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Plant roots interact with rhizosphere microorganisms to accelerate soil organic matter (SOM) mineralization for nutrient acquisition. Root‐mediated changes in SOM mineralization largely depend on root‐derived carbon (root‐C) input and soil nutrient status. Hence, intraspecific competition over plant development and spatiotemporal variability in the root‐C input and nutrients uptake may modify SOM mineralization. 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 and determined in situ soil C‐ and N‐mineralization by 13C‐natural abundance and 15N‐pool dilution approaches. From heading to ripening, soil C‐ and N‐mineralization rates exhibit similar unimodal trends and were tightly coupled. The C‐to‐N‐mineralization ratio (0.6 to 2.6) increased with N availability, indicating that an increase in N‐mineralization with N depletion was driven by microorganisms mining N‐rich SOM. With the intraspecific competition, plants increased specific root lengths as an efficient strategy to compete for resources. Root morphologic traits rather than root biomass per se were positively related to C‐ and N‐mineralization. Overall, plant phenology and intraspecific competition controlled the intensity and mechanisms of soil C‐ and N‐ mineralization by the adaptation of root traits and nutrient mining.

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-toN -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.

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