Development of biological soil crusts at the soil surface influences the persistence and movement of crop protection products

Laboratory soil biodegradation studies required for approval of crop protection products (CPPs) are performed under continuous darkness, nullifying any potential contributions of algal and moss dominated biological soil crusts (BSC). There is growing evidence for metabolism of CPPs by phototrophic microorganisms under laboratory conditions, but limited data is available under field conditions. In this study we investigated the impact of the BSC on the fate of two ¹⁴C-fungicides under semi-field conditions using different light filters to alter formation of the BSC by exclusion/transmission of UV and photosynthetically active (PAR) wavelengths. Attenuation of PAR light significantly reduced formation of a BSC, which resulted in a significant slowing of the dissipation of benzovindiflupyr, which is known to be susceptible to phototrophic metabolism in aquatic systems, with 12-14 % more parent compound remaining at the end of the study when BSC development was impeded. For paclobutrazol, however, no significant difference in dissipation rate was observed. For both compounds there was significantly less non-extractable residue (NER) formation when BSC development was impeded (4-9 % reduction). Additionally, for both fungicides, the presence of a viable BSC resulted in 10-20 % more movement through the surface 5 mm of the soil, although this effect was limited to the period immediately after application and was likely due to increased porosity of the surface layer. This study confirms that the presence of phototrophs can significantly impact the environmental fate of CPPs on the surface of agricultural fields, either directly through metabolism or indirectly by altering the properties of the surface layer.