Martin A. Locke

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index ofN availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N 0 , k (mineralization rate), and N 0 * (N 0 estimated with a fixed k equal to 0.054 wk -1 ). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCI (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N 0 (r > 0.85) and had linear regressions with r 2 > 0.60. None of the indices were good predictors ofk. Correlations between indices and N 0 * improved compared with N 0 , ranging from r = 0.90 to 0.95. Total N and Hush of CO 2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N 0 (R 2 = 0.85) while the best combination for predicting N 0 * (R 2 = 0.94) included TN, Fl_CO2 Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Nutrient mitigation capacity in Mississippi Delta, USA drainage ditches

Eutrophication and hypoxia within aquatic systems are a serious international concern. Various management practices have been proposed to help alleviate nutrient loads transported to the Gulf of Mexico and other high-profile aquatic systems. The current study examined the nutrient mitigation capacity of a vegetated (V) and non-vegetated (NV) agricultural drainage ditch of similar size and landform in the Mississippi Delta. While no statistically significant differences in ammonium, nitrate, or dissolved inorganic phosphorus mitigation between the two ditches existed, there were significant differences in total inorganic phosphorus percent load reductions (V: 36% +/- 4; NV: 71% +/- 4). However, both agricultural drainage ditches were able to mitigate nutrients, thus reducing the load reaching downstream aquatic receiving systems. Further studies examining ecosystem dynamics within drainage ditches such as sediment and plant nutrient partitioning, as well as microbial processes involved, are needed to provide a better understanding of natural nutrient variability, seasonality and flux.

Microbial adaptation for accelerated atrazine mineralization/ degradation in Mississippi Delta soils

Abstract Most well-drained Mississippi Delta soils have been used for cotton production, but corn has recently become a desirable alternative crop, and subsequently, atrazine use has increased. Between 2000 and 2001, 21 surface soils (0 to 5 cm depth) with known management histories were collected from various sites in Leflore, Sunflower, and Washington counties of Mississippi. Atrazine degradation was assessed in 30-d laboratory studies using 14C-ring–labeled herbicide. Mineralization was extensive in all soils with a history of one to three atrazine applications with cumulative mineralization over 30 d ranging from 45 to 72%. In contrast, cumulative mineralization of atrazine from three soils with no atrazine history was only 5 to 10%. However, one soil with no history of atrazine application mineralized 54 and 29% of the atrazine in soils collected in 2000 and 2001, respectively. Methanol extracted 15 to 23% of the 14C-atrazine 7 d after treatment in soils having two applications within the past 6 yr, whereas 65 to 70% was extracted from no-history soils. First-order kinetic models indicated soil with 2 yr of atrazine exposure exhibited a half-life of less than 6 d. Most probable number (MPN) estimates of atrazine-ring mineralizing-microorganisms ranged from 450 to 7,200 propagules g−1 in atrazine-exposed soils, and none were detected in soils with no history of atrazine use. Although most soils exhibited rapid atrazine mineralization, analysis of DNA isolated from these soils by direct or nested polymerase chain reaction (PCR) failed to amplify DNA sequences with primers for the atzA atrazine chlorohydrolase gene. These results indicate that microbial populations capable of accelerated atrazine degradation have developed in Mississippi Delta soils. This may reduce the weed control efficacy of atrazine but also reduce the potential for off-site movement. Studies are continuing to identify the genetic basis of atrazine degradation in these soils. Nomenclature: Atrazine; cyanazine; DEA, de-ethyl atrazine; DIA, de-isopropyl atrazine; corn, Zea mays L.; cotton, Gossypium hirsutum L.

Cover crop, tillage, and herbicide effects on weeds, soil properties, microbial populations, and soybean yield

Abstract A field study was conducted during 1997 to 2001 on a Dundee silt loam soil at Stoneville, MS, to examine the effects of rye and crimson clover residues on weeds, soil properties, soil microbial populations, and soybean yield in conventional tillage (CT) and no-tillage (NT) systems with preemergence (PRE)-only, postemergence (POST)-only, and PRE plus POST herbicide programs. Rye and crimson clover were planted in October, desiccated in April, and tilled (CT plots only) before planting soybean. Both cover-crop residues reduced density of barnyardgrass, broadleaf signalgrass, browntop millet, entireleaf morningglory, and hyssop spurge but did not affect yellow nutsedge at 7 wk after soybean planting (WAP) in the absence of herbicides. Densities of these weed species were generally lower with PRE-only, POST-only, and PRE plus POST applications than with no-herbicide treatment. Total weed dry biomass was lower when comparing CT (1,570 kg ha−1) with NT (1,970 kg ha−1), rye (1,520 kg ha−1) with crimson clover (2,050 kg ha−1), and PRE plus POST (640 kg ha−1) with PRE-only (1,870 kg ha−1) or POST-only (1,130 kg ha−1) treatments at 7 WAP. Soils with crimson clover had higher organic matter, NO3–N, SO4–S, and Mn, and lower pH compared with rye and no–cover crop soils. Total fungi and bacterial populations and fluorescein diacetate hydrolytic activity were higher in soil with crimson clover, followed by rye and no cover crop. Soybean yields were similar between CT (1,830 kg ha−1) and NT (1,960 kg ha−1), no cover crop (2,010 kg ha−1) and rye (1,900 kg ha−1), and rye and crimson clover (1,790 kg ha−1), but they were higher in PRE plus POST (2,260 kg ha−1) than in PRE-only (1,890 kg ha−1) or POST-only (1,970 kg ha−1) treatments. Nomenclature: Acifluorfen; bentazon; clethodim; flumetsulam; metolachlor; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash BRAPP; browntop millet, Brachiaria ramosa (L.) Stapf. PANRA; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray IPOHG; hyssop spurge, Euphorbia hyssopifolia L. EPHHS; yellow nutsedge, Cyperus esculentus L. CYPES; crimson clover, Trifolium incarnatum L. ‘Dixie’; rye, Secale cereale L. ‘Elbon’; soybean, Glycine max (L.) Merr.

Mitigation of two pyrethroid insecticides in a Mississippi Delta constructed wetland

Constructed wetlands are a suggested best management practice to help mitigate agricultural runoff before entering receiving aquatic ecosystems. A constructed wetland system (180 m x 30 m), comprising a sediment retention basin and two treatment cells, was used to determine the fate and transport of simulated runoff containing the pyrethroid insecticides lambda-cyhalothrin and cyfluthrin, as well as suspended sediment. Wetland water, sediment, and plant samples were collected spatially and temporally over 55 d. Results showed 49 and 76% of the studys measured lambda-cyhalothrin and cyfluthrin masses were associated with vegetation, respectively. Based on conservative effects concentrations for invertebrates and regression analyses of maximum observed wetland aqueous concentrations, a wetland length of 215 m x 30 m width would be required to adequately mitigate 1% pesticide runoff from a 14 ha contributing area. Results of this experiment can be used to model future design specifications for constructed wetland mitigation of pyrethroid insecticides.

Interactions of tillage and soil depth on fluometuron degradation in a Dundee silt loam soil.

Fluometuron (N,N-dimethyl-N 0 -[3-(trifluoromethyl)phenyl]urea) is the most widely used soil applied herbicide in cotton (Gossypium hirsutum) production in the Southeastern United States. Thus, there is a need to understand the persistence of this herbicide under various crop management systems in regards to both herbicide efficiency and environmental fate. The effects of long-term no-tillage practices on fluometuron degradation were studied in a Dundee silt loam under laboratory conditions. Soil was collected at four depths (0‐2, 2‐5, 5‐10, and 10‐25 cm) from field plots following 11 years of continuous no-tillage (NT) or conventional tillage (CT). Organic carbon content, microbial biomass, and fluorescein diacetate (FDA)-hydrolytic activity were 48, 106, and 127% greater, respectively, in the upper 2 cm of NT soil compared to CT soil. Microbial biomass and FDA-hydrolytic activity were 39‐66 and 34‐99% greater in the CT compared to NT soil, respectively, in the 2‐5 and 5‐ 10 cm depths. Fluometuron degradation was assessed under laboratory conditions using 14 C-ring labeled fluometuron. Fluometuron was degraded more rapidly at depths between 0 and 10 cm of the CT soil relative to the NT soil. Fluometuron degradation was described as first-order (ka 0:0754, 0.0608, 0.0273, and 0.0074 per day in the 0‐2, 2‐5, 5‐10, and 10‐25 cm depths of CT soils compared to 0.035, 0.0368, 0.0145, and 0.0138 per day in respective depths of NT soil). Although the surface 0‐2 cm of the NT soil had greater microbial activity and biomass, higher fluometuron sorption in the surface 0‐2 cm NT soil compared to CT (Freundlich coefficient âKfa NTa 7:12 versus CTa 1:88) reduced fluometuron solution concentration in NT soil and may have impeded degradation. Interactions between soil biological activity and fluometuron soil solution concentrations, mediated by sorption, may determine the potential for fluometuron degradation in soils under long-term NT practices. Published by Elsevier Science B.V.

Integrating soil conservation practices and glyphosate-resistant crops : impacts on soil

BACKGROUND Conservation practices often associated with glyphosate-resistant crops, e.g. limited tillage and crop cover, improve soil conditions, but only limited research has evaluated their effects on soil in combination with glyphosate-resistant crops. It is assumed that conservation practices have similar benefits to soil whether or not glyphosate-resistant crops are used. This paper reviews the impact on soil of conservation practices and glyphosate-resistant crops, and presents data from a Mississippi field trial comparing glyphosate-resistant and non-glyphosate-resistant maize (Zea mays L.) and cotton (Gossypium hirsutum L.) under limited tillage management. RESULTS Results from the reduced-tillage study indicate differences in soil biological and chemical properties owing to glyphosate-resistant crops. Under continuous glyphosate-resistant maize, soils maintained greater soil organic carbon and nitrogen as compared with continuous non-glyphosate-resistant maize, but no differences were measured in continuous cotton or in cotton rotated with maize. Soil microbial community structure based on total fatty acid methyl ester analysis indicated a significant effect of glyphosate-resistant crop following 5 years of continuous glyphosate-resistant crop as compared with the non-glyphosate-resistant crop system. Results from this study, as well as the literature review, indicate differences attributable to the interaction of conservation practices and glyphosate-resistant crop, but many are transient and benign for the soil ecosystem. CONCLUSIONS Glyphosate use may result in minor effects on soil biological/chemical properties. However, enhanced organic carbon and plant residues in surface soils under conservation practices may buffer potential effects of glyphosate. Long-term field research established under various cropping systems and ecological regions is needed for critical assessment of glyphosate-resistant crop and conservation practice interactions.

Degradation of 2,4-D and fluometuron in cover crop residues

Degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was studied in hairy vetch (Viciavillosa Roth) and rye (Secale cereale L.) residues. Transformation offluometuron (1,1-dimetbyl-3-(ct,~ct-triliuoro-m-tolyl)urea)was also evaluated in annual ryegrass (Lolium multiforum Lain.) residues. Microflora associated with herbiddcdesiccated ~ vetch and rye residues were 100-fold or greater than soils. Microbial activity (fluorescein diacetate hydrolysis and respiration) were 6-fold or greater in hairy vetch and rye residues than soil. In 14-d studies, 78 to 82% versus 28 to 40% of 14C-carboxyl-labelled 2,4-D and 48 to 60% versus 5 to 17% ofl4C-dng labelled 2,4-D were mineralized in soil and crop residues, respectively. Fhometuron can be degraded by Ndemethylation in ryegrass residues at rates similar to soil, however, high moisture content was required. Degradation ofherbiddes in cover crop residues was most likely due to limited bioavailability rather than biological activity. ©1998 Elsevier Science Ltd. All rights reserved

Annualized Agricultural Non-Point Source model application for Mississippi Delta Beasley Lake watershed conservation practices assessment

The Annualized Agricultural Non-Point Source (AnnAGNPS) model has been developed to quantify watershed response to agricultural management practices. The objective of this study was to identify critical areas where conservation practices could be implemented and to predict their impact on Beasley Lake water quality in the Mississippi Delta using AnnAGNPS. Model evaluation was first performed by comparing the observed runoff and sediment from a US Geological Survey gauging station draining 7 ha (17 ac) of Beasley Lake watershed with the AnnAGNPS simulated runoff and sediment. The model demonstrated satisfactory capability in simulating runoff and sediment at an event scale. Without calibration, the Nash-Sutcliffe coefficient of efficiency was 0.81 for runoff and 0.54 for sediment; the relative error was 0.1 for runoff and 0.18 for sediment, and the Willmott index of agreement was 0.94 for runoff and 0.80 for sediment. The quantity of water and sediment produced from each field within the Beasley Lake watershed, quantity of water and sediment reaching Beasley Lake, and the potential impact of various USDA Natural Resources Conservation Service conservation programs on water quality were then simulated. High sediment-producing areas for nonpoint source pollution control were identified where sediment loads could be reduced by 15% to 77% using conservation practices. Simulations predicted that converting all cropland to no-till soybeans (Glycine max [L.] Merr.) would reduce sediment load by 77% whereas no-till cotton (Gossypium hirsutum L.) would reduce it 64%. The approach taken in this study could be used elsewhere in applying AnnAGNPS to ungauged watersheds or watersheds with limited field observations for conservation program planning or evaluation.

Mineralization of the allelochemical sorgoleone in soil.

The allelochemical sorgoleone is produced in and released from the root hairs of sorghum (Sorghum bicolor). Studies have confirmed that it is the release of sorgoleone that causes the phytotoxic properties of sorghum, and sorgoleone has a potential to become a new natural herbicide, or the weed suppressive activity of sorghum can be utilized in integrated weed management. Since sorgoleone is released into soil, knowledge of the fate of sorgoleone in soil is essential if it is to be utilized as an herbicide. Fate studies will characterize the persistence and mobility of the compound. Three types of radioactively labelled sorgoleone were produced and used to study mineralization (complete degradation to CO(2)) of this lipid benzoquinone in four soils, two from the United States of America (Mississippi) and two from Denmark. The studies showed that sorgoleone was mineralized in all soils tested. The methoxy group of sorgoleone was readily mineralized, whereas mineralization of the remaining molecule was slower. Mineralization kinetics indicated that microorganisms in American soils were able to use sorgoleone as a source of energy.