Bromoxynil degradation in a Mississippi silt loam soil

BACKGROUND: The objectives of these laboratory experiments were: (1) to assess bromoxynil sorption,mineralization, bound residue formation and extractable residue persistence in a Dundee silt loam collected from 0\u20132 cm and 2\u201310 cm depths under continuous conventional tillage and no-tillage; (2) to assess the effects of autoclaving on bromoxynilmineralization and bound residue formation; (3) to determine the partitioning of non-extractable residues; and (4) to ascertain the effects of bromoxynil concentration on extractable and bound residues andmetabolite formation. RESULTS: BromoxynilKd values rangedfrom 0.7 to1.4 Lkg−1 andwerepositively correlatedwith soilorganic carbon.Cumulative mineralization (38.5% \ub1 1.5), bound residue formation (46.5% \ub1 0.5) and persistence of extractable residues (T1/2 < 1 day) in non-autoclaved soils were independent of tillage and depth. Autoclaving decreased mineralization and bound residue formation 257-fold and 6.0-fold respectively. Bromoxynil persistence in soil was rate independent (T1/2 < 1 day), and the majority of non-extractable residues (87%) were associated with the humic acid fraction of soil organicmatter. CONCLUSIONS: Irrespective of tillage or depth, bromoxynil half-life in native soil is less than 1 day owing to rapid incorporation of the herbicide into non-extractable residues. Bound residue formation is governed principally by biochemical metabolite formation and primarily associated with soil humic acids that are moderately bioavailable for mineralization. These data indicate that the risk of off-site transport of bromoxynil residues is low owing to rapid incorporation into non-extractable residues

Soil depth and tillage effects on glyphosate degradation

The use of glyphosate-resistant crops facilitated the widespread adoption of no-tillage (NT) cropping systems. The experimental objectives were to determine glyphosate sorption, mineralization, and persistence at two depths [0-2 cm (A) and 2-10 cm (B)] in a silt loam managed under long-term conventional tillage (CT) or NT soybean. Relative to the other soils, organic carbon (OC) and fluorescein diacetate (FDA) hydrolytic activity were at least 1.4-fold higher in NT-A. Glyphosate Kd values ranged from 78.2 to 48.1 and were not correlated with OC. Cumulative glyphosate mineralized after 35 days was highest in NT-A soil (70%), intermediate in CT-A and CT-B (63%), and least in NT-B (51%). Mineralization was positively correlated with OC and FDA activity, but negatively correlated with Kd, indicating that sorption decreased bioavailability. Independent of tillage and depth, the half-lives for 0.01 N CaCl2 and 0.1 N NaOH extractable residues (bioavailable residues and residues bound to iron and aluminum oxides, respectively) were e1.2 h and e14.2 days, respectively. These data indicate that glyphosate sorption and persistence are similar between the surface of NT and CT soils and that the adoption of NT will likely have minimal impact on the risk for nontarget effects of glyphosate on soil microflora or transport in surface runoff

Aspergillus flavus occurence and expression of biosynthesis genes in soil

The carcinogen, aflatoxin B1 (AFB1) produced by Aspergillus flavus, is a major food safety concern in crops. However, information on AFB1 occurrence in soil and crop residue is scarce. A series of experiments investigated the occurrence of AFB1 in soil and corn residues, and ascertained the ecology of A. flavus in a Dundee silt loam soil. Samples of untilled soil (0 to 2 cm) and residues were collected in March 2007 from plots previously planted with a corn isoline containing the Bacillus thurengiensis endotoxin gene (Bt) or the parental non-Bt isoline. Aflatoxin B1 levels were significantly different in various corn residues. The highest AFB1 levels were observed in cobs containing grain, with 145 and 275 ng g-1, in Bt and non-Bt, respectively (Pr ≥ F = 0.001). Aflatoxin levels averaged 3.3 and 9.6 ng g-1 in leaves/stalks and cobs without grain, respectively. All soils had AFB1 ranging from 0.6 to 5.5 ng g-1 with similar levels in plots from Bt and non-Bt corn. Based on cultural methods, soil contained from log (10) 3.1 to 4.5 A. flavus CFU g-1 with about 60% of isolates producing aflatoxin. Laboratory experiments demonstrated that AFB1 is rapidly degraded in soil at 28oC (half-life ≤ 5 days). The potential of the soil A. flavus to produce aflatoxins was confirmed by molecular methods. Transcription of five aflatoxin biosynthesis genes including afllD, aflG, aflP, aflR and aflS were detected by RT-PCR analysis in soil. Although AFB1 appears to be transient in soils, it is clear that AFB1 is produced in surface soil in the presence of corn residues as indicated by A. flavus CFU levels, AFB1 detection, and expression of aflatoxin biosynthetic genes

Atrazine dissipation in s-triazine-adapted and non-adapted soil from Colorado and Mississippi: implications of enhanced degradation on atrazine fate and transport parameters

Soil bacteria have developed novel metabolic abilities resulting in enhanced atrazine degradation. Consequently, there is a need to evaluate the eff ects of enhanced degradation on parameters used to model atrazine fate and transport. Th e objectives of this study were (i) to screen Colorado (CO) and Mississippi (MS) atrazine-adapted and non-adapted soil for genes that code for enzymes able to rapidly catabolize atrazine and (ii) to compare atrazine persistence, Q10, β, and metabolite profi les between adapted and non-adapted soils. Th e atzABC and/or trzN genes were detected only in adapted soil. Atrazine\u2019s average half-life in adapted soil was 10-fold lower than that of the non-adapted soil and 18-fold lower than the USEPA estimate of 3 to 4 mo. Q10 was greater in adapted soil. No diff erence in β was observed between soils. Th e accumulation and persistence of mono-Ndealkylated metabolites was lower in adapted soil; conversely, under suboptimal moisture levels in CO adapted soil, hydroxyatrazine concentrations exceeded 30% of the parent compounds\u2019 initial mass. Results indicate that (i) enhanced atrazine degradation and atzABC and/or trzN genes are likely widespread across the Western and Southern corn-growing regions of the USA; (ii) persistence of atrazine and its mono- N-dealkylated metabolites is signifi cantly reduced in adapted soil; (iii) hydroxyatrazine can be a major degradation product in adapted soil; and (iv) fate, transport, and risk assessment models that assume historic atrazine degradation pathways and persistence estimates will likely overpredict the compounds\u2019 transport potential in adapted soil

Dynamics of mycotoxin and Aspergillus flavus levels in aging Bt and non-Bt corn residues under Mississippi no-till conditions

Mycotoxin and Aspergillus flavus levels in soil-surface corn debris left by no-till agriculture methods (stover, cobs, and cobs with grain) were determined during the December-March fallow period for near-isogenic Bt and non-Bt hybrid corn. By December, average mycotoxin levels in non-Bt corn were many times higher in cobs with grain than in grain harvested in September (total aflatoxins, 774 vs 211 ng/g; total fumonisins, 216 vs 3.5 μg/g; cyclopiazonic acid, 4102 vs 72.2 μg/g; zearalenone, 0.2 vs < 0.1 μg/g). No trichothecenes were detected. Levels of mycotoxins and A. flavus propagules were ∼10- to 50-fold lower in cobs without grain and stover, respectively, for all mycotoxins except zearalenone. Mycotoxin levels in corn debris fractions decreased during winter but began to rise in March. Levels of all mycotoxins and A. flavus propagules were lower in harvested grain and debris from Bt than non-Bt corn, but differences were significant (p < 0.05) only for aflatoxins