COS 64-1 - Climate, soil mineralogy, and mycorrhizal fungi influence soil organic matter fractions in forests across the National Ecological Observatory Network

Background/Question/Methods

Biotic and abiotic factors are known to influence soil organic matter pools in forests, yet we lack an understanding the relative importance of these drivers across broad-scale edapho-climate gradients. Recent studies have reported that soils beneath trees with arbuscular mycorrhizal (AM) fungi may store more soil organic matter (SOM) in mineral-associated pools (hereafter “MAOM”) and less SOM in particulate organic matter pools (hereafter POM) relative to soils beneath trees associating with ectomycorrhizal (ECM) fungi. However, climate and soil mineralogy can also influence POM and MAOM formation, and it is unclear how variation in environmental conditions may interact with tree mycorrhizal type to determine SOM distributions in forest soils. We used density fractionation to isolate mineral-associated and particulate carbon and nitrogen from mineral soil horizons collected in seven forests across the National Ecological Observatory Network. The NEON sites span a gradient of mean annual temperature and soil mineralogy (e.g., clay fraction and oxalate-extractable iron (Fe-Ox) concentrations) and include multiple soil orders from spodosols to ultisols. Within each site, soil samples were collected from plots with differing mycorrhizal dominance, estimated by the percentage of basal area from AM- and ECM-associated tree species in each plot.

Results/Conclusions

Tree mycorrhizal associations impacted the proportion of total C in the MAOM vs. POM fractions, while climate and soil mineralogy influenced concentrations of C and N in these fractions. The relative amount of C in MAOM was lower under ECM tree species in nearly all sites (t_7,46= -2.74, p=0.006), but the effect of mycorrhizal dominance on MAOM N proportions depended on climate; negative relationships between MAOM N and ECM dominance were strongest in colder sites and weaker in warmer sites (t_7,46= 2.25, p= 0.024). POM fractions generally followed the opposite pattern, with larger proportions of particulate C in ECM-dominated plots. MAOM [C] and [N] increased with % Fe-Ox (C: t_7,46= 3.54, N: t_7,46= 5.18; p=< 0.001), and [C] and [N] of all three soil fractions was lower in warmer, wetter sites. Finally, the C:N ratio of all three soil fractions tended to increase with ECM dominance. These results suggest that the influence of biological communities on SOM dynamics should be considered in the context of local environmental conditions, highlighting a need to consider the broad influence of climate and soil mineralogy along with fine-scale variation in vegetation and microbial communities when modeling patterns in SOM pools across large geographic areas.