Sharkey Clay Soil Research Articles

Effect of biochar amendment on sorption-desorption and dissipation of 17α‑ethinylestradiol in sandy loam and clay soils

Animal manure application in agricultural land has caused the release of steroid estrogens in the soil environment and further movement to aquatic systems. The objective of this study was to investigate the effects of biochar addition on sorption-desorption and dissipation behaviors of 17α‑ethinylestradiol (EE2) in two different textured soils. A Commerce sandy loam and a Shakey clay were selected and subjected to sterilization. Soil samples with and without sterilization were reacted with a series of EE2 solutions of different concentrations for sorption followed by desorption and quantification using HPLC-MS/MS. Long-term dissipation of EE2 in the same soils was also evaluated over a 30-d incubation. Biochar amendment increased the maximum EE2 sorption capacity but decreased its water desorption in both sandy loam and clay soils. On other hand, biochar addition increased the Koc in the clay soil which had low EE2 sorption efficiency but decreased Koc in the sandy loam which had high EE2 sorption efficiency. Biochar did significantly increase both desorbable and non-extractable fractions of EE2, while it reduced the bioavailability of EE2 to microbial degradation. The dissipation of EE2 in non-sterilized soils fit to the first-order kinetic model, whereas it was better described by zero-order kinetic for sterilized soil. Biochar increased the half-life of EE2 dissipation in non-sterilized Commerce sandy loam soil by 48% (from 3.63 to 5.37 d) and in non-sterilized Sharkey clay soil by 67% (from 2.28 to 3.81 d). Overall, this study demonstrated positive impacts of biochar on the retention of estrogen hormones in soils.

Selectivity of Very‐Long‐Chain Fatty Acid‐Inhibiting Herbicides in Rice as Influenced by Application Timing and Soil Texture

Core Ideas Alternative herbicide sites of action are desperately needed in US rice. Rice exhibited tolerance to the currently non‐labeled herbicides acetochlor and pethoxamid when applied at the one‐ to two‐leaf growth stage on a silt loam and clay soil. Delaying application of very‐long‐chain fatty acid‐inhibiting herbicides until drill‐seeded rice has completely emerged enhanced crop tolerance. Very‐long‐chain fatty acid (VLCFA)‐inhibiting herbicides include pyroxasulfone, S‐metolachlor, acetochlor, and pethoxamid. Currently, no VLCFA‐inhibiting herbicides are labeled for use in US rice production; however, if rice tolerance can be established they would provide an alternative herbicide site of action (SOA). In 2015 and 2016, pyroxasulfone (Zidua) at 0.14 lb a.i. ac−1, S‐metolachlor (Dual Magnum) at 0.95 lb a.i. ac−1, acetochlor (Warrant) at 0.95 lb a.i. ac−1, and pethoxamid at 0.75 lb a.i. ac−1 were applied delayed preemergence (DPRE) and to spiking and one‐ to two‐leaf rice on a Dewitt silt loam and a Sharkey clay soil. Trials were conducted at separate locations to determine the effect differing soil properties may have on the tolerance of rice to VLCFA‐inhibiting herbicides. Substantial rice injury was observed on the Dewitt silt loam soil following all pyroxasulfone and S‐metolachlor treatments, ranging from 20 to 100% injury 3 weeks after rice flooding. On the Dewitt silt loam, treatments containing acetochlor and pethoxamid applied to one‐ to two‐leaf rice resulted in minimal injury and the highest yields observed among VLCFA‐inhibiting herbicides. In general, rice injury ratings were less on the Sharkey clay soil. Pethoxamid and acetochlor applied to rice at any application timing resulted in ≤ 1% injury 2 weeks after treatment. Hence, contingent on a label, pethoxamid and acetochlor can be applied under similar conditions as early as DPRE on the Sharkey clay soil and not until the one‐ to two‐leaf rice stage on the Dewitt silt loam at the rates assessed.

Effect of Irrigation and Silicon Fertilizer on Total Rice Grain Arsenic Content and Yield

Core Ideas Arsenic content in rice was reduced by not flooding fields. Silicon fertilizer did not affect arsenic content in rice. Cultivar and hybrid selection did not affect rice arsenic content. Field tests were conducted at three locations for 2 yr with rice grown with different irrigation systems and rates of calcium silicate fertilizer to determine the effects on brown rice arsenic (As) levels and rice yields. Irrigation systems were sprinkler irrigated using a center‐pivot system, intermittent flooding, and continuous flooding beginning at the first‐tiller rice growth stage. Pelletized calcium silicate fertilizer, a byproduct from steel mills, was applied at 0, 121, and 242 kg Si ha−1 in subplots to bare soil and incorporated. After harvest, rice was de‐hulled to make brown rice grain and tested for total As content. Silicon (Si) fertilizer and cultivar/hybrid selection did not impact rice grain As levels in any of the irrigation programs. Weed control was more difficult to maintain without flooding, which reduced yields. Rice yields grown in aerobic soil conditions with sprinkler irrigation were increased significantly with 60 kg Si ha−1. The high to low order of total As content in rice grain from plots was continuous‐flood > intermittent flood > sprinkler irrigated. The test site with sprinkler irrigation was on a different soil than intermittent and continuous flood systems. Flooded rice on Overcup silt loam soil averaged 210 µg As kg−1 total As. On Sharkey clay soil, rice with intermittent‐flood irrigation contained 27 µg As kg−1 total As in grain compared with 77 µg As kg−1 in continuous flood, but yield was not significantly different from continuous‐flooded rice. Sprinkler‐irrigated rice on Tiptonville silt loam soil averaged 13 µg As kg−1.

Establishing Soil Silicon Test Procedure and Critical Silicon Level for Rice in Louisiana Soils

ABSTRACTCalibration of crop responses to applied silicon (Si) serves as a basis for developing Si fertilizer recommendation guidelines. A greenhouse experiment was set up in a randomized complete block design with five replications, two sources of Si (wollastonite and slag) and four Si rates (0, 170, 340 and 680 kg ha−1) to calibrate plant-available Si for growing rice in Louisiana soils. Silicon concentrations were determined in soils using seven different extraction procedures. Based on a quadratic model (p < 0.05), the estimated soil Si critical level using 0.01 M calcium chloride (CaCl2) for Sharkey clay soil was 110 mg kg−1 while for Crowley silt loam and Commerce silt loam, levels were 37 and 43 mg kg−1, respectively. These results suggest that suitability of an extractant that gives the best estimate of plant-available Si could considerably depend on soil type and it is unlikely that there is a universal extractant for all soils.

Agricultural practices altered soybean seed protein, oil, fatty acids, sugars, and minerals in the Midsouth USA.

Information on the effects of management practices on soybean seed composition is scarce. Therefore, the objective of this research was to investigate the effects of planting date (PD) and seeding rate (SR) on seed composition (protein, oil, fatty acids, and sugars) and seed minerals (B, P, and Fe) in soybean grown in two row-types (RTs) on the Mississippi Delta region of the Midsouth USA. Two field experiments were conducted in 2009 and 2010 on Sharkey clay and Beulah fine sandy loam soil at Stoneville, MS, USA, under irrigated conditions. Soybean were grown in 102 cm single-rows and 25 cm twin-rows in 102 cm centers at SRs of 20, 30, 40, and 50 seeds m-2. The results showed that in May and June planting, protein, glucose, P, and B concentrations increased with increased SR, but at the highest SRs (40 and 50 seeds m-2), the concentrations remained constant or declined. Palmitic, stearic, and linoleic acid concentrations were the least responsive to SR increases. Early planting resulted in higher oil, oleic acid, sucrose, B, and P on both single and twin-rows. Late planting resulted in higher protein and linolenic acid, but lower oleic acid and oil concentrations. The changes in seed constituents could be due to changes in environmental factors (drought and temperature), and nutrient accumulation in seeds and leaves. The increase of stachyose sugar in 2010 may be due to a drier year and high temperature in 2010 compared to 2009; suggesting the possible role of stachyose as an environmental stress compound. Our research demonstrated that PD, SR, and RT altered some seed constituents, but the level of alteration in each year dependent on environmental factors such as drought and temperature. This information benefits growers and breeders for considering agronomic practices to select for soybean seed nutritional qualities under drought and high heat conditions.

Chamber Size Effects on Methane Emissions from Rice Production

Quantifying methane (CH4) emissions from cultivated rice (Oryza sativa L.) production in the field has received increased attention recently due to methane’s importance as a greenhouse gas. The enclosed-headspace chamber technique is the standard methodology for field assessments of trace gas emissions. However, to our knowledge, no direct comparisons of measured CH4 fluxes and emissions from field-grown rice among differing chamber sizes have been reported. Therefore, the objective of this study was to evaluate the effect of chamber size [15.2- and 30.4-cm inside diameter (id)] on CH4 fluxes and season-long emissions from rice grown on a clay soil in Arkansas. Chamber size did not affect (P > 0.05) CH4 fluxes on 10 sampling dates during the flooded portion of the rice growing season and only affected fluxes on one of four sampling dates after flood release. Total season-long CH4 emissions from optimally N-fertilized rice were 32.6 and 35.6 kg CH4-C ha&#451&#183season&#451, which did not differ, and from bare clay soil were 0.74 and 1.75 kg CH4-C ha&#451&#183season&#451, which also did not differ, from the 15.2- and 30.4-cm chambers, respectively. Chamber size (i.e., 15.2- or 30.4-cm id) did not result in differences in cumulative CH4 emissions from this flooded-rice study that was conducted on a Sharkey clay soil in northeast Arkansas. Results indicate that both 15.2- and 30.4-cm diameter chambers were similarly adequate for measuring CH4 fluxes and emissions from the clay soil investigated. The similarity in emissions results between chamber sizes also indicates that the 15.2-cm diameter chambers adequately facilitated the quantification of CH4 emissions in this study.

Nitrification and Yield for Delayed‐Flood Rice as Affected by a Nitrification Inhibitor and Coated Urea

Laboratory experiments were conducted in 2010, 2011, and 2012 in Stoneville, MS, utilizing urea fertilizer to quantify the nitrification potential of southern soils, and to evaluate a nitrification inhibitor aimed to reduce nitrification rates on clay soils. Field research was conducted from 2011 to 2013 evaluating the effectiveness of dicyandiamide (DCD) and sulfur‐polymer coated urea at reducing N loss compared to urea alone on clay soil. The number of days when half the total recovered inorganic N was in the NH4+–N form (half‐life for ammonium) for each soil and stabilizer for the nitrification potential and N stabilizer experiments was determined. Nitrogen was applied to the soil at 0.115 g kg−1 for both laboratory experiments while DCD was applied at three different rates for the N stabilizer experiment. These studies confirmed that the Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquerts) at Stoneville, MS, had significantly higher nitrification potential compared to three other soil types studied and DCD increased half‐life for ammonium by approximately threefold when compared to non‐amended urea. Field experiment data indicated that increasing DCD rate to 17 kg ha−1 increased rice (Oryza sativa L.) grain yield response and resulted in yields comparable to the standard treatment, which was urea applied 1 day before flood establishment (dbf). The sulfur‐polymer coated urea applied 12 dbf reduced nitrification; however, it yielded less than the standard treatment. Products that limit nitrification and subsequent denitrification can optimize N efficiency, resulting in greater potential for increased yields and decreased production costs.

Adsorption and Desorption of Atrazine Retention by Sugarcane Residue and Soils

Quantifying herbicide retention in the soil environment is essential for minimizing their runoff and contamination of groundwater resources. In this study, atrazine retention characteristics were carried out for sugarcane (Saccharum spp. Hyb.) residue grown on Sharkey clay and Commerce loam soils. The residue that covers the soil surface following combine harvesting was sampled over several growing seasons. Batch methods were used to quantify adsorption and desorption for a wide range of atrazine concentrations and reaction times. Desorption was carried out using successive dilutions. Atrazine retention by the residue exhibited linear adsorption where the partitioning coefficient (Kd) increased over time of retention. Adsorption-desorption exhibited strong hysteresis indicative of time-dependent retention and slow release by the residue. Limited atrazine sorption kinetics by the two soils was observed, and their Kd values were an order of magnitude lower than that for the residue. A multireaction model that accounts for kinetic- and equilibrium-type retention was successful in describing the time-dependent atrazine adsorption and desorption behavior by the sugarcane residue over a wide range of applied concentrations. Atrazine was strongly sorbed by the residue, where some 40% of the amount adsorbed was nonextractable by methanol. Moreover, retention results indicated that a decreasing or increasing trend of atrazine retention by the sugarcane residue with time of decay was not observed. The use of an average retention (Kd) value to represent atrazine retention over an entire growing season is recommended.

Effects of Spring Post-planting Flooding on Early Soybean Production in Mississippi

Planting soybean in April to avoid late-summer drought is a common management practice in Mississippi. However, soybeans planted on Sharkey clay soils are often exposed to waterlogged conditions during the early spring. Experiments were conducted at the Delta Research and Extension Center (DREC) at Stoneville, MS, in 2003 and 2004 to assess the effects of induced flooding on soybean seedling emergence, stand establishment, and associated yield loss. In 2003, post-planting flood treatments of 0 (control), 24, or 48 h were applied before seedling emergence or at the V1 stage. In 2004, similar treatments were applied except the timings of treatment were 0, 36, and 72 h. Results indicated that flooding prior to seedling emergence reduced number of plants and plant height, whereas only plant stand was affected when flooding was applied at the V1 stage. Number of pods per plant and number of seeds per pod were higher for the non-flooded control compared with the 48-h to 72-h flood duration at the V1 stage, but the final seed yield was not affected in both years. The data suggest that waterlogged conditions lasting two to three days may not have significant effect on seed yield.

EFFECT OF IONIC STRENGTH AND SODIUM ADSORPTION RATIO ON THE FLOCCULATION/DISPERSION OF TWO SURFACE SOILS FROM EASTERN ARKANSAS

Poor tilth and, often, a surface crust characterize eastern Arkansas soils. Such soils have very low water infiltration and high water runoff rate leading to increase turbidity in rivers and soil erosion. Sharkey clay and Desha silt loam soils from Delta region of eastern Arkansas were used to find the effect of sodium adsorption ratio (SAR) and ionic strength (IS) on the dispersion/flocculation behaviors of the soils. Percent light transmissions at a wavelength of 530 nm were related to the concentration of clay in suspension. Each soil was equilibrated with a series of solutions having different IS and containing Na + and Ca 2+ in various proportions. At different IS and SAR, the two soils exhibited different behaviors. Clay dispersibility was a more sensitive function of SAR at low IS than at high IS.

Soil deformation and its spectral signature

AbstractSpatial variation of the surface micro‐relief in a Sharkey clay soil was investigated. The micro‐relief measurements of the soil surface were obtained using an automated infrared laser system and evaluated with methods of Fourier analysis. The authors illustrate the spectral technique with a simple example and then use the technique to interpret the surface micro‐relief patterns of soil samples subjected to three simulated rainstorm intensities. The periodicity of the soil surface micro‐relief in the longitudinal direction was 80–120 mm. The periodic surface micro‐relief patterns immediately following the rainstorm are shown to be an early manifestation of the cracking pattern. Published in 2006 by John Wiley &amp; Sons, Ltd.

Influence of Large Amounts of Nitrogen on Nonirrigated and Irrigated Soybean

Nitrogen supplied by N2 fixation to soybean [Glycine max (L.) Merr.] may not be sufficient to maximize yield. Field studies were conducted in 2002, 2003, and 2004 on Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objective was to determine the effect of high rates of N applied as a replacement for N2 fixation in nonirrigated and irrigated environments. Eight cultivars ranging from Maturity Group II to IV were planted on 17 Apr. 2002, 2 Apr. 2003, and 25 Mar. 2004. Not all cultivars were evaluated in all 3 yr. Glyphosate herbicide was used in all 3 yr and a non‐glyphosate herbicide treatment was applied in 2002. Cultivars grown in 2003 were also evaluated under an application of 21.3 kg ha−1 of Mn. All cultivar, herbicide, and Mn treatments were evaluated in irrigated and nonirrigated environments with fertilizer N (PlusN treatment) or without fertilizer N (ZeroN treatment). In the PlusN treatment, granular NH4NO3 was surface applied at soybean emergence at rates of 290 kg ha−1 in 2002, 310 kg ha−1 in 2003, and 360 kg ha−1 in 2004. When analyzed over all management practices (years, cultivars, herbicide, and Mn treatments), the PlusN treatment resulted in significantly decreased ureide concentration (57.2 and 53.5% reduction) and significantly increased biomass accumulation (14.1 and 16.7%), N accumulation (12.8 and 28.1%), and seed yield (7.7 and 15.5%) for the irrigated and nonirrigated environments, respectively. The majority of the yield increase in each environment resulted from increased number of seed (9.5% irrigated and 16.2% nonirrigated). These results confirm the sensitivity of N2 fixation to drought and indicate that N2 fixation may limit yield of soybean grown in both irrigated and nonirrigated environments of the midsouthern USA, and that N2 fixation deficiencies occur before the beginning of processes that determine number of seed.

Effects of Flooding Regime and Seedling Treatment on Early Survival and Growth of Nuttall Oak

Abstract Effects of flooding on survival and growth of three different types of Nuttall oak (Quercus texana Buckl.) seedlings were observed at the end of third and fifth growing seasons at Yazoo National Wildlife Refuge, Mississippi, U.S.A. Three types of seedlings were planted in January 1995 in a split‐plot design, with four replications at each of two elevations on floodprone, former cropland in Sharkey clay soil. The lower of the two planting elevations was inundated for 21 days during the first growing season, whereas the higher elevation did not flood during the 5‐year period of this study. The three types of 1‐0 seedlings were bareroot seedlings, seedlings grown in containers (3.8 × 21–cm plastic seedling cones), and container‐grown seedlings inoculated with vegetative mycelia of Pisolithus tinctorius (Pers.) Coker. Survival of all the three seedling types was greatest at the lower, intermittently flooded elevation, indicating that drought and related effects on plant competition were more limiting to seedling survival than flooding. At the lower elevation, survival of mycorrhizal‐inoculated container seedlings was greater than that of noninoculated container seedlings. Survival among bareroot seedlings and inoculated container seedlings was not significantly different at either elevation. At the higher, nonflooded elevation, however, bareroot seedling survival was greater than the survival of container seedlings without inoculation. Differences were significant among the inoculated and the noninoculated container seedlings, with higher survival of inoculated seedlings at both elevations, though differences were only significant in year 3. At the end of the fifth year, height of bareroot seedlings was significantly greater than the heights of both types of container‐grown seedlings at both planting elevations. Because seedlings grown in the plastic seedlings cones did not survive better than the bareroot seedlings at either planting elevation, the bareroot stock appear to be the economically superior choice for regeneration in Sharkey soil.

Predicting Pesticide Transport in Mulch‐Amended Soils

With adoption of combine harvest technology, massive sugarcane (Saccharum Spp. Hyb.) residue is left on the soil surface following each harvest, which could affect the environmental fate and efficacy of applied pesticides in subsequent cropping seasons. The objective of this study was to quantify the retention characteristics and mobility of a commonly used herbicide, atrazine (2‐chloro‐4‐ethylamine‐6‐isopropylamino‐S‐triazine), in soils when sugarcane mulch residue was present. Specifically, adsorption and desorption kinetic batch experiments were performed to quantify atrazine retention by the mulch residue over time. Atrazine sorption exhibited time‐dependent behavior and was followed by slow release regardless of input concentration. A kinetic—equilibrium model based on a second‐order two‐site (SOTS) formulation was successful in describing atrazine adsorption versus time by the mulch residue. One set of model parameters was capable of describing atrazine release based on six successive desorption steps. As a test for the applicability of the model, data sets from two other experiments where sorption and release was measured for extended time periods (1348 and 2476 h) were successfully predicted by the SOTS model. Miscible displacement methods were used to measure the mobility of atrazine in packed columns where the mulch residue was mixed with a reference sand material or a Sharkey clay soil (very fine, smectitic, thermic Chromic Epiaquerts). Use of batch‐measured model parameters did not adequately describe atrazine mobility in the sand‐mulch column. In contrast, for a Sharkey‐mulch soil column, a two‐compartment SOTS model was successful in predicting atrazine breakthrough results where independently estimated parameters for the mulch residue and the soil matrix were used as the input parameters. Results also showed that mixing of mulch with soil created physical non‐equilibrium condition in the columns, which was responsible for errors in model predictions.

Denitrification and N2O emission from forested and cultivated alluvial clay soil

Restored forested wetlands reduce N loads in surface discharge through plant uptake and denitrification. While removal of reactive N reduces impact on receiving waters, it is unclear whether enhanced denitrification also enhances emissions of the greenhouse gas N2O, thus compromising the water-quality benefits of restoration. This study compares denitrification rates and N2O:N2 emission ratios from Sharkey clay soil in a mature bottomland forest to those from an adjacent cultivated site in the Lower Mississippi Alluvial Valley. Potential denitrification of forested soil was 2.4 times of cultivated soil. Using intact soil cores, denitrification rates of forested soil were 5.2, 6.6 and 2.0 times those of cultivated soil at 70, 85 and 100% water-filled pore space (WFPS), respectively. When NO3 was added, N2O emissions from forested soil were 2.2 times those of cultivated soil at 70% WFPS. At 85 and 100% WFPS, N2O emissions were not significantly different despite much greater denitrification rates in the forested soil because N2O:N2 emission ratios declined more rapidly in forested soil as WFPS increased. These findings suggest that restoration of forested wetlands to reduce NO3 in surface discharge will not contribute significantly to the atmospheric burden of N2O.

EFFECT OF SOYBEAN STEM GROWTH HABIT ON HEIGHT AND NODE NUMBER AFTER BEGINNING BLOOM IN THE MIDSOUTHERN USA

There are two broad types of stem growth habit in soybean [Glycine max (L.) Merr.]: indeterminate [generally maturity group (MG) IV and earlier] and determinate (generally MG V and later). Field studies of irrigated April and May plantings were conducted on Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (lat. 33°26′N), to determine if current expectations for stem growth habit are valid in genetic backgrounds of indeterminate MG IV and determinate MG V cultivars that are used in the early soybean production system (ESPS) in the midsouthern USA. All cultivars increased height and node number between beginning bloom (R1) and stem termination (ST), but the increases were greater for MG IV cultivars. Height and node number were greater at R1 for MG V than for MG IV cultivars, and greater for MG IV cultivars at ST. Averaged across planting date and year, MG V cultivars increased height and node number after R1 by 23 cm and 3.7 nodes, respectively. These results indicate that determinate MG V cultivars are capable of producing significant increases in height and node number after R1.

Influence of Early-Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate-Resistant and Susceptible Soybean

Field studies were conducted on Sharkey clay soil (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate-resistant (GR) and non-GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre-emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non-GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non-GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest-yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non-GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non-GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST-only was not necessary for achieving greatest yield or net return with either non-GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.

Effect of tillage on microbial characteristics and herbicide degradation in a Sharkey clay soil

Field and laboratory studies were conducted at Stoneville, MS, from 1996 to 1998 to determine the influence of subsoiling (SS) and conventional tillage (CT) of a Sharkey clay soil on microbial characteristics and herbicide degradation. Soil samples obtained from imazaquin-treated and nontreated plots from the soybean row and interrow position were analyzed. Because only the row position is actually disturbed by SS, a comparison of row and interrow position on the parameter was conducted. Imazaquin (preemergence, 140 g ai ha−1) had no effect on microbial populations, microbial enzyme activity (fluorescein diacetate [FDA] hydrolysis and triphenyl-tetrazolium chloride [TTC] dehydrogenase), and organic carbon content. Estimates of microbial activity based on FDA hydrolysis and TTC dehydrogenase activity indicated greater activity under CT; however, microbial biomass and organic carbon were not affected by tillage or row position. A laboratory study assessed the degradation of carboxyl- and ring-labeled 2,4-D as influenced by tillage and row position. Soils from CT plots had an initially higher mineralization rate of 14C carboxyl-labeled 2,4-D compared to soils from SS plots; however, no effect of tillage or row position was observed on the cumulative amount of 14CO2 evolved 14 d after treatment (DAT) in 1996 and 18 DAT in 1998. In studies with ring-labeled 2,4-D, a higher 14CO2 evolution was detected in soils obtained from SS plots, regardless of row position, whereas a greater amount of radioactivity was observed in the unextractable fraction from CT soils. Because differences in 2,4-D mineralization between tillage regimes were minimal, adoption of SS as a tillage practice for heavy clay soils in the Mississippi Delta may have a limited effect on microbial characteristics and biodegradation of soil-applied herbicides.

Imazaquin mobility and persistence in a Sharkey clay soil as influenced by tillage systems

Field studies were conducted at Delta Research and Extension Center, Stoneville, MS, in 1996, 1997, and 1998 to assess the effect of tillage systems (conventional tillage and subsoiling) on the environmental fate of imazaquin in a Sharkey clay soil. Imazaquin was applied preemergence at 140 g ai ha−1. Subsoiling in the fall did not affect imazaquin dissipation, total volume of runoff, imazaquin concentration in runoff, or imazaquin concentration in soil, as determined by chemical extraction. A corn root bioassay revealed no differences due to tillage systems in plant-available imazaquin in soil. Imazaquin concentration measured by chemical extraction or bioassay diminished over time, with a half-life ranging from 8 to 25 d. A field bioassay utilizing cotton and corn was conducted in 1997 and 1998 using plots that had received imazaquin the previous year. In 1997, 2 wk after planting, cotton and corn injury ranged from 3 to 15%, whereas no injury was observed in 1998. Injury symptoms declined over time, with no injury 5 wk after planting in either year. Although early-season cotton stunting and slight discoloration of corn was apparent in 1997, imazaquin residues did not affect subsequent vegetative and reproductive growing patterns of cotton or corn. In 1998, corn and cotton height were significantly greater in subsoiled plots compared to conventional tillage.

Predicting Solute Transport in Soils Second‐Order Two‐Site Models

Failure of numerous efforts to describe the movement of reactive solutes in soils is often due to inaccurate identification of solute–soil interactions or to the lack of independently derived model parameters. The main objective of this work was to test the applicability of chemical vs. physical nonequilibrium approaches in describing the transport of solutes in soils. The models evaluated are based on second‐order two‐site approaches (SOTS) with and without consideration of physical nonequilibrium in soils. The capability of these approaches for predicting the transport of metolachlor [2‐Chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐1‐methylethyl)acetamide] in Sharkey clay (very‐fine, smectitic, thermic Chromic Epiaquerts) soil columns of different aggregate sizes (&lt;2, 2–4, and 4–6 mm) was examined. Moreover, two sets of model parameters were independently derived from the kinetic retention experiments. The first set was based on kinetic adsorption isotherms, and the second set utilized both adsorption and desorption kinetic retention (batch) results. Judging from data on the total root mean square error, parameters based on adsorption and desorption batch results provided breakthrough curve (BTC) predictions that are improved over those of parameters from adsorption kinetics only. The coupled physical and chemical nonequilibrium model (SOTS plus mobile–immobile [MIM]: SOTS–MIM) considerably improved BTC predictions for the 2‐ to 4‐ and 4‐ to 6‐mm soil aggregate sizes. We conclude that the modified SOTS and SOTS–MIM methods provided an improved description of metolachlor transport in the Sharkey soil. Based on total root mean square errors, the modified SOTS–MIM with parameters derived from adsorption and desorption kinetic retention experiments provided best overall BTC predictions.