فصلنامه
نوع مقاله : مقالات پژوهشی
نویسندگان
1 دانشجوی دکتری، گروه علوم خاک، دانشکده کشاورزی ،دانشگاه فردوسی مشهد
2 گروه علوم خاک، دانشکده کشاورزی، دانشگاه فردوسی مشهد، ایران
3 گروه علوم دامی، دانشکده کشاورزی دانشگاه فردوسی مشهد، ایران
کلیدواژهها
عنوان مقاله English
نویسندگان English
Introduction
The increasing use of fungicides and antibiotics in agricultural systems has raised concerns regarding their unintended effects on soil biological processes. Benomyl, a benzimidazole fungicide, and tetracycline, a broad-spectrum antibiotic, are frequently introduced into soil through plant protection practices and the application of animal manures. Once in soil, these compounds may alter microbial growth, metabolic activity, and enzyme-mediated nutrient cycling, thereby affecting essential soil functions.
Soil microorganisms are central to soil fertility and ecosystem functioning, and parameters such as microbial respiration, microbial biomass, and enzyme activities are widely used as sensitive indicators of soil disturbance. The simultaneous presence of fungicides and antibiotics may result in interactive effects that differ from those observed for individual compounds, potentially intensifying or prolonging biological stress.
Soil organic matter, particularly humic substances, plays a critical role in regulating the fate and bioavailability of organic contaminants. Humic acid can reduce the toxicity of pesticides and antibiotics through adsorption and complexation, while simultaneously improving physicochemical conditions that support microbial activity. Despite this, information on the combined effects of benomyl and tetracycline on soil microbial activity in the presence of humic acid, and their temporal dynamics, remains limited.
The objective of this study was therefore to assess the individual and combined effects of benomyl and tetracycline on soil microbial respiration, enzyme activities, and microbial biomass carbon, with particular emphasis on the moderating role of humic acid and time-dependent microbial responses.
Materials and Methods
A laboratory incubation experiment was conducted using a soil classified as Typic Haplocambids. The experiment followed a completely randomized design with eight treatments: control (C), benomyl (F), tetracycline (T), benomyl + tetracycline (FT), humic acid (H), benomyl + humic acid (FH), tetracycline + humic acid (TH), and benomyl + tetracycline + humic acid (FTH).
Benomyl and tetracycline were applied at rates of 12 and 50 mg kg⁻¹ soil, respectively, and humic acid was added at 500 mg kg⁻¹ soil. Soil moisture content was adjusted to 60% of water-holding capacity, and samples were incubated at 25 °C for 30 days. Destructive sampling was performed after 1, 7, and 30 days of incubation.
Microbial activity was assessed using substrate-induced respiration with glucose and cellulose as carbon substrates. Microbial biomass carbon (MBC) was determined using the fumigation–extraction method. Enzyme activities, including dehydrogenase, acid phosphomonoesterase (ACP), and alkaline phosphomonoesterase (ALP), were measured using standard colorimetric assays. Data were analyzed by analysis of variance (ANOVA), and treatment means were compared using Tukey’s test at a significance level of P < 0.01.
Results and Discussion
Application of benomyl and tetracycline significantly affected soil microbial activity, with the strongest inhibitory effects observed under their combined application (FT). Substrate-induced respiration with glucose was reduced in soils treated with benomyl and tetracycline during the early incubation period, while the FT treatment consistently exhibited the lowest CO₂ production across all sampling times, indicating a pronounced suppressive effect on microbial metabolic activity.
Respiration responses to cellulose were lower than those observed for glucose, suggesting greater resistance of complex carbon decomposition processes to chemical disturbance. Nevertheless, cellulose-induced respiration was persistently reduced in the FT treatment throughout the incubation period, reflecting impaired hydrolytic capacity of the microbial community.
Microbial biomass carbon declined markedly in treatments without humic acid, particularly in the FT treatment, on day 1 of incubation. Although partial recovery of MBC was observed by day 30, biomass levels in the FT treatment remained significantly lower than in the control, indicating incomplete microbial adaptation.
Dehydrogenase activity exhibited a transient increase in some benomyl-treated soils, especially in the presence of humic acid (FH), followed by a gradual decline over time. This pattern reflects an initial stress-induced stimulation followed by metabolic inhibition. In contrast, humic acid–amended soils generally maintained higher and more stable dehydrogenase activity throughout the incubation period.
Alkaline phosphatase activity showed an inducible response, particularly in the FH and TH treatments during the early stages of incubation, likely reflecting increased phosphorus demand under chemical stress. Acid phosphatase activity responded more variably depending on treatment and incubation time. In soils amended with humic acid, temporal fluctuations in enzyme activities were reduced, indicating enhanced functional stability; however, the inhibitory effect of tetracycline remained evident in the TH treatment.
Conclusion
Benomyl and tetracycline, particularly when applied simultaneously, exerted inhibitory effects on soil microbial respiration, biomass, and enzyme activities. The presence of humic acid mitigated these negative effects by enhancing microbial stability and reducing biological stress over time. Although partial microbial adaptation occurred during the incubation period, the combined fungicide–antibiotic treatment caused persistent disturbances in soil microbial functioning. Overall, humic acid contributes to improved biological stability of soil exposed to bioactive contaminants and represents a potential management strategy for mitigating chemical stress.
کلیدواژهها English
