carbon accumulation

Greater Soil Carbon Sequestration under Nitrogen-Fixing Trees Compared with Eucalyptus Species

Resh, SC, D. Binkley, and JA Parrotta. 2002. Greater soil carbon sequestration under nitrogen-fixing trees compared with eucalyptus species RID A-2703-2010. Ecosystems 5 (3) (APR): 217-31.

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 C4 land use (SOC4) and the gain of new soil organic C from the C3, N-fixer, and non-N-fixer plantations (SOC3). Soils beneath N-fixing trees sequestered 0.11 + 0.07 kg m-2 y-' (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 under N-fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

The potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands

Silver, W.L. et al. (2000) The potential for carbon sequestration
through reforestation of abandoned tropical agricultural and pasture
lands. Rest. Ecol. 8, 394–407

Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of aboveground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha−1yr−1 over a 100-year period, and at faster rates during the first 20 years (1.30 Mg carbon ha−1 yr−1). Past land use affects the rate of both above- and belowground carbon sequestration. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites that were cleared but not developed, and on pasture sites. Our results indicate that tropical reforestation has the potential to serve as a carbon offset mechanism both above- and belowground for at least 40 to 80 years, and possibly much longer. More research is needed to determine the potential for longer-term carbon sequestration for mitigation of atmospheric CO2 emissions.
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