## 4 Discussion  
### Characteristics of 4 and FRP in the Study Forest  
This study showed that the contribution of \(d\) was 25% between T3 and T4 in the differential method (Figure 5). The ratio was comparable to that in a growing secondary oak forest (27.1%, [10]) but higher  
Figure 4: Fine root biomass (\(\Delta\)B), fine root necromass (\(\Delta\)N), decomposed dead fine roots (d) and fine root production (FRP) over various sampling times using (**a**) the differential method and (**b**) the simple method. Different letters indicate significant differences among sampling times (\(p\) < 0.05). Note that, as \(\Delta\)B between T2 and T3 were negative, they were assigned as zero, because negative values are not possible in ecology [9].  
Figure 5: Ratio of decomposed dead fine roots (d) to fine root production (FRP) estimated by (**a**) the differential method and (**b**) the simple method for various sampling times.  
than that in a temperate coniferous forest (14%, [9]) and a secondary oak forest (8.0\(-\)12.8%, [34]). Thus, the contribution of decomposed fine roots was not consistently higher than that of other forests, including tropical rainforests, located at higher latitudes where lower decomposition ratios were reported [11; 35; 36]. \(d\) was calculated from fine root necromass and the decomposition ratio (\(\gamma_{ij}\), Equations (2) and (3)). The \(\gamma_{ij}\) between T3 and T4 in the present study (0.30 \(\pm\) 0.07) was lower than that in a secondary Mongolian oak (0.93 \(\pm\) 0.22, [10]). In comparison, necromass in the ingrowth cores at the sampling interval of T3 was more than twice that of the cores 9 months after installation in [PERSON] et al. [10]. The lower decomposition ratio but higher necromass, in the present study was partly attributed to the similar contribution of decomposed fine roots. The \(\gamma_{ij}\) between T3 and T4 in the present study was comparable to the global mean for broadleaf (0.35, \(n=40\), [9]), including boreal, temperate and tropical forests. The decomposition rate generally increases with mean annual temperature or latitude [11; 35; 36], suggesting a relatively low decomposition ratio in the present study. Thus, the contribution of decomposed fine roots in the present study might be lower than that in other tropical rainforests.  
NPP\({}_{\text{fr}}\) and the ratio of NPP\({}_{\text{fr}}\) to NPP\({}_{\text{total}}\) in the present study were relatively higher than values reported in the Neotropics (Table 1). These results correspond to a previous study reporting higher belowground carbon allocation in this forest type. Belowground carbon was used for high rhizosphere production and/or respiration, probably because of poor nutrient availability [19]. However, the ratio of NPP\({}_{\text{fr}}\) to total belowground carbon flux (19.63 MgC ha\({}^{-1}\) year\({}^{-1}\), [19]) was just 0.22 in our study forest. The averaged ratio of NPP\({}_{\text{fr}}\) to TBCF among forest ecosystems ranged between 0.42 and 0.53 [37]. Thus, large quantities of carbon might be used for rhizosphere respiration, root exudation, production of volatile organic compounds and carbon allocation to symbionts, none of which were measured in the study forest. In addition, previous studies (see Table 1) used different ingrowth core methods to the present study. For instance, ingrowth cores were collected every 3 months and root-free soil was re-installed inside the cores and placed to the same holes [5; 28; 29; 30; 31; 32]. The effect of repeated disturbance caused by use of the same hole has not been quantified, along with the effect on \(d\). To compare FRP across various forests, it is necessary to use consistent methods, even when using ingrowth cores.