Edaphic and Climatic Controls On Soil Organic Matter Storage and Dynamics in Eastern Deciduous Forests.

Forest soils represent a significant pool for C sequestration and storage, but the factors controlling soil C cycling are not well constrained. We used density fractionation and radiocarbon measurements to assess differences in soil organic matter (SOM) cycling amongst four eastern deciduous forests that vary in climate, soil type and parent materials, and soil ecology and are part of the AmeriFlux network. We collected mineral soil from 0-5 cm and 5-15 cm depth at Harvard Forest (HAF) in central Massachusetts, Bartlett Experimental Forest (BEF) in New Hampshire, the University of Michigan Biological Station (UMBS), and Baskett Wildlife Recreation and Education Area in the Missouri Ozarks (MOZ). We fractionated soil samples by density into free light (unprotected SOM), occluded light (physically protected SOM), and dense (mineral-protected) fractions using sodium polytungstate (1.65 g ml-1), measured C concentration and radiocarbon in bulk soil and fractions, and used a three-pool steady-state model to determine radiocarbon-based turnover times for fractions.

The northeastern sites, HAF and BEF, had higher bulk soil C (65 and 40 g C kg soil-1, respectively) than MOZ or UMBS (20 and 10 g C kg soil-1). Bulk soil radiocarbon values (Δ14C) decreased with depth and were lower at northeastern sites than Midwestern sites (36, 8, 113, and 65 ‰ for 0-5 cm at HF, BEF, MOZ, and UMBS, respectively).

Soil C distribution amongst fractions was similar at HAF, BEF, and MOZ with the unprotected free light fraction containing about 40 % of bulk soil C for 0-5 cm and 20 % of bulk soil C for 15-20 cm. At these three sites, the physically protected occluded light fraction contained about 10 % of bulk soil C, with the mineral-protected dense fraction containing the remaining 50-70 %. In contrast, UMBS, the site with the sandiest soil, had a greater portion of bulk soil C recovered in the unprotected free light fraction and very little C recovered in the occluded light fraction.

Radiocarbon-based SOM turnover times for the sites suggest that soil carbon pools in all three fractions turn over much more quickly at MOZ, the warmest site, than the other sites. In addition, turnover times for free and occluded light fractions are slower at UMBS and BEF, the coolest sites, than at HAF and MOZ.

These results suggest that soil type and climate interact to control soil organic matter cycling. Specifically, soil organic matter decomposition is slower in cooler than warmer climates and C stabilization in protected soil organic matter pools is greater in soils of finer texture, at least at the scale encompassed by our study.