Nutrient Cycling and Nutrient Use Efficiency in Short Rotation,High Density Central Himalayan Tarai Poplar Plantations

This paper deals with the pattern of nutrient cycling and nutrient use efficiency in four (1–4 years old) poplar (Populus deltoidesMarsh) plantations previously investigated for dry matter dynamics. The present plantations were planted at 3×5 m spacing after clear felling of natural sal (Shorea robustaDipterocarpaceae) mixed broad-leaved forests in central Himalayan Tarai. The nutrient concentrations (N, P and K) in different layers of vegetation were in the order: tree>shrub>herb; whereas the standing state of nutrients were in the order: tree>herb>shrub. Soil, litter and vegetation, respectively accounted for 81–96, 2–4 and 2–15% of the total nutrients in the system. Considerable reductions (trees, 50–68; shrubs, 35–40; and herbs 18–26%) in the concentration of nutrients in leaves occurred during senescence. The uptake of nutrients by the vegetation, and also by the different components, with and without adjustment for internal recycling, was calculated separately. Annual transfer of litter nutrients to the soil by vegetation was 91–148 N, 8–15 P and 70–99 K kg ha^?1yr^?1. The turnover rate for different nutrients ranged between 0.83 and 0.92 yr^?1. The nutrient use efficiency of poplar plantations ranged from 151 to 174 kg ha^?1yr^?1for N, 1338 to 1566 kg ha^?1yr^?1for P, and 313 to 318 kg ha^?1yr^?1for K. Compared with low density eucalypt and older poplar stands, there was a higher proportion of nutrient retranslocation in present poplars, largely because of higher tissue nutrient concentrations. This indicates lower nutrient use efficiency as compared to eucalypt plantations. Compartment models for nutrient dynamics have been developed to represent the distribution of nutrient pools and net annual fluxes within the system.