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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°C) to evaluate changes in SOM accumulation in native Australian grassland. In this system, competitive interactions appear to favor C₄ over C₃ 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 C₃- and C₄-dominated vegetation. We used physical fractionation to quantify particulate organic matter (POM) and long-term incubation to assess potential decomposition rates. Aboveground production of C₄ grasses increased in response to FACE, but total root biomass declined. Across treatments, C : N ratios were higher in leaves, roots and POM of C₄ vegetation. CO₂ and temperature treatments interacted with FT to influence SOM, and especially POM, such that soil carbon was increased by warming under C₄ 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 C₃-C₄ vegetation responses to future climate conditions will strongly influence SOM storage in temperate grasslands. |
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