratory incubation experiments. Effect of concentration on the overall behavior of alachlor was similar in both soils. Based on percent of applied chemical, persistence of alachlor increased with increasing concentration. Mineralization and formation of degradation products and bound residues decreased at higher concentrations. At 10 000 mg kg−1, alachlor was extremely persistent, with estimated 50% dissipation times (DT50) of 12.6 and 13.5 yr in the Webster and Estherville soil, respectively. Although the DT50 increased with increasing concentration, significant amounts of alachlor, in absolute mass, degraded at higher concentration. However, mineralization at 1000 and 10 000 mg kg−1 was similar to that at 100 mg kg−1. Specific alachlor biodegradation mechanisms, limited water solubility, and kinetics of dissolution of precipitated alachlor and alachlor desorption are postulated as the rate‐limiting factors for the degradation of alachlor at elevated levels. A field experiment also indicated alachlor dissipation at high concentration was extremely slow over the first 6 mo after application. Degradation and leaching increased by the following spring. It would appear that an effective remediation means to detoxify alachlor‐contaminated soil would be to land‐spread the soil, thereby diluting the concentration of alachlor in the soil to <100 mg kg−1.

Traffic Pattern and Tillage System Effects on Corn Root and Shoot Growth

AbstractControlled wheel traffic is one way to manage compaction in no‐till and ridge‐till systems. This study was conducted from 1990 to 1992 at Kanawha, IA, on a Webster silty clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) to examine the effect of a wheel traffic pattern on corn (Zea mays L.) root distribution, shoot growth, and yield in no‐till, ridge‐till, and chisel‐plow tillage systems. The wheel traffic pattern was configured so that some rows would have wheel tracks on both sides, on one side, or on neither side. Bulk density, hydraulic conductivity, root length density, shoot dry weight, and yield were measured at several positions across the traffic pattern. In general, the effect of tillage systems was not significant averaged across positions. Position relative to the traffic pattern had some effect, however, on all measured parameters. Bulk density was greatest in trafficked interrows (1.36 Mg m~3) and least in untrafficked interrows (1.09 Mg m−3). Hydraulic conductivity near saturation was less in trafficked (39.4 μm s−1) than in untrafficked (104.7 μm s−1) interrows. Root length density in trafficked interrows was on average one‐third of that in untrafficked interrows. Root length density in a particular interrow also was influenced by the traffic pattern in the adjacent interrows. In 2 of the 3 yr, yields of rows with a trafficked interrow on only one side were 7% less than those of rows without trafficked interrows on either side. The wheel traffic pattern, and not just the presence or absence of wheel traffic, affected corn root growth and yield.

Yield and Botanical Composition of Legume‐Interseeded vs. Nitrogen‐fertilized Switchgrass

AbstractSwitchgrass (Panicum virgatum L.) has a relatively high N requirement for high yields of quality forage. It is not clear what role legumes can play in supplying this N and in improving herbage yield when grown in association with switchgrass. To evaluate cool‐season legume renovation vs. N fertilization of established switchgrass, 10 forage legumes and a legume mixture were compared with 0, 60, 120, and 240 kg N ha−1. Forage yield and botanical composition of basal (<20 cm) and upper (>20 cm) canopy were compared at Ames, IA, on a Webster silty clay loam (mesic Typic Haplaquoll). Legumes were no‐till interseeded in early April; N was applied before mid‐May. Legume renovation did not affect June yield during the establishment year (Yl), but produced 9% greater yields than 0‐N grass in July. N fertilization increased uppercanopy grass yield 2.4‐fold compared with 0 N and legume renovation during Yl. During the second year (Y2) of 1991 seedings, all legume treatments except crownvetch (Coronilla varia L.) produced more totalseason upper‐canopy yield than grass fertilized with 240 kg N. For 1992 seedings, birdsfoot trefoil (Lotus corniculatus L.), Mammoth red clover (Trifolium pratense L.), and trefoil‐red clover mixture had Y2 yields that equaled or exceeded yield for 240 kg N. Mean legume composition of Y2 upper canopy for June, July, and August was 84, 70, and 51%, respectively, in 1991 seedings and 53, 28, and 27% in weather‐damaged stands of 1992 seedings. Y2 yields for interseeded legumes provided significant improvement over 120 or 240 kg ha−1 N, so cool‐season legumes can substitute for N fertilization after the seeding year. Adequate defoliation in early June is important to minimize legume competition to established switchgrass. Livestock producers should renovate only a portion of switchgrass pastures in a single year, because of a shortfall in forage supply during legume establishment compared with that of N‐fertilized grass.

Root Growth and Distribution are Affected by Corn‐Soybean Cropping Sequence

AbstractCorn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yield better when grown in rotation than when grown in monoculture, even under high inputs. The explanation for this yield increase due to rotation is not yet known. The objective of this study was to evaluate the effect of cropping sequence on seasonal root development of corn and soybean. We hypothesized that there would be more roots on rotated crops than on crops held in a continuous monoculture. The research was conducted in 1991 and 1992 at the Univ. of Minnesota Southwest Experiment Station on a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll). Destructive root sampling via soil cores was performed at the seedling and flowering stages of both crops. Nondestructive root monitoring was performed weekly and recorded on video cassette tapes via an agricultural research video camera inserted into minirhizotron tubes that had been placed into the soil beneath crop rows. Our hypothesis that there would be more roots on first‐year corn than continuous corn was generally supported. Minirhizotron monitoring suggested that corn grown in rotation had greater root length density than continuous corn, with the exception of the top 12.5 cm, where continuous corn had greater root length density early in the season. Soil cores confirmed 22% more seedling roots in the top 12.5 cm on continuous corn. Our hypothesis that there would be more roots on first‐year soybean than continuous soybean was generally not supported. Minirhizotron monitoring indicated that when there were differences, continuous soybean had more roots than rotated soybean, with the single exception of depths of 37.5 to 50 cm, where rotated soybean had more than twice as many roots at flowering in 1992. Root length density data obtained from soil cores also indicated that rotated soybean had approximately twice as many roots at depths of 36 and 48 cm at flowering. Growth and distribution of both corn and soybean roots were thus affected by cropping sequence. Fewer roots in continuous corn may have been the result of autotoxins from decomposing roots of the previous corn crop. We do not have an explanation for increased soybean roots under monoculture.

Test of the LEACHP Model for Predicting Atrazine Movement in Three Minnesota Soils

AbstractLeaching Estimation and Chemistry Model‐pesticide (LEACHP) was evaluated for predicting atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) movement in sandy loam (sandy, Typic Hapludoll), silt loam (fine‐silty, Typic Hapludoll), and clay loam (fine‐loamy, Typic Haplaquoll) soils under moldboard and chisel plow tillage practices. Atrazine distribution in the root zone was measured by analyzing soil cores. Model predictions were compared with observed atrazine concentrations in the soil profiles on various dates after application (two to three growing seasons). Model performance was evaluated by testing whether model predictions fall within a specified factor of true values and using Goodness‐of‐Fit tests (maximum error, root mean squared error, coefficient of determination, modelling efficiency, coefficient of residual mass). LEACHP predicted depth of peak atrazine concentration in the soil profile accurately in all three soils (under both moldboard [MB] and chisel plow [CP] treatments and at all sampling dates). It predicted depth of atrazine movement and atrazine concentrations in the soil profile reasonably well in the Estherville soil (sandy, mixed, mesic Typic Hapludoll) except some disparities between measured and predicted concentrations in 1988 because of the difficulty in recovering representative samples. In Port Byron (fine‐silty, mixed, mesic Typic Hapludoll) and Webster soils (fine‐loamy, mixed, mesic Typic Haplaquoll), depth of atrazine movement and atrazine concentrations in the soil profile were predicted reasonably well during relatively dryer years (1988 and 1989). In a relatively wet year (1987), however, the model did not predict significant amounts of atrazine at depths >45 cm, whereas, 2.5 to 27.5 µg kg−1 of atrazine were detected in soil samples during earlier sampling dates (43, 73 DAA in Port Byron and 14, 43 DAA in Webster soils). Overall performance of the model was similar in both tillage treatments (MP or CP). Considering the broad range in soil properties and climatic conditions used in testing, the model performed well.

Soil and Phosphorus Loss from Conservation and Conventional Tillage in Corn Production

AbstractConservation tillage is encouraged in southwestern Ontario by concern for soil erosion and compaction. The contribution of agriculture to eutrophication of the Great Lakes by P is also at issue. Soil loss and ortho‐P transport were measured from a conventional and two conservation tillage treatments (zero and ridge tillage) from January 1988 to 30 Sept. 1990 to evaluate their impact on meeting Great Lakes water quality objectives for P. Sediment concentration from the poorly drained, Brookston clay loam (clayey, mixed, mesic Typic Haplaquolls), cropped to corn (Zea mays L.) was 2.1 times larger in surface runoff than tile discharge (0.20 g L−1) but tile discharge contributed 44 to 65% of the soil loss probably from preferential flow. Conservation tillage reduced average soil loss 49% from conventional tillage (899 kg ha−1). Conservation tillage increased ortho‐P concentrations in runoff 2.2 times from conventional tillage (0.25 mg L−1). Orthophosphate transport decreased in the order zero>ridge>conventional tillage. Average ortho‐P loss was 1.7 to 2.7 times greater from conservation than conventional tillage (559 g ha−1 yr−1). Subsurface drainage accounted for 55 to 68% of the ortho‐P transported. Transport of total soluble P and total P (sum of sediment‐attached P and soluble P, only measured in 1990) increased 2.2 and 2.0 times, respectively, with conservation than conventional tillage. Dissolved P accounted for 84 to 93% of the P transported from the three tillage treatments. Sediment‐attached P constituted 7 to 16% of total P loss. Conservation tillage effectively reduced soil erosion but increased P loss.