Contemporary increases in nitrogen deposition can have wide-ranging effects on community structure and ecosystem function. Of particular importance are the effects of nitrogen deposition on soil microbial communities, which can mediate cascading geochemical and biological responses. Although prior work has focused on mesic ecosystems, arid ecosystems could show high sensitivity because of their relatively low levels of N and historical sources of N via fixation by biological soil crusts. Additionally, these effects can be mediated by local edaphic characteristics such as soil texture, water availability, and C:N. In this study, we investigated the indirect effects of experimental N addition on dominant grass performance via shifts in soil surface (crust) and rhizospheric microbial community composition across a soil texture gradient. Live soil inocula were collected from a N fertilization field experiment in Arches National Park, Utah, USA. Of each inoculum sample, half was sterilized to isolate the microbial effect, added to the soil surface of conetainer pots filled with sterile sand (inocula 20% total volume). Indian ricegrass (Achnatherum hymenoides) seeds were germinated in each pot, grown for 10 months in the greenhouse, and harvested. We examined above/belowground plant biomass, root fungal colonization, and soil C:N at the end of the experiment.
Results/Conclusions
Our results showed no difference in microbial effects on plant performance of inocula from different N treatments in the field. However, we found facilitative effects of microbial inocula from the sandiest site on plant performance, compared to neutral microbial effects of inocula from other sites. This pattern was reflected in root fungal colonization of live-inoculated plants: total root colonization was lower in plants inoculated with soil from the sandiest site, and these plants had much higher rates of colonization of dark septate endophytes (DSE) compared to the plants inoculated with soils from the other sites. It is likely that increased DSE in the soil community from the sandiest site contributed to the observed facilitative plant-microbe interaction outcomes. We also found higher soil C in pots inoculated with soil microbes from the least sandy site. In conclusion, the results of this study suggest that an increase in atmospheric nitrogen deposition, which can be a big driver of plant and microbial dynamics, is unlikely an important driver of dynamics in this ecosystem. Instead, our results point to heterogeneity in edaphic properties as a more important determinant of plant-soil microbial interaction outcomes.