Do tree plantations or cultivated fields have the same ability to maintain soil quality as natural forests?

Human activities, such as tree cutting and land-use change, have significantly altered the forests of Iran's Zagros Mountains. In plantation projects within these forests, maintaining soil structure and fertility is necessary for sustainable forest management. Ideally, the soil quality of tree plantations should be as good as that of natural forests. In this study, we investigated the long-term impact of land-use change, i.e. conversion of natural oak forest to tree plantations or cultivated fields, on soil quality (i.e., physicochemical, biological and biochemical soil properties) in the forests of the Zagros Mountains. We found that the conversion of natural oak forest to tree plantations (in this case Cupressus arizonica, Cedrus libani, Elaeagnus angustifolia, Quercus brantii (persica), Robinia pseudoacacia and Fraxinus rotundifolia) and agriculture caused declines in soil organic carbon content, especially in the cultivated fields where it fell by around 60%. Overall, the lowest contents of soil nutrients, except Cu, were measured in agricultural soils. Interestingly, the plantation of a non-native tree (Cedrus libani) sequestrated more atmospheric carbon and nitrogen than native Persian oak forests. The impact on soil biological activity remains unclear: the conversion of natural forests into cultivated fields reduced soil biological activity, while conversion from forest to tree plantations had little influence. Finally, the conversion of natural forest to tree plantations had less impact on soil properties than conversion to cultivated fields. In conclusion, to maintain soil fertility, forest managers should avoid forest conversion to agriculture. Moreover, we do not recommend plantation of Cupressus arizonica in the Zagros region for the purpose of carbon and nitrogen sequestration and the increase of soil productivity.