Field evaluation of the cotton-strip assay for quantifying decomposition rates in extensive green roof substrates

Extensive green roof (EGR) substrates and vegetation layer are critical components to ensure EGR multifunctionality. However, little is known on EGR substrate biological functioning, especially nutrient cycling that is crucial for supporting life in soils. This work aimed at investigating for the first time the effectiveness of an affordable and easy-to-use standardized method, the cotton-strip assay, to assess decomposition rates in ten-year old EGR. Three metrics were evaluated following 34 to 110 days of cotton-strip incubation into substrates: tensile strength, weight and surface area losses. Tensile strength loss was the most sensitive index after 34 days while weight loss over 110 days confirmed elevated decomposition rates in EGR substrates compared to gravel rooftop or common urban park soil. This could be related to positive influence of slightly acidic pH and vegetation cover. Finally, semi-intensive but thin green roof substrates may be an alternative to intensive ones to sustain ecological processes supporting long-term vegetation growth, as long as there is no net carbon loss. Moreover, broader assessment of decomposition into different green roof substrates configurations could pave the way for a better understanding of green roof ecosystems as a whole and the design of more resilient green infrastructures.