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Microorganisms help govern soil organic carbon (SOC) turnover and accumulation. Whilst it is increasingly clear that microbial necromass is a precursor of SOC formation, the relationship between living microorganisms, necromass turnover and SOC persistence remains elusive. In this study, we used phospholipid fatty acids and amino sugars to quantify living versus dead microbial carbon concentrations and evaluated the utility of each pool as an indicator of SOC persistence across a range of climates (low‐, mid‐ and high‐latitude sites) and ecotypes (old‐growth forests vs. managed croplands). We found that microbial necromass was higher in forest than in cropland soils and was positively correlated with soil moisture, SOC and total nitrogen (TN). However, the flow of microbial biomass into necromass and SOC was decoupled in forest sites, likely because the high soil SOC/TN ratio accelerated necromass turnover and recycling by living microorganisms. In contrast, microbial biomass and necromass pools were tightly coupled in croplands and influenced by multiple environmental and biological factors (e.g., necromass concentrations exhibited greater variability in soils with more bacteria than fungi, and those with more gram‐positive than gram‐negative taxa). Contrasting our expectations, the proportion of microbially‐derived necromass in SOC was decoupled from soil properties and microbial biomass in both ecotypes. Whilst SOC and pH appear to be universal drivers of necromass cycling, feedbacks between living biomass, necromass and SOC are shaped by local factors. Our results contribute to ecological theory by highlighting the environmental and biological factors underpinning SOC formation and turnover that can be used to inform land‐management practices that optimize below‐ground carbon sequestration. [ABSTRACT FROM AUTHOR]

Tasmanian eucalypt forests are among the most carbon‐dense in the world, but projected climate change could destabilize this critical carbon sink. While the impact of abiotic factors on forest ecosystem carbon dynamics have received considerable attention, biotic factors such as the input of animal scat are less understood. Tasmanian devils (Sarcophilus harrisii)—an osteophageous scavenger that can ingest and solubilize nutrients locked in bone material—may subsidize plant and microbial productivity by concentrating bioavailable nutrients (e.g., nitrogen and phosphorus) in scat latrines. However, dramatic declines in devil population densities, driven by the spread of a transmissible cancer, may have underappreciated consequences for soil organic carbon (SOC) storage and forest productivity by altering nutrient cycling. Here, we fuse experimental data and modeling to quantify and predict future changes to forest productivity and SOC under various climate and scat‐quality futures. We find that devil scat significantly increases concentrations of nitrogen, ammonium, phosphorus, and phosphate in the soil and shifts soil microbial communities toward those dominated by r‐selected (e.g., fast‐growing) phyla. Further, under expected increases in temperature and changes in precipitation, devil scat inputs are projected to increase above‐ and below‐ground net primary productivity and microbial biomass carbon through 2100. In contrast, when devil scat is replaced by lower‐quality scat (e.g., from non‐osteophageous scavengers and herbivores), forest carbon pools are likely to increase more slowly, or in some cases, decline. Together, our results suggest often overlooked biotic factors will interact with climate change to drive current and future carbon pool dynamics in Tasmanian forests. [ABSTRACT FROM AUTHOR]