Belowground carbon fluxes in an old-field ecosystem are sensitive to climate change
While it's well-known that roots play a central role in carbon (C) and nutrient cycling, remarkably little is known about how root processes respond to climate change, and the ecosystem consequences of these responses. We investigated the sensitivity of fine root production and rhizodeposition to changes in climate in an old-field ecosystem at the Boston-Area Climate Experiment. In this experiment, plant communities have been exposed to three precipitation (ambient, -50% of ambient, and +50% of ambient) and two warming (ambient and +4°C above ambient) treatments since 2008. Fine root production and rhizodeposition were quantified over a one-year period (fall, 2013 to fall, 2014) using ingrowth cores (1000 µm mesh) filled with a mix of sand (80% by volume) and isotopically-distinct “C4 soil” (20% by volume). Inputs of rhizodeposition were estimated by calculating the net change in 13C inside the cores resulting from root inputs from the C3 plants. Ingrowth cores containing the same sand-soil mix, but with a 50 µm, were also inserted into each plot in order to partition root- and mycorrhizal-derived C inputs.
Overall, warming effects on belowground C fluxes were contingent on soil moisture. In plots receiving ambient precipitation, warming had no effect on fine root productivity relative to unwarmed plots (P > 0.05), but increased rhizodeposition by a factor of two (P < 0.0001). In plots receiving reduced precipitation, warming decreased fine root productivity by 66% relative to unwarmed plots (P < 0.05) and decreased rhizodeposition by 50% (P < 0.05). In plots receiving additional precipitation, warming had no effect on root productivity or rhizodeposition relative to unwarmed plots (P > 0.05 for both). Notably, the percentage of fine root C released as rhizodeposition varied strongly by treatment from ~30% in the unwarmed plots to ~80% in the warmed plots. Mycorrhizal inputs of C generally responded in the same direction as root inputs - but these inputs were 1-2 orders of magnitude less than the root inputs.
Collectively, our results indicate that while belowground processes may be highly sensitive to climate, the differential response of fine root growth and rhizodeposition to environmental drivers may alter whether changes in C storage occur in plant biomass vs. soils. Thus, ecosystem models that consider both root growth and rhizodeposition may lead to improved predictions of potential climate change feedbacks.