(Wharf Street Basin, Josh Byrne& Associates. Image: Danika Zuks)

Soil stores over three times the carbon found in the atmosphere globally, making reversing soil carbon depletion a critical element in carbon emission reduction and climate change mitigation

It is widely understood that development resulting in deforestation contributes to anthropogenic carbon emissions by returning the carbon sequestered in biomass to the atmosphere, as well as the loss of potential carbon sequestration through photosynthesis. However, the impact of development on soil carbon is not as widely understood, but is critical for climate change mitigation. When it is acknowledged that soil stores over three times the carbon found in the atmosphere, the magnitude of the risk of soil carbon depletion becomes apparent. Of the carbon sequestrated in terrestrial ecosystems, 80% is stored in soil with the remaining 20% of carbon being found in plants and animals[1]. This makes understanding soil carbon storage critical to climate change mitigation.

Soil carbon sequestration involves the fixing of atmospheric carbon dioxide into plant matter through photosynthesis. When the plant biomass decomposes, some of the carbon dioxide is returned to the atmosphere as respiration and some is retained in the soil as soil organic carbon which gives soil its dark colour. When carbon input from photosynthesis exceeds carbon lost through respiration, soil organic carbon increases. Cool, wet conditions typically increase soil organic carbon. Carbon is also stored in soil as inorganic carbon derived from light coloured carbonate minerals such as calcium carbonate (for example, limestone) that have eroded and mixed with the soil.

Organic and inorganic soil carbon levels are at risk from development/human activity and climate change. Land clearing, wetland draining, and coastal reclamation have greatly reduced soil carbon stocks [2]. As well as the risks posed by development, there is a high risk of loss of soil carbon from climate change[3]. The impacts of climate change include higher temperatures, which when accompanied by lower rainfall will reduce the rate of photosynthesis and carbon fixing[4]. Soil carbon is disrupted by soil erosion, irrigation, fertilisation and soil acidification[5]. Soil acidification through acid rain, industrial pollution, and intense agricultural production impacts inorganic soil carbon levels. Acid dissolves calcium carbonate, dissolving it in water or releasing it as carbon dioxide gas. Disruption of inorganic carbon can undermine acidity and nutrient regulation, plant growth and the stabilisation of organic carbon[6]. Development can impact soil carbon levels in many ways and so development projects need to consider very carefully the impact of design proposals on soil carbon levels and how design can mitigate soil carbon loss or increase soil carbon levels.

Blue and teal systems sequester more carbon than terrestrial systems and at faster rates[7]. Blue carbon describes the carbon storage of vegetated coastal ecosystems (mangrove, saltmarsh, seagrass). Teal Carbon refers to the carbon stored in rivers and wetlands. Both definitions refer to carbon sequestered in the vegetation and the soil. The carbon stored in the soil of blue and teal systems is significantly higher than that stored their plant biomass[8] . Wetland and coastal soils store more carbon than terrestrial soils because waterlogged soils reduce oxygen levels which reduces decomposition (decomposition leads to carbon being returned to the atmosphere). Disruption of the soil carbon in these systems includes draining of wetlands, development of floodplains, reclamation of land adjacent to rivers and oceans, and removal of riparian and coastal vegetation and replacement with engineered edges.

Poor understanding of environmental impact has resulted in environmentally inappropriate development that has depleted soil carbon. However, greater understanding of the carbon cycle can enable us to adopt regenerative design strategies that not only mitigate further loss, but increase soil carbon.

A key aim of sustainable design is to reduce carbon emissions, which as a starting point requires limiting further soil carbon depletion by avoiding deforestation, tree removal, land reclamation, and wetland drainage. However, the risks of climate change necessitate we go beyond limiting further harm to enacting regenerative approaches by increasing the rate at which carbon is accumulated in the soil. Regenerative design strategies to increase soil carbon storage include:

• Reforestation/restoration of vegetated areas.
• Restoration of wetland areas, riparian systems, and coastal ecosystems.
• Maximisation of vegetative cover of soil to reduce soil erosion and moisture loss and increase photosynthesis.
• Increasing areas of vegetation. In constrained urban areas, this could include roof gardens, vertical greening, temporary uses for vacant land, and urban agriculture.
• Water Sensitive Urban Design (WSUD) strategies to remediate existing water bodies or provide an alternative to conventional stormwater management.
• Improving soil cover with mulch or leaf litter to reduce wind and water erosion.
• Improving depleted soils with soil amendment such as biochar and organic matter, but limiting fertiliser use.
• Reducing soil disturbance and tilling (tillage/ripping exposes previously protected organic matter to decomposition).

In urban and peri urban areas, urban drainage systems present significant opportunity for soil carbon remediation. Conventional approaches to stormwater management typically involve engineered concrete infrastructure that depletes soil carbon. In some instances, natural waterways have been replaced with concrete culverts. WSUD offers the potential to increase soil carbon by using vegetated elements to manage stormwater such as roof gardens, vegetated swales, and constructed wetlands. Vegetated systems enable increased carbon storage as well as controlling erosion and sediment movement and mitigating the loss of soil moisture in a drying climate.

Co-benefits of these strategies to increase soil carbon include improved soil health, increased plant growth, nutrient retention, water retention, reduced soil erosion, reduced need for fertiliser input, urban heat mitigation, resilient ecosystems, and community amenity. Further benefits of coastal and wetland restoration include water pollutant removal, and coastal protection. These regenerative design strategies also offer opportunities to reconnect with Country and enhance biodiversity by developing holistic design solutions that acknowledge the interconnectedness of natural systems. In order to respond to climate change risk, regenerative design strategies must become the ‘new normal’.

References:

[1] Onti TA, Schulte LA. Soil Carbon Storage. Nature Education Knowledge. 2012;3(10): 35.

[2] Fitch P, Battaglia M, Lenton A, Ferron P, Gao L, Hortle A, et al. Australia’s carbon sequestration potential: A stocktake and analysis of sequestration technologies. 2022. https://www.csiro.au/en/research/environmental-impacts/emissions/carbon-sequestration-potential; Nazir MJ, Li G, Nazir MM, Zulfiqar F, Siddique KHM, Iqbal B, et al. Harnessing soil carbon sequestration to address climate change challenges in agriculture. Soil and Tillage Research. 2024;237: 105959. https://doi.org/10.1016/j.still.2023.105959.

[3] Fitch P, Battaglia M, Lenton A, Ferron P, Gao L, Hortle A, et al. Australia’s carbon sequestration potential: A stocktake and analysis of sequestration technologies. 2022. https://www.csiro.au/en/research/environmental-impacts/emissions/carbon-sequestration-potential

[4] Onti TA, Schulte LA. Soil Carbon Storage. Nature Education Knowledge. 2012;3(10): 35.

[5] Canadell P, Wang Y, Huang Y. Trillions of tonnes of carbon locked in soil has been left out of environmental models – and it’s on the move. The Conversation. http://theconversation.com/trillions-of-tonnes-of-carbon-locked-in-soil-has-been-left-out-of-environmental-models-and-its-on-the-move-227597 [Accessed 22nd May 2024].

[6] Canadell P, Wang Y, Huang Y. Trillions of tonnes of carbon locked in soil has been left out of environmental models – and it’s on the move. The Conversation. http://theconversation.com/trillions-of-tonnes-of-carbon-locked-in-soil-has-been-left-out-of-environmental-models-and-its-on-the-move-227597 [Accessed 22nd May 2024].

[7] Valach AC, Kasak K, Hemes KS, Anthony TL, Dronova I, Taddeo S, et al. Productive wetlands restored for carbon sequestration quickly become net CO2 sinks with site-level factors driving uptake variability. PLoS ONE. 2021;16(3): e0248398. https://doi.org/10.1371/journal.pone.0248398.

[8] Fitch P, Battaglia M, Lenton A, Ferron P, Gao L, Hortle A, et al. Australia’s carbon sequestration potential: A stocktake and analysis of sequestration technologies. 2022. https://www.csiro.au/en/research/environmental-impacts/emissions/carbon-sequestration-potential; Kavehei E, Jenkins GA, Adame MF, Lemckert C. Carbon sequestration potential for mitigating the carbon footprint of green stormwater infrastructure. Renewable and Sustainable Energy Reviews. 2018;94: 1179–1191. https://doi.org/10.1016/j.rser.2018.07.002.

By Jen Lorrimar-Shanks, AILA

Submitted on behalf of Australian Institute of Landscape Architects