How do management techniques affect carbon stock in intensive hardwood plantations?

Recent studies in temperate regions have shown that agroforestry systems, especially silvopastoral systems, have greater carbon (C) sequestration potential than monocropping systems or pastures, or even forest plantations. In Europe, there is growing interest in establishing high quality wood plantations with intensive management comprising irrigation, fertilisation and chemical weed control to reduce rotation length. However, these operations can have major environmental impacts similar to the effects of intensive agriculture, such as impoverishment of soil C. The aim of this study is to identify optimum management practices for intensive systems of quality wood production to optimise soil C stock and plantation productivity. An experiment was conducted in Extremadura, mid-west Spain, from 2011 to 2014, in a 13-year-old hybrid walnut (Juglans major × regia mj 209xra) plantation with a density of 333 trees ha−1. Two essays were established: one with three techniques to control competition from herbaceous strata beneath trees - mowing, ploughing and sheep grazing (1 sheep ha−1) - and the other to test implementation of legumes (mixture of Trifolium michelanium and Ornithopus compressus complemented by the same quantities of phosphorous and potassium as mineral treatment) as an alternative to traditional mineral fertilisation (40 kg N ha−1, 40 kg P2O5 ha−1 and 50 kg K2O ha−1). The C stock estimate was based on soil organic carbon (SOC) and aboveground (tree trunks and branches) and belowground biomass (tree and pasture roots). Most of the C stock was contained in SOC, at 50% in the uppermost soil layer (0-25 cm), followed by aboveground biomass. The response of SOC in each treatment was higher than the other parameters analysed, suggesting that SOC is a more sensitive pool to management techniques. Grazing as control of herbaceous vegetation and legume implementation as nitrogen supply are suitable techniques for optimising soil C stocks and also achieve adequate tree growth in the longer term.