(1) Root nodule age-class transition, seasonal proportion of biomass contribution of the three age-classes of active nodules and annual nodule production were estimated in plantation stands of Alnus nepalensis D. Don in the Kalimpong forest division of the Eastern Himalayas. (2) The proportion of nodule age-class transition from young to medium and medium to old and, finally, to inactive nodules were functions of the season, nodule age and their nitrogen-fixing potential. Seasonal variation in proportion of biomass contribution of the three age-classes of active nodules in the five stands were distinct. The nodule biomass of 149-457 kg ha-1 and annual nodule production of 244-492 kg ha-1 contributed significantly to the below-ground production. (3) Seasonal rates of decomposition were distinct, with highest rate in the first 3 months (warm rainy season) followed by subsequent seasons of the year. Among different stands it was highest in the 7-year stand. The pattern of nutrient release (K > Ca > P > N) was the same in all the stands but the actual amount of nutrient release decreased in older stands. The energy content loss in 12 months per unit area was highest (179 kJ m-2) in the 7-year stand. (4) It is concluded that under the age sequence of A. nepalensis stands, root nodules significantly contribute to the below-ground production. Decomposition rate, nutrients and energy release were highest in the youngest (7-year) stand which had the shortest turnover time. Decomposition of nodules was related to their mineral element concentrations in nodules and environmental factors.
