Abstract:
We investigated the role of carbon, nitrogen and phosphorus as limiting factors of microorganisms and microbial grazers
(testate amoebae) in a montane tropical rain forest in southern Ecuador. Carbon (as glucose), nitrogen (as NH4NO3) and
phosphorus (as NaH2PO4) were added separately and in combination bimonthly to experimental plots for 20 months.
By adding glucose and nutrients we expected to increase the growth of microorganisms as the major food resource of
testate amoebae. Th e response of microorganisms to experimental treatments was determined by analysing microbial biomass
(SIR), fungal biomass and microbial community composition as measured by phospholipid fatty acids (PLFAs). We
hypothesized that the response of testate amoebae is closely linked to that of microorganisms. Carbon addition strongly
increased ergosterol concentration and, less pronounced, the amount of linoleic acid as fungal biomarker, suggesting that
saprotrophic fungi are limited by carbon. Microbial biomass and ergosterol concentrations reached a maximum in the
combined treatment with C, N and P indicating that both N and P also were in short supply. In contrast to saprotrophic
fungi and microorganisms in total, testate amoebae suff ered from the addition of C and reached maximum density by the
addition of N. Th e results indicate that saprotrophic fungi in tropical montane rain forests are mainly limited by carbon
whereas gram positive and negative bacteria benefi t from increased availability of P. Testate amoebae suff ered from increased
dominance of saprotrophic fungi in glucose treatments but benefi ted from increased supply of N. Th e results show that testate
amoebae of tropical montane rain forests are controlled by bottom?up forces relying on specifi c food resources rather
than the amount of bacterial biomass with saprotrophic fungi functioning as major antagonists. Compared to temperate
systems microbial food webs in tropical forests therefore may be much more complex than previously assumed with trophic
links being rather specifi c and antagonistic interactions overriding trophic interactions.
