INTERNAL CYCLING OF NITRATE IN SOILS OF A MATURE CONIFEROUS FOREST

Gross rates of N mineralization, immobilization, and nitrification were measured by N-15 isotope dilution in a 10-yr-old conifer plantation and in a mature conifer forest. Gross rates revealed nutrient cycling characteristics that differ from expectations based on more common measures of net rates. Although net mineralization rates were somewhat higher in the young forest than in the old forest, gross mineralization rates in the old forest were 2-3 times as high as gross mineralization rates in the young forest, indicating more rapid turnover of inorganic-N pools in the old forest. Net mineralization rates were < 14% of gross mineralization rates. Smaller NO3- pool size and lower net nitrification rates in the old forest than the young forest might lead to the conclusion that the old forest is a non-nitrifying ecosystem and that nitrate is important only in the N cycle of the young forest. However, gross nitrification rates were similar in both young and old forests. Microbial assimilation of NO3- was also significant in both forests, indicating a rapid turnover of a small but important NO3- pool. Microbial assimilation may be an important pathway for NO3- retention in forest ecosystems.