Water stable soil aggregates promote soil carbon (C) storage. Forest soils are an important global C sink, but it remains unclear how ectomycorrhizal tree species affect the formation of soil aggregates. We assessed the role of two common functional groups of trees and their ectomycorrhizal fungi on soil aggregation using two experimental manipulations at the Cedar Creek LTER, MN, USA. We established 6 blocks in each of 3 forest stands: (1) dominated by oak (Quercus ellipsoidalis), (2) dominated by pine (Pinus strobis), and (3) a mixture of both species. To assess the role of ectomycorrhizae on soil aggregation, one 4m2 plot within each block was trenched biweekly to remove ectomycorrhizal fungi, while a paired plot was left untrenched. After two years of ectomycorrhizal exclusion, we collected the top 10 cm of mineral soil and performed a wet-sieving procedure to measure water stable aggregate composition by diameter size (>2000µm, 2000-250µm, 250-53µm, <53µm diameter). We performed the same wet-sieving procedure on soils from a tree biodiversity experiment. Here we compared soil aggregation in pine monocultures (P. strobis, P. resinosa, or P. banksiana) with oak monocultures (Q. ellipsoidalis, Q. rubra, or Q. macrocarpa), then compared aggregate size fractions to soil C stocks.
Results/Conclusions
We found no clear effect of mycorrhizal exclusion on aggregate function after two years. This was despite the fact that trenching had clear effects on soil fungal communities, and previous work from these plots has shown trenching effects on rates of litter decomposition and nitrogen cycling. This suggests that ectomycorrhizal fungal effects on aggregation are either minimal or very slow. Unlike trenching effects, we found clear differences in aggregate fractions across the different tree communities. Soils under oak trees contained a greater proportion of macroaggregates (250-2000µm and 53-250µm) than soils under pine species (P<0.001). Results from the common garden experiment supported this finding: oak monocultures contained more macroaggregates than pine monocultures (P=0.02). Macroaggregates in these plots were correlated with soil C pools (P<0.001). Our results highlight clear tree species effects on soil aggregate formation over short timescales (6 years), with the potential to impact rates of soil C sequestration.