###### Abstract
Fine roots are the drivers of a significant C and nutrient pool and are closely associated with the soil, thus contributing to the formation of soil organic matter (SOM) [15]. Globally, fine roots contribute about 33% of the NPP and account for 3-30% of the total root biomass in terrestrial ecosystems. Consequently, this leads to competition amongst species with a substantial effect on the below-ground competition via fine roots [16]. Many plant species such as pineapple, ginger (_Zingiber officinal_), turmeric (_Curcuna longa_), and tea (_Camellia sinensis_) are widely cultivated as intercrops in rubber-based agroforestry systems [17; 18]. Intercropping with nitrogen-fixing species (NFS) [19; 20; 21] can enrich the soil quality and productivity of rubber plantations [22]. The present study focused on the intercropping legume _Flemingia macrophylla_ (Wild.) Kuntze ex Merr. as it is an excellent NFS cover crop with dense growth, low decomposition rate and accumulation of SOM, capability to resist occasional flooding, and moderate drought tolerance. A previous study also showed that intercropping of _F. macrophylla_ in rubber-based agroforestry systems enhanced soil C and N storage and conserved water [23]. Rubber-_F. macrophylla_ intercropping systems have become widespread in Southwestern China [24]. However, so far, no studies have been attempted to understand how intercropping of _F. macrophylla_ can change fine root biomass dynamics in different aged rubber plantations and how rubber-_F. macrophylla_ systems impact nutrient cycling in rubber plantations. Hence, assessment of fine root dynamics and nutrient status of rubber-_F. macrophylla_ intercropping can benefit in understanding the below-ground competition and how the species diversity (monoculture rubber plantation Vs intercropping) affects below-ground nutrient (C and N) storage. Though fine root biomass is crucial in C and N cycling in plantation systems, growth and biomass production is significantly affected by the plantation stand age. Several studies have demonstrated an increasing trend in soil N concentrations with stand age [25]. However, other studies of secondary forest ecosystems have found that C sequestration decreases with the stand age [26; 27; 28], suggesting that some forests and plantations might become less productive over time, possibly due to the depletion of soil nutrients [29; 30]. Therefore, it is unclear how the stand age affects the fine root biomass and whether there is an interaction between different stand ages and soil nutrients across different soil depths. Further, it is unclear how _F. macrophylla_ intercropping alters spatiotemporal dynamics of fine root biomass in a rubber plantation. Hence, in the present study, we attempted to explore the seasonal variations in fine root biomass dynamics and nutrient stoichiometry across different stand ages in rubber monoculture and rubber-intercropping systems. The main objectives of this study were (i) to examine the influence of the intercropping of _F. macrophylla_ on different-aged (12, 15, and 24 years) rubber monoculture plantations on fine root biomass rates at 0-60 cm soil depth (ii) to evaluate the seasonal variations in fine root biomass and soil available soil nutrients (C, N, P, and K) to know the relationship between fine root biomass and nutrients in different stand age development. It was hypothesized that rubber intercropping systems would accumulate more fine roots than rubber monoculture. We also hypothesized that the seasonal changes in monoculture and intercropping systems would influence available soil nutrients.