Evidence for 2,4-D mineralisation in Mediterranean soils: impact of moisture content and temperature

The 2,4-D degradation ability of the microbiota of three arable Mediterranean soils was estimated. The impact of soil moisture and temperature on 2,4-D degradation was investigated.

The microbiota of the three soils regularly exposed to 2,4-D were able rapidly to mineralise this herbicide. The half-life of 2,4-D ranged from 8 to 30 days, and maximum mineralisation of (14)C-2,4-D ranged from 57 to 71%. Extractable (14)C-2,4-D and (14)C-bound residues accounted for less than 1 and 15% respectively of the (14)C-2,4-D initially added. The highest amounts of (14)C-2,4-D bound residues were recorded in the soil with the lowest 2,4-D-mineralising ability. Although all three soils were able to mineralise 2,4-D, multivariate analysis revealed that performance of this degrading microbial activity was dependent on clay content and magnesium oxide. Soil temperature affected the global structure of soil microbial community, but it had only a moderate effect on 2,4-D-mineralising ability. 2,4-D-mineralising ability was positively correlated with soil moisture content. Negligible 2,4-D mineralisation occurred in all three soils when incubated at 10 or 15% soil moisture content, i.e. within the range naturally occurring under the Mediterranean climate of Algeria.

This study shows that, although soil microbiota can adapt to rapid mineralisation of 2,4-D, this microbial activity is strongly dependent on climatic parameters. It suggests that only limited pesticide biodegradation occurs under Mediterranean climate, and that arable Mediterranean soils are therefore fragile and likely to accumulate pesticide residues.