Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees

Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m² more soil N and 2000 g/m² more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and "old" sugarcane-derived C. Deeper soil C (50-100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Qur results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixedspecies afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.