Energetics of nitrogen accretion was studied in five different ages of Alnus nepalensis D. Don (alder) plantations in the Kalimpong forest division of the Eastern Himalayas. The percentage contributions of biological N2 fixation to annual total nitrogen uptake were 33%, 19%, 15%, 13% and 11% in 7-, 17-, 30-, 46- and 56-year-old plantations, respectively. Nitrogen accretion through fixation reduced with the decrease in nitrogen demand as trees aged (117 kg ha-1 yr-1 in 7-year stand to 29 kg ha-1 yr-1 in 56-year stand). Low N2 fixation efficiency and low net energy allocation in root nodules with increase in tree age caused sharp reduction in nitrogen accretion through fixation in older stands, while nitrogen uptake from soil remained nearly the same in young to old plantations. Nitrogen build up strategy is based on sound energy conservation dynamics and the alder replenishes nitrogen in erosion prone hill slopes of the Himalayas.
