Carbon sequestration potential in perennial bioenergy crops: the importance of organic matter inputs and its physical protection

To date, only few studies have compared the soil organic carbon (SOC) sequestration potential between perennial woody and herbaceous crops. The main objective of this study was to assess the effect of perennial woody (poplar, black locust, willow) and herbaceous (giant reed, miscanthus, switchgrass) crops on SOC stock and its stabilization level after 6 years from plantation on an arable field. Seven SOC fractions related to different soil stabilization mechanisms were isolated by a combination of physical and chemical fractionation methods: unprotected (cPOM and fPOM), physically protected (iPOM), physically and chemically protected (HC-ls + c), chemically protected (HC-ds + c), and biochemically protected (NHC-ds + c and NHC-ls + c). The continuous C input to the soil and the minimal soil disturbance increased SOC stocks in the top 10 cm of soil, but not in deeper soil layers (10–30; 30–60; and 60–100 cm). In the top soil layer, greater SOC accumulation rates were observed under woody species (105 g m 2 yr-1) than under herbaceous ones (71 g m 2 yr-1) presumably due to a higher C input from leaf-litter. The conversion from an arable maize monoculture to perennial bioenergy crops increased the organic C associated to the most labile organic matter (POM) fractions, which accounted for 38% of the total SOC stock across bioenergy crops, while no significant increments were observed in more recalcitrant (silt- and clay-sized) fractions, highlighting that the POM fractions were the most prone to land-use change. The iPOM fraction increased under all perennial bioenergy species compared to the arable field. In addition, the iPOM was higher under woody crops than under herbaceous ones because of the additional C inputs from leaflitter that occurred in the former. Conversion from arable cropping systems to perennial bioenergy crops can effectively increase the SOC stock and enlarge the SOC fraction that is physically protected within soil microaggregates.