## 3 Discussion
### Fine Root Biomass and Nutrients Dynamics in Two Management Systems  
The rubber-_F. macrophylla_ intercropping system did not affect the total fine root biomass. Hence, there is no direct evidence of tree species diversity and intercropping system's effects on fine root biomass under the rubber-_F. macrophylla_ system, thus rejecting our first hypothesis. [PERSON] et al. (2018) also found no significant difference in fine root biomass among different tree species diversity systems (one, three, or five dominant tree species). However, [PERSON] et al. (2018) reported that intercropping of deciduous species in monoculture plantations produced high fine root biomass in southern Finland. Similarly, [PERSON] et al. (2018) found a positive effect on fine root biomass with _Castanea dentata_ introduced to the forest. As an indicator of the total soil space-filling of fine roots in the monoculture systems, fine root biomass heterogeneity contained higher above-ground biomass than the mixed-species stands ([PERSON] et al., 2018). [PERSON] et al. (2018) also described that fine root biomass increases with species diversity compared to a monoculture plantation. Thus, we conclude that less  
Figure 2: Correlational matrix of total fine root biomass with different species in rubber monoculture ((**A**) = Correlogram of 12 years, **(B)** = Correlogram of 15 years, and **(C)** = Correlogram of 24 years) and rubber-_F. macrophylla_ intercropping agroforestry system (**(D)** = Correlogram of 12 years, **(E)** = Correlogram of 15 years, and **(F)** = Correlogram of 24 years). The acronyms represent fine roots of rubber (RFR), _F. macrophylla_ (FFR), other (non-tress) (OFR), dead (DFR), and available nutrients (N, P, and K). The colored legend gradients represent correlation coefficients of r values from dark green (+1.0) to light red (\(-\)1.0). All coefficients were analyzed through the Pearson correlation at significance level: ***, \(p\) < 0.001; **, \(p\) < 0.01, *, \(p\) < 0.05 for the possible pair variables in the matrix. fine root biomass in mixed forests in our study could be explained by more effective soil resource uptake per unit of fine root biomass under intercropping system over the previous years; for example, _F. macrophylla_ was planted in 2010, eight years before our samplings. In the present study, the effect of competition for N under the intercropping system was not clear. In contrast, soil P and K concentrations differed significantly between the cropping systems. Due to the high mobility of N compounds within the soil, P and K are leached easily through preferential flows during the rainy season [36]. The significant increase in the P and K concentrations in the rubber monoculture (Table 3) suggests enhanced nutrient acquisition by the rubber roots biomass. However, the low soil P and K concentration in the rubber-_F. macrophylla_ intercropping implies that intense interspecific competition may result in less P and K input or output within the soil. The effects of competition for nutrients are weakly reflected in the soil P concentration because of its low mobility [37]. The rubber trees had a significant advantage in competing for soil P because their roots were more developed than the other intercrops. Similarly, K was substantially higher under the young stand age of rubber monoculture (Table 3), which depends on the type of clay content (mainly smectites) in soil and the level of leaching corresponding with other studies [38]. The previous research demonstrated that multi-cropping agroecosystems showed increased soil organic matter and C and N contents compared to monoculture [39]. However, we found rubber monoculture exhibited the highest soil total C, available P, and K concentrations across the soil profile, whereas C and N loss under rubber-_F. macrophylla_ intercropping could be due to a higher decomposition rate and leaching. Moreover, _F.