Abstracts on Global Climate Change

Nov 2004

Dynamics of carbon sequestration in a coastal wetland using radiocarbon measurements

Choi, YH Wang, Y


[ 1] Coastal wetlands are sensitive to global climate change and may play an important role in the global carbon cycle. However, the dynamics of carbon ( C) cycling in coastal wetlands and its response to sea level change associated with global warming is still poorly understood. In this study, we estimated the long-term and short-term rates of C accumulation, using C and C isotopic measurements of peat cores collected along a soil chronosequence, in a coastal wetland in north Florida. The long-term C accumulation rates determined by examining the C inventory and the radioactive decay of radiocarbon as a function of depth in the peat cores decrease with time from -130 +/- 9 g C/m(2)/yr over the last century to -13 +/- 2 g C/m(2)/yr over the millennium timescale. The short-term C accumulation rates estimated by examining the differences in the radiocarbon and C contents of the surfacial peat between archived ( 1985, 1988) and present ( 1996 and 1997) samples range from 42 to 193 g C/m(2)/yr in low marsh, from 18 to 184 g C/m(2)/yr in middle marsh, and from -50 to 181 g C/m(2)/yr in high marsh. The high end-values of our estimated short-term C accumulation rates are comparable to the estimated rates of C sequestration in coastal wetlands reported by Chmura et al. [ 2003], but are significantly higher than our estimated long-term rates in the marshes and are also much higher than the published rates of C sequestration in northern peatlands. The higher recent rates of C accumulation in coastal marshes, in comparison with the longer-term rates, are due to slow but continuous decomposition of organic matter in the peat over time. However, other factors such as increased primary production in the coastal wetland over the last decades or century, due to a rise in mean sea level and/or CO2 and nitrogen fertilization effect, could also have contributed to the large difference between the recent and longer-term rates. Our data indicate that salt marshes in this area have been and continue to be a sink for atmospheric carbon dioxide. Because of higher rates of C sequestration and lower CH4 emissions, coastal wetlands could be more valuable C sinks per unit area than other ecosystems in a warmer world.

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Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

de Graaff, MA Six, J Harris, D Blum, H van Kessel, C


Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate-soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha(-1) yr(-1)), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the C-13-CO2 signature, reflected the turnover times of SOM-C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate-SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.

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