Abstract
Bioenergy crops have a secondary benefit if they increase soil organic C (SOC) stocks through capture and allocation below-ground. The effects of four genotypes of short-rotation coppice willow (Salix spp., ‘Terra Nova’ and ‘Tora’) and Miscanthus (M. × giganteus (‘Giganteus’) and M. sinensis (‘Sinensis’)) on roots, SOC and total nitrogen (TN) were quantified to test whether below-ground biomass controls SOC and TN dynamics. Soil cores were collected under (‘plant’) and between plants (‘gap’) in a field experiment on a temperate agricultural silty clay loam after 4 and 6 years’ management. Root density was greater under Miscanthus for plant (up to 15.5 kg m−3) compared with gap (up to 2.7 kg m−3), whereas willow had lower densities (up to 3.7 kg m−3). Over 2 years, SOC increased below 0.2 m depth from 7.1 to 8.5 kg m−3 and was greatest under Sinensis at 0–0.1 m depth (24.8 kg m−3). Miscanthus-derived SOC, based on stable isotope analysis, was greater under plant (11.6 kg m−3) than gap (3.1 kg m−3) for Sinensis. Estimated SOC stock change rates over the 2-year period to 1-m depth were 6.4 for Terra Nova, 7.4 for Tora, 3.1 for Giganteus and 8.8 Mg ha−1 year−1 for Sinensis. Rates of change of TN were much less. That SOC matched root mass down the profile, particularly under Miscanthus, indicated that perennial root systems are an important contributor. Willow and Miscanthus offer both biomass production and C sequestration when planted in arable soil.
Highlights
There has been an increase in the use of dedicated biomass crops to exploit photosynthesis for bioenergy production over recent decades to address two pressing global concerns: C emission reduction and energy security [1, 2]
In England in 2015, there were 2885 ha under short rotation coppice (SRC), yielding 17–35 Gg of dry biomass, of which 15 Gg was used in power stations, and 6905 ha under Miscanthus, yielding 69–104 Gg of dry biomass, of which 33 Gg was used in power stations [6]
Significant genotype differences were apparent, but these were mainly restricted to upper layers, where density was often greater for Miscanthus ( Sinensis) than willow varieties
Summary
There has been an increase in the use of dedicated biomass crops to exploit photosynthesis for bioenergy production over recent decades to address two pressing global concerns: C emission reduction and energy security [1, 2]. Two dedicated low-input bioenergy crops frequently planted in temperate regions, such as the UK, are willow (Salix spp.) in short rotation coppice (SRC) systems and species of the perennial grass genus Miscanthus [3]. Commercial willow plantations produce 9–12 Mg ha−1 year−1 of biomass in 2–4-year SRC harvest rotations typically [1, 4,5,6]. Miscanthus is an annually-harvested perennial rhizomatous grass originating from Asia which has C4 physiology and can produce biomass yields of 12–15 Mg ha−1 year−1 in the UK [1, 2, 6, 7].
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