Can the conversion to perennial cereal crops simultaneously promote SOC formation and stimulate microbial N-mining?

Enhancing soil carbon (C) storage is critical for climate mitigation, and perennial systems for cereal agriculture have emerged as a promising strategy due to their sustained root-derived C inputs. However, an increased supply of labile C may also lead to a higher demand for nitrogen (N), whereby microbes decompose existing soil organic matter (SOM) to acquire N, termed N-mining, potentially triggering a priming effect that offsets C storage. Whether perennial cropping primarily promotes microbial C assimilation and subsequent production of SOM or accelerates SOM mineralization remains uncertain. Moreover, the stand age of perennial crops can substantially modify root-exudate C, thereby altering microbial C availability and shifing microbial decomposition strategies. But how perennial stand age regulates these coupled plant-soil-microbe processes is still poorly understood.

Here, we examined how converting annual crops to perennial intermediate wheatgrass (Thinopyrum intermedium, Kernza®) influences microbial decomposition dynamics and N-mining. Soils were collected from the annual cropping system, the first-year Kernza stand, and the ninth-year stand. Root-exudate inputs were simulated by semi-continuous additions of ¹³C-glucose over 20 days, applied at the daily exudate-C level of the perennial crop and at a five-fold higher intensity. We quantified the real-time soil organic C mineralization, organic N mineralization with the ¹⁵N pool dilution method, and microbial growth and biomass to resolve the balance between C storage and SOC loss, N mining from SOM, and its microbial response underpinning the simulated rhizosphere. We hypothesized that the conversion to perennial crops would enhance microbial N-mining and priming effects, particularly in young stands, whereas older stands progressively shift toward more efficient microbial C utilization and higher SOM stabilization potential. Based on the results, we found that glucose applied at levels matching those in the perennial crop rhizosphere induced fast (within days) and sustained (for weeks) priming response. Across addition levels, young perennial crops exhibited consistently higher cumulative priming than older perennial crops. These temporal patterns best matched responses in bacterial growth, suggesting a bacterial control of the young perennial rhizosphere priming effect and indicating a greater need for bacteria to acquire organic N there.