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 (SOC 4 ) and the gain of new soil organic C from the C 3 , N-fixer, and non-N-fixer plantations (SOC 3 ). Soils beneath N-fixing trees sequestered 0.11 Ϯ 0.07 kg m Ϫ2 y Ϫ1 (mean Ϯ one standard error) of total soil organic carbon (SOC T ) compared with no change under Eucalyptus (0.00 Ϯ 0.07 kg m Ϫ2 y Ϫ1 ; P ϭ 0.02). About 55% of the greater SOC T sequestration under the N-fixers resulted from greater retention of old SOC 4 , and 45% resulted from greater accretion of new SOC 3 . Soil N accretion under the N-fixers explained 62% of the variability of the greater retention of old SOC 4 under the N-fixers. The greater retention of older soil C under N-fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

Intensively managed plantations of trees occupy vast areas of the tropics. The productivity of these forests depends strongly on nutrient supply, and nutrient supply may change rapidly under intensive management regimes. We documented changes in a Hawaiian soil after 32 mo of development of a plantation of eucalyptus [Eucalyptus saligna (Sm.)]. Soil C did not change significantly (average = ‐23 g C m‐2 yr‐1 to 30 cm; 95% confidence −139 to +93 g C m‐2 yr‐1). This lack of change in soil C resulted from a rapid loss of older soil C derived from sugarcane (−191 g C m‐2 yr‐1) and a rapid gain of new soil C from eucalyptus (160 g C m‐2 yr‐1). Soil N declined by 19 g N m‐2 yr‐1 (P = 0.08), despite fertilizer additions of 31 to 70 g m‐2. Large reductions in exchangeable Ca and Mg probably resulted from dissolution and leaching of residual lime from prior agricultural management. We conclude that intensive sampling regimes may detect relatively small changes in tropical forest soils, and that expectations of C accumulation in soils following afforestation may need to be reconsidered.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m2 more soil N and 2000 g/m2 more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and "old" sugarcane-derived C. Deeper soil C (50-100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Our results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixedspecies afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.

This study develops a feasible method for evaluating coarse root biomass (roots >2 mm diameter) of well established plantations of eucalypts and then examines coarse root biomass variability across tree age and size, fertilization treatment, species and site for Eucalyptus globulus and E. nitens in Tasmania, Australia. The most efficient sampling protocol consisted of rootball excavation and soil coring for bulk coarse roots, which when compared with total tree excavation estimated total coarse root biomass contained inside the sampled area to within 10%. Across all treatments, an average of 76% of the coarse root biomass was located within the rootball. The majority (>65%) of the coarse roots outside the rootball were located in the surface 20 cm of soil. When size class distribution was examined, 75% of coarse root biomass was found to occur in the larger (20+ mm) diameter size class, a size class that displayed considerable spatial heterogeneity. At the stand level, coarse root biomass ranged from 2.18 to 7.38 kg m 2 depending primarily on tree size but also on fertilization treatment, species and site. It is estimated that allocation to coarse root biomass production was around 0.2 kg m 2 year 1 (around 6% of estimated NPP) for the E. nitens stands examined in this study and around 1 kg m 2 year 1 (around 20% of estimated NPP) for the E. globulus stand examined. Robust relationships using above-ground parameters could be used to predict coarse root biomass regardless of fertilization or site, but species changed the relationship.