Dynamics of soil microbial carbon storage compounds in low fertility landscapes

Intracellular storage of carbon (C) by soil micro-organisms is emerging as a key process that influences soil biogeochemical cycling and the broader function of terrestrial ecosystems. One likely role of intracellular C storage is to serve as a stoichiometric buffer against nutritional imbalances in the microbial substrate. Such a function would make storage compounds vital to the long-term function of ecosystems associated with strongly weathered, low fertility soils, yet there have been few studies of intracellular carbon storage in such ecosystems. We examined the dynamics of two putative storage compounds (triacylglycerol [TAG] and polyhydroxybutyrate [PHB]) across two natural soil fertility gradients in eastern Australia. Across all sites and samples, absolute quantities of storage compounds ranged from 0 to 173 µg C g soil^-1 in the case of TAG and 0 to 56 µg C g soil^-1 for PHB. When standardized to total soil organic C, quantities of storage compounds tended to be markedly higher than those observed in prior studies of temperate and/or agricultural soils. Allocation to storage compounds followed strong trends across natural gradients of soil fertility and tended to peak in phosphorus-deficient and/or retrogressive ecosystems. Across soils of differing parent material, allocation to C storage was highest in infertile soils derived from phosphorus-depleted sandstone and ironstone compared to soils derived from shale and basalt. Likewise, allocation to C storage increased throughout ~700k years of soil development across a strongly weathered podzolic dune chronosequence. Dynamics of community-level C storage allocation were evidently underpinned by a combination of assemblage-level processes, most notably changes in the relative abundance of TAG-rich, C-limited fungal taxa, and physiological plasticity on the level of individual P-limited bacterial cells. Our findings are largely consistent with the surplus/reserve storage framework and highlight the importance of storage compounds for the function of oligotrophic ecosystems and as a major pool of C in soil.