Carbon Sequestration by Perennial Energy Crops: Is the Jury Still Out?

Abstract

Soil organic carbon (SOC) changes associated with land conversion to energy crops are central to the debate on bioenergy and their potential carbon neutrality. Here, the experimental evidence on SOC under perennial energy crops (PECs) is synthesised to parameterise a whole systems model and to identify uncertainties and knowledge gaps determining PECs being a sink or source of greenhouse gas (GHG). For Miscanthus and willow (Salix spp.) and their analogues (switchgrass, poplar), we examine carbon (C) allocation to above- and belowground residue inputs, turnover rates and retention in the soil. A meta-analysis showed that studies on dry matter partitioning and C inputs to soils are plentiful, whilst data on turnover are rare and rely on few isotopic C tracer studies. Comprehensive studies on SOC dynamics and GHG emissions under PECs are limited and subsoil processes and C losses through leaching remain unknown. Data showed dynamic changes of gross C inputs and SOC stocks depending on stand age. C inputs and turnover can now be specifically parameterised in whole PEC system models, whilst dependencies on soil texture, moisture and temperature remain empirical. In conclusion, the annual net SOC storage change exceeds the minimum mitigation requirement (0.25 Mg C ha−1 year−1) under herbaceous and woody perennials by far (1.14 to 1.88 and 0.63 to 0.72 Mg C ha−1 year−1, respectively). However, long-term time series of field data are needed to verify sustainable SOC enrichment, as the physical and chemical stabilities of SOC pools remain uncertain, although they are essential in defining the sustainability of C sequestration (half-life >25 years).

Species and Genotype Effects of Bioenergy Crops on Root Production, Carbon and Nitrogen in Temperate Agricultural Soil

“…Our estimates of root biomass are comparable with other studies. Ferchaud et al [22] reported root biomass of 4.1 Mg ha −1 , assuming a C content of 43% [15], under a 5-year stand of Giganteus in northern France under similar climatic conditions, similar to our measurement (2.9-7.1 Mg ha −1 ) when root density is expressed as a stock. The rhizome biomass of 18.1 Mg ha −1 given by Ferchaud et al [22] is within the wide range of that calculated for the 4-year-old Giganteus herein (6.4 in gap and 109.3 Mg ha −1 in plant).…”

“…This would appear to support the differences in root biomass described above. Willow roots were finer than Miscanthus roots which may increase their turnover rate [15], although mean root diameter under all genotypes increased in size over time, and RLD did not vary much over the measurement period. Despite measurable differences in some above-ground traits of willows [5], these did not appear to be manifest in differences in the root traits we measured in this study.…”

“…Perennial bioenergy crops allocate nutrients and C below ground to the root system during senescence in readiness for the following growing season [1,12], and some crops extend their root systems deep into the subsoil, especially Miscanthus [8,13,14]. In their comprehensive review, Agostini et al [15] found that root C stocks as an input were greater for willow (1.0 Mg ha −1 year −1 ) than Miscanthus (0.5 Mg ha −1 year −1 ), but that the latter had a greater mean residence time (1.3 years) than the former (1.8 years), based on the finer nature of willow roots and their fast turnover. The importance of root inputs to SOC is likely dependent on their physical (diameter, association with soil structure) and chemical (greater or lesser 'labile' compounds) characteristics.…”

“…Also important is the age of the stand, as others have reported an establishment phase as crop yields increase [19] where resident SOC turns over before full replacement by new SOC deriving from the bioenergy crop [16], particularly where the former land use was under perennial grass [18,20,21]. Therefore, a full assessment of the potential for bioenergy crops to sequester significant amounts of C is still far from certain [8,15,22,23] and is reliant on monitoring the dynamics of SOC in well-designed field experiments. It has been estimated that sequestration under bioenergy crops needs to be at least 0.25 Mg SOC ha −1 year −1 for the system to be truly C-neutral [15,24].…”

“…Therefore, a full assessment of the potential for bioenergy crops to sequester significant amounts of C is still far from certain [8,15,22,23] and is reliant on monitoring the dynamics of SOC in well-designed field experiments. It has been estimated that sequestration under bioenergy crops needs to be at least 0.25 Mg SOC ha −1 year −1 for the system to be truly C-neutral [15,24].…”