Microbial Carbon Turnover and Dynamics From Biochemically Diverse Microbial Groups in Temperate and Tropical Forest Soils.

Microorganisms represent an important source of actively cycling carbon (C) in soils, yet little is known of the effect of microbial community structure and biochemistry on soil C dynamics.  This project utilized ¹³C- labeled microbial Treatments including fungi, gm (+) bacteria, gm (-) bacteria, and actinomycetes as substrates in a reciprocal transplant experiment in a temperate forest in the Sierra Nevada and a tropical forest in Puerto Rico. The temperate site is dominated by fungi and the tropical site is dominated by bacteria, which provides information about how different microbial communities assimilate the labeled C. Microbes were isolated from each site, grown with ¹³C media, autoclaved and lypholized, and residues were added back to soils at both sites. Temperate and tropical soils were sampled over a 3 and 2 year period, respectively. Mean residence time (MRT) for labeled-C in California was 5.21 ± 1.11 years, and in Puerto Rico was 2.22 ± 0.45. There were no significant differences in MRT among Treatments in California; whereas in Puerto Rico there were differences, with temperate fungi, temperate Gram (+) bacteria, and tropical actinomycetes exhibiting a significantly longer MRT as compared with tropical fungi and temperate Gram (-).Physical fractionation of soil indicate differences between sites in the partitioning of microbial C among light fraction (LF), occluded fraction (OF), and mineral fraction (MF).  While the temperate site retained a substantial portion of input microbial in the light fraction and mineral fraction, the tropical site retained substantially more of the treatments within the mineral fraction.  These results also correspond to the overall C pool sizes for the two sites among fractions. Recovery within the OF was significantly less than LF and MF for both sites, but exhibits a slower turnover rate relative to the LF and MF. Compound-specific turnover of input microbial residues associated with biomarkers from pyrolysis-gas chromatography-mass spectrometry-isotope ratio mass spectrometry (Py-GC-MS-IRMS) for the two sites will be discussed.