Abstract
This dissertation summarizes results of measurements of the below- and above ground biomass production, soil organic carbon (SOC) changes, soil CO<sub>2 </sub>effluxes (F<sub>c<</sub>), and carbon balances in short rotation willow biomass crops. Two treatments were applied: willow plots that were harvested and stems were allowed to regrow (continuous production) and willow plots that were harvested, and then stools and roots were chipped and ground into the soil (tear-out). The study utilized a chronosequence of shrub willow biomass crops (SWBC) in production for 19, 14, 12, 5, and 0 years. Four manuscripts were presented in this dissertation. In the first manuscript, it was found that SWBC is a net source of C sequestration with a net GHG potential of -31.2 Mg CO<sub>2</sub> eqv ha<super>-1</super> without fine root biomass, and -42.9 Mg CO<sub>2</sub> eqv ha<super>-1</super> with standing fine root biomass. The second manuscript showed that total SOC contents to 45-cm soil depth remained constant across different age classes (p=0.02). However, SOC contents significantly decrease (P<0.0005) at the top 15-cm soil depth during the first five years after SWBC establishment. No interaction effects were detected between age and soil depth (p = 0.42). The third manuscript demonstrated that across different age classes, F<sub>c</sub> rates were not significantly different (p = 0.53). However, the age of crop showed significant interaction with treatments (p=0.005). The continuous production (CP) treatment also showed a significantly greater F<sub>c</sub> rates than the tear-out (TO) treatment (P=0.02). The proportion of root respiration relative to total F<sub>c</sub> values ranged from 18 to 33% across the five different ages. Finally, the fourth manuscript revealed that cumulative F<sub>c</sub> values, computed as an average of two years continuous measurements, were large, ranging from 27.1 to 35.4 Mg CO<sub>2</sub> ha<super>-1</super> yr<super>-1</super> for CP and from 26.7 to 29.3 Mg CO<sub>2</sub> ha<super>-1</super> yr<super>-1</super> for TO. Cumulative F<sub>c</sub> values in the CP were offset by the C sequestration in foliage and fine roots. Establishment of new willow fields after termination of willows may offset about two-third of the total cumulative F<sub>c</sub> values in the TO treatment.