First-rotation changes in soil carbon and nitrogen in a Eucalyptus plantation in Hawaii

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 C ₃-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.