Rhizosphere engineering traits among invasive and native C3 grass species: Plant nitrogen uptake shapes rhizosphere microbial assemblages

Background/Question/Methods

Plant rhizospheres are considered ‘hot zones’ where carbon released from roots is quickly assimilated by soil microbes, consequently influencing microbial community assemblages as well as microbial C, N and P enzyme acquisition activities. However, the specific mechanisms used by plants to induce shifts in rhizosphere microbial community structure during the growing season are still not well understood. Thus, gaining a better understanding of how plant rhizoengineering mechanisms facilitate microbial community associations is essential to elucidate belowground plant competition strategies. For example, many studies have determined that when the invasive C₃ Bromus tectorum invades, its rhizosphere microbial communities tend to have higher N mineralization rates relative to those associated with native plants. Therefore, understanding soil N dynamics within the rhizosphere of B. tectorum could provide a valuable mechanistic understanding to help resolve belowground competitive advantages that influence microbial community properties associated with this highly successful invasive grass species. Here, we assessed plant species specific -microbial relationships among the rhizospheres of one invasive C₃ annual grass species B. tectorum and three native C₃ grass species in a controlled greenhouse environment at three different time points throughout the plant growing season to assess temporal plant influences on rhizosphere microbial community structural and functional characteristics.

Results/Conclusions

Rhizosphere microbial community structural and functional properties (phylogenetic, biomass C and N, and enzyme activities) continued to shift over time. During the first two sampling periods (day 28 and 78), rhizosphere microbial and soil nutrient characteristics demonstrated few differences among specific plant species.  However, throughout the growing season and during plant senescence, the invasive grass (B. tectorum) was consistently distinct (from P. smithii in particular and was associated with the highest plant N uptake during the plant growing season (i.e. days 28 and 76). Elevated rates of plant N uptake in the rhizosphere of B. tectorum corresponded with declines in microbial biomass (i.e. C and N) along with significant microbial community phylogenetic shifts and declining microbial community diversity. This finding suggests that during ‘high N demand’ periods of plant growth, plant N uptake may initiate a rhizosphere microbial community ‘selection’ event to ultimately restructure community composition simply by filtering certain microbial groups or individuals that cannot tolerate the plant-imposed environmental conditions.  This phylogenetic shift is robustly illustrated in our results and follows a linear trajectory across the plant growing season to convincingly suggest microbial community selection as influenced by higher plant N uptake and MBN declines over time.