Microbial utilisation of maize rhizodeposits applied to an agricultural soil at a range of concentrations

Rhizodeposition fuels carbon (C) and nutrient cycling in soil. However, the dynamics of microbial growth on rhizodeposits in relation to the distance from the root have not been well studied. This study investigates microbial growth on individual organic components of rhizodeposits and on maize root-derived exudates and mucilage from an agricultural soil. By creating a gradient of substrate concentrations, we simulated reduced microbial access to rhizosphere C with increasing distance to the root surface. We identified distinct C-thresholds for the activation of microbial growth, and these were significantly higher for rhizodeposits compared to singular, simple sugars. In addition, testing for stoichiometric constraints of microbial growth by supplementing N and P showed accelerated and increased microbial growth by activating a larger proportion of the microbial biomass. Early and late season exudates triggered significantly different microbial growth responses. The mineralisation of early season exudates was induced at a high C-threshold, whereas the mineralisation of late season exudates showed 'sugar-like' properties, with a low C-threshold, high substrate affinity, and a reduced maximum respiration rate. Mucilage exhibited the highest C-threshold for the activation of microbial growth, although with a short lag-period and with an efficient mucilage degradation comparable to that of sugars. By determining kinetic parameters and turnover times for different root-derived substrates, our data enable the upscaling of micro-scale processes to the whole root system, allowing more precise predictions of how rhizodeposits drive microbial C and nutrient dynamics in soil.