Mechanisms of soil carbon storage during tropical reforestation.
Marin-Spiotta, Erika*,1, Silver, Whendee1, Ostertag, Rebecca2, Swanston, Christopher3, Torn, Margaret4, Burton, Sarah5, 1 U.C. Berkeley, Berkeley, CA2 U. of Hawaii, Hilo, HI3 Lawrence Livermore National Laboratory, Livermore, CA4 Lawrence Berkeley National Laboratory, Berkeley, CA5 Pacific Northwest National Laboratory, Richland, WA
ABSTRACT- Land-use change can affect soil C storage by affecting the quantity and quality of inputs as well as the controls on soil C turnover. Our research examines mechanisms of soil C storage during forest regrowth in Puerto Rico. Using a long-term successional chronosequence, we sampled replicate pastures, primary forests, and forests regrowing on pastures abandoned 10, 20, 30, 60 and 80 years ago. An increase in aboveground biomass with secondary succession was not reflected belowground. The loss of pasture-derived C4-C was compensated by a gain in new forest C3-C, resulting in no net change in soil C content up to 1 m with land-use and land-cover change. The turnover times of C4-C increased with soil depth, from 60 years in the top 10 cm to three times that in the 50-60 cm depth. While changes in bulk soil C stock were undetectable, we found differences in the C content of soil fractions. We used a density fractionation scheme to separate soil C into: a free light fraction (LF), an occluded LF after the physical disruption of soil aggregates, and a mineral-associated, heavy fraction (HF). Forested sites had a greater proportion of their C pool in free LF than did pastures. We analyzed 13C-NMR spectra of free LF and occluded LF. The two differed significantly from plant litter precursors in the O-alkyl, aromatic and carboxylic regions. The signal of the alkyl region (representing non-polar compounds derived from plant waxes and other lipids) tended to increase in intensity from fresh plant litter, to free LF to occluded LF. While free and occluded LFs did not differ greatly chemically, radiocarbon concentration analyses suggest turnover rates of these fractions are significantly affected by physical location in the soil matrix. The occluded LF is older than C in the free LF, and has similar 14C concentrations to the mineral-associated HF. These results indicate that physical aggregation plays an important role in the protection of near surface soil C at our sites.
Key words: soil carbon, turnover, aggregation, reforestation
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