Tropical forest trees differ in belowground carbon allocation to fungi

Background/Question/Methods

The pathways of belowground C cycling in are relatively unexplored in secondary tropical forests. In these forests, tree species may differentially allocate C belowground to members of the soil microbial community. Two potentially large belowground C allocation pathways are: (1) direct allocation to arbuscular mycorrhizal (AM) fungi, and (2) allocation to saprotrophic, free living microbes through fine root production and root exudates. We reasoned that by pathway 1, AM fungal biomass would be highest and production of P- and possibly N-acquiring enzymes would be elevated. Also, AMF would dominate the composition of fungal communities. Alternatively, by pathway 2, saprotroph biomass would be greatest and production of all enzymes, (C-, N- and P-degrading) would be high. Saprotrophs would also make up the majority of the community. We tested these hypotheses in a replicated experimental site in the tropical forest at La Selva Biological Station, Costa Rica. The experiment contained 24-yr-old monodominant stands of four tree species, Hieronyma alchorneoides, Virola koschnyi, Vochysia guatemalensis, or Pentaclethera macroloba,  24-yr-old secondary forests and mature forests. Fungal biomass, composition and enzyme activities were characterized in both wetter and drier portions of the year. 

Results/Conclusions

Carbon allocation to AM fungi was supported by high levels of P-acquiring enzymes in soils of all stands (P < 0.05) and larger colonization rates of AM fungi in the roots of all tree species relative to saprotrophic fungi (P < 0.05). Allocation of C to AM fungi may differ between species, as Vochysia guatemalensis stands had higher activities of both P- and N-acquiring enzymes relative to other tree species and other enzymes (P < 0.05). However, fungal biomass in bulk soils did not directly support preferential C allocation to either pathway as AM fungal and saprotroph biomass was equivalent. While overall patterns in C allocation did not vary between sampling points, the magnitude of C allocation to belowground microbes may vary temporally. Enzyme activity and fungal biomass belowground varied the most over time, with the wetter period supporting 3 times more fungi and 9 times higher enzyme activities. Because both tree species and season can affect C allocation belowground, considering secondary forest composition and seasonality can inform ecosystem-level rates of C storage and flux to the atmosphere.