Growth rate and carbon biomass
The growth rate (F = 59.65, p < 0.001) and C biomass (F = 286.9, p < 0.001) ofScenedesmus were significantly different among the culture media prepared from 11 types of leaf litter leachates in the control treatment (Fig. 1a). The addition of micronutrients to the culture medium had a significant positive effect on the algal growth rate of in the leaf litter leachates of oak, Japanese elm, and Japanese hemlock. However, the micronutrient amendment had a negative effect on growth in the culture medium prepared from the leaf litter leachate of hinoki cypress. For the algal C biomass, a significant positive effect of micronutrient amendment was detected only in the culture medium prepared from the leaf litter leachate of Japanese hemlock (Fig. 1b). However, algal C biomass was not affected by the micronutrient amendment in culture media prepared from other leaf litter leachates.
Because TDN and TDP concentrations varied among the different types of leaf litter leachates, we examined whether variations in the algal growth rate and C biomass of Scenedesmus were related to these nutrient concentrations in the culture media prepared from the leaf litter leachate. Due to the large variation of growth rate at TDN concentrations < 5 μM, the growth rate was not related to TDN concentration regardless of the micronutrient amendment (Fig. 2a and b). However, the algal growth rate tended to increase with TDP concentration in the amended treatment but not in the control, although the growth rate also varied widely and decreased when TDP concentration was < 2 μM (Fig. 2c and d).
Unlike the growth rate, the algal C biomass increased significantly with TDN in the control treatment, although the increasing trend was attenuated when TDN exceeded 5 μM, as shown by the segmented regressions with breakpoints at 5.31 μM (Fig. 2e and f). In the amended treatment, the relationship between the algal C biomass and TDN was not improved. It was almost the same as in the control treatment, as indicated by similar values in the segmented regressions (Fig. 2e and f). Similarly, algal C biomass tended to increase with TDP regardless of the micronutrient amendment, although the increasing trend was not significant due to low C biomass values relative to TDP in some culture media, such as those prepared from leaf litter leachates of Siebold’s beech, Japanese elm and Japanese red pine (species ID 5, 6, 9 in Fig. 2g and h).