EFFECT OF CHRONIC NITROGEN ADDITIONS ON SOIL-NITROGEN FRACTIONS IN RED SPRUCE STANDS

The responses of temperate and boreal forest ecosystems to increased n itrogen (N) inputs have been varied, and the responses of soil N pools have been difficult to measure. In this study, fractions and pool siz es of N were determined in the forest floor of red spruce stands at fo ur sites in the northeastern U.S. to evaluate the effect of increased N inputs on forest floor N. Two of the stands received 100 kg N ha(-1) yr(-1) for three years, one stand received 34 kg N ha(-1)yr(-1) for si x years, and the remaining stand received only ambient N inputs. No di fferences in total N content or N fractions were measured in samples o f the Oie and Oa horizons between treated and control plots in the thr ee sites that received N amendments. The predominant N fraction in the se samples was amino acid N (31-45 % of total N), followed by hydrolyz able unidentified N (16-31 % of total N), acid-soluble N (18-22 % of t otal N), and NH4+-N (9-13 % of total N). Rates of atmospheric depositi on varied greatly among the four stands. Ammonium N and amino acid N c oncentrations in the Oie horizon were positively related to wet N depo sition, with respective r(2) values of 0.92 and 0.94 (n = 4, p < 0.05) . These relationships were somewhat stronger than that observed betwee n atmospheric wet N deposition and total N content of the forest floor , suggesting that these pools retain atmospherically deposited N. The NH4+-N pool may represent atmospherically deposited N that is incorpor ated into organic matter, whereas the amino acid N pool could result f rom microbial immobilization of atmospheric N inputs. The response of forest floor N pools to applications of N may be masked, possibly by t he large soil N pool, which has been increased by the long-term input of N from atmospheric deposition, thereby overwhelming the short-term treatments.