Certain Soil Surfactants Could Become a Source of Soil Water Repellency after Repeated Application.

Repeated application of soil surfactants, or wetting agents, is a common practice for alleviating soil water repellency associated with soil organic coatings. However, wetting agents are organic compounds that may also coat soil particle surfaces and reduce wettability. For this experiment, hydrophobic sands from the field and fresh, wettable sands were collected and treated with either a polyoxyalkylene polymer (PoAP) or alkyl block polymer (ABP) wetting agent, or water only treatments served as a control. Following repeated treatment application and sequential washings, dissolved and particulate organic carbon (OC) were detected in the leachates of both sand systems. The total amount of OC recovered in leachates was 88% or less than the OC introduced by the wetting agents, indicating sorption of wetting agent monomers to soil particle surfaces regardless of soil hydrophobicity status. While ABP treatment did not alter solid phase organic carbon (SOC) in the sands studied, PoAP application increased SOC by 16% and 45% which was visible in scanning electronic microscopy images, for hydrophobic and wettable sands, respectively. PoAP application also increased the hydrophobicity of both sands that were studied. In contrast, ABP treatment increased the wettability of hydrophobic sand. Our results provide strong evidence that certain wetting agents may increase soil hydrophobicity and exacerbate wettability challenges if used repeatedly over time.

Sampling and Handling of Soil to Identify Microorganisms with Impacts on Plant Growth.

Sampling and handling of soils and rhizosphere soil are very critical steps for obtaining representative microbial cultures and genomic material from these environments. Attention to position in the landscape of a sampling site, previous management of the site, time of year, and depth of sampling is important to assure representative samples. Detailed protocols are provided to assist environmental microbiologists and molecular biologists in proper sampling and handling of soils that serve as the source of cultures and DNA for subsequent use in important experiments carried out in the laboratory.

Veterinary Antibiotic Effects on Atrazine Degradation and Soil Microorganisms.

Veterinary antibiotics (VAs) in manure applied to agricultural lands may change agrichemical degradation by altering soil microbial community structure or function. The objectives of this study were to investigate the influence of two VAs, sulfamethazine (SMZ) and oxytetracycline (OTC), on atrazine (ATZ) degradation, soil microbial enzymatic activity, and phospholipid fatty acid (PLFA) markers. Sandy loam soil with and without 5% swine manure (w/w) was amended with 0 or 500 µg kgC radiolabeled ATZ and with 0, 100, or 1000 µg kg SMZ or OTC and incubated at 25°C in the dark for 96 d. The half-life of ATZ was not significantly affected by VA treatment in the presence or absence of manure; however, the VAs significantly ( < 0.05) inhibited ATZ mineralization in soil without manure (25-50% reduction). Manure amendment decreased ATZ degradation by 22%, reduced ATZ mineralization by 50%, and increased the half-life of ATZ by >10 d. The VAs had limited adverse effects on the microbial enzymes ß-glucosidase and dehydrogenase in soils with and without manure. In contrast, manure application stimulated dehydrogenase activity and altered chlorinated ATZ metabolite profiles. Concentrations of PLFA markers were reduced by additions of ATZ, manure, OTC, and SMZ; adverse additive effects of combined treatments were noted for arbuscular mycorrhizal fungi and actinobacteria. In this work, the VAs did not influence persistence of the ATZ parent compound or chlorinated ATZ metabolite formation and degradation. However, reduced CO evolved from VA-treated soil suggests an inhibition to the degradation of other ATZ metabolites due to an altered soil microbial community structure.

Long-term agroecosystem research in the central Mississippi river basin: introduction, establishment, and overview.

Many challenges currently facing agriculture require long-term data on landscape-scale hydrologic responses to weather, such as from the Goodwater Creek Experimental Watershed (GCEW), located in northeastern Missouri, USA. This watershed is prone to surface runoff despite shallow slopes, as a result of a significant smectitic clay layer 30 to 50 cm deep that restricts downward flow of water and gives rise to a periodic perched water table. This paper is the first in a series that documents the database developed from GCEW. The objectives of this paper are to (i) establish the context of long-term data and the federal infrastructure that provides it, (ii) describe the GCEW/ Central Mississippi River Basin (CMRB) establishment and the geophysical and anthropogenic context, (iii) summarize in brief the collected research results published using data from within GCEW, (iv) describe the series of papers this work introduces, and (v) identify knowledge gaps and research needs. The rationale for the collection derives from converging trends in data from long-term research, integration of multiple disciplines, and increasing public awareness of increasingly larger problems. The outcome of those trends includes being selected as the CMRB site in the USDA-ARS Long-Term Agro-Ecosystem Research (LTAR) network. Research needs include quantifying watershed scale fluxes of N, P, K, sediment, and energy, accounting for fluxes involving forest, livestock, and anthropogenic sources, scaling from near-term point-scale results to increasingly long and broad scales, and considering whole-system interactions. This special section informs the scientific community about this database and provides support for its future use in research to solve natural resource problems important to US agricultural, environmental, and science policy.

Stimulated rhizodegradation of atrazine by selected plant species.

The efficacy of vegetative buffer strips (VBS) in removing herbicides deposited from surface runoff is related to the ability of plant species to promote rapid herbicide degradation. A growth chamber study was conducted to compare C-atrazine (ATR) degradation profiles in soil rhizospheres from different forage grasses and correlate ATR degradation rates and profiles with microbial activity using three soil enzymes. The plant treatments included: (i) orchardgrass ( L.), (ii) smooth bromegrass ( Leyss.), (iii) tall fescue ( Schreb.), (iv) Illinois bundle flower (), (v) perennial ryegrass ( L.), (vi) switchgrass ( L.), and (vii) eastern gamagrass (). Soil without plants was used as the control. The results suggested that all plant species significantly enhanced ATR degradation by 84 to 260% compared with the control, but eastern gamagrass showed the highest capability for promoting biodegradation of ATR in the rhizosphere. More than 90% of ATR was degraded in the eastern gamagrass rhizosphere compared with 24% in the control. Dealkylation of atrazine strongly correlated with increased enzymatic activities of ß-glucosidase (GLU) ( = 0.96), dehydrogenase (DHG) ( = 0.842), and fluorescein diacetate (FDA) hydrolysis ( = 0.702). The incorporation of forage species, particularly eastern gamagrass, into VBS designs will significantly promote the degradation of ATR transported into the VBS by surface runoff. Microbial parameters widely used for assessment of soil quality, e.g., DHG and GLU activities, are promising tools for evaluating the overall degradation potential of various vegetative buffer designs for ATR remediation.

Dissipation of sulfamethazine and tetracycline in the root zone of grass and tree species.

The detection of veterinary antibiotics (VAs) in drinking water resources resulting from manure disposal operations has raised public health concerns. Previous studies have demonstrated the benefits of using multispecies vegetated buffer strips (VBS) to reduce agrichemical transport from agroecosystems. However, VA fate and subsequent effects of VAs on microbial activities in the root zone ofVBS have not been well documented. A growth chamber study was conducted to investigate dissipation of two commonly administered VAs, sulfamethazine (SMZ) and tetracycline (TC), and the relationship between VA dissipation and soil enzyme activities in the root zone of selected plant species. Switchgrass, eastern gammagrass, orchardgrass, and a hybrid poplar tree were grown in pots containing a Mexico silt loam/sand mixture for 3 mo, followed by plant biomass removal and collection of root zone soil. Radiolabeled (3H) SMZ or TC was applied to the soils and samples were incubated in the dark for 5 wk. Among the plant species studied, hybrid poplar showed enhanced capability for promoting SMZ dissipation. The half-lives of SMZ in soil planted to the poplar tree were significantly reduced by the enhanced enzymatic activity. Comparison of soil enzymatic activities between the antibiotic treatments revealed that fluorescein diacetate hydrolytic and glucosaminidase enzyme activities were significantly lower in TC-treated soils than in SMZ-treated soils. The beta-glucosidase activities were similar between the two VA treatments. Correlation analyses showed that the half-life of SMZ in the soil was negatively correlated with enzymatic activity. Enhanced SMZ dissipation in soil planted to hybrid poplar suggests that incorporation of this plant species in VBS may mitigate deleterious effects of SMZ in the environment.

Glyphosate affects seed composition in glyphosate-resistant soybean.

The cultivation of glyphosate-resistant (GR) soybeans has continuously increased worldwide in recent years mainly due to the importance of glyphosate in current weed management systems. However, not much has been done to understand eventual effects of glyphosate application on GR soybean physiology, especially those related to seed composition with potential effects on human health. Two experiments were conducted to evaluate the effects of glyphosate application on GR soybeans compared with its near-isogenic non-GR parental lines. Results of the first experiment showed that glyphosate application resulted in significant decreases in shoot nutrient concentrations, photosynthetic parameters, and biomass production. Similar trends were observed for the second experiment, although glyphosate application significantly altered seed nutrient concentrations and polyunsaturated fatty acid percentages. Glyphosate resulted in significant decreases in polyunsaturated linoleic acid (18:2n-6) (2.3% decrease) and linolenic acid (18:3n-3) (9.6% decrease) and a significant increase in monounsaturated fatty acids 17:1n-7 (30.3% increase) and 18:1n-7 (25% increase). The combined observations of decreased photosynthetic parameters and low nutrient availability in glyphosate-treated plants may explain potential adverse effects of glyphosate in GR soybeans.

Evaluation of PCR-based quantification techniques to estimate the abundance of atrazine chlorohydrolase gene atzA in rhizosphere soils.

There are many challenges in the accurate quantification of bacterial genes, such as the atrazine-degrading enzyme antA from Pseudomonas sp. strain ADP, from soil samples. We compared four quantitative methods for enumeration of atrazine-degrading bacteria in rhizosphere environments and utilized the optimal probe-based real-time polymerase chain reaction (PCR)-based method in an ongoing bioremediation experiment to monitor atzA copy number over time. We compared three quantitative PCR (qPCR) based methods--quantitative competitive PCR and two real-time qPCR methods--to traditional dilution-plate counting techniques. The optimal real-time qPCR assay was then used to monitor atzA copy number over time in the robust atrazine-degrading Pseudomonas sp. strain ADP-spiked rhizosphere environment. The use of sensitive and reliable probe-based real-time qPCRs for the enumeration of bacterial catabolic genes allows for their detection from soil samples and monitoring of potential degradative populations over time. The addition of arrazine-biodegrading bacteria into arrazine-contaminated sites to remove entrapped atrazine is a promising approach for mitigating atrazine pollution and its metabolites. The methodology contained herein will allow for optimal monitoring of atzA in rhizosphere soil with or without the addition of biodegradative Pseudomonas sp. strain ADP of bacteria.

Effects of glyphosate and foliar amendments on activity of microorganisms in the soybean rhizosphere.

A field study was conducted to determine the effects of glyphosate on microbial activity in the rhizosphere of glyphosate-resistant (GR) soybean and to evaluate interactions with foliar amendments. Glyphosate at 0.84 kg ae ha(-1) was applied GR soybean at the V4-V5 development stages. Check treatments included a conventional herbicide tank mix (2003 study only) and no herbicides (hand-weeded). Ten days after herbicide application, a commercially available biostimulant and a urea solution (21.0% N) were applied to soybean foliage at 33.5 mL ha(-1) and 9.2 kg ha(-1), respectively. Soil and plant samples were taken 0, 5, 10, 15, 20 and 25 days after herbicide application then assayed for enzyme and respiration activities. Soil respiration and enzyme activity increased with glyphosate and foliar amendment applications during the 2002 growing season; however, similar increases were not observed in 2003. Contrasting cumulative rainfall between 2002 and 2003 likely accounted for differences in soil microbial activities. Increases in soil microbial activity in 2002 suggest that adequate soil water and glyphosate application acted together to increase microbial activity. Our study suggests that general soil microbial properties including those involving C and N transformations are not sensitive enough to detect effects of glyphosate on rhizosphere microbial activity. Measurements of soil-plant-microbe relationships including specific microbial groups (i.e., root-associated Fusarium spp.) are likely better indicators of impacts of glyphosate on soil microbial ecology.

Bacterial diversity in rhizospheres of nontransgenic and transgenic corn.

Bacterial diversity in transgenic and nontransgenic corn rhizospheres was determined. In greenhouse and field studies, metabolic profiling and molecular analysis of 16S rRNAs differentiated bacterial communities among soil textures but not between corn varieties. We conclude that bacteria in corn rhizospheres are affected more by soil texture than by cultivation of transgenic varieties.

Management of Weed Seed Banks with Microorganisms.

Successful weed management in agroecosystems centers on manipulating the weed seed bank in soil, the source of annual weed infestations. Despite advances in aboveground weed control and decreases in the production of new seed, weed infestations continue to be generated from a small portion of the seed bank that persists as a result of dormancy and resistance to decay. Depletion of the persistent seeds using soil-applied chemicals to stimulate germination has received much attention while the search for microorganisms selective for seed decay has been largely overlooked. This paper provides an overview of the effects of microorganisms on weed seed viability relative to seed bank depletion, and how this information can be applied to weed management. Limited studies indicate that microorganisms associated with weed seeds can contribute to seed bank depletion through attraction to seeds by chemotaxis, rapid colonization of the spermosphere and production of enzymes and/or phytotoxins to kill seeds prior to germination. It is recognized, however, that the best opportunity for success will be through integration of selected microorganisms or microbial products with other approaches including germination stimulation, application of low rates of herbicides, manipulation of the soil environment (e.g., solarization), and biological control agents for effectively eliminating dormant, persistent seeds from soil. To achieve success, more in-depth research on microbial factors affecting weed seed banks is required.