Greater soil carbon sequestration under nitrogen-fixing trees compared with Eucalyptus species

Forests with nitrogen-fixing trees (N-fixers) typically accumulate more carbon (C) in soils than similar forests without N-fixing trees. This difference may develop from fundamentally different processes, with either greater accumulation of recently fixed C or reduced decomposition of older soil C. We compared the Soil C Pools under N-Fixers with Eucalyptus (non-N-fixers) at four tropical sites: two sites on Andisol soils in Hawaii and two sites on Vertisol and Entisol soils in Puerto Rico. Using stable carbon isotope techniques, we tracked the loss of the old soil organic C from the previous C-4 land use (SOC4) and the gain of ne v soil organic C from the C-3, N-fixer, and non-N-fixer plantations (SOC3). Soils beneath N-fixing trees sequestered 0.11 +/- 0.07 kg m(-2) y(-1) (mean +/- one standard error) of total soil organic carbon (SOCT) compared with no change under Eucalyptus (0.00 +/- 0.07 kg m(-2) y(-1); P = 0.02). About 55% of the greater SOCT Sequestration under the N-fixers resulted from greater retention of old SOC4 and 45% resulted from greater accretion of new SOC3. Soil N accretion under the N-fixers explained 62% of the variability of the greater retention of old SOC4 under the N-fixers. The greater retention of older soil C tinder N-fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.