Historical contingency and priority effects impact the decomposition of dead microbial biomass

Abstract

Differences in species arrival order can significantly impact both the structure and functioning of ecological communities. These so-called ‘priority effects’ can occur when the early arriving species modify their environment to make it more or less suitable for subsequent colonizers (i.e., niche modification) or when the early arriving species exhaust resources and thus limit subsequent species establishment (i.e., niche preemption). Here, I examined how priority effects impacted the decomposition of dead fungal mycelium (i.e., necromass), a major component of soil carbon (C) and nitrogen (N) in forest soils. In Chapter I, I assessed how fungi and bacteria affected necromass decay using laboratory microcosms. I found that fungi rather than bacteria, drive the early decomposition of fungal necromass. Nevertheless, I observed that bacteria were capable of growing on necromass as their main C-source when co-inoculated with a fungal partner. In Chapter II, I conducted a paired laboratory- and field-based experiment to test the effect of species arrival order on necromass decay, using both fungal and bacterial strains. I found strong priority effects in terms of necromass remaining and pH between fungi. Even when bacteria did not affect necromass decay, they still benefited from the early-arrival of a fungal partner. Lastly, in Chapter III, I tested whether having a harsher environment would reduce the strong priority effects observed in Chapter II. To do so, I used melanized necromass as a proxy of environmental harshness and tested the effects of species arrival order using fungi in a laboratory and field-based experiment. I found that priority effects in terms of necromass remaining prevailed, but melanized necromass reduced the differential niche modification of fungi in terms of pH. In the field-decomposition experiments from Chapters II and III, I observed that priority effects on necromass remaining attenuated over time, regardless of necromass chemistry (i.e., melanin content). Yet, pre-colonization with a fungal strain modified the alpha- and beta-diversity of fungal communities in decaying necromass. Collectively, my research highlights the importance of priority effects in shaping the structure and functioning of the fungal necrobiome and demonstrates that fungi can use multiple mechanisms to alter community assembly processes.