Potential of the slow pyrolysis products birch tar oil, wood vinegar and biochar in sustainable plant protection: pesticidal effects, soil improvement and environmental risks

The increased use of pesticides and their impacts on environment have become a matter of considerable concern in recent decades. The use of pesticides, especially synthetic ones, is suggested to be replaced by compensatory substances that exert a lower risk to the environment. At the same time, global climate change will lead to an increase in temperature and rainfall in some areas, which could enhance the growth of several pest populations. For example, populations of the molluscs Arianta arbustorum and Arion lusitanicus have increased in many parts of northern Europe in recent years.

Plant-derived products may have a significant role in sustainable plant protection when functioning as compensatory substances for synthetic pesticides, or alternatively by affecting the behaviour of synthetic pesticides in the soil. This thesis research investigated the suitability of birch (Betula sp.) -derived slow pyrolysis products birch tar oil (BTO), wood vinegar (WV) and biochar in sustainable plant protection. The aims of the study were to (i) explore the efficiency of birch derived pyrolysis liquids in mollusc control and (ii) investigate the environmental risks related to their use. In addition, (iii) the effects of biochar and wood vinegar on the environmental fate of glyphosate, one of the most commonly used herbicide, was examined.

BTO and WV proved to be ineffective in eliminating snails. Instead, BTO and the mixture of BTO and WV exhibited a clear repellent effect against molluscs. The effect of WV on non-target organisms was assessed in several toxicity tests and risk assessment calculations. Soil organisms were more tolerant of WV than aquatic organisms. No long-term effects on soil microbes, nematodes or enchytraeids were found. The initial risk assessment indicated the risks of WV (PIENEMPI 400 L ha-1) to soil and aquatic organisms to be negligible.

Based on preliminary data, biochar reduced the leaching of glyphosate from the soil. The effects of WV on glyphosate leaching was inconsistent. Soils treated with a mixture of biochar and WV showed the lowest glyphosate leaching. Neither WV nor biochar alone had clear effects on glyphosate degradation in the soil. The results show, for the first time, that biochar has the potential to influence the fate of glyphosate in the soil by reducing its leaching from soil.

This thesis provides strong evidence for the potential of birch-derived pyrolysis liquids as an effective, non-costly and environmental friendly method against molluscs. As WV is only slightly toxic or non-toxic to most non-target organisms, the environmental risk due to synthetic pesticides could be diminished by including WV as part of a pest control protocol. Biochar could also play a role in pesticide risk reduction, particularly in preventing contamination of the aquatic environment. Based on the results of this thesis, the birch-derived slow pyrolysis liquids and biochar appear to have potential for use in sustainable plant protection.