Belowground carbon cycling in three temperate forests of the eastern United States

Rates of carbon (C) cycling in soils are an important component in the equation of forest carbon balance yet at present they are poorly quantified. This research focuses on quantifying rates of soil C cycling at three temperate forests in the eastern United States along a latitudinal gradient from Maine to Tennessee. Measurements of C and radiocarbon (14C) stocks and fluxes are our principal measurement tools.; Total soil C stocks (to 80 cm depth) decrease along the gradient and are 14.6, 8.4 and 4.9 kgC m−2 at Howland ME, Harvard Forest MA, and Walker Branch TN, respectively. Much of this trend is due to decreasing C stocks in the organic (O) horizons. Howland and Harvard Forest both have large humified C stocks in the O horizon with long turnover times (15–50 years), while at Walker Branch this humified component is largely absent and turnover times are much shorter (7–15 years). On timescales of human interest, significant C accumulation in these soils will happen only in the O and A horizons, which have large C stocks that cycle on decadal and centennial timescales. The deeper mineral horizons, despite their large stocks, have long turnover times (200–2000 years) and can store C effectively only over millennia. Soils of northern sites such as Howland and Harvard Forest have greater C storage potential than the southern site of Walker Branch. Currently, well-drained soils account for an uptake of 5–50 gC m−2 y−1 or 1–25% of the measured net ecosystem C uptake at each of the sites.; Measurements of the 14C in respired CO₂ from incubations of soil organic matter combined with C and 14C mass balance allow partitioning of soil respiration into heterotrophic and autotrophic sources. Heterotrophic respiration contributed from 44–84% of total soil respiration for well-drained soils at Howland and Harvard Forest in 1999. The average age of heterotrophic respiration at all three sites is 8–9 years. The amount of total soil respiration from C fixed >1–2 years ago decreases along the latitudinal gradient and is 55, 42 and 33% at the Howland, Harvard Forest, and Walker Branch, sites respectively for 1998.