A Survey of Organic Carbon Stocks in Mineral Soils of Eucalyptus globulus Labill. Plantations under Mediterranean Climate Conditions

Abstract

The main aim of this study was to assess the amount of carbon (C) stored in the upper 30 cm layer of mineral soils in eucalypt plantations in Portugal, with a Mediterranean-type climate. Soil sampling data (2468 samples), field evaluations (soil profile description) and relevant information on the particle size distribution, climate, bedrock and reference soil group were accomplished. Bulk density per sample was assessed using pedo-transfer functions and soil C stock was estimated. The results showed an average of 41.2 t C ha−1 stored in the soil. In the northern regions of Portugal, the coldest and wettest areas of the country with better stand productivity, a higher soil organic carbon (SOC) is achieved (median SOC of 39.2 g kg−1 and soil C stock of 55 t ha−1) than in southern and inland regions, with a warmer and drier climate (median SOC of 15.2 g kg−1 and soil C stock of 28 t ha−1). The assessment of mean soil C stock per bedrock type revealed higher C stored in granites followed by conglomerates, coal shales and clay shales. Regarding soil type, the results showed a higher C stock in Cambisols, Leptosols and Fluvisols (>50 t C ha−1), whereas Regosols and Luvisols stored less, following the same trend presented for reference soil groups in Europe. Comparing the geographic distribution of the C stock in the upper layer of the mineral soils with the amount of C in eucalyptus stands (root and aboveground biomass—data from national forest inventory), the mineral soil pool can represent more than two-thirds of the total C stored in eucalyptus plantations in Portugal. Further studies should focus on the evolution of C stocks in eucalypt plantations during different stages of stand growth and under different management practices.