Rapid accumulation and turnover of soil carbon in a re-establishing forest

Present understanding of the global carbon cycle is limited by uncertainty over soil-carbon dynamics(1-6). The clearing of the world's forests, mainly for agricultural uses, releases large amounts of carbon to the atmosphere (up to 2 x 10(15) g yr(-1)), much of which arises from the cultivation driv ing an accelerated decomposition of soil organic matter(1-4). Although the effects of cultivation on soil carbon are well studied, studies of soil-car bon recovery after cultivation are limited(4-11). Here we present a four-de cade-long field study of carbon accumulation by pine ecosystems established on previously cultivated soils in South Carolina, USA(7). Newly accumulate d carbon is tracked by its distinctive C-14 signature, acquired around the onset of forest growth from thermonuclear bomb testing that nearly doubled atmospheric (CO2)-C-14 in the 1960s. Field data combined with model simulat ions indicate that the young aggrading forest rapidly incorporated bomb rad iocarbon into the forest floor and the upper 60 cm of underlying mineral so il. By the 1990s, however, carbon accumulated only in forest biomass, fores t floor, and the upper 7.5 cm of the mineral soil. Although the forest was a strong carbon sink, trees accounted for about 80%, the forest floor 20%, and mineral soil <1%, of the carbon accretion. Despite high carbon inputs t o the mineral soil, carbon sequestration was limited by rapid decomposition , facilitated by the coarse soil texture and low-activity clay mineralogy.