The intersection between climate change, energy transitions and the circular economy highlight the opportunities and contestations between different efforts to mitigate the complex environmental challenges we face. The energy we use to extract, manufacture, remanufacture and dispose of our material world is a major contributor to diverse climate impacts, an issue which is compounded by linear economic models that necessitate eternal extraction. Yet many of the materials we depend upon are exceptionally efficient at enabling functions that facilitate social, economic and environmental sustainability. This dichotomy is arguably most acutely debated in the world of polymers and plastics. While recycling has long been touted as a solution space for plastic sustainability, a plethora of chemists, biologists and engineers have more recently expanded global research in this direction. The resultant proliferation of terms like ‘up-’ or ‘down-’ or ‘re-’cycling that frame these opportunities are often poorly defined as value propositions. The danger lies in directions acting as a barrier to circularity, or even greenwashing transformations. Herein, we explore the value judgements and verifications of this directionality, investigate how we can better define these value judgements from a systems sustainability perspective and evaluate different proposed approaches and their barriers across different supply chains and sectors.

Biodegradable microplastics (BMPs) are reported to have a priming effect on soil organic matter (SOM) decomposition. However, their impact on the turnover of specific SOM components, especially nitrogen (N)-containing ones, remains unclear. Given the wide use of poly(butylene adipate-co-terephthalate) (PBAT) in agricultural films and the crucial role of amino sugar N-acetylglucosamine (NAG) in microbial necromass, the effects of PBAT BMPs on NAG decomposition in soil were investigated. We found that PBAT accelerated the decomposition of NAG, with specific effects varying considerably in the two soils examined (Yingtan soil and Nanjing soil). Microbial biomass and metagenomic sequencing analyses revealed that, in Yingtan soil with low available N (6 mg kg-1), PBAT promoted the incorporation of NAG into living microbial biomass, and increased the abundance of NAG phosphorylation and isomerization genes (amgK and glmM). In Nanjing soil with high available N (127 mg kg-1), chitin synthase gene (CHS1) abundance decreased and there was no significant change in microbial biomass, indicating the extra NAG decomposed in PBAT-treated soils might mainly enter the glycolysis pathway to generate energy rather than synthesizing new cells. Potential PBAT degraders enriched were also NAG degraders, suggesting carbon-rich PBAT selected for microbes that could obtain N from amino sugars.

One of the fundamental challenges for the UN plastics treaty is to shift the current linear plastic economy into a more circular plastic economy. Transitioning to a circular plastic economy requires a profound transformation of socio-technical systems, and research suggests that disruptive policies must simultaneously destabilize the entrenched linear system and cultivate a new regime that supports circular business models. A major barrier to this transformation lies in the artificially low cost of primary plastics, maintained by substantial subsidies for fossil fuels and plastic production. These subsidies, alongside the failure to internalize negative externalities – such as extensive health impacts and environmental damage – mask the true cost of plastic use, thereby undermining the economic case for innovation in sustainable alternatives. The upcoming UN plastics treaty presents a unique opportunity to realign market incentives and drive the necessary transition toward a circular, regenerative plastic economy.