Balancing carbon persistence and N₂O emissions: Structural control of roots on soil organic matter dynamics

The persistence of soil organic carbon (SOC) and the production of nitrous oxide (N₂O) are tightly coupled through microscale soil structure, yet are often studied in isolation. Here we synthesize results from recent studies to show that particulate organic matter (POM), and in particular its spatial embedding, provides a mechanistic link between long-term carbon storage and denitrification-driven N₂O emissions. X-ray μCT–based pore network analysis combined with isotopic tracing, demonstrated that root-derived structural inputs are not only major precursors of SOC accrual but also key regulators of microscale redox dynamics.

In a study on Silphium perfoliatum, extensive root systems promoted the formation of organic-rich porous matrices and generated large amounts of root-derived POM. A substantial fraction of this material became embedded deeper within the soil matrix and spatially disconnected from air-filled pores, forming “distal POM” that overlaps operationally with occluded POM but is precisely defined by its spatial position within the pore network. These distal POM particles were decisive for SOC accrual.

In a complementary study focusing on denitrification, we show that the same distal POM exerts strong control over N₂O production. While POM located close to air-filled pores predominantly fueled aerobic respiration, microbial oxygen consumption around distal POM created transient anoxic microsites that favored denitrification. Denitrification hotspots emerged from the interaction between limited O₂ supply and high local O₂ demand of the distal POM.

Together, these findings highlight a fundamental trade-off: soil structural conditions that favor SOC stabilization via distal POM formation can simultaneously enhance denitrification and N₂O emissions.