It has been reported that ground-penetrating radar (GPR) is a nondestructive tool that can be used to detect coarse roots in forest soils. However, successful GPR application for root detection has been site-specific and numerous factors can interfere with the resolution of the roots. We evaluated the effects of root diameter, root volumetric water content, and vertical and horizontal intervals between roots on the root detection of Cryptomeria japonica in sand using 900-MHz GPR. We found that roots greater than 19 mm in diameter were clearly detected. Roots having high volumetric water content were easily detected, but roots with less than 20% water content were not detected. Two roots that were located closely together were not individually distinguished. These results confirm that root diameter, root water content, and intervals between roots are important factors when using GPR for root detection and that these factors lead to an underestimation of root biomass.

Fine root respiration is a significant component of carbon cycling in forest ecosystems. Although fine roots differ functionally from coarse roots, these root types have been distinguished based on arbitrary diameter cut-offs (e.g., 2 or 5 mm). Fine root morphology is directly related to physiological function, but few attempts have been made to understand the relationships between morphology and respiration of fine roots. To examine relationships between respiration rates and morphological traits of fine roots (0.15-1.4 mm in diameter) of mature Quercus serrata Murr., we measured respiration of small fine root segments in the field with a portable closed static chamber system. We found a significant power relationship between mean root diameter and respiration rate. Respiration rates of roots<0.4 mm in mean diameter were high and variable, ranging from 3.8 to 11.3 nmol CO2 g(-1) s(-1), compared with those of larger diameter roots (0.4-1.4 mm), which ranged from 1.8 to 3.0 nmol CO2 g(-1) s(-1). Fine root respiration rate was positively correlated with specific root length (SRL) as well as with root nitrogen (N) concentration. For roots<0.4 mm in diameter, SRL had a wider range (11.3-80.4 m g(-1)) and was more strongly correlated with respiration rate than diameter. Our results indicate that a more detailed classification of fine roots<2.0 mm is needed to represent the heterogeneity of root respiration and to evaluate root biomass and root morphological traits.

To determine the means and variations in CH4 uptake and N2O emission in the dominant soil and vegetation types to enable estimation of annual gases fluxes in the forest land of Japan, we measured monthly fluxes of both gases using a closed‐chamber technique at 26 sites throughout Japan over 2 years. No clear seasonal changes in CH4 uptake rates were observed at most sites. N2O emission was mostly low throughout the year, but was higher in summer at most sites. The annual mean rates of CH4 uptake and N2O emission (all sites combined) were 66 (2.9–175) µg CH4‐C m−2 h−1 and 1.88 (0.17–12.5) µg N2O‐N m−2 h−1, respectively. Annual changes in these fluxes over the 2 years were small. Significant differences in CH4 uptake were found among soil types (P < 0.05). The mean CH4 uptake rates (µg CH4‐C m−2 h−1) were as follows: Black soil (95 ± 39, mean ± standard deviation [SD]) > Brown forest soil (60 ± 27) ≥ other soils (20 ± 24). N2O emission rates differed significantly among vegetation types (P < 0.05). The mean N2O emission rates (µg N2O‐N m−2 h−1) were as follows: Japanese cedar (4.0 ± 2.3) ≥ Japanese cypress (2.6 ± 3.4) > hardwoods (0.8 ± 2.2) = other conifers (0.7 ± 1.4). The CH4 uptake rates in Japanese temperate forests were relatively higher than those in Europe and the USA (11–43 µg CH4‐C m−2 h−1), and the N2O emission rates in Japan were lower than those reported for temperate forests (0.23–252 µg N2O‐N m−2 h−1). Using land area data of vegetation cover and soil distribution, the amount of annual CH4 uptake and N2O emission in the Japanese forest land was estimated to be 124 Gg CH4‐C year−1 with 39% uncertainty and 3.3 Gg N2O‐N year−1 with 76% uncertainty, respectively.

Very fine roots (<0.5 mm in diameter) of forest trees may serve as better indicators of root function than the traditional category of <2 mm, but how these roots will exhibit the plasticity of species-specific traits in response to heterogeneous soil nutrients is unknown. Here, we examined the vertical distribution of biomass and morphological and physiological traits of fine roots across three narrow diameter classes (<0.5, 0.5-1.0, and 1.0-2.0 mm) of Quercus serrata and Ilex pedunculosa at five soil depths down to 50 cm in a broad-leaved temperate forest. In both species, biomass and the allocation of very fine roots were higher in the surface soil but lower below 10-cm soil depth compared to values for larger roots (0.5-2.0 mm). When we applied these diameter classes, only very fine roots of Q. serrata exhibited significant changes in specific root length (SRL; m g À1 ) and root nitrogen (N) concentrations with soil depth, whereas the N concentrations only changed significantly in I. pedunculosa. The SRL and root N concentrations of larger roots in the two species did not significantly differ among soil depths. Thus, very fine roots may exhibit species-specific traits and change their potential for nutrient and water uptake in response to soil depth by plasticity in root biomass, the length, and the N in response to available resources.

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