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DIVISION S-7—FOREST & RANGE SOILS

First-Rotation Changes in Soil Carbon and Nitrogen in a Eucalyptus Plantation in Hawaii

^a Dep. of Forest, Rangeland, and Watershed Stewardship and Graduate Degree Program in Ecology, Colorado State Univ., Ft. Collins, CO 80523
^b Dep. of Biology and Center for Environmental Studies, Arizona State Univ., Tempe, AZ 85287
^c Rocky Mountain Res. Stn., 240 West Prospect, Fort Collins, CO 80526

^* Corresponding author (dan{at}cnr.colostate.edu)

We measured soil changes through a full rotation of a Eucalyptus saligna (Sm.) plantation. We hypothesized that accretion of C from Eucalyptus trees (C₃–derived carbon, C₃–C) would be balanced by an equal loss of older soil C derived from sugarcane (Saccharum officinarum L.) agriculture (C₄–derived C, C₄–C). We also hypothesized that large additions of N-containing fertilizer would increase C accretion by increasing the rate of C addition and decreasing the rate of C loss. The low spatial variability of the soil and the intensive sampling design provided precise tests of these hypotheses. Soil C averaged 13.8 kg m^–2 for the O horizon plus the 0- to 45-cm depth mineral soil, with no change through the rotation [95% confidence interval (CI) ±0.057 kg m^–2 yr^–1], supporting the first hypothesis. Significant gains of C₃–C (0.136 kg m^–2 yr^–1) balanced the losses of C₄–C (0.144 kg m^–2 yr^–1). The second hypothesis was tested in the field using three levels of repeated, complete fertilization (including N at rates of 300, 700, and 1600 kg N ha^–1), and in laboratory incubations with N addition. Addition of N had no effect on the accumulation of soil N and C₃–C, nor on the rate of loss of older C₄–C, refuting the second hypothesis. This first-rotation forest plantation was not able to increase soil C, even with heavy fertilization. These results contrast markedly from the soil changes under the influence of N-fixing trees, indicating that the effect of N fixation on soil C derives from factors other than N supply.

Abbreviations: ¹³C, the per mil difference between the carbon-13 content of the sample and the Pee Dee belemnite standard • C₃–C, C₃–derived carbon • C₄–C, C₄–derived carbon • CI, confidence interval