The dynamics of carbon, nitrogen and soil organic matter in Populus plantations

Short-rotation intensive-culture forestry is similar to agricultural systems requiring increased nutrient input and management. The expense and environmental concerns associated with fertilizers have raised questions about the sustainability of these ecosystems. Sustainability of production oriented ecosystems can be aided by understanding the mineralization-immobilization potential of the soil microbial biomass. The soil microbial biomass is central to a complex system of soil organic fractions that control soil fertility, production and environmental contamination.; The lack of root turnover studies has led to an inadequate understanding of the role of root turnover as substrate soil microbial biomass and organic matter formation. The current study was designed to determine: (i) the role of below-ground production and turnover in nutrient cycling processes; (ii) the contribution of leaf and root litter as substrate for the maintenance of soil organic matter; and (iii) relate soil microbial biomass and organic matter dynamics to plant carbon and nitrogen allocation patterns.; Two-year-old Populus euramericana cv. Eugenei trees were labeled with {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N in the field. The {dollar}sp{lcub}15{rcub}{dollar}N was injected into the stem to label leaves and roots without labeling the soil. The {dollar}sp{lcub}14{rcub}{dollar}C labeling was done in a Plexiglas chamber under ambient conditions. The tree-soil system was sampled for one year. Labeled leaf litter was placed onto unlabeled tree plots to differentiate the contribution of leaf and root derived carbon and nitrogen to soil over a two year period.; The {dollar}sp{lcub}14{rcub}{dollar}C required two weeks to stabilize in the root system and averaged 20% of soil respiration. One year latter, 32% of applied {dollar}sp{lcub}14{rcub}{dollar}C and 33% of the injected {dollar}sp{lcub}15{rcub}{dollar}N was recovered. Reserves in the root system were sufficient to replace fine roots one and one half times. This represented significantly less C than leaf litter, yet similar amounts of {dollar}sp{lcub}14{rcub}{dollar}C were found in soil from both litters. Leaf litter appeared to dominate N cycling since {dollar}sp{lcub}15{rcub}{dollar}N was not detected in root labeled soil. Kinetic analysis of incubated soil showed a greater contribution of C and N to soil organic fractions from leaf litter. The turnover of labile soil {dollar}sp{lcub}14{rcub}{dollar}C in both leaf and root litter labeled soil was 14-64 days. The {dollar}sp{lcub}14{rcub}{dollar}C in pools of intermediate resistance had a turnover time of 2-16 years and increased with soil depth.