Biochar addition rate determines contrasting shifts in soil nematode trophic groups in outdoor mesocosms: An appraisal of underlying mechanisms

Abstract The medium-term impact of the application of a pine gasification biochar at low and high rates (5 and 30 tons ha−1) on nematode communities was assessed in outdoor mesocosms cropped to barley during the spring and the fall of the third agronomical year following the application. Impacts were measured as shifts in community taxon and functional composition, and ecological indices were related to soil physicochemical and microbiological properties. Biochar addition did not affect soil properties, total nematode abundance, richness or biomass, but different community shifts were observed in the two biochar addition treatments (5 and 30 tons ha−1) when compared with controls (with no biochar added). Communities shifted both taxonomically and functionally: the low application rate promoted predator and plant-parasitic nematodes, while the high application rate favored bacterivores and fungivores. An appraisal of the plausible main mechanisms behind those shifts was carried out, based on the various mechanisms proposed in the available literature about shifts in nematode communities after organic amendments, as well as the scarce biochar studies published to date. This study suggests that biochar additions show a capacity for regulating plant-parasitic nematodes in agroecosystems, but also that this regulation is dependent on the addition rate. There is a need to validate this work with further studies to identify the mechanisms behind this suppression and determine the necessary rates of biochar addition to mitigate unintended effects.