Tamm Review: Fine root biomass in the organic (O) horizon in forest ecosystems: Global patterns and controlling factors

Considerable mass of fine roots occurs in the organic (O) horizon, which features greater availabilities of water and nutrients in forest ecosystems. Although global patterns of fine root biomass in mineral soils have been well studied, global patterns of fine root biomass in the O horizon remain unclear. By synthesizing 109 observations from 38 studies worldwide, we calculated Fo (fine root biomass per unit area in the O horizon), Ft (sum of fine root biomass in the O horizon and whole mineral soil), F20 (the same as Ft but for the standardized sampling depth of 0?20 cm in mineral soil), Rt (Fo/Ft) and R20 (Fo/F20). We then analyzed how Fo, Rt and R20 varied among forest biomes of tropical, temperate, and boreal forests, and assessed their relationships with stand (O horizon thickness, stand age and density) and environmental factors (latitude, elevation, mean annual temperature (MAT), mean annual precipitation (MAP), MAT/MAP). Our results showed that, Fo could be well predicted by Ft and F20, each of which explained about 85% of the variation. Fo was significantly greater in tropical forests (726 g m- 2) than in temperate forests (243 g m- 2) and boreal forests (176 g m- 2). Rt was larger in boreal forests (45.7%) and tropical forests (44.2%) than in temperate forests (36.6%); but the greatest R20 was observed in tropical forests. Compared with Rt and R20, Fo exhibited closer associations with environmental and stand factors. Fo, Rt and R20 were all significantly positively correlated with the O horizon thickness but negatively correlated with MAT/MAP. Additionally, Fo and Rt increased as stand age increased, while R20 increased with MAP. These findings highlight the crucial role that fine roots in the O horizon play in belowground resource acquisition and carbon cycling across forest biomes, and should be considered in assessment of ecosystem functions in forests.