Arbuscular mycorrhizal fungal (AMF) composition influences the functioning of the symbiosis with plants. However, how species composition of AMF will change in response to environmental change is poorly understood. We enumerated responses of the AMF community to nitrogen (N) fertilization and drought in coastal sage scrub of southern California. We hypothesized that: predominantly extra-radical (edaphophilic, e.g. Gigasporaceae) AMF would be more abundant in native shrubs and respond negatively to drought and N addition; generally intra-radical (rhizophilic, e.g. Glomeraceae) AMF would be more abundant in invasive grasses and with N addition; AMF without preferential allocation to extra- or intra-radical colonization (ancestral, e.g. Acaulosporaceae) would not respond to treatments. We hypothesized that increasing N and precipitation leads to higher plant allocation of carbon to AMF, resulting in more spore and hyphal production.
We sampled roots and soils from a long-term experiment manipulating precipitation (ambient, added and reduced) and N (ambient and added), in both summer and winter. We measured percent root infection, soil hyphal lengths and spore abundance across two seasons. We extracted DNA from roots collected in winter, amplified SSU rDNA using AMF specific primers, and used these environmental sequences to measure responses of AMF composition to precipitation and N treatments.
Results/Conclusions Drought reduced sporulation in winter but not summer and did not affect root colonization or hyphal activity. Beta-diversity did not respond to watering treatments, although drought reduced some edaphophilic AMF and increased the abundance of some ancestral AMF. Nitrogen addition increased root colonization, but did not affect sporulation, community composition or hyphal length. Host-plant identity regulated root colonization, soil hyphal lengths, beta-diversity and abundance of some ancestral AMF taxa. Alpha-diversity did not differ across treatments, suggesting that differences in beta-diversity were changes in AMF species identity and not abundance.
These data suggest that increasing aridity in southern California may reduce the abundance of edaphophilic AMF, altering plant diversity and functioning. While additional N increased root colonization by AMF, this did not affect species composition. This contrasts with past research in coastal sage scrub finding an increased abundance of rhizophilic AMF and a decline in root colonization with N deposition. The observed structuring of the AMF community by host-plants suggests that ongoing change in coastal sage scrub composition through invasion by grasses could markedly change the identity and biomass allocation patterns of the AMF community.