Soil type modifies response of soil carbon pools to an atmospheric CO2 gradient

Background/Question/Methods

Literature suggests that as atmospheric CO₂ rises, soil carbon will cycle more rapidly as plants input greater amounts of labile carbon into the soil.  This labile carbon may stimulate the decomposition of more slowly-cycling soil organic matter through microbial priming.  We test these hypotheses in a prairie exposed to a preindustrial-to-future gradient (250-500ppm) of CO₂ for four growing seasons.  The experiment contains two contrasting soil types, allowing us to ask (1) How does the CO₂ gradient affect the sizes of active versus slow-cycling soil organic carbon (SOC) pools? And 2) Does the CO₂ effect differ in a clay-rich versus a sandy soil?

To investigate these questions, we incubated soil from the CO₂ gradient for one year in the lab, under constant temperature and moisture.  Carbon mineralization rate (Cmin) was measured throughout the year, and modeled with a two-pool exponential decay curve to estimate the size of SOC pools.

Results/Conclusions

The clay-rich Houston soil supported the hypothesis that SOC cycles more rapidly under elevated CO₂, but the sandy Bastrop soil did not.  Based on the exponential decay model, the size of the active C pool increased linearly with increasing CO₂ in the clay-rich Houston soil, but did not change in the sandy Bastrop soil.  The decay model also estimated that the size of the slow C pool did not change with CO₂ treatment in either soil.  This does not support the microbial priming hypothesis, which predicts a slight decline in the slow SOC pool at elevated CO₂.

Although Houston soil had roughly three times higher SOC concentration (26.7 g/kg) than Bastrop soil (8.2 g/kg), Bastrop SOC was more labile.  Cmin rate per gram SOC, an indicator of lability, was at least twice as large in Bastrop compared to Houston soil.  The decay model produced similar results, estimating that the active SOC pool constituted a greater proportion of total SOC in Bastrop soil (0.67%) than Houston soil (0.22%).

Given its influence on active SOC accumulation with CO2 treatment, soil type may be an important factor in predicting soil C sequestration under future CO₂ levels.