Recent climate models suggest a possible increase of precipitation in the southeastern United States. To understand the response of ecosystems to increased rainfall, it is necessary to identify both above and belowground mechanisms that regulate carbon (C) and nutrient cycles. Specifically, shifts in the relative C allocation to mycorrhizal fungi can have important implications for forest dynamics. We investigated the role of mycorrhizal fungi in belowground C cycling of a longleaf pine savannah located in southern Georgia (USA).  Vegetation is dominated by ectomycorrhizal C3 trees (longleaf pine and turkey oak) and by an arbuscular mycorrhizal C4 plant (wiregrass). Carbon isotopes (∂¹³C) differ between C3 and C4 plants. The contribution of C₃ and C4 symbioses to belowground carbon can be quantified using this isotopic difference. Contributions of ECM and AM fungi can also be separated by analyzing the ∂¹³C of fungal biomarkers such as PLFA’s. Soil samples were collected in plots subjected to natural rainfall or to artificially enhanced rainfall after 8 years of treatment, in 2009.  

Results/Conclusions

In 2009, soil C% in the first 20 cm was higher in the treated plot (1.13%) than in the untreated plot (0.84%). This relative increase of 35% between treated and untreated plots suggests that increased precipitation greatly enhanced both above and belowground production. Soil ∂¹³C in the upper soil did not change with treatment (-23.5‰ on average), indicating that the relative contribution of C3 (62%) and C4 (38%) symbioses did not change with increased precipitation. C% and ∂¹³C in the lower soil did not change with treatment being respectively 0.4% and -23.0‰ on average. Aboveground, increased precipitation significantly lowered the ∂¹³C signature of pine (-28.6 to -30.6) and wiregrass (-12.0 to -14.0), but less so in oak (-30.1 to -30.6).  These results indicate an effect of precipitation regimes on plant ∂¹³C possibly leading, on a longer term, to a change of soil ∂¹³C. More detailed investigations using ∂¹³C are being conducted to specifically untangle the effect of precipitation on belowground C partitioning.