PS 32-184 - A process-based metamodel analysis of plant-microbe growth promotion pathways

Abstract: Soil microbes are integral to plant growth promotion, but their mechanisms and interactions are complex, dynamic, and difficult to predict. Here we observed Wisconsin Fast Plant (Brassica rapa) grown under initially inoculated (with a growth promoting soil from a Wyoming site, "SBC" ) or sterilized ("BLANK") soils for a two-week period in a green house. Projected leaf area was recorded throughout the study and simulated using a process-based model for above and below ground mechanisms. A metamodel was employed to interpolate the best fit distributions of leaf growth and soil microbe parameters (a scalar for setting initial microbial biomass pools) based on 10000 randomly tested parameter sets compared to each treatment for five potential mechanistic pathways. Each pathway is defined by which parameters (growth or soil microbe scalar) are held constant, as fit to one of the treatments, while the other parameter is fit directly, except in the last pathway where all parameters are fit to both treatments. The first pathway, 'nutrient only stimulation', represents solely an increase of nutrients supplied to the SBC plants stimulated by the microbes. The second pathway, 'nutrient only inhibition', represents inhibition of nutrient uptake due to the absence of soil microbes. Third, 'non-nutrient only stimulation', represents a pathway for growth promotion other than nutrients supplied. Fourth, 'non-nutrient only inhibition', represents a pathway for growth inhibition other than through changes to nutrients supplied. Finally, 'nutrient and non-nutrient growth control', represents a combination of the other pathways. Results show 'nutrient and non-nutrient growth control' pathway providing the best fit followed, in order, by 'non-nutrient only stimulation', 'non-nutrient only inhibition', 'nutrient only stimulation', and 'nutrient only inhibition'.

The presence of soil microbes causing growth promotion, rather than absence causing inhibition, is supported by our findings. These results suggest that improved nutrient uptake is not the only important factor for soil microbe induced growth promotion and that other interactions need to be considered. Additionally, our results demonstrate the utility of process-based models and metamodeling techniques in exploring potential mechanistic pathways that are otherwise obscured by the complex and dynamic interactions of plants and soil microbes. Our method also holds promise for exploring other, equally complex, below ground interactions that have proven difficult to study directly.