Carbon Allocation and Cation Uptake Affected by Ectomycorrhizal Fungus in Nutrient Poor Settings, Using 13CO2 Pulse Labeling

Ectomycorrhizal fungi play a major role in cation uptake of pine trees from base poor soils and it is known that large amounts of photosynthate carbon are allocated belowground to maintain the ectomycorrhizal association. Such allocation may affect soil nutrient status and carbon accumulation on seasonal to ecosystem-development timescales. However, it is largely unknown what is the carbon cost of cation uptake by ectomycorrhizal weathering of forest tree species. We have compared chemical weathering fluxes and pine cation uptake with carbon allocation to pine and microbial biomass of ectomycorrhizal and non-ectomycorrhizal pine seedlings in a laboratory growth study. Red pine seedlings with or without ectomycorrhiza were grown in a base poor medium in a year-long growth chamber experiment. Chemical weathering and denudation fluxes were estimated by mass-balance of Ca²⁺, K⁺ and Mg²⁺ in input and output solutions, in plant biomass and on exchangeable cation sites of the growth medium at the end of the experiment. Before harvest the seedlings were exposed to two pulses of ¹³CO₂, the carbon allocation was estimated by measuring the ¹³C uptake of pine biomass and phospholipid fatty acids (PLFAs) in the growth medium using a quadruple gas chromatograph mass spectrometer. Ectomycorrhizal treatments accumulated the most biomass and the highest amounts of cations in biomass, which resulted in the highest weathering fluxes and the largest weathering-to-denudation ratios among treatments. These treatments also allocated the largest amounts of labeled carbon into their biomass and to their microbial associates. The PLFAs were only measurable in the ectomycorrhizal treatments. These results suggest that the larger cation uptake of ectomycorrhizal seedlings contributes to larger carbon capture, growth rate and carbon usage efficiency under nutrient poor conditions.