Nadelhoffer et al.1 use 15N-tracer studies in nine northern forests to argue that increasing inputs of combined nitrogen from the atmosphere are unlikely to cause the increase in forest growth that has been postulated as the ‘missing sink’ for atmospheric CO2. Only about 20% of the tracer ended up in the trees and about 70% remained in the organic and mineral layers of the soil. If only 20% of the nitrogen input from the atmosphere were available for tree growth, then not enough combined nitrogen would be coming into the northern forests each year to explain the missing carbon sink.