Microplastic disguising as soil carbon storage

Research on microplastic pollution has been an established research topic in marine ecology for well over a decade (1), with the potential implications for sustainability of our oceans becoming increasingly clear. Microplastics, that is pieces of plastic < 5mm, are now increasingly recognized also as a problematic pollutant in soils and terrestrial ecosystems globally (2,3). Research is currently underway to understand the effects of these seemingly ubiquitous and persistent particles on soil biodiversity and functions, and there is a global effort underway to develop methods for quantifying concentrations of microplastics present in soils.

However, one question that is specific to soils has not been asked: is microplastic already making a hidden contribution to soil carbon storage? Microplastic particles, either already produced as such for industrial purposes, or fragmented from larger plastic following exposure in the environment, are extremely slow to decompose and thus likely accumulate in our soils as relatively persistent pollutants. These materials become incorporated into the soil, for example by soil biota or by plowing. They are then integrated into soil aggregates along with other pieces of organic matter. As such, microplastics become ‘invisible’, but we will nevertheless detect them as carbon by currently used methods to quantify soil organic carbon. In addition, microplastic content may also affect another key soil parameter, soil bulk density, which is important for extrapolating carbon concentrations to stocks, for example in kg C ha^-1.

In the scientific literature, microplastic is currently considered in terms of number of particles or mass (4). A connection to carbon content has not been made, despite the fact that plastics are mostly carbon (e.g. polystyrene or polyethylene are almost 90% carbon). Given the large amounts of microplastic we are inadvertently adding to our managed agricultural soils (including as sewage sludge, as a contaminant of compost, in other residues or through plastic mulching), may this have contributed to appreciable amounts of apparent soil carbon storage? The question attains particular relevance given policy initiatives such as the ‘4 per mille’ goal and considering the central importance of measuring soil carbon to estimate a key ecosystem service.

Microplastic could span quite a range in terms of percentage of soil C. At the low end, microplastics in Swiss riparian soils (5) in nature protected areas contain up to 55 mg kg^-1 (very likely still a substantial underestimate despite the elaborate methods used). Assuming microplastic to be 90% C, and with Swiss riparian soils containing from 0.11 to 5.62 g 100 g^-1 total organic C (6), this amounts to 0.1 to 5% microplastic-C in these soils relatively removed from direct human influence. On the other end of the spectrum, Australian industrial soils contained anywhere from 300 mg kg^-1 to over 67,000 mg kg^-1 (that is 6.7% microplastic per total soil weight). Many agricultural or urban soils will likely have microplastic-C contents somewhere in between.

Microplastic in soil could thus be a unique case of an environmental pollutant posing as an ecosystem service, i.e. soil carbon storage, a hallmark of sustainable soil management. As a consequence, we should re-evaluate what constitutes ‘true’ carbon storage: the mere presence of carbon-containing organic molecules may simply no longer suffice. Contributions to soil carbon made by microplastic (and potentially other pollutants, including soot) should be subtracted to arrive at a fair estimate of carbon sequestration.

Alternatively, are we prepared to accept microplastic as a non-natural portion of soil carbon? After all, this material will likely be in our soils for the foreseeable future and does represent organic C. We should not, since this material is likely not functionally similar to soil organic matter, probably interacts differently with soil microbes, and it reflects a completely different production path and carbon origin. Either way, we should know its contribution to soil carbon, and this means: investing into methods for quantifying microplastic, and microplastic-carbon in soils globally; understanding its persistence and behavior in soil; and reaching a consensus towards adjusting established standard soil science methods to account for microplastic-derived carbon in the future (perhaps similar to current protocols for removing carbonate). An important driver of such actions could be policy actions to exclude microplastic-derived carbon as a portion of soil carbon.