Tracking 13C into soil organic matter through the mycorrhizal fungal pathway under nutrient addition

Arbuscular mycorrhizal (AM) fungi are ubiquitous plant symbionts that mediate soil organic matter (SOM) formation through nutrient exchange and the accumulation of their own biomass and necromass. Recent studies suggest that microbial necromass, including fungal necromass, may account for upwards of half of SOM carbon, but the specific contribution of AM necromass remains unknown. Additionally, how land management impacts AM-mediated SOM is not well understood, especially in grasslands where AM fungi are prevalent and play a key role in regulating plant diversity and ecosystem function. We grew Lolium perenne with four treatments: (1) an inoculation of AM spore-rich sandy soil, (2) an application of nitrogen-phosphorus-potassium (NPK) fertilizer, (3) a combination of spore-rich soil and NPK, and (4) a control. We isotopically labelled plants with ¹³C-CO2 before incubating the roots and AM hyphae in place, and are measuring ¹³C in SOM, plant and microbial pools, and released CO₂ at multiple timepoints over 6 months to track C through decomposition. Our preliminary results show that both ¹³C and ¹³C-CO₂ respiration were lower for fertilizer treatments compared to control and inoculation-only treatments during the first month of incubation. This demonstrates that even a small one-time NPK application may influence subsequent decomposition of root and AM tissues at the end of the growing season. The lower amounts of ¹³C respiration may be due to differences in plant carbon to nitrogen ratios, leading to higher microbial carbon use efficiency, or NPK addition may have inhibited the growth of AM hyphae, leading to decreased available ¹³C in the soil. These hypotheses will be investigated further with additional measurements as outlined above. By using stable isotope tracing into biomarkers, SOM pools, and soil respiration, our study will shed light on the contribution of AM and associated root necromass to SOM carbon and provide needed insight for conscious grassland management.