Carbon sequestration in soil beneath long-term Miscanthus plantations as determined by 13C abundance

Miscanthus is a perennial rhizomatous warm-season grass with C4-photosynthesis. It shows considerable production potentials (10– dry matter ha−1) under NW European growth conditions and plantations of Miscanthus are established to provide biomass for energy. The plant senesces in the autumn in response to adverse climatic conditions, but harvest is normally postponed until spring when the biomass is more suitable for combustion. Total pre-harvest and harvest losses may account for as much as two-thirds of autumn standing biomass and these losses provide a significant carbon input to the soil. In this study, we examine soil organic carbon (SOC) storage and turnovers beneath 9 and 16 year old Miscanthus plantations established at Hornum, Denmark (56°50′N, 09°26′E). The soil is a loamy sand (Typic Haplumbrept, coarse loamy, mixed, mesic) with a C3 vegetation history. Soil was sampled at 0–20, 20–50 and 50– depth in the Miscanthus plantations and in two reference sites under C3-plants. The 0– samples were divided into fine soil , particulate organic matter (POM; 250–), rhizomes/stubbles and coarse roots. All samples were analysed for carbon content and / ratio. Rhizomes/stubbles accounted for 10.9– and coarse roots for 3.2– at 0– depth. No rhizomes and coarse roots were observed in the deeper soil layers. Concentrations of SOC were higher at all soil depths under the 16 year old Miscanthus whereas 9 years of Miscanthus and reference sites showed similar SOC concentrations. in 0– reference soil averaged −27.6‰ while soil beneath 9 and 16 year Miscanthus showed −25.6‰ and −22.8‰, respectively. Difference in between reference and Miscanthus soils was smaller at greater soil depths. SOC inventories at 0– ranged from 91– in reference and 9 year Miscanthus to under 16 years of Miscanthus growing. The main part of the SOC was at 0–20 and 20– soil with 30– in each layer. Although changes in the overall SOC storage were less significant, 13% and 31% of the SOC present in 0– soil was derived from Miscanthus beneath 9 and 16 year plantations, respectively. The organic matter recovered in POM contained 48–65% of Miscanthus derived C. At 20–50 and 50– depth, the fractions of SOC from Miscanthus were 6–9% and 1.3–6%, respectively. It was estimated that 26–29% of the cumulated C input from Miscanthus had been retained in the soil.