PS 30-109 - Divergent soil priming by mycorrhizal types in response to nitrogen availability

Background/Question/Methods

Temperate forests are dominated by two types of mycorrhizal fungi—arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (EMF). EMF are hypothesized to compete with saprotrophic microbes for organic nitrogen (N), while AMF scavenge inorganic N, promoting a fast-cycling inorganic N economy. An important mechanism in these different mycorrhizal N-acquisition strategies is the propensity to exude plant-derived sugars to the free-living microbial community to prime further decomposition and N release. Some research shows that with increasing N availability, EMF decrease while AMF may increase exudation, which could promote both native soil carbon (C) loss and mineral-associated organic matter (MAOM) formation. We hypothesized that the tradeoff between new MAOM formation and native C loss would differ between mycorrhizal types and with N availability. We grew four AMF and four EMF-associated tree species in a ¹³C-labeled CO₂ atmosphere. Each pot contained three mesh soil pouches (450um, 50um – excludes roots, 1um - excludes roots/hyphae). Seedlings received three ¹⁵N-labeled NH₄⁺/NO₃^- treatments. Potting mix soil was analyzed for microbial biomass and NO₃^- and pouch soil for new C inputs and native C loss. Fungal communities from pouch soil and root tips were characterized by DNA sequencing, qPCR, and root tip colonization.

Results/Conclusions

Initial results show that microbial biomass decreased with increasing NO₃^- availability in EMF but not AMF pots. Additionally, bulk soil ¹³C was positively associated with plant biomass in EMF but not AMF associated soils. These findings indicate different relationships between N availability and exudation to the rhizosphere between mycorrhizal types. Because some research has found that EMF but not AMF decrease exudation to the rhizosphere with increasing inorganic N availability, we expect that more ¹³C-labeled exudates will be found in EMF soil with increasing plant N demand, resulting in more native C loss. We predict that this will correspond with increasing EMF-associated plant tissue ¹⁵N content as plants were likely to rely on native organic sources of N more when inorganic N was scarce. Our results will elucidate how exudation and priming may contribute to the observed differences in C storage between AMF and EMF forests.