Linking plant species and their rhizosphere traits with soil carbon priming in situ

Background/Question/Methods

While it is understood that plants and particularly roots can alter soil organic matter dynamics, with potential impacts on the ecosystem’s C balance, we have little understanding of how variation in plant traits drives this response. In the available literature, observed indications of soil organic matter enhancement or suppression (priming) with environmental changes are indirect and only secondarily linked to specific plant traits. A robust examination of plant traits and soil C priming in undisturbed, in-situ conditions is lacking.  Because of the particular way in which the combination of plants traits modifies the rhizosphere environment, plant rhizospheres could vary in terms of how conducive they are to the priming of C if exposed to an increase in available C, for example with increased atmospheric CO₂. How conducive a species’ rhizosphere environment is to priming could be assessed by looking at the in-situ response of soil organic matter respiration to the addition of a  C source. We (1) assessed how ‘primable’ soil organic matter was under eight selected species present in a native North American mixed grass prairie, including grasses, forbs, legumes, C3, C4, native and weedy species, and under bare soil (via the addition of an isotopically labeled C source); and (2) characterized the rhizosphere environment for these species (belowground plant traits, chemistry of soil solution, microbial and exoenzyme stoichiometry).

Results/Conclusions

We detected considerable variation among the rhizosphere environments of the plant species and between these and the bare soils (where no influence of roots exists). After the addition of identical amounts of the C tracer to equivalent rhizosphere volumes, the amount, isotopic composition and temporal behaviour of the respiration of soil C and the freshly added substrate varied substantially across plant species. This suggests a strong role of root presence/activity on C decomposition and notably, high species dependency. The magnitude, direction and temporal dynamics of soil C priming did not appear closely driven by plant stoichiometric traits or microbial/enzymatic stoichiometry (C:N, C:P, N:P) or, by plant functional groups. Instead it was the species specific combination of traits that drove the variation in priming of soil C. Our results indicate that understanding the links between plant traits and priming should be useful for predicting changes in soil C cycling and ecosystem C balance with shifts in plant community composition.