Rapid increase in soil pH solubilises organic matter, dramatically increases denitrification potential and strongly stimulates microorganisms from the Firmicutes phylum

We used potassium hydroxide (KOH) to induce rapid soil pH changes and then observed microbial community change over 48 hours in anaerobic conditions before measuring denitrification enzyme activity (DEA). Soil pH was elevated from 4.7 to 6.7, 8.3 or 8.8, straddling the range of localized pH changes likely to be observed in soil after deposition of livestock urine or urea fertiliser. Up to 240-fold higher dissolved organic matter (DOM) was mobilized by KOH compared to the controls. This increased microbial metabolism but there was no correlation between DOM concentrations and CO2 respiration nor N-metabolism rates. Microbial communities became dominated by Firmicutes bacteria within 16 hours, while few changes were observed in the fungal communities. Changes in N-biogeochemistry were rapid and DEA increased up to 25-fold with the highest rates occurring in microcosms at pH 8.3 that had been incubated for 24-hour prior to measuring DEA. Nitrous oxide reductase (N2O-R) was inactive in the pH 4.7 controls but at pH 8.3 the reduction rates exceeded of 3000 ng N2-N g-1 h-1 in the presence of native DOM. Evidence for DNRA and/or organic matter mineralisation was observed with ammonium (NH4+) increasing to concentrations up to 10 times the original native soil concentrations while significant concentrations of nitrate (NO3-) were utilised. Pure isolates from the microcosms were predominantly Bacillus spp. and exhibited varying NO3- reductive potential.