Spatial and temporal environmental drivers of microbial carbon storage in soil

Abstract Soils are critical stocks of terrestrial organic carbon (C), with microbial processes mediating key transformations of plant-derived C inputs. While microbial replicative growth and respiration have been widely studied for decades, a part of the microbial C budget, intracellular C storage, that does not directly contribute to microbial activities, has been largely neglected. Recent evidence suggests that intracellular C storage is a significant and active pool. Polyhydroxybutyrate (PHB) is a microbial storage compound typically formed in response to stoichiometric imbalance, but the factors that govern its abundance in soil are not known. We hypothesized that PHB is an inter-seasonal storage, such that its content in soil and its allocation (PHB-C relative to microbial biomass carbon) would follow a seasonal cycle, increasing in summer when belowground C inputs are high due to high net primary productivity (NPP), and decreasing in winter as microbes draw down their stores (H1); that PHB-C allocation would be positively related to short-term NPP (H2), reflecting short-term changes in belowground C input; and finally, that PHB-C allocation would be controlled by nitrogen (N) availability and C/N stoichiometry of the soil and thus would be higher at locations with lower N availability (H3). PHB dynamics and spatial variation were observed across a temperate grazed grassland over one year, combined with the estimation of NPP from remote-sensing and measurements of soil chemistry and extractable microbial biomass. This revealed that PHB formation was dynamic at the small scale but varied only weakly across seasons and with temporal changes in NPP. Instead, PHB-C allocation was strongly linked to total soil N content and soil C/N stoichiometry, as well as spatial variation in annual NPP, with highest storage in N-poor locations. Soil microbial PHB storage is evidently not seasonally determined at the field scale but rather driven by soil conditions at small spatial scales, most notably soil nutrient availability and stoichiometry.