Modelling changes in soil organic matter after planting fast-growing Pinus radiata on Mediterranean agricultural soils

The Kyoto Protocol explicitly allows the storage of carbon (C) in ecosystems resulting from afforestation to be offset against a nation's carbon emissions and paves the way for carbon storage in soils to be eligible as carbon offsets in the future. More information is required about how afforestation affects carbon storage, especially in the soil. We report a study in which soil carbon storage in first-rotation Mediterranean Pinus radiata plantations, established on former cereal fields and vineyards, was measured and modelled. Measurements were made on plantations of several ages, as well as repeat measurements at the same site after 5 years. We tested the ability of two widely used soil organic matter models (RothC and Century) to predict carbon sequestration in Mediterranean forest soils. Increases in the top 5 cm of soil of about 10 g C m(-2) year(-1) were observed after afforestation of former vineyards, but nitrogen (N) either remained the same or decreased slightly. During afforestation, most organic matter accumulated in the ectorganic layers at a rate of 19 g C m(-2) year(-1) in former vineyards and 41 g C m(-2) year(-1) in former cereal fields. The RothC and Century models were sensitive to previous land use and estimated a carbon sequestration potential over 20 years of 950 and 700 g C m(-2), respectively. The accurate simulation of the dynamics of soil organic matter by RothC, together with measured above-ground inputs, allowed us to calculate below-ground inputs during afforestation. The Century model simulated total C and N, including the ectorganic horizons, well. Simulations showed a depletion of N in the below-ground fractions during afforestation, with N limitation in the former vineyard but not on former cereal land. The approach demonstrates the potential of models to enhance our understanding of the processes leading to carbon sequestration in soils.