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13C AND 15N IN MICROARTHROPODS REVEAL LITTLE RESPONSE OF DOUGLAS-FIR ECOSYSTEMS TO CLIMATE CHANGE
Citation:
HOBBIE, E. A., P. T. RYGIEWICZ, M. G. JOHNSON, AND A. R. MOLDENKE. 13C AND 15N IN MICROARTHROPODS REVEAL LITTLE RESPONSE OF DOUGLAS-FIR ECOSYSTEMS TO CLIMATE CHANGE. GLOBAL CHANGE BIOLOGY. Blackwell Publishing, Malden, MA, 13:13886-1397, (2007).
Impact/Purpose:
To better understand ecosystem carbon and nitrogen cycling under global change
Description:
Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO2) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO2 increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n = 4, r2 = 0.96, P = 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8¿1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25¿26) and litter (36¿48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO2 increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.