Nitrogen turnover in fresh Douglas fir litter directly after additions of moisture and inorganic nitrogen

The effects of wetting and drying and inorganic nitrogen (N) addition on carbon (C) and N turnover in fresh Douglas fir litter (Speuld forest, the Netherlands) were investigated. Litter was incubated for 9 days in the laboratory, receiving different moisture and N addition treatments. Following the additions, a series of reactions were observed of which most notable were a rapid retention of added ammonium and nitrate (NO3-) and a sudden increase in CO2 respiration. For the rewetted-and-moist incubations, respiration levels remained elevated, N was net immobilized and nitrous oxide (N2O) production increased throughout the experiment. About 80% of the NO3- produced was lost again as N2O. In the rewetted-and-dried incubations, respiration decreased during the drying phase; no clear patterns in N mineralization were detected; and N2O production remained at constant levels, but still resulted in gaseous loss for half of the NO3- net produced. The experiments thus revealed two important NO3- sinks in LF1 litter, namely rapid retention of added NO3- and gaseous loss as N2O. The maximum NO3- loss via these sinks was estimated at 2 kg-N ha-1 yr-1, which is small compared to annual NO3- leaching at 90 cm soil depth (31 kg-N ha-1 yr-1).