Influence of fumigants on soil microbial diversity and survival of E. coli O157:H7

Abiotic conditions outweigh microbial origin during bacterial assembly in soils

Understanding the processes guiding microbial community assembly in soils is essential for predicting microbiome structure and function following soil disturbance events like agricultural soil fumigation. However, assembly outcomes are complex and variable, being affected by both selective abiotic forces and by the history of colonizing microorganisms. To untangle the interactions between these factors, we conducted a controlled microcosm study tracking bacterial assembly in cleared soils over 7 weeks. We used mesh bags to connect five unsterilized source soils, differing in land use history (forested, agricultural, or fallow), with four sterile recipient soil treatments, differing in abiotic conditions (no soil additives, salt addition, urea addition, or mixed salt/urea addition). We found that 59%-96% of bacterial colonizers after 1 week were Firmicutes, but by 7 weeks Actinobacteria and Bacteroidetes were also dominant. Salt and nitrogen additions reshaped bacterial assembly by constraining alpha diversity by up to half and biomass accumulation by up to an order of magnitude. Within-treatment dispersion was significantly lower for salt and nutrient addition microcosms, suggesting deterministic selective pressures. In contrast, source soil origin had little impact on assembly trajectories. These results suggest that abiotic conditions can overshadow microbial source history in shaping community assembly outcomes.

Effect of soil fumigants on degradation of abamectin and their combination synergistic effect to root-knot nematode

BACKGROUND

Root-knot nematode (Meloidogyne spp., RKN) causes a disease that significantly reduces the yield of greenhouse cucumber crops year after year. Chemical control based on a single pesticide is now unreliable mainly due to pest resistance. Fumigant and non-fumigant pesticide combinations can potentially result in effective and economic RKN control.

RESULTS

Combining the insecticide abamectin (ABM) with fumigants dazomet (DZ) or chloropicrin (CP) significantly extended the half-life of ABM by an average of about 1.68 and 1.56 times respectively in laboratory trials, and by an average of about 2.02 and 1.69 times respectively in greenhouse trials. Laboratory experiments indicated that all the low rate ABM combination treatments controlled RKN through a synergistic effect. ABM diffused into the nematode epidermis more rapidly when ABM was combined with DZ and CP, giving effective nematode control and an increase cucumber total yield, compared to the use of these products alone. ABM combined with CP or DZ produced significantly higher total cucumber yield than when these products were used alone.

CONCLUSIONS

A low concentration of ABM combined with DZ in preference to CP would be an economic and practical way to control nematode and soilborne fungi in a greenhouse producing cucumbers.

Nontarget effects of chemical pesticides and biological pesticide on rhizospheric microbial community structure and function in Vigna radiata

Intensive agriculture has resulted in an indiscriminate use of pesticides, which demands in-depth analysis of their impact on indigenous rhizospheric microbial community structure and function. Hence, the objective of the present work was to study the impact of two chemical pesticides (chlorpyrifos and cypermethrin) and one biological pesticide (azadirachtin) at two dosages on the microbial community structure using cultivation-dependent approach and on rhizospheric bacterial communities involved in nitrogen cycle in Vigna radiata rhizosphere through cultivation-independent technique of real-time PCR. Cultivation-dependent study highlighted the adverse effects of both chemical pesticide and biopesticide on rhizospheric bacterial and fungal communities at different plant growth stages. Also, an adverse effect on number of genes and transcripts of nifH (nitrogen fixation); amoA (nitrification); and narG, nirK, and nirS (denitrification) was observed. The results from the present study highlighted two points, firstly that nontarget effects of pesticides are significantly detrimental to soil microflora, and despite being of biological origin, azadirachtin exerted negative impact on rhizospheric microbial community of V. radiata behaving similar to chemical pesticides. Hence, such nontarget effects of chemical pesticide and biopesticide in plants' rhizosphere, which bring out the larger picture in terms of their ecotoxicological effect, demand a proper risk assessment before application of pesticides as agricultural amendments.

Evaluation of methyl bromide alternatives efficacy against soil-borne pathogens, nematodes and soil microbial community

Methyl bromide (MB) and other alternatives were evaluated for suppression of Fusarium spp., Phytophthora spp., and Meloidogyne spp. and their influence on soil microbial communities. Both Fusarium spp. and Phytophthora spp. were significantly reduced by the MB (30.74 mg kg-1), methyl iodide (MI: 45.58 mg kg-1), metham sodium (MS: 53.92 mg kg-1) treatments. MS exhibited comparable effectiveness to MB in controlling Meloidogyne spp. and total nematodes, followed by MI at the tested rate. By contrast, sulfuryl fluoride (SF: 33.04 mg kg-1) and chloroform (CF: 23.68 mg kg-1) showed low efficacy in controlling Fusarium spp., Phytophthora spp., and Meloidogyne spp. MB, MI and MS significantly lowered the abundance of different microbial populations and microbial biomass in soil, whereas SF and CF had limited influence on them compared with the control. Diversity indices in Biolog studies decreased in response to fumigation, but no significant difference was found among treatments in PLFA studies. Principal component and cluster analyses of Biolog and PLFA data sets revealed that MB and MI treatments greatly influenced the soil microbial community functional and structural diversity compared with SF treatment. These results suggest that fumigants with high effectiveness in suppressing soil-borne disease could significantly influence soil microbial community.

Integrated management of root-knot nematodes on tomato in glasshouse production using nematicides and a biocontrol agent, and their effect on soil microbial communities

Nematicides are the main method for managing root-knot nematodes but they are not environmentally benign. Biological control is one alternative that has a lower or no environmental impact. This study was conducted to evaluate the efficacy of two nematicides (Fosthiazate and Dazomet), a biocontrol agent (Purpureocillium lilacinum(=Paecilomyces lilacinus) YES-2), their combination on controlling root-knot nematodes on tomato plants and their effects on the rhizosphere microbial community in long-term glasshouse experiments. The gall index and numbers of second-stage juveniles (J2) were significantly reduced by the individual treatments of Fosthiazate, Dazomet orP. lilacinumYES-2.Purpureocillium lilacinumYES-2 combined with a half dose of Fosthiazate showed the most efficient control as indicated by gall index and number of J2 compared with the control treatment in glasshouse experiments conducted over a 5-year period (2007-2011). The bacterial and fungal biodiversity in soil measured using Biolog ECO MicroPlates and denaturing gradient gel electrophoresis (DGGE) fingerprint was higher in the treatments ofP. lilacinumYES-2 alone or its combination with chemicals than in the treatments of Dazomet or Fosthiazate (in 2011 only). This study suggested a synergistic effect on the control of root-knot nematode byP. lilacinumYES-2 combined with Fosthiazate, and the contribution of these two treatments to the microbial communities in the soil.

Quantification of the effects of various soil fumigation treatments on nitrogen mineralization and nitrification in laboratory incubation and field studies

Better quantification of nitrogen mineralization and nitrification after fumigation would indicate if any adjustment is needed in fertilizer application. The effects of chloropicrin (Pic), 1,3-dichloropropene (1,3-D), dimethyl disulfide (DMDS) and metham sodium (MS) fumigation on soil nitrogen dynamics were evaluated in lab incubation and field studies. Although some differences were observed in NH(4)(+)-N and NO(3)(-)-N concentrations in lab incubation and field experiments, both studies led to the same conclusions: (1) Soil fumigation was shown to increase soil mineral nitrogen only during the first 2 weeks after fumigation (WAF). In particular, Pic significantly increased soil mineral nitrogen in both studies at 1 WAF. However, for all fumigant treatments the observed effect was temporary; the soil mineral content of treated samples recovered to the general level observed in the untreated control. (2) All the fumigation treatments depressed nitrification temporarily, although the treatments exhibited significant differences in the duration of nitrification inhibition. In both studies, for a limited period of time, Pic showed a stronger inhibitory effect on nitrification compared to other fumigant treatments. An S-shaped function was fitted to the concentrations of NO(3)(-)-N in lab incubation samples. The times of maximum nitrification (t(max)) in DMDS and MS treatments were 0.97 week and 1.03 week, which is similar to the untreated control (t(max)=1.02 week). While Pic has the longest effect on nitrifying bacteria, nitrification appears to restart at a later time (t(max)=14.37 week).