Comparison of forest soil carbon dynamics at five sites along a latitudinal gradient

Abstract

The aim of this study was to compare the turnover time of labile soil carbon (C), in relation to temperature and soil texture, in several forest ecosystems that are representative of large areas of North America. Carbon and nitrogen (N) stocks, and C:N ratios, were measured in the forest floor, mineral soil, and two mineral soil fractions (particulate and mineral-associated organic matter, POM and MOM, respectively) at five AmeriFlux sites along a latitudinal gradient in the eastern United States. Sampling at four sites was replicated over two consecutive years. With one exception, forest floor and mineral soil C stocks increased from warm, southern sites (with fine-textured soils) to cool, northern sites (with more coarse-textured soils). The exception was a northern site, with less than 10% silt-clay content, that had a soil organic C stock similar to the southern sites. A two-compartment model was used to calculate the turnover time of labile soil organic C (MRT U) and the annual transfer of labile C to stable C (k 2) at each site. Moving from south to north, MRT U increased from approximately 5 to 14 years. Carbon-13 enrichment factors (ε), that described the rate of change in δ 13C through the soil profile, were associated with soil C turnover times. Consistent with its role in stabilization of soil organic C, silt-clay content was positively correlated (r = 0.91; P ≤ 0.001) with parameter k 2. Latitudinal differences in the storage and turnover of soil C were related to mean annual temperature (MAT, °C), but soil texture superseded temperature when there was too little silt and clay to stabilize labile soil C and protect it from decomposition. Each site had a relatively high proportion of labile soil C (nearly 50% to a depth of 20 cm). Depending on unknown temperature sensitivities, large labile pools of forest soil C are at risk of decomposition in a warming climate, and losses could be disproportionately higher from coarse textured forest soils.