Labile carbon inputs support the recovery of bacterial communities, but not fungal communities, from a simulated bovine urine event

Inputs of carbon to soil may be used to stimulate microbial growth and immobilize excess nitrogen from sources such as livestock urine. However, the growth responses of microbial taxa to carbon inputs under conditions of excess soil nitrogen remain poorly understood. Using DNA metabarcoding and a field-based soil lysimeter experiment, we characterised the temporal responses (up to 112 days) of bacterial and fungal communities to a simulated bovine urine event plus inputs of labile carbon (sucrose) at two concentrations. Fungal communities were impacted more strongly than bacterial communities by carbon inputs following the simulated urine event, with more variable responses among taxa. Chytridiomycota and Glomeromycota richness were most negatively affected, and Tremellomycetes richness most positively affected, by carbon inputs. A minority of fungal ASVs had greatly increased proportional abundances in response to carbon, while fungal trophic composition became highly dominated by saprotrophs by the experiment end. Bacterial taxa showed consistent trends of declining (to about 14 days) and recovering (to 112 days) richness in response to urine and carbon inputs, but carbon-related evenness and proportional abundance trends varied between taxa. Proportional abundances of Actinobacteria, Bacteroidetes, Betaproteobacteria, and Gammaproteobacteria increased in response to carbon, whereas proportional abundances of Acidobacteria, candidate division WPS-1, Planctomycetes, Deltaproteobacteria, and Verrucomicrobia decreased. These results show that labile carbon inputs to limit nitrate leaching support the recovery of bacterial communities to bovine urine events but may have long-term impacts on fungal community composition and function, with potential consequences for soil food webs, carbon sequestration, and agricultural productivity.