Elevated CO2 and N addition alter rhizosphere priming of soil organic matter decomposition

Background/Question/Methods

As the largest terrestrial pool of carbon (C), even relatively small changes in soil C could have large implications for atmospheric carbon dioxide (CO₂) concentrations. Rhizosphere priming of soil organic matter decomposition is an important source of soil C loss, and could respond to changing environmental conditions. Specifically, increasing atmospheric CO₂ concentrations and nitrogen (N) deposition could alter plant allocation of C resources belowground, resulting in changes in priming and C release through decomposition. We tested the effects of elevated CO₂ and N addition on soil CO₂ flux and the amount of CO₂-C specifically released from soil by rhizosphere priming of soil organic matter decomposition. To do this, we used a mesocosm study within the Biodiversity, CO₂, and Nitrogen Experiment at the Cedar Creek Ecosystem Science Reserve in central Minnesota, USA. We harnessed differences in stable isotope chemistry between plants and soil (based on C3/C4 differences) to partition the amount of total CO₂ originating from SOM, and the specific amount decomposed as a result of rhizosphere processes.

Results/Conclusions

Overall, elevated CO₂ increased the soil C loss from rhizosphere priming of decomposition by 34-39% on average (on a per g soil and per g soil C basis, respectively), and N addition reduced the soil C loss from priming by 29% on average (on a per g soil C basis, there was no N effect on the rhizosphere priming effect when expressed per g soil). We also found substantial differences in the amount of C lost from rhizosphere priming of decomposition between the two C3 grass species studied – either 340% or 460% depending on the metric of C flux used (on a per g soil and per g soil C basis, respectively). Finally there was no mediation of the elevated CO₂-induced rhizosphere priming effect with N addition (i.e. no CO₂*N interaction), as we expected if plant nutrient status is a primary driver of the rhizosphere priming CO₂ effect. These findings support the hypothesis that increasing concentrations of CO₂ could result in greater loss of old, soil organic matter-derived C from grassland soils.