Accurate and efficient estimation of soil C is vital to understanding and monitoring the role of afforestation in C sequestration. Here, we focused on the potential of mixed-species plantings, for which there is negligible information but expanding investment due to their added environmental benefits. We surveyed soil C and N over a representative chronosequence (5–29 years old) of existing plantings, including measurements in the adjacent pastures to account for differences in soil type and land-use history among properties. Vegetation characteristics of the tree plantings were measured to identify potential surrogates for rapid assessment of soil C. Soil C was highly heterogeneous under the plantings and the adjacent pastures, with up to eight cores required to sample adequately a plot of 400 m2. Vegetation surrogates had limited success in predicting soil C after afforestation, with the only strong predictors being tree density and planting age. Three decades of afforestation with mixed species had not led to substantial changes in C concentration or content of the soil. The C:N ratio of soils increased with planting age suggesting that the C becomes more resistant to decomposition after afforestation. Over longer time scales, tree plantings are likely to have larger impacts on the amount and forms of soil C.