Diverse microbial communities show uniform respiration response to nitrogen addition in simulated early-stage leaf litter decomposition experiments

Plant litter decomposition by microbes plays a major role in soil CO₂ and dissolved organic carbon (DOC) production. Adding reactive nitrogen species is predicted to affect these fluxes, but it’s unclear to what extent since both microbial community composition and abiotic soil and litter chemistry will influence outcomes. We therefore sought to understand broad geochemical trends across diverse microbial assemblages during the initial stages of litter decomposition by incubating 10 diverse soil communities in identical geochemical conditions – sterile sand and leaf litter with water, inorganic nitrogen (ammonium nitrate), organic nitrogen (urea), or both – in the dark for 48 days at 25 °C. The headspace was regularly purged with ambient air, and we measured CO₂ production, microbial biomass C:N, bacterial and fungal community composition, DOC and dissolved total nitrogen (DTN) concentrations, respiratory quotients (RQ), and carbon use efficiency (CUE). Overall, we find across all communities and treatments that most grass litter carbon (~ 30%) is aerobically respired as CO₂ while some (~ 6%) is consumed as DOC and less than 1% becomes biomass. Compared to the water control, nitrogen treatment – particularly organic nitrogen – increases CO₂ production and DOC consumption by roughly 10% and 1% respectively, but CUE, RQ, and biomass C:N are unchanged. By constraining the litter and chemical composition of our incubations, we show reactive nitrogen addition enhances rather than alters existing microbial pathways of CO₂ production during the early stages of leaf litter decomposition.