MICROBIAL BIOMASS AND ACTIVITY IN CANOPY ORGANIC-MATTER AND THE FOREST FLOOR OF A TROPICAL CLOUD FOREST

In some tropical forests, organic matter derived from epiphytic plants accumulates as mats on tree branches. Microbial characteristics relating to the cycling of C and N were studied in this canopy organic matter (COM) and in the forest floor of a tropical montane forest in Costa Rica. COM was found to have high microbiat activity which was. in general, similar to the forest floor H (FF-H) horizon. The concentration of microbial biomass C was 2650, 2670 and 1950 μg g-1 soil in COM, FF-H and FF-Al, respectively. Biomass N, laboratory CO2 evolution and laboratory N mineralization were also higher or similar for COM relative to the forest floor. In contrast to the H and Al horizons of the forest floor, nitrification was not detectable or only very low in COM during laboratory and field incubations. The potential for denitrification, as evidenced by the decline in added NO-1 relative to NH+4 during laboratory incubation, was higher in the forest floor than in the canopy. Microbial respiration and biomass in COM and the forest floor responded more to C (as glucose) than N (as NH4NO1) amendments. Microbial immobilization of added N appeared to be higher in the forest floor than in COM, although the microbial population in all materials readily immobilized inorganic N in the presence of glucose. The total pool of COM in this forest is estimated to be between 1000 and 2000 kg ha-1. From this, a preliminary estimate of 0.7 kg ha- of N immobilized in microbial tissue can be calculated. Because of their high biomass and activity, microorganisms in the canopy could be important in regulating N availability in this forest. © 1990.