Interactive effects of plant-mediated controls and substrate quality on fungal necromass decay

Background and aims: A significant portion of grassland plant productivity is directed belowground, fueling microbial biomass production and influencing soil carbon (C) cycling. However, it's unclear how plant interactions with foliar fungal pathogens and environmental nutrient supply alter these plant-derived soil organic carbon (SOC) inputs and the subsequent breakdown of microbial necromass, a major SOC pool. In this study, we used a long-term grassland experiment manipulating foliar fungi and soil nutrients to test contrasting effects on fungal necromass decomposition. Methods: We assessed the independent and interactive effects of soil and necromass resource quality on microbial decomposer communities by deploying fungal necromass substrates of varying C quality in fertilized and unfertilized grassland plots with and without fungicide application. Specifically, we quantified necromass decay rates along with decomposer abundance (qPCR) and community composition (high-throughput sequencing) at early and later stages (days 6 and 32) of decay. We also characterized plant community above- and belowground biomass and functional group representation. Results: High quality necromass decay increased with C3 plant biomass as well as mould and yeast relative abundance. Conversely, low quality necromass loss was regulated directly by soil moisture and pH, while its recalcitrant fraction responded to opposing controls: increasing under C3 grasses but decreasing with fertilization. Conclusion: We show that bottom-up and top-down factors interact with necromass chemistry to govern necromass decay. Our findings support the critical role of plants in coupling aboveground fungal communities with belowground decomposer communities and their collective influence on soil C cycling.