Probing the pump: Soil carbon dynamics, microbial carbon use efficiency and community composition in response to stoichiometrically-balanced compost and biochar

The accumulation of soil organic matter (SOM) is influenced by the ecophysiological traits of soil microbes. Amending soils with stoichiometrically-balanced inputs can optimise microbial resource acquisition and subsequent carbon (C) stabilisation pathways. However, this mechanism has been poorly translated into practice. Wastewater biosolids can be transformed into value-added soil amendments that return balanced ratios of C and nutrients from urban environments to agroecosystems. To investigate drivers of the microbial C pump in freshly-amended soils, we modelled microbial C use efficiency from stoichiometry theory (CUEST), respiration, physiological traits, community structure and soil C dynamics. Agricultural soil was amended with an equal C mass from composted biosolids, biosolids biochar or glucose supplemented with inorganic nitrogen (N), phosphorus (P) and sulphur (S), such that element ratios were analogous to stable SOM. Treatments were incubated in temperature-controlled microcosms, alongside a control with no C or nutrients added. Soil chemical characteristics, greenhouse gas evolution, microbial community composition and extracellular enzymes were measured at 0, 1, 2, 3, 7, 28 and 56 days. First order kinetics estimated total soil C at 2.60% (control), 2.54% (glucose + NPS), 2.93% (composted biosolids) and 2.92% (biochar). Soil C emissions increased by 702% (glucose + NPS), 22% (composted biosolids) and 203% (biochar) compared to the control. CUEST increased over time (p < 0.001) but was negatively affected by composted biosolids (p < 0.001) and glucose + NPS (p < 0.001). CUEST was linked to soil pH and correlated positively to microbial diversity for both composted biosolids (p < 0.001, r2 = 0.37) and biochar (p < 0.001, r2 = 0.43). Composted biosolids retained the most soil C, at the cost of microbial CUEST, whereas biochar maintained CUEST, but increased CO2 emissions. We provide insights into ecophysiological drivers of the microbial C pump and reveal how novel soil amendments differentially optimise the delivery of C and nutrients to the soil ecosystem.