Soil carbon storage under simulated climate change is mediated by plant functional type

The stability of soil organic matter (SOM) pools exposed to elevated CO2 and warming has not been evaluated adequately in long-term experiments and represents a substantial source of uncertainty in predicting ecosystem feedbacks to climate change. We conducted a 6-year experiment combining free-air CO2 enrichment (FACE, 550 ppm) and warming (+2 degrees C) to evaluate changes in SOM accumulation in native Australian grassland. In this system, competitive interactions appear to favor C-4 over C-3 species under FACE and warming. We therefore investigated the role of plant functional type (FT) on biomass and SOM responses to the long-term treatments by carefully sampling soil under patches of C-3- and C-4-dominated vegetation. We used physical fractionation to quantify particulate organic matter (POM) and long-term incubation to assess potential decomposition rates. Aboveground production of C-4 grasses increased in response to FACE, but total root biomass declined. Across treatments, C : N ratios were higher in leaves, roots and POM of C-4 vegetation. CO2 and temperature treatments interacted with FT to influence SOM, and especially POM, such that soil carbon was increased by warming under C-4 vegetation, but not in combination with elevated CO2. Potential decomposition rates increased in response to FACE and decreased with warming, possibly owing to treatment effects on soil moisture and microbial community composition. Decomposition was also inversely correlated with root N concentration, suggesting increased microbial demand for older, N-rich SOM in treatments with low root N inputs. This research suggests that C-3-C-4 vegetation responses to future climate conditions will strongly influence SOM storage in temperate grasslands.