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 (C 3 –derived carbon, C 3 –C) would be balanced by an equal loss of older soil C derived from sugarcane ( Saccharum officinarum L.) agriculture (C 4 –derived C, C 4 –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 C 4 –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 C 3 –C, nor on the rate of loss of older C 4 –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.