Nitrogen mineralization and nitrification in a mixed-conifer forest in southern California: controlling factors, fluxes, and nitrogen fertilization response at a high and low nitrogen deposition site

摘要

Net fluxes of nitrogen (N) mineralization and nitrification were measured in situ on a monthly basis for 3 years at a high (HN) and low (LN) N deposition site in the San Bernardino Mountains, California, Mean N mineralization fluxes in the forest floor and top 10 cm of mineral soil were 19.0 and 59.8 kg N-ha~(-1) year~(-1) at LN and HN, respectively. Mean net nitrification fluxes were 11.2 and 5.5.9 kg N-ha~(-1)-year~(-1) at LN and HN, respectively, Relative nitrification (the percent N mineralized that was nitrified) was generally lower under Pinus ponderosa Dougl. ex P. & C. Laws, (or Pinus jeffreyi Grev. & Balf.) canopies than under Quercus kelloggii Newb. or open canopies. The rate of net N mineralization was the key factor for predicting the rate of net nitrification, Fertilization with 50 and 150 kg N-ha~(-1) at LN significantly increased the rates of net mineralization and net nitrification. At HN fertilization had no significant effect on net nitrification. We conclude that at low-deposition sites increased nitrification occurs in the short term in response to added N, but that sustained elevated net nitrification is driven by the accumulation of N-enriched litter and soil organic matter in conjunction with chronic throughfall N depositioninputs.