Compost amendment alters kale (Brassica oleracea) susceptibility to green peach aphids (Myzus persicae) in a soil microbiome-dependent manner

While soil microbial communities are known to enhance plant resilience to abiotic and biotic stressors, it is still poorly understood how specific farming practices and soil microbiomes interact to influence plant resilience outcomes. The goal of this study was to evaluate the impact of compost amendment on soil microbiome-mediated plant resistance using kale (Brassica oleracea) and green peach aphids (Myzus persicae) as a model system. Over two consecutive years, kale plants were grown in soil that had been inoculated with distinct soil microbiome extracts from farms across New York. Each pot was amended with commercial compost or left unamended. After four weeks, plants were used in insect bioassays, and the resulting soil microbial communities were sequenced. Insect bioassays revealed that compost amendment did not consistently affect kale resistance to aphids when plants were grown in different soil microbiomes. In some microbiomes compost enhanced plant resistance (fewer aphid progeny), while in others it had no effect or even reduced resistance. The effect of microbiomes with compost were consistent across sampling years from some farms, while others changed over time. Despite this, compost consistently increased microbial community alpha diversity in all microbiome treatments and reduced beta diversity between treatments. The relative abundance of several compost-associated microbial taxa that were linked to increases or decreases in kale resistance, suggesting these microbes may act as defense inducers or suppressors. Our findings demonstrate that although compost is often associated with enhanced insect resistance, its effects are context-dependent and shaped by the existing soil microbiome.