Plant intraspecific competition and growth stage alter carbon and nitrogen mineralization in the rhizosphere

Plant roots interact with rhizosphere microbes to accelerate soil organic matter (SOM) mineralization and promote nutrient acquisition. Root-mediated changes in SOM turnover largely depend on root-C input and soil nutrient availability. Hence, interspecific competition and nutrient uptake dynamics over plant development stages as well as spatiotemporal variability in C input may modify SOM turnover. To investigate the effect of intraspecific competition on SOM mineralization at three growth stages (heading, flowering and ripening), we grew maize (C4 plant) under three planting densities on a C3 soil. 13C-natural abundance and 15N-pool dilution were applied in situ to determine C- and N-mineralization rates. Soil C- and N-mineralization rates were tightly coupled and peaked at maize flowering. However, the C-to-N-mineralization ratio increased with N, indicating that microbes mineralize N-rich components to mine SOM for N. Furthermore, intraspecific competition did not affect root biomass; instead, plants shaped root morphology towards higher specific root length as an efficient strategy competing for nutrient. Hence, root morphologic traits rather than root biomass per se were positively related to C- and N-mineralization. Overall, plant competition for nutrients controlled the intensity and mechanisms of soil C- and N-turnover by the adaptation of root traits and nutrient depletion.