The underlying mechanism of how shifts in plant detritus input impact soil organic carbon (SOC) dynamics is not fully understood. Here we investigated the soil C content in different fractions (aggregates, labile and recalcitrant pool) after two years of detritus input manipulations (i.e. detritus input and removal treatment–DIRT: control, CK; double litter, DL; no litter, NL; no roots, NR; no aboveground litter and no roots, NRNL) in a coniferous forest (Platycladus orientalis (Linn.) Franco) ecosystem in subtropical China. The root exclusion treatments (NR and NRNL) significantly decreased the macro-aggregate (> 2000 μm) fraction by 23.4%–27.7% compared to CK after two-year detritus input manipulation, and accordingly decreased the C content in macroaggregates.
In contrast, the aboveground litter removal and addition (DL and NL) did not significantly impact those properties. Labile SOC significantly declined in the detritus removal treatments, while recalcitrant SOC remained relatively stable. In addition, root exclusion significantly reduced total microbial biomass and most of the taxonomic biomarkers compared to the CK. Fungal biomass were positively correlated with the proportion of macroaggregates across treatments. Principal component analysis revealed the separation of root exclusion treatments from the other detritus input manipulations was based on lower soil C content and proportion of
macroaggregates. Overall, our results suggest that future shifts in plant detritus input, especially decreases in belowground litter inputs, can strongly and rapidly reduce SOC pools by reducing the proportion of macroaggregates and the C content in acroaggregates, highlighting the importance of root in regulating soil C sequestration in response to future climatic scenarios.