Soil Carbon Modelling in Salix Biomass Plantations: Variety Determines Carbon Sequestration and Climate Impacts

Abstract

Short-rotation coppice (SRC) Salix plantations have the potential to provide fast-growing biomass feedstock with significant soil and climate mitigation benefits. Salix varieties exhibit significant variation in their physiological traits, growth patterns and soil ecology—but the effects of these variations have rarely been studied from a systems perspective. This study analyses the influence of variety on soil organic carbon (SOC) dynamics and climate impacts from Salix cultivation for heat production for a Swedish site with specific conditions. Soil carbon modelling was combined with a life cycle assessment (LCA) approach to quantify SOC sequestration and climate impacts over a 50-year period. The analysis used data from a Swedish field trial of six Salix varieties grown under fertilized and unfertilized treatments on Vertic Cambisols during 2001–2018. The Salix systems were compared with a reference case where heat is produced from natural gas and green fallow was the land use alternative. Climate impacts were determined using time-dependent LCA methodology—on a land-use (per hectare) and delivered energy unit (per MJheat) basis. All Salix varieties and treatments increased SOC, but the magnitude depended on the variety. Fertilization led to lower carbon sequestration than the equivalent unfertilized case. There was no clear relationship between biomass yield and SOC increase. In comparison with reference cases, all Salix varieties had significant potential for climate change mitigation. From a land-use perspective, high yield was the most important determining factor, followed by SOC sequestration, therefore high-yielding fertilized varieties such as ‘Tordis’, ‘Tora’ and ‘Björn’ performed best. On an energy-delivered basis, SOC sequestration potential was the determining factor for the climate change mitigation effect, with unfertilized ‘Jorr’ and ‘Loden’ outperforming the other varieties. These results show that Salix variety has a strong influence on SOC sequestration potential, biomass yield, growth pattern, response to fertilization and, ultimately, climate impact.

Optimized utilization of Salix—Perspectives for the genetic improvement toward sustainable biofuel value chains

“…The climate impact of six varieties of Salix grown under fertilized and unfertilized conditions (Table 1, subset of 2) was analyzed and compared using LCA methodology, using the data generated from a Salix field study at Pustnäs, Uppsala, Sweden (Table 1, subset of 2). The study system involved the steps from field site preparation to end use of the Salix biomass as biofuel for an incineration plant, with heat energy as the end product, described in Kalita et al (2021). The system did not include the distribution of heat and transportation of ash.…”

“…Soil organic carbon stocks under the different Salix plantations in the 0–20 cm soil layer after 50 years as calculated using ICBM soil carbon model (modified from Kalita et al, 2021). …”

“…This indicates that cultivation of selected Salix varieties under the given set of conditions in central Sweden for bioenergy production to substitute fossil energy would have a cooling effect on the climate, and thus a potential to mitigate climate change. The climate impacts of the different Salix biomass‐to‐bioenergy systems after 50 years under the system conditions including substitution effects of the reference scenario are shown in Figure 4 with two functional units; namely, 1 ha of land and one MJ of heat (Kalita et al, 2021). Even when looking at the Salix systems alone, that is, without substitution effects, all except fertilized Björn lead to negative emissions over the lifecycle under the given system conditions.…”

“…Climate impacts of Salix (SRC) systems after 50 years including substitution effects. The climate impacts are given in terms of GWP100 and Δ T s per hectare of land and per MJ of heat (modified from Kalita et al, 2021). …”

“…The results from Kalita et al's (2021) study show that Salix genotype influences the biomass yield, response to fertilization, SOC sequestration rate, and ultimately the climate impact. It is important to account for these genotype effects in systems studies and environmental impact assessments.…”