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Action of two herbicides on the microbial activity of soil cultivated with common bean (Phaseolus vulgaris)in conventional-till and no-till systems J B SANTOS*, A JAKELAITIS*, A A SILVA*, M D COSTA ,AMANABE &MCS SILVA *Departamento de Fitotecnia and Departamento de Microbiologia, Universidade Federal de Vic¸ osa (UFV), Brazil Received 23 August 2005 Revised version accepted 22 February 2006 fomesafen and by the commercial mixture of the two Summary herbicides. Mycorrhizal colonisation was affected by the The effects of application of the herbicides fluazifop-p- herbicides only at 12 DAA under CTS; however, in both butyl and fomesafen and the commercial mixture of these periods, the highest value was found under NTS. All the herbicides on the microbial activity of a soil, cultivated herbicides caused a decrease in the MBC and qMIC with common bean under no-till (NTS) and conven- values at 51 DAA; the qCO2, which is related to the soil tional-till (CTS) systems, were evaluated. Microbial system stability, indicated a greater NTS balance over respiration was monitored for 63 days after application CTS. The herbicide fomesafen induced lower stability in (DAA) of the herbicides, and the following evaluated at the system. Lower grain yield was obtained without weed 12 and 51 DAA: microbial biomass carbon (MBC), control (no herbicides) and with fomesafen-only treat- microbial quotient (qMIC), metabolic quotient (qCO2), ments, which may be attributed to the high weed percentage of bean root colonisation by mycorrhizal infestation in the experimental area. fungi and grain yield at the end of the cycle. A greater Keywords: fluazifop-p-butyl, fomesafen, arbuscular microbial respiratory rate was observed under NTS, with mycorrhiza, no-till system, microbial quotient, meta- fluazifop-p-butyl providing the lowest respiration. At 12 bolic quotient. DAA, MBC and qMIC were most affected negatively by SANTOS JB, JAKELAITIS A, SILVA AA, COSTA MD, MANABE A&SILVA MCS (2006) Action of two herbicides on the microbial activity of soil cultivated with common bean (Phaseolus vulgaris) in conventional-till and no-till systems. Weed Research 46, 284–289. Introduction functions in the soil, such as decomposition and accumulation of organic matter or changes involving Increased soil contamination by herbicides in intensive mineral nutrients or xenobiotic compounds. Microbial agricultural areas has attracted the interest of research- quotient (qMIC), which indicates the quality of organic ers in evaluating biological indicators of soil quality and matter by reflecting additions of it to the soil, estimates has shown that soils can be affected. Understanding soil the efficiency of converting organic C into MB carbon or quality attributes allows progress towards sustainable carbon losses in soil (Sparling, 1992); also, metabolic soil management, as by monitoring the changes in soil quotient (qCO2), established through the relation quality one will be able to determine sets of sustainable- between accumulated CO2 and total MB carbon, estab- oriented practices. lishes that, as MB efficient use of the resources increases, An accurate and consistent evaluation of soil quality less C is lost as CO2 through respiration, with the may be accomplished by indicators such as microbial possibility of the latter being incorporated into the biomass (MB), responsible for the control of essential microbial tissues (Anderson & Domsch, 1985). Thus, Correspondence: Dr J Santos, Universidade Federal de Vic¸osa – Fitotecnia, Av. PH Rolfs s/n Campus – UFV Vic¸osa, Minas Gerais, 36570000 Brazil. Tel: +55 031 38992611; Fax: +55 031 38992611; E-mail: jbarbosasantos@yahoo .com.br Ó 2006 The Authors Journal compilation Ó 2006 European Weed Research Society 46, 284–289 Herbicide action on soil microbial activity 285 lower qCO2 means higher MB stability, i.e. greater Table 1 Chemical and textural composition of a layer of 0–10 cm system stability. MB, which has a comparatively rapid depth of clayey soil derived from no-till (NTS) and conventional- formation and decomposition rate of 1 or 2 years till (CTS) systems used in the experiment (Jenkinson & Ladd, 1981), may be proposed as a more CTS NTS sensitive measure of soil disturbances resulting from Mineral fraction (%) herbicide application, as its increase or decrease affects Fine sand 15 10 the total amount of organic matter in the soil. Coarse sand 29 16 Under a no-till system (NTS), a greater retention of Silt 11 19 organic compounds has been observed with the Clay 45 55 increased aggregation of particles (Beare et al., 1995); Texture Clayey Clayey Chemical analysis these protect the organic matter physically by forming pH 5.2 5.3 barriers which isolate the microorganisms in the sub- (mg dm)3) strate, also affecting MB cycling (Elliott & Coleman, P 13 14.1 1988). Thus, the use of herbicides under this system will K+ 7.1 14.2 )3 have a different impact from that observed under the (cmolc dm ) H + Al 4.62 4.29 conventional-till system (CTS). Al3+ 0.1 0.1 Fomesafen is one of the most recommended herbi- Ca2+ 1.4 2 cides for bean cultivation, controlling several dicotyle- Mg2+ 0.4 0.8 donous weeds. However, this herbicide is persistent in CEC (total) 6.6 7.45 the soil, with a half-life from 2 to 6 months (Rodrigues (%) V3042 & Almeida, 2005). Normally, fomesafen is used in a m53 mixture with fluazifop-p-butyl for the simultaneous OM (dag kg)1) 2.90 3.82 control of dicotyledonous and grass weeds (Silva et al., 2003). Studies have shown that some herbicides affect Analyses carried out at the Soil Physical and Chemical Analysis MB negatively (Edwards, 1989; Wardle, 1994; Pers- Laboratories of the Department of Soils of the UFV. CEC, cation exchange capacity; OM, organic matter; V, base saturation; m, chbacher et al., 1997; Kinney et al., 2005). However, aluminium saturation. these studies have not analysed climate conditions and tropical soils under NTS. This work evaluated the effects of the herbicides fluazifop-p-butyl and fomesafen were split into split–split plots with the evaluation applied alone or in a mixture (RobustÒ) on the timings. microbial activity of a soil in contrasted NTS and CTS Microbial respiration was estimated based on the of common bean (Phaseolus vulgaris L.) cultivation. amount of CO2 evolved from the samples of 100 g of soil, which were placed in flasks containing 100 mL of NaOH (0.25 mol L)1) under a continuous air flow Material and methods system (without CO2 and moisture), at 5, 12, 28, 51 The experiment was carried out in the field and the and 63 days after application (DAA) of the herbicides. analyses in the Laborato´rio de Herbicidas of the After the soil samples were sieved in a two-mesh sieve, Universidade Federal de Vic¸osa (UFV), using samples air-dried and their water content determined, they were of a Red-Yellow Argisol collected at the depth of weighed and incubated in Erlenmeyer flasks at a water 0–10 cm in areas cultivated with bean (cv. Ouro content of 80% of field capacity (approximately Vermelho) under NTS and CTS. In both areas, bean )0.5 bar or )50 kPa). This was followed by indirect had been intercropped with maize (Zea mays L.) over titration of sodium hydroxide with HCl (0.25 mol L)1) 6 years. The chemical and textural characterisation of so that the remaining NaOH that did not react with the the soil, in each area, is shown in Table 1. evolved CO2 was quantified (Anderson, 1982). The experiment was arranged in split–split plots in a In addition to microbial respiration, the following randomised block design, with four repetitions. The was determined at 12 and 51 DAA: (a) microbial main plots constituted the planting systems (NTS and biomass carbon (MBC) by the method described by CTS) and the split plots constituted the treatments Vance et al. (1987), replacing-chloroform (fumigation) composed of the herbicides FusiladÒ (fluazifop-p-butyl by microwave oven (irradiation) (Islam & Weil, 1998); ) – 187.5 g a.i. ha 1) and FlexÒ (fomesafen – 250 g (b) microbial quotient (qMIC), through the relation ) a.i. ha 1) and their commercial mixture, RobustÒ,at between MBC and the organic C multiplied by 100; (c) )1 the doses of 140 g a.i. ha of fluazifop-p-butyl + 175 g metabolic quotient (qCO2), through the relation between )1 )1 a.i. ha of fomesafen, applied 30 days after bean the accumulated CO2 (mg 100 g soil) and MB total C sowing, plus a control without herbicide. Sub-plots (lgg)1 soil); and (d) the percentage of bean roots Ó 2006 The Authors Journal compilation Ó 2006 European Weed Research Society 46, 284–289 286 J B Santos et al. colonised by mycorrhizal fungi was evaluated according mixture at 12 DAA. Regardless of the herbicide treat- to Giovannetti and Mosse (1980). ment, MBC was higher in the NTS soil samples at 51 At 63 DAA, the bean was harvested and grain yield DAA (Table 2). determined. The data were submitted to variance Regardless of the tillage system used, herbicide analysis by the F-test and the treatment averages for application reduced MBC in both periods evaluated. the traits evaluated were compared by the Tukey test at At 12 DAA, fomesafen and the commercial mixture 5% probability. Regression curves were also fitted for elicited the greatest reductions in MBC in CTS soil. evolution of the accumulated CO2 during the evaluation Under NTS, MB decrease was higher under application period. of the herbicide alone than with the mixture. At 51 DAA, a greater MBC reduction was provided by the et al. Results and discussion commercial herbicide mixture (Table 2). Aon (2001) verified that fungal and bacterial biomass treated Regardless of the treatment applied, the CO2 evolution with the herbicides atrazine, glyphosate, 2,4-D, 2,4-DB, response was linear (P <1%byt-test) throughout the EPTC and flumetsulan were less affected and more sampled period (Fig.

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