Effect of different types of plastic incorporation on soil temperature: first results from a field-experiment
Résumé
The impact of microplastics on soil physical properties is not well studied. This study was carried out to assess if (micro)plastics incorporated into the soil can act as heat retainers and if they can subsequently have adverse impacts on soil microbial activity. Three types of plastic films were used for this study in a field-experiment carried out during six months in an agricultural soil: 1) a white/transparent greenhouse plastic, 2) black plastic from mulch and 3) a grey oxo-biodegradable plastic from rubbish bags. Both greenhouse film and plastic mulch were collected from the soil surface in the study area, and thus aged in the environment. The oxo-biodegradable plastic was obtained from a local supermarket. For each treatment, three replicates consisting ofsoil surface plots of 50x50 cm were prepared in the field. The spontaneous vegetation was manually removed from the soil surface of each soil plot at the beginning and biweekly over the experimental period. All plastics were manually cut to obtain a mix of micro- and macroplastics (from 4 mm to 50 mm), as plastics debris in agricultural soils can have several sizes and shapes. For each replicate, 100 plastic items were buried in the first five centimetres of the topsoil. No plastic items were added to the control plots. At the beginning and at the end of the experimental time (August and December 2019), soil samples from each replicate were collected to assess their physicochemical properties (e.g. pH, organic matter, electrical conductivity, soil density, soil aggregates), biochemical parameters (arylsulfatase, cellulase dehydrogenase, acid phosphatase, urease, nitrogen mineralization and potentially nitrification) and microbial community colonizing the plastics. Besides, infrared thermography was applied to the soil squares to assess the thermal patterns in each plot. Despite the differences in surface temperatures achieved for the different treatments, it was not possible to find a trend in the results, since the surface temperatures were on average very similar to the control without plastics, although with different thermal patterns. These differences on thermal patterns can be related to soil surface roughness, the sun’s shift on the horizon during the experimental measurements or the aleatory position of plastics buried in soils. Thus, any impact on soil microbial parameters can be related to changes in soil temperature.
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