Rhizosphere priming regulates soil organic carbon and nitrogen mineralization: The significance of abiotic mechanisms

The effect of living roots on the co-mineralization of soil organic carbon (SOC) and nitrogen (SON) and driving mechanisms of the rhizosphere priming effect (RPE) remains unclear. Moreover, it is still poorly understood whether the abiotic mechanisms, whereby roots accelerate soil organic matter (SOM) loss by destabilizing organo-mineral associations, are involved. Biotic and abiotic processes involved in the RPE and gross N mineralization (GNM) were investigated using three paddy soils (C3) under maize (C4 plant) cultivation. The soils had high and low total N (1.79 and 3.3 g kg−1 soil) as well as iron- (Fe-) (hydr-) oxide (1.1 and 2.2 g kg−1 soil) contents, which gave the following combinations: high-Fe/low-N, low-Fe/low-N, and low-Fe/high-N. The RPEs and GNM were measured using a 13C-natural abundance approach and 15N-pool dilution technique, respectively, on day 86 of maize cultivation. Living roots enhanced native SOC mineralization by 70.4–204% and GNM by 118–382%. As expected, biotic mechanisms contributed to RPEs (‘microbial activation’ and ‘microbial N-mining’) that was supported by the increase of soil microbial biomass C and extracellular enzyme activities in presence of plant, and the lower C/N ratio of primed SOM than the original SOM in unplanted soils. Higher RPEs and relative primed N mineralization via stronger ‘microbial N-mining’ were found in low-Fe/low-N vs low-Fe/high-N soils. In contrast to expectations, the RPEs and relative primed N mineralization were greater in high-Fe/low-N versus low-Fe oxide soils. The strongest decrease in SOM-derived Fe-bound C and increase in root-derived Fe-bound C were observed in the high-Fe soil, because root exudates liberated more C from Fe-bound SOM and co-precipitated with Fe oxides. This shows that the abiotic process was involved in RPEs whereby root exudates promoted soil C loss by releasing it from Fe-organic complexes. Thus, this study demonstrated that coupled biotic-abiotic processes could regulate RPEs.