Physicochemical and microbial soil quality indicators as affected by the agricultural management system in strawberry cultivation using straw or black polyethylene mulching

- Less favourable soil eco-physiological parameters are observed under plastic film.
- Soil bacteria and in particular soil fungi are significantly reduced under plastic.
- Mycotoxin production as consequence of environmental stress conditions is observed.

The aim of this study was to understand the differences in physicochemical and microbial soil quality resulting from the use of either wheat straw or black polyethylene, two widely used practices in strawberry cultivation. Soil samples were collected from strawberry crops used during 4 years as mono-cropping system, covered by either straw or black polyethylene (plastic mulching). Soil physicochemical properties included water content, pH, effective cation exchange capacity, elemental analysis of total carbon and nitrogen, dissolved and total organic carbon, and soil stability by percentage of water-stable aggregates (>0.2 mm). Soil microbial analysis comprised soil microbial biomass (Cmic and Nmic and DNA concentrations) and estimation of soil eco-physiological conditions Cmic:Corg and Nmic:Ntot. Soil bacteria and a fraction of cultivable fungi were studied respectively by molecular analysis and counting of colony-forming units (CFU values). Mycotoxin concentration in soil (deoxynivalenol) was used as an indirect indicator of fungal stress. The plastic mulching system showed positive effects on soil physicochemical properties as compared to straw mulching: Higher soil carbon content and better aggregate stability were observed in soils under plastic film, and the values for soil microbial biomass were comparable in both systems. Yet, soil eco-physiological conditions under plastic mulching were less appropriate compared to straw, as reflected by a reduced Cmic:Corg ratio (1.3 ± 0.3%), followed by a decline of the number of bacteria (18%), a six-fold reduction of the biomass of soil cultivable fungi, and finally by a two-times higher deoxynivalenol concentration in soil (mean 2.2 ± 2.4 μg kg−1). This indicates that the plastic mulching system led to less favourable soil conditions and that the production of mycotoxins can be understood as a stress induced response by fungi. Further information is therefore needed to assess how the quality of soil is restricted by the shift of soil microbial communities and by the production of mycotoxins, especially in intensive agriculture with long-term plastic coverage.