Scaling rhizosphere respiration and priming effect from single plants to field ecosystems

Background/Question/Methods

Most knowledge of key rhizosphere processes comes from laboratory studies using single plant species. The main objective of our study was to determine whether rhizosphere processes are scalable among different plant species, different growth conditions in various soil types, and at different temporal and spatial scales by finding convergent scaling relationships. To address this objective we performed both greenhouse and field experiments. In the greenhouse we continuously labeled plants with ¹³C-depleted CO₂. The greenhouse experiment included four grassland species and unplanted controls in two different container sizes. In the field experiment we grew two C₄ plants, corn and sudan grass, in a C₃ organic farm soil to achieve differentiated δ¹³C values for soil and plant-derived respiration. The farm experiment also included both plant species and unplanted controls in field plots and in buried containers identical to those used in the greenhouse experiment. In both experiments we partitioned belowground respiration into soil and plant-derived components and evaluated rhizosphere priming effects on soil organic matter decomposition. We also measured shoot and root biomass, soil microbial biomass and turnover rate, and extracellular enzyme activities for both experiments.
Results/Conclusions

Our results will test two hypotheses: (1) rhizosphere respiration rate scales isometrically with live root N content, and is independent of plant taxonomic groupings or growth conditions; and (2) there exists an invariant scaling relationship between the rate of rhizosphere-primed soil organic matter decomposition and live root N content, live root biomass, and /or plant productivity. Initial results indicate that some rhizosphere processes can be scaled spatially. In the greenhouse experiment, there was no significant container-size effect on total soil respiration per unit of soil mass. Furthermore, the rates of total soil respiration per unit of soil mass were similar among different plant species in the greenhouse experiment. In the field natural ¹³C tracer experiment the rate of total soil respiration in the field plots without plant was largely comparable to the rate in the buried pots without plants indicating that the respiration rates can be reliably scaled across spatial scales using our methods. Similar soil respiration rates were also found between the two plant species. However, soil respiration rates in buried pots were significantly higher than the rates in the field plots, most likely due to the much higher root densities found in the buried pots.