Measuring and Monitoring Carbon Source/Sink Relationship In Short-Rotation Afforestation Systems.

There is increasing interest in exploiting carbon © sequestration benefits of short-rotation woody crops (SRWC) planted on agricultural soils to mitigate Canada’s emissions of greenhouse gases. The fast-growing and high-yielding nature of SRWC under intensive management allows large amounts of C to be sequestered in long-term storage pools, i.e., wood biomass and soil organic matter. Although the potential of SRWC to sequester C is recognized, the net C benefits of these systems are unclear, particularly as it is expected that they will be C sources during the first few years following land use change. Understanding the C dynamics occurring in these systems is essential to accurately assess full rotation forecast of C accumulations potential of these plantations under different soil type, particularly in response to climatic variations and management regimes.

 In Canada, we used the eddy covariance and biometric techniques to assess the long-term carbon source /sink strength of the of hybrid poplar (HP) plantations growing under different soil type and management conditions in Alberta and Ontario.  The specific objectives were: (1)  to determine the C source-sink relationships in a chronosequence of HP plantations, (2) to study the influence of biophysical variables and soil type on above- and below-ground biomass accumulation, as well as the soil CO₂ emissions over the life cycle of HP plantations . 

 The eddy covariance technique showed how net ecosystem productivity (NEP) and ecosystem water use (E) responded diurnally and seasonally to climate variables. We found that monthly NEP was highest in the month of October when photosynthesis rates were maintained but respiration decreased due to mild temperature. Seasonal E responded more to precipitation and its maxima coincided with maximum monthly precipitation in the month of July. Cumulative E was in good agreement with that calculated using the field water balance method with both following closely cumulative precipitation, indicating little deep drainage of water below 150-cm depth. Comparing C sequestration by HP stands in Alberta showed that Site Class 1 soil (Chernozem) sequestered 10 g C m^-2 y^-1 while Site Class 3 soil (Luvisol) lost 190 g C m^-2 y^-1 in the second year of growth.

 Using the biometric approach, we found that HP grown on Site Class 3 soil in Ontario sequestered significantly more C per year than either Site Class 1 or 3 soils in Alberta. The differences in sequestration potential may be attributable to the differences in climate (Ontario being warmer and receiving much more precipitation than Alberta) and length of the growing season. However, C loss through soil respiration was also higher for Ontario but greater biomass productivity more than compensated greater soil C loss. In Alberta, soil respiration in the first eight years after plantation averaged 8.23 Mg C ha^-1 yr^-1 (range 6.04-10.92 Mg C ha^-1 yr^-1) for site class 1 which was nearly twice of an average of 4.41 Mg C ha^-1 yr^-1 (range 3.18-7.11 Mg C ha^-1 yr^-1) for Site Class 3 in the first 12 years after plantation . 

 Early results using both eddy covariance and biometric techniques suggest that hybrid poplar plantation on former agriculture soils become a sink within 2 to 5 years of plantation.  Sites across Canada does show a significant difference in C capture/sequestration potential based on site quality, bio-geo-climatic zone and management regime and intensity.