Soil Organic Carbon is Increased in Mixed-Species Plantations of Eucalyptus and Nitrogen-Fixing Acacia
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Soil organic carbon (soil C) sequestration in forests is often higher under nitrogen (N2)-fixing than under non-N2-fixing tree species. Here, we examined whether soil C could be increased using mixed-species plantations compared to monocultures, which are less productive aboveground than mixtures. In addition, we compared soil C sequestration under N2-fixing trees with non-N2-fixing trees that received N fertilizer. Monocultures of Eucalyptus globulus (E) and the N2-fixing Acacia mearnsii (A) and mixtures of these species were planted in a replacement series: 100%E, 75%E + 25%A, 50%E + 50%A, 25%E + 75%A and 100%A. Soil samples were also collected from fertilized monoculture treatments (100%EFer) of E. globulus (250 kg N ha−1). Total organic C, N and phosphorus were determined at age 8 years at two soil depths (0–10 cm and 10–30 cm) and three density fractions of soil organic matter (SOM) were quantified for 0–5 cm depth. Soil C was highest in the 50%E + 50%A mixed stand and was highly correlated with aboveground biomass, not to the percentage of A. mearnsii in mixtures. This was largely due to soil C at 10–30 cm because there were no treatment effects on soil C at 0–10 cm. All density fractions of SOM at 0–5 cm increased with the percentage of A. mearnsii. In E. globulus monocultures, N fertilization did not increase soil C when compared with unfertilized stands. These results indicate that the inclusion of N2-fixing trees into eucalypt plantations may increase soil C stocks through increased productivity.
Keywordssoil carbon sequestration organic matter fractionation decomposition nitrogen fixation
The authors wish to thank John Smith and Vijay Koul for assistance in the field and with soil processing. The experiment was established by the Commonwealth Scientific and Industrial Research Organisation, Division of Forestry, with the Department of Natural Resources and Environment, Victoria, providing the site and the Australian Centre for International Agricultural Research providing some financial support. The authors also thank two anonymous reviewers who provided useful comments that improved the manuscript.
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