Fine root biomass and production regarding root diameter in pinus densiflora and quercus serrata forests: Soil depth effects and the relationship with net primary production

This study aimed to examine the effects of soil depth on fine root biomass (FRB) and production (FRP), and determine the relationship between FRP and net primary production (NPP) across two root diameter classes (<1 and 1–2 mm) in Pinus densiflora and Quercus serrata forests. FRB and FRP were investigated from April 2016 to March 2017 using the soil sequential coring and ingrowth core methods. In P. densiflora and Q. serrata forests, mean FRB < 1 mm (2.22 ± 0.23 and 2.63 ± 0.23 Mg ha^–1) and annual FRP < 1 mm (0.97 ± 0.09 and 1.55 ± 0.16 Mg ha^–1 year^–1) were higher than mean FRB 1–2 mm (0.63 ± 0.12 and 1.72 ± 0.38 Mg ha^–1) and annual FRP 1–2 mm (0.26 ± 0.14 and 0.20 ± 0.06 Mg ha^–1 year^–1) at 0–30 cm depth. Soil properties, such as soil moisture, organic matter, and inorganic nitrogen, decreased with soil depth (0–30 cm). The gradient of soil properties corresponding with soil depth could directly and indirectly influence FRP <1 mm, resulting in higher FRB <1 mm and FRP <1 mm at topsoil (0–10 cm). Additionally, only FRP <1 mm showed significant relationships with NPP and litter production. Although very fine roots (<1 mm in diameter) occupied a small percentage of NPP (7.5%), they may be an important factor for predicting forest NPP, since NPP would be regulated by the capability of roots to absorb water and nutrients. Our findings indicate that finer roots are more sensitive to soil conditions than thicker roots, and they could be a forest productivity indicator.