Deep soil sorption dynamics across fertility and precipitation gradients in lowland tropical forests

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Emily K. Blackaby, Daniela Cusack and Lee H. Dietterich, Ecosystem Science & Sustainability, Colorado State University, Fort Collins, CO, Karis McFarlane, Lawrence Livermore National Laboratory, Livermore, CA, William C. Hockaday, Geology, Baylor University, Waco, TX, Nicholas J. Bouskill, Earth and Environmental Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA, Stephany S. Chacon, Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA

Background/Question/Methods
Tropical soils are among the most carbon(C)-rich on Earth, with large stocks of relatively stable C likely residing in deep subsoils. Soil fertility and rainfall are likely to influence the capacity for deep soil C storage among tropical forests. To explore deep soil characteristics related to C stock sizes, we assessed a set of distinct lowland tropical forests across soil fertility and rainfall gradients in Panama. We assessed the sorption capacity of soils to retain additional dissolved organic carbon (DOC) for 13 Panama forests, using soils from 0-10 cm, 10- 25 cm, 25-50 cm, 50 -75 cm. and 75-100 cm depths. 13C nuclear magnetic resonance and a 14C radiocarbon analysis were used to characterize the organic carbon present at each site. We conducted batch experiments, adding DOC extracted from native litter to these soils after removing extant C. The native DOC solution was mixed with .25 g of soil and shaken for 24 hrs. The samples were then filtered through a 0.9 μm membrane filter to isolate the DOC component. Both the stock DOC solution and the filtrates were analyzed for total organic C to determine the amount of DOC sorption to the mineral surfaces at each of the sites and depths.
Results/Conclusions
Deep soils of strongly weathered, infertile soils had the largest subsoil C stocks among sites, explaining larger overall C stocks to 1m depth in these sites relative to our more fertile sites. Sorption dynamics varied among sites. Studying deep C storage and sorption dynamics will inform our understanding for forest-climate feedbacks.