Soil symbionts as filters and their environmental consequences

A critical question in ecology is the extent to which ecosystem functions can be influenced by the interaction of plant and soil microbial communities and how disturbance might alter those ecologically complex relations. Arbuscular mycorrhizal fungi (AMF) are widespread and associate with the majority of plant species in terrestrial ecosystems, influencing plant growth, local biodiversity and ecosystem function, in particular stabilization of soil aggregates. This dissertation presents a collection of studies that explore the effects of disturbance on plant - AMF interactions and soil aggregation. The first study revealed that anthropogenic disturbance consistently reduced water-stable aggregates in North American grasslands. Experimental studies suggest that this reduction is potentially driven by non-native plants and the degradation of the native soil microbial community. The second study of a chronosequence of twelve ex-arable fields in the Bolivian Altiplano, observed that fields with the longest time since disturbance have a greater proportion of water stable aggregates than more recently abandoned fields and corroborates that the soil fungal community is strongly correlated with the increase of water stable aggregates after cessation of cultivation. The third study, presents a description of two new AMF species detected on the Galapagos Islands and Brazilian sand dunes. The fourth study explores anthropogenically disturbed and naturally invaded fields from the highlands of Santa Cruz Island and investigates the mycorrhizal status of the local flora. Introduced plant species were more often of families that typically associate with AMF compared to native plant species. This result is consistent with a plant growth experiment, which revealed that introduced plants are more responsive to AMF inoculation than native species. The surveys of soil aggregate stability, vegetation and AMF showed that sites dominated by introduced species have a higher proportion of soil stable aggregates, mycorrhizal inoculum potential and AMF species richness. Together, invasive plants and AMF introductions could increase the success of invasions and alteration of ecosystem properties. The response of plant and soil microbial interactions varies due to taxon origin and geographical location, and disturbance may alter those relationships that feed back to important ecosystem properties.