Afforestation of agricultural land to mitigate climate change and other environmental degradation has been increasingly implemented around the world. However, the effects of afforestation on soil carbon and particularly nitrogen content remain unclear. We conducted a hierarchical Bayesian meta-analysis of published data on the effects of afforestation of pastures on soil carbon and nitrogen stocks in a Mediterranean climate. We found no evidence for substantial changes in soil carbon, nitrogen or C:N ratio across three decades of afforestation. However, a lack of data on the carbon content of the litter underestimates the potential for afforestation to sequester carbon in the soil. We investigated soil carbon content under remnant woodlands and found that it was higher than under the afforested pastures suggesting that afforestation has the potential to increase soil carbon over longer time scales. Targeted studies that measure carbon content and form within soils over an extended chronosequence are much needed to improve predictions of the potential for afforestation to increase soil carbon storage.