Influence of biochar-triggered bottom-up effects on soil enzymes, soybean defenses, and the fitness of Spodoptera litura and its parasitoid

Bottom-up effects are major ecological forces in plant-herbivore-parasitoid multitrophic interactions. Biochar amendment in the field alters the bottom-up effects, as it can directly affect the soil environment and plant nutrient uptake, which in turn, indirectly affects herbivores and their natural enemies. However, such effects are poorly documented. We examined the influence of biochar on soil enzyme activity, soybean defense chemicals, and fitness of Spodoptera litura and its parasitoid Meteorus pulchricornis. As the biochar application rate increased, soil urease and sucrase activities, soybean trypsin protease inhibitor content, and polyphenol oxidase activity gradually increased. However, S. litura survival rate and adult body weight showed an initial increase, followed by a decrease, and then remained steady. Soil phosphatase activity initially increased and then stabilized, while M. pulchricornis cocoon emergence rate and fecundity showed an initial increase, followed by a decrease. Spo-doptera litura adults preferred to oviposit on soybean at the 10 g/kg biochar level, whereas the parasitoids preferred the hosts at the 20 g/kg biochar level. Our results indicate that the biochar amendment of soils increased soil enzymes activity and soybean defenses, which had a positive effect on S. litura and M. pulchricornis at low levels, but became detrimental at high levels. The bottom-up effects triggered by biochar seemingly affected the second and third trophic levels, resulting in increased defense potential against herbivorous insects. Thus, biochar showed potential to enhance direct plant defenses against pests and indirectly influenced defenses mediated by natural enemies.