FERTILIZED SHORT ROTATION PLANTATIONS OF HYBRID ASPEN (POPULUS TREMULOIDES MICHX. x POPULUS TREMULA L.) FOR ENERGY WOOD OR MITIGATION OF GHG EMISSIONS

Agroforestry, an ecologically and environmentally sustainable type of land use, offers great opportunity to sequestrate carbon in above-and below-ground carbon pools. Aspens (Populus spp.) are accepted as an agriculture energy crop with rotation period up to 5 years in Latvia since 2011. The research aim was to determine the productivity and removals of CO2 in a hybrid aspen (Populus tremula L. x P. tremuloides Michx.) plantation during the 5 years rotation period after managing the plantation as an agroforestry system together with perennial crops - reed canary grass (Phalaris arundinacea L.), festulolium (x Festulolium pabulare) and fodder galega (Galega orientalis Lam.), as intercrop, and fertilized with biogas production residues (30 tonnes.ha(-1)), wastewater sludge (10 tonnes(DM).ha(-1)) and wood ash (6 tonnes(DM).ha(-1)). The most important impact on the plantation's productivity has selection of the clone, although relevant impact on the tree growth was observed for fertilizer application and the type of intercrop. The most significant impact on the growth of the most productive clone No 4 demonstrated fertilization with biogas production residues and waste water sludge - 0.3-4.2 fresh tonnes of additional biomass compared to control without intercrop and without fertilizer. Comparing different intercrops, the best results were obtained with galega and red canary grass - 3.3 and 3.7 fresh tonnes of biomass higher stock in comparison to control unfertilized plots. The best combination was clone No 4 fertilized with wastewater sludge or biogas fermentation residues in combination with reed canary grass (Phalaris arundinacea L.) intercrop - 16.4 fresh tonnes of biomass (42 MWh of primary energy per hectare) or 16.5 tonnes.ha(-1) of CO2 removed in above ground biomass (8.5 tonnes(DM).ha(-1)). This study demonstrated that soil bulk density and organic carbon stock in upper soil layers decreased significantly during the 5 years after establishment of the fertilized hybrid aspen plantation in carbon rich mineral agricultural soil.