Plant and Soil 87,195-208 (1985). Ms. FA-07 9 1985 Martinus. Ni]hoffPublishers, Dordrecht. Printed in the Netherlands. Nitrogen cycling in dense plantings of hybrid poplar and black alder B. COTE and C. CAMIRI~ D@artement des sciences forestibres, Facult6 de foresterie, Universitb Laval, Ste-Foy, Qua., Canada, GIK 7P4 Key words Alnus glutinosa Black alder Dinitrogen fixation Hybrid poplar Nitrogen Nitrogen cycling Short-rotation plantation Summary Nitrogen cycling was studied during the third growing season in pure and mixed plantings (33 X 33 cm spacing) of hybrid poplar and black alder in southeastern Canada. After 3 years, hybrid poplar growth and N content of living tissues in a plot and of individual hybrid poplar plants increased with the proportion of black alder in a planting. No differences were detected among N contents of individual alder plants regardless of plot treatment. Black aider allocated a larger portion of its N to roots than hybrid poplar. Symbiotic nitrogen fixation was estimated to account for 80% of the nitrogen in aboveground alder tissues in the pure treatment using natural lSN dilution. N return in leaf litter was estimated to be 70 kg ha -~ in the pure alder treatment and decreased to a minimum of 20kg ha -~ in the pure hybrid poplar plots. No difference was detected among treatments for throughfall N content. Nitro- gen concentration in roots and leaf litterfall of black alder was higher than hybrid poplar. Significant soil N accretion occurred in mixed plantings containing two alders to one poplar and pure black alder plantings. Nitrogen availability (NO 3-N) increased with the amount of black aider in a plot. Results suggest that the early increase in nitrogen accumulation of hybrid poplar in mixed treatments can be attributed to an increase of total soil N availability resulting from the input of large amounts of N from easily mineralizable alder tissue. Introduction Intensive forest management is gaining attention in southeastern Canada primarily because of a predicted downturn in timber supply. One of the strategies being considered to compensate for this down- ward trend is short-rotation culture of hybrid poplar I . However, the nitrogen requirements of these highly-productive plantations can be very high ~8 . Black alder, a nonleguminous nitrogen fixing tree has the ability to increase the growth of associated hybrid poplar 8,~9,33 , and the total yield of mixed plantations 12 with black cottonwood (Populus tricho- carpa Torr and Gray). Increased poplar growth is sometimes associated with an increase in total soil nitrogen, as noted by DeBell and Rad- wan t2 . Increased availability of soil nitrogen has been suggested as an explanation for increased poplar productivity in mixed plantings 19, but the exact mechanisms involved are still unclear. Higher nitrogen content 7'39 and nitrogen concentration 49 have been observed in leaf litter of alder than in litter of other species. Throughfall and stemflow of alder stands have been reported to be rich in nitrogen 7. Root 195