Persistence of soil organic matter as an ecosystem property

The mechanisms underpinning soil carbon stability are complicated. The future response of soil carbon to climate change is uncertain but crucial, given that the carbon pool in soils is three times greater than that of the atmosphere. In a Perspective, Michael Schmidt and an international team of collaborators discuss how our understanding of soil carbon cycling has been changing. Rather than being mostly a function of molecular structure, as has been assumed, soil organic carbon stability is an ecosystem property. This means that it arises from complex interactions among many biotic and abiotic factors that are not fully understood. This fact must be more rigorously addressed in a new generation of experiments and soil carbon models, say Schmidt et al., if we are to improve our attempts to understand this vital component of the Earth system. Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.