Soil microorganisms are among the most diverse component of terrestrial ecosystems [1] where they, among other, play a role in the quality of agricultural soils. It was suggested that the key to understanding soil functioning is the description of the composition and the biodiversity of soil microbial communities [2]. However, the hypothetical interrelation between soil microbial biodiversity and soil ecological functioning is poorly documented. This underlines the need to develop approaches that allow explicit links to be established between the presence of specific microorganisms and the processes they catalyse [3].

Main results

The impact of organic amendment (sewage sludge or waste water) used to fertilize agricultural soils was estimated on the atrazine- degrading activity (figure 1), the atrazine-degrading genetic potential and the bacterial community structure of soils continuously cropped with corn (figure 2).

Figure 1 : Kinetics of degradation of 14C ring-labelled atrazine in [panel A] the soil of Couhins (U) non-amended, amended with (FM) farmyard manure, (SS10) sewage sludge (10 ton ha-1 year-1) and (SS100) sewage sludge (100 ton ha-1 year-2); [panel B] the soil of Pierrelaye amended with (LP) lightly polluted, (MP) moderately polluted or (HP) highly polluted waste water; [panel C] the soil of La Bouzule (U) nonamended, (LDSS) amended with lightly dehydrated sewage sludge, (LDCSS) amended with lightly dehydrated composted sewage sludge and (LDCSS+PAH) amended with lightly dehydrated composted sewage sludge added with polyaromatic hydrocarbon.

Figure 2 : Quantitative PCR analysis of atzA, B and C sequences from DNA samples extracted from [panel A] the soil of Couhins U, FM, SS10 and SS100; [panel B] the soil of Pierrelaye LP, MP or HP; [panel C] the soil of La Bouzule U, LDSS, LDCSS and LDCSS+PAH.

Long-term application of organic amendment did not modify atrazine-mineralizing activity, which was found to essentially depend on the soil type. It also did not modify atrazine-degrading genetic potential estimated by quantitative PCR targeting atzA, B and C genes, which was shown to depend on soil type. The structure of soil bacterial community determined by RISA fingerprinting was significantly affected by organic amendment. These results showed that modification of the structure of soil bacterial community in response to organic amendment is not necessarily accompanied by a modification of atrazine-degrading genetic potential or activity. In addition, these results revealed that different soils showing similar atrazine-degrading genetic potentials may exhibit different atrazine-degrading activities.

References

1. Ekschmitt, K. and Griffiths, B.S. (1998) Soil biodiversity and its implications for ecosystem functioning in a heterogeneous and variable environment. Appl. Soil Ecol. 10, 201–215.

2. Schloter, M., Lebuhn, M., Heulin, T. and Hartmann, A. (2000) Ecology and evolution of bacterial microdiversity. FEMS Microbiol. Rev. 24, 647–660.

3. Gray, N.D. and Head, I.M. (2001) Linking genetic identity and function in communities of uncultured bacteria. Environ. Microbiol. 3, 481–492.

Affiliations

a INRA-CMSE, UMR 1229 INRA-Université de Bourgogne, Microbiologie et Géochimie des Sols, 17 rue Sully, BP 86510, 21065 Dijon Cedex, France

b ENSAIA-INPL/INRA, Laboratoire Sols et Environnement UMR 1120, 2, avenue de la Forêt de Haye, BP 172, F-54505 Vandoeuvre-les-Nancy, France