Investigating soil nitrous oxide fluxes along a harvested red spruce forest chronosequence

Forest harvesting can alter soil biogeochemical cycles, shifting the fluxes of greenhouse gases to the atmosphere on the scale of months to decades. Clearcut harvesting is a standard management practice in many temperate forest regions, yet our current understanding of how this alters the balance of production and consumption of nitrous oxide (N2O) in these soils is poorly understood and quantified. This study investigates soil N2 O fluxes across a red spruce forest chronosequence in eastern Canada with six post-harvest sites (stand ages 0 - 83 years) and one uncut control old growth (125+ yrs) stand. Nitrous oxide surface fluxes along with soil temperature and moisture were measured biweekly through the growing season, and supplemented with laboratory based experiments designed to understand the controls of moisture, temperature and N-status upon early post-harvest soil cores. Over the chronosequence, soils acted as a net sink during the 0 - 0.5 year period immediately following harvesting (-1.42 +/- 1.76 mug N2O-N/m2/hr), followed by a net source in stands aged 0.5 - 5 years (5.36 +/- 1.58 mug N2O-N/m2/hr), with fluxes declining to background control forest levels (-0.13 +/- 1.15 mug N2O-N/m2/hr) within 20 years. Overall, high spatial and temporal variability in field fluxes was documented, with negative fluxes observed at all stands. Significant correlations were seldom observed between N2O fluxes and soil temperature and moisture in the field data. Post harvest fluxes measured in controlled laboratory experiments pointed to N availability as the major variable controlling flux magnitudes. Nitrogen amendments resulted in positive soil fluxes that increased with elevated temperature and under optimal moisture conditions. Without N amendment however, enhanced sink activity was observed at elevated temperatures.