Plant species-specific stoichiometry of soil nutrients and extracellular enzymes in a semi-arid grassland ecosystem

Background/Question/Methods

In terrestrial ecosystems, plant tissue stoichiometry has been shown to vary in response to limiting nutrients as well as latitudinal shifts. At the ecosystem scale, the stoichiometry of plants, soils, and microbes may vary widely within a landscape. Microbes can control their internal stoichiometry by altering their allocation of resources towards production of enzymes that degrade C, N, P, and other soil nutrients. If different plant species favor distinct microbial communities, under natural selection we might expect genotypic variations controlling plant physiology to influence rhizosphere microbial communities. To examine whether stoichiometric characteristics vary consistently among plant species, we collected plants with intact root systems and rhizosphere soil for eight different plant species in a semi-arid grassland. We examined nutrient stoichiometry for soil enzyme nutrient acquisition activities, microbial biomass, mineral soil, and plant roots and shoots to address fundamental questions: Does components of the rhizosphere stoichiometry (i.e. soil and / or microbial properties) (1) differ among plant species, and (2) correlate to plant species tissue stoichiometry? This study is among the first to explore whether rhizosphere stoichiometry might be used to relate plant community composition to belowground C, N and P dynamics as an assessment of overall ecosystem functional properties.

Results/Conclusions

Here, we assessed ecological stoichiometry patterns at a finer spatial scale within individual plant rhizospheres to provide additional insight into the role of plant community composition in driving ecosystem function. Overall, our findings demonstrated variable, but strict stoichiometric homeostasis (i.e. consumer:resource stoichiometry ≠ 1:1) among plant stoichiometry and  rhizosphere components; thus not consistent with red-field type relationships at the plant - species specific level. While the majority of plant species rhizospheres demonstrated relatively similar stoichiometric patterns, B. dactyloides (C₄ grass) exhibited significantly wider soil and leaf C:N and significantly narrow soil and leaf N:P compared to the legume A. laxmannii. Recent studies have shown that plant species shifts can influence multiple above and belowground ecosystem processes. The belowground positive vs. negative feedbacks in that exist among plant species interactions is highly deterministic in the coexistence of competing plant species. Contrasting stoichiometric properties among plant rhizosphere zones may illustrate how soil P or N limitations could follow shifts in A. laxmannii or B. dactyloides abundances (respectively). Therefore, these plant species specific stoichiometry observations are useful to link potential belowground ecosystem function to changes in plant species distributions.