Land Use Change Effect On Ecosystem Carbon Balance: From Agriculture to Hybrid Poplar Plantation.

We studied the effects of land use change on net ecosystem productivity (NEP) and changes in ecosystem carbon (C) storage 2-4 and 9-11 years after converting an agricultural land use (planted to canola) to a hybrid poplar (Populus deltoides x Populus x petrowskyana var. Walker) plantation in north central Alberta, Canada. In C terms, NEP across land uses ranged between -2 and 11 Mg C ha^-1 yr^-1; while changes in C storage over the course of two years (2006-2008) ranged between 0 and 7 and between -2 and 4 Mg C ha^-1 yr^-1, as biomass and soil organic C, respectively. When agriculture was converted to hybrid poplar plantation, the plantations were initially large sources of C. As cultivation ceased and trees grew bigger, the soils under plantations became C sinks by year 7, and recovered soil C lost from the early plantation establishment phase by year 11, indicating that growing hybrid poplars on rotations longer than 11 years would sequester significant quantities of C. At the ecosystem level, however, hybrid poplar plantations were a source of C in the first 4 years. However, due to the fast growing nature of the hybrid poplars, plant biomass C accretion (11 Mg ha^-1 yr^-1) caused the plantation to become C sink by year five. Quantifying the net C balance of short-rotation woody crop plantations is necessary for validating the C sequestration potential of these systems and for understanding the role of these ecosystems in mitigating future climate change.