Ectomycorrhizal and non-mycorrhizal rhizosphere fungi increase root-derived C input to soil and modify enzyme activities: A 14C pulse labelling of Picea abies seedlings

Consequences of interactions between ectomycorrhizal fungi (EcMF) and non-mycorrhizal rhizosphere fungi (NMRF) for plant carbon (C) allocation belowground and nutrient cycling in soil remain unknown. To address this topic, we performed a mesocosm study with Norway spruce seedlings [Picea abies (L.) H. Karst] inoculated with EcMF, NMRF, or a mixture of both (MIX). (CO2)-C-14 pulse labelling of spruce was applied to trace and visualize the C-14 incorporation into roots, rhizohyphosphere and hyphosphere. Activities and localization of enzymes involved in the C, nitrogen (N) and phosphorus (P) cycling were visualized using zymography. Spruce seedlings inoculated with EcMF and NMRF allocated more C to soils (EcMF: 10.7%; NMRF: 3.5% of total recovered C) compared to uninoculated control seedlings. The C-14 activity in the hyphosphere was highest for EcMF and lowest for NMRF. In the presence of both, NMRF and EcMF (MIX), the C-14 activity was 64% lower compared with EcMF inoculation alone. This suggests a suppressed C allocation via EcMF likely due to the competition between EcMF and NMRF for N and P. Furthermore, we observed 57% and 49% higher chitinase and leucine-aminopeptidase activities in the rhizohyphosphere of EcMF compared to the uninoculated control, respectively. In contrast, beta-glucosidase activity (14.3 nmol cm(-2) h(-1)) was highest in NMRF likely because NMRF consumed rhizodeposits efficiently. This was further supported by that enzyme stoichiometry in soil with EcMF shifted to a higher investment of nutrient acquisition enzymes (e.g., chitinase, leucine-aminopeptidase, acid phosphatase) compared to NMRF inoculation, where investment in beta-glucosidase increased. In conclusion, the alleviation of EcMF from C limitation promotes higher activities of enzymes involved in the N and P cycle to cover the nutrient demand of EcMF and host seedlings. In contrast, C limitation of NMRF probably led to a shift in investment towards higher activities of enzymes involved in the C cycle.

Automated summary

The consequences of interactions between ectomycorrhizal fungi (EcMF) and non-mycorrhizal rhizosphere fungi (NMRF) for plant carbon allocation and nutrient cycling remain unclear. This study found that spruce seedlings inoculated with EcMF and NMRF allocated more carbon to soils compared to uninoculated control seedlings. Furthermore, there was competition between EcMF and NMRF for nitrogen and phosphorus, which resulted in suppressed carbon allocation via EcMF. Additionally, inoculation with EcMF promoted higher activities of enzymes involved in nitrogen and phosphorus cycling, while inoculation with NMRF led to higher activities of enzymes involved in carbon cycling.