Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere

Northern mid-latitude forests are a large terrestrial carbon sink(1-4). Ign oring nutrient limitations, large increases in carbon sequestration from ca rbon dioxide (CO2) fertilization are expected in these forests(5). Yet, for ests are usually relegated to sites of moderate to poor fertility, where tr ee growth is often limited by nutrient supply, in particular nitrogen(6,7). Here we present evidence that estimates of increases in carbon sequestrati on of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic(8). In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritional ly poor site, and the stimulation at a nutritionally moderate site was tran sient, stabilizing at a marginal gain after three years. However, a large s ynergistic gain from higher CO2 and nutrients was detected with nutrients a dded. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus , fertility can restrain the response of wood carbon sequestration to incre ased atmospheric CO2. Assessment of future carbon sequestration should cons ider the limitations imposed by soil fertility, as well as interactions wit h nitrogen deposition.