The effects of pesticides on bacterial nitrogen fixers in peanut-growing area

Short-term responses of identified soil beneficial-bacteria to the insecticide fipronil: toxicological impacts

Pesticides including insecticides are often applied to prevent distortion posed by plant insect pests. However, the application of these chemicals detrimentally affected the non-target organisms including soil biota. Fipronil (FIP), a broad-spectrum insecticide, is extensively used to control pests across the globe. The frequent usage calls for attention regarding risk assessment of undesirable effects on non-target microorganisms. Here, laboratory-based experiments were conducted to assess the effect of FIP on plant-beneficial bacteria (PBB); Rhizobium leguminosarum (Acc. No. PQ578652), Azotobacter salinestris (Acc. No. PQ578649) and Serratia marcescens (Acc. No. PQ578651). PBB synthesized growth regulating substances were negatively affected by increasing fipronil concentrations. For instance, at 100 µg FIPmL-1, a decrease in indole-3-acetic acid (IAA) synthesis by bacterial strains followed the order: A. salinestris (95.6%) S. marcescens (91.6%) > R. leguminosarum (87%). Also, exposure of bacteria cells to FIP hindered the growth and morphology of PBB observed as distortion, cracking, and aberrant structure under scanning electron microscopy (SEM). Moreover, FIP-treated and propidium iodide (PI)-stained bacterial cells displayed an insecticide dose-dependent increase in cellular permeability as observed under a confocal laser microscope (CLSM). Colony counts (log10 CFU mL-1) and growth of A. salinestris was completely inhibited at 150 µg FIPmL-1. The surface adhering ability (biofilm formation) of PBB was also disrupted/inhibited in a FIP dose-related manner. The respiration loss due to FIP was coupled with a reduction in population size. Fipronil at 150 µgmL-1 decreased cellular respiration in A. salinestris (72%) S. marcescens (53%) and R. leguminosarum (85%). Additionally, biomarker enzymes; lactate dehydrogenase (LDH), lipid peroxidation (LPO), and oxidative stress (catalase; CAT) induced by FIP represented significant (p ≤ 0.05) toxicity towards PBB strains. Conclusively, fipronil suggests a toxic effect that emphasizes their careful monitoring in soils before application and their optimum addition in the soil-plant system. It is high time to prepare both target-specific and slow-released agrochemical formulation for crop protection with concurrent safeguarding of soils.

Temporal dynamics of total and bioavailable fungicide concentrations in soil and their effect upon nine soil microbial markers

Pesticides constitute an integral part of today's agriculture. Their widespread use leads to ubiquitous contamination of the environment, including soils. Soils are a precious resource providing vital functions to society - thus, it is of utmost importance to thoroughly assess the risk posed by widespread pesticide contamination. The exposure of non-target organisms to pesticides in soils is challenging to quantify since only a fraction of the total pesticide concentration is bioavailable. Here we measured and compared the bioavailable and total concentrations of three fungicides - boscalid, azoxystrobin, and epoxiconazole - and evaluated which concentration best predicts effects on nine microbial markers. The experiments were performed in three different soils at five time points over two months employing nearly 900 microcosms with a model plant. The total and bioavailable concentrations of azoxystrobin and boscalid decreased steadily during the trial to levels of 25 % and 8 % of the original concentration, respectively, while the concentration of epoxiconazole in soil nearly remained unchanged. The bioavailable fraction generally showed a slightly faster and more pronounced decline. The microbial markers varied in their sensitivity to the three fungicides. Specific microbial markers, such as arbuscular mycorrhizal fungi, and bacterial and archaeal ammonia oxidizers, were most sensitive to each of the fungicide treatments, making them suitable indicators for pesticide effects. Even though the responses were predominantly negative, they were also transient, and the impact was no longer evident after two months. Finally, the bioavailable fraction did not better predict the relationships between exposure and effect than the total concentration. This study demonstrates that key microbial groups are temporarily susceptible to a single fungicide application, pointing to the risk that repeated use of pesticides may disrupt vital soil functions such as nutrient cycling in agroecosystems.

Effects of cloransulam-methyl and diclosulam on soil nitrogen and carbon cycle-related microorganisms

Cloransulam-methyl and diclosulam are applied to soybean fields to control broad-leaved weeds. These herbicides have become a focus of attention because of their low application dose and high-efficiency advantages. However, the effects of these two herbicides on soil microorganisms are unknown. The present study investigated the effects of 0.05, 0.5, and 2.5 mg kg^-1 of cloransulam-methyl or diclosulam on soil microbes after 7, 14, 28, 42, and 56 days of exposure. The results showed that the two herbicides increased the abundances of functional bacteria related to pesticide degradation. Based on the genetic expression results, we speculated that 0.05 mg kg^-1 of these two herbicides inhibited the nitrification reaction but promoted the denitrification reaction. Diclosulam at a concentration of 0.5 mg kg^-1 may enhance the ability of microbes to fix carbon. β-glucosidase activity was activated by the two herbicides at a concentration of 2.5 mg kg^-1. Diclosulam had a positive effect on urease, but cloransulam-methyl activated urease activity only at concentrations of 0.05 and 0.5 mg kg^-1. The results of the integrated biomarker response showed that the toxicity of diclosulam was greater than that of cloransulam-methyl. Our research provides data for evaluating the environmental risks of cloransulam-methyl and diclosulam.

The Challenge of Combining High Yields with Environmentally Friendly Bioproducts: A Review on the Compatibility of Pesticides with Microbial Inoculants

Inoculants or biofertilizers aiming to partially or fully replace chemical fertilizers are becoming increasingly important in agriculture, as there is a global perception of the need to increase sustainability. In this review, we discuss some important results of inoculation of a variety of crops with rhizobia and other plant growth-promoting bacteria (PGPB). Important improvements in the quality of the inoculants and on the release of new strains and formulations have been achieved. However, agriculture will continue to demand chemical pesticides, and their low compatibility with inoculants, especially when applied to seeds, represents a major limitation to the success of inoculation. The differences in the compatibility between pesticides and inoculants depend on their active principle, formulation, time of application, and period of contact with living microorganisms; however, in general they have a high impact on cell survival and metabolism, affecting the microbial contribution to plant growth. New strategies to solve the incompatibility between pesticides and inoculants are needed, as those that have been proposed to date are still very modest in terms of demand.

Successive chlorothalonil applications inhibit soil nitrification and discrepantly affect abundances of functional genes in soil nitrogen cycling

Broad-spectrum fungicide chlorothalonil (CTN) is successively applied into intensive agriculture soil. However, the impacts of successive CTN applications on soil nitrification and related microorganisms remain poorly understood. A microcosm study was conducted to reveal the effects of successive CTN applications on soil nitrification and functional genes involved in soil nitrogen (N) cycling. The CTN at the dosages of 5 mg kg^-1 dry soil (RD) and 25 mg kg^-1 dry soil (5RD) was successively applied into the test soil at 7-day intervals which resulted in the accumulations of CTN residues. After 28 days of incubation, CTN residues in the RD and 5RD treatments were 3.14 and 69.7 mg kg^-1 dry soil respectively. Net nitrification rates in the RD and 5RD treatments were lower than that obtained from the blank control (CK). Real-time PCR analysis revealed that AOA and AOB amoA gene abundances were significantly decreased by CTN applications. Moreover, CTN applications also discrepantly decreased the abundances of functional genes involved in soil denitrification, with the exception of nosZ gene. Principal component analysis further supported the observation that successive CTN applications could result in enhanced ecological toxicity.

Non-target effects of repeated chlorothalonil application on soil nitrogen cycling: The key functional gene study

The widespread and increasing application of chlorothalonil (CTN) raises concerns about its non-target impacts, but little information is available on the effect of CTN on the key functional genes related to soil nitrogen (N) cycling, especially in the case of repeated applications. In the present study, a microcosm incubation was conducted to determine CTN residues and the impacts on the abundances of key functional genes related to N cycling after repeated CTN applications. The results demonstrated that repeated CTN applications at the recommended application rate and five times the recommended rate led to the accumulation of CTN residue in soil at concentrations of 5.59 and 78.79 mg kg(-1), respectively, by the end of incubation. Real time PCR (RT-PCR) revealed that repeated CTN applications had negative effects on the chiA and aprA gene abundances. There were significantly negative correlations between CTN residues and abundances of AOA and AOB genes. In addition, the abundances of key functional genes involved in soil denitrification were declined by repeated CTN applications with the sole exception of the nosZ gene. This study suggests that repeated CTN applications could lead to the accumulation of CTN residue and generate somewhat inconsistent and erratic effects on the abundances of key functional genes related to soil N cycling.