Nitrifying and Denitrifying Microorganisms Respond to Fertilization and Mounding in Regenerating Forest Stands.

Nitrifying and denitrifying microorganisms respond to fertilization and mounding in regenerating forest stands David Levy-Booth and Sue Grayston, University of British Columbia, Department of Forest & Conservation Sciences, 2424 Main Mall, Vancouver, BC, V6T 1Z4 Fertilization and mounding have been proposed in British Columbia (BC), Canada to enhance tree growth, sequester soil carbon and reduce rotation times in wet forest ecosystems. However, fertilization may increase soil N2O emissions, and mounding may increase CO2 emissions. This study quantified functional genes in the pathways that produce N2O to elucidate the effects of fertilization and mounding the microbial community responsible for greenhouse gas fluxes in regenerating forest stands. Research was conducted in a wet-cool sub-boreal spruce stand near Prince George, BC. Following harvest the site was mechanically mounded, planted with spruce, and fertilized with 200 kg N, 100 kg P, 100 kg K, 50 kg S ha-1 in June 2012. Soil and gas sampling over the next four months were used to investigate the effects of these practices on the soil microbial community, including nitrifiers and denitrifying microorganisms. Abundance of bacterial 16S, Archaeal ammonia monooxygenase (amoA), bacterial amoA, nitrite reductase (nirS, nirK) and nitrous oxide reductase (nosZ) genes was estimated with quantitative real-time PCR and compared to CO2 and N2O fluxes. Principal component analysis and path modeling found that amoA, nirS, nirK and nosZ correlated with N2O and demonstrated that fertilization increased abundance of these genes, leading to an increase in N2O efflux. Total bacterial 16S abundance were correlated with CO2 and were greater in unmounded plots than in mounded plots in June 2012 (p=0.00086). This suggests mounding suppresses soil bacteria, likely due to the disturbance of forest floor layers. In fertilized plots, nirSK-nosZ and AOB amoA were greater than in unfertilized plots during June (p=0.0196 and 0.0033, respectively) and August (p=0.0378 and 0.00012 respectively). These genes can provide an estimate of the ability of the microbial community to emit N2O. Microbial functional genes can be used as indicators of GHG flux potential in regenerating forest stands.