Quantifying the soil organic carbon sequestration performance and carbon emissions offset potential of the City of Calgary’s Willow Biomass and Marginal Land Reclamation Demonstration Project

Abstract

The primary objective of this study was to measure the soil organic carbon (SOC) sequestration performance of The City of Calgary Dewatered Biosolids Land Application Program – Willow Biomass and Marginal Land Reclamation Demonstration Project (hereafter, “the demonstration Project”) after five years of operation. The second objective was to assess the demonstration Project’s potential to earn soil-based carbon offset revenue through the Alberta Emissions Offset System in the future. To accomplish the first objective, SOC stocks were measured at three sampling locations subject to different combinations of recommended management practices (RMP) for SOC sequestration by the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change (COP21). The agricultural Crops + No Biosolids (Control), agricultural Crops + Biosolids (C+BS), and Willows + Biosolids (W+BS) sampling locations were subject to zero, one (organic amendments), and three (no-till, agroforestry, and organic amendments) of the COP21 RMPs respectively. When comparing SOC stocks between the Control and RMP treatment sampling locations the results were not consistent - which aligns with expectations of this study. At the 0-15 cm differences in SOC stocks between sampling locations at the 0-15 cm depth interval was not significant. In contrast, the differences in SOC stocks between the W+BS and Control sampling locations at the 15-30 cm depth interval was larger (4.5 ± 1.6 Mg C ha-1) and significant. A post hoc pairwise comparison (Games-Howell test) analysis indicated that the SOC stocks at the 15-30 cm depth interval of the W+BS sampling location were significantly higher than both the C+BS and the Control sampling locations. When converting the difference in SOC stock between the W+BS and Control sampling locations at the 15-30 cm depth interval to a carbon offset equivalent, the estimated carbon offset performance of the demonstration Project within the first five years of operation was 16.5 ± 5.9 Mg CO2e ha-1. The expected (and observed) inconsistency of between the 0-15 cm sampling depth and the 15-30 cm sampling depth intervals aligned with the results of peer literature for similarly designed SOC monitoring studies at local and global scales. This is because the dynamics of SOC flux rates are influenced by numerous heterogeneous limiting (e.g. climate, and vegetation community), determining (e.g. clay content, pH, and mineral content), and reducing factors (e.g. microbial community) that shift across time, lateral distance, and depth. Therefore, differences in land management study outcomes are expected when experimental designs do not control for all the same variables. To accomplish the second objective, the knowledge gained from this study was used to develop recommendations for approaching future SOC stock monitoring studies of Willow + Biosolids systems more strategically by (1) providing guidance on conducting baseline studies to screen and compare proposed SOC sink project sites for their SOC sequestration potential, and (2) proposing more cost effective and statistically powerful study designs. This study has demonstrated that it is possible to verify measurable changes in SOC stocks at Willow + Biosolids project sites, and that SOC stocks at the demonstration Project site are likely to continue increasing and eventually plateau within the next 15 to 20 years. To move forward with developing a custom quantification protocol and gain approval within the Alberta Emissions Offset System, further research and documentation efforts are required to address the remaining key issues associated with biogenic based carbon offset protocols including; additionality, permanence, transparency, and leakage.