| Land use and land use change strongly influence the carbon (C) dynamics within ecosystems. This study quantified four aspects of land use and land use change effects: (1) ecosystem C stocks and distribution; (2) soil respiration; (3) soil C mineralization; and (4) net ecosystem productivity. Land use systems studied include agriculture (AG), 2-yr- and 9-yr-old hybrid poplar plantations (2HP and 9HP, respectively), grassland (GRA), and native forest stand (NAT). Ecosystem C stock in NAT (223 Mg C ha-1) was similar to 9HP (174 Mg C ha-1) and both were significantly greater than AG (122 Mg C ha-1), GRA (121 Mg C ha-1 ), and 2HP (110 Mg C ha-1). Cumulative soil C loss via soil respiration averaged over two growing seasons was in the order of: NAT (7.81+/-0.40 Mg C ha-1) > 9HP (5.51+/-0.31 Mg C ha-1) > GRA (5.23+/-0.30 Mg C ha-1 ) > AG (5.02+/-0.24 Mg C ha-1) > 2HP (4.28+/-0.20 Mg C ha-1). Depending on land use, seasonal heterotrophic and autotrophic respiration had respective contributions to soil respiration of up to 35 and 83%. Soil C mineralization of bulk soil across the land uses ranged between 2 to 5% of initial total organic C (Ci), with mineralization rates ranging from 0.06 to 0.12 microg C mg-1 C i d-1 and mean residence times ranging from 30 to 51 yrs. Across particle size fractions, soil C mineralization was in the order of: AG > HPs > GRA > NAT of which the coarse fractions, representing labile C, were the main source of mineralized C (79%). Mineralization increased when NAT was converted to AG; and decreased when AG was converted to HP or GRA. Net ecosystem productivity across land uses, expressed in terms of C, ranged between -2 (AG) and 11 Mg C ha-1 yr -1 (older HP). Conversion from AG to GRA increased net ecosystem productivity three-fold. When AG was converted to HP, the plantation was a C source in the first four years and became a C sink by year five. Results obtained from this study are relevant to modeling efforts designed at determining the impact of future climate change on a variety of land uses. |
