Biomass and nitrogen dynamics of fine roots of poplar under differential N and P levels in an agroforestry system in Punjab

The understanding of root distribution in the soil profile is useful for proper management of agroforestry systems. Depthwise (0-15, 15-30, 30-45 and 45-75 cm) distribution of fine root biomass (FRB), concentration and accumulation of N in the fine roots of poplar (Populus deltoides Bartr.) were determined during four months (April, July, October and January) in a five year old poplar plantation under different levels of N and P in an agroforestry system in Punjab. Leaf N concentration was also determined and correlated to root N concentration. Significant variations (P < 0.0001) were observed for these parameters among soil depths, months and nutrient levels. Total FRB and N accumulation (0-75 cm depth) were the lowest in unfertilized trees (33.7 g m(-2) and 191 mg m(-2), respectively) and the highest at the highest level of nutrient application (95.7 g m(-2) and 1290 mg m(-2), respectively). More than 35 % FRB and 42 % N accumulation were confined to 0-15 cm soil depth which declined with increasing depth. Fine root biomass and N accumulation were significantly higher in July and October than April and January whereas root and leaf N concentration were the highest in April (1.04 and 1.19 %, respectively) and the lowest in January (0.72 and 0.90 %, respectively). Root and leaf N concentration also increased significantly (P < 0.0001) with the application of nutrients. It may be concluded that the root competition between tree and intercrop roots would be higher in summer season crops (sown in June/July and harvested in September/October) than the winter crops (sown in November and harvested in April).