Effects of atrazine on microbial activity in semiarid soil
Introduction
Atrazine [2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine] is a herbicide widely used to control broad-leaved weeds especially in corn (Zea mays L.) and sorghum (Sorghum bicolor L. Moensch) production. Due to its widespread use, its relatively long half-life in agricultural soil and moderate soil mobility it has been detected in many environmental compartments, especially in surface water (Wenk et al., 1997). In recent years, atrazine use has decreased in favor of other s-triazine herbicides, and in many European countries it has been recommended to use atrazine only at rates below 1.5 kg of active ingredient/ha per annum in agricultural crops (Seiler et al., 1992).
Soil microorganisms and the processes that they govern are essential for long-term fertility of soil, and there is therefore, concern about the effects of herbicides on soil microbial biomass and activity (Perucci et al., 2000). Exposure to some xenobiotic organic compounds may change the resources which soil microorganisms use to obtain energy and nutrients, especially when the soil organic matter is scarce and these compounds are bioavailable in the media (Alexander, 1994) Specific microorganisms are able to detoxicate atrazine by N-dealkylation or dehalogenation reactions and this may imply the development of microbial communities that can utilize the N in the triazine ring (Haney et al., 2002). The concentration of atrazine in soil is an important factor which affects its biodegradation and the microbial response (Dzantor and Felsot, 1991, Gan et al., 1996). Moreover, the conditions of a semiarid soil (low water content, and scarce nutrient content) may modify the atrazine degradation rate by microorganisms in comparison with other soil conditions. Several authors (e.g. Assaf and Turco, 1994, Gebendinger and Radosevich, 1999) have reported that the concentration and forms of C and N are major determinants of atrazine mineralization. Soil moisture and temperature directly affect many biological processes including microbial degradation of chemicals and could also influence atrazine persistence (Weber et al., 1993). However, literature on the effect of atrazine on microbial activity in semiarid soils is scarce.
In this study, the effects of a range (from 0.2 to 1000 mg kg−1 soil) of atrazine concentrations on different microbial activity parameters of a semiarid soil, and the influence of time on the response of soil microbial activity to the herbicide were assessed.
Section snippets
Soil treatment
In June 2002, the 0–10 cm depth layer of an agricultural soil from SE Spain, which had been abandoned several years ago after intensive utilization, was sampled. The climate regime of this region is semiarid, and it is characterized by sporadic rainfalls (annual precipitation: 200–300 mm) that in most cases are torrential, and moderate temperatures (19 °C of annual average). The plant cover of this soil was scarce and thus, the organic matter content was very low. The main characteristics of this
Results
The atrazine concentration in soil decreased with the incubation time (Table 2). After 6 h, the degradation of this herbicide was minimal for most treatments, with 50–90% of the atrazine added being recovered from the treated soils. After 16 days incubation only 2–50% of atrazine was recovered, and after 45 days, residues of this herbicide were undetectable in most cases.
The effect of atrazine concentration (treatment) on microbial biomass C (Cmic), soil microbial respiration (SMR), ATP content
Discussion
Atrazine persistence in soil is characterized by a moderate, short half-life of 35 to 50 days (Topp, 2001), although faster atrazine mineralization has been observed in soils which had been exposed to this herbicide as a normal agricultural practice (Pussemier et al., 1997, Houot et al., 2000). In our study, after 16 days of incubation there was, at a maximum, 50% of the added atrazine remaining. The biodegradation of atrazine was carried out by the native semiarid soil population under optimum
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