MINERAL BIOLOGICAL CYCLING OF DIFFERENT A GED POPLAR (POPULUS CV ROBUSTA) PLANTATIONS (HAINAUT, BELGIUM)

Mineral biological cycling of different aged poplar (Populus cv. robus ta) plantations (Hainaut, Belgium). - In this paper are presented nutr ients contents, mineralomasses (pool of total nutrients) and biologica l cycling of minerals in different aged (31, 24 and 8 years) poplar po pulations in the limoneous countryside of Hainaut (central Belgium). T hese results complete those on the phytomasses and primary productivit ies presented in this volume. Nutrients' concentrations are in general ly high, especially in N, K and Ca. The bark of boles and branches imm obilizes a great part of the wood's mineralomass. Leafs retain tempora rily from 14 to 43% of poplars' total mineralomass. Nutrients' mineral omasses reach 2.5 t/ha in the 31 years old plantation, 1.0 t/ha in tha t of 24 years and 0.5 t/ha in that of 8 years. There is an important a nnual nutrient cycling in the ecosystem: annual absorption represents respectively 36, 52 and 94% of mineralomasses in the plantations of 31 , 24 and 8 years old. Storage in wood phytomass increases them respect ively for 14, 13 and 4%. Restitution is also similar in the three case s (0.4-0.6 t/ha/year). However, the part of poplar alone is quite diff erent: 0.35 t for the 31 years old plantation, 0.17 for 24 years and 0 .03 t/ha/year for 8 years. This is due to the different participation of poplars in each plantation (in that of 8 years, the herbaceous laye r is dominant).