Changes in soil carbon of pastures after afforestation with mixed species: Sampling, heterogeneity and surrogates

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 m(2). 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. (C) 2012 Elsevier B.V. All rights reserved.