Toko Tanikawa

Intraspecific variation in fine root respiration and morphology in response to in situ soil nitrogen fertility in a 100-year-old Chamaecyparis obtusa forest.

Soil N fertility has an effect on belowground C allocation, but the physiological and morphological responses of individual fine root segments to variations in N availability under field conditions are still unclear. In this study, the direction and magnitude of the physiological and morphological function of fine roots in response to variable in situ soil N fertility in a forest site were determined. We measured the specific root respiration (Rr) rate, N concentration and morphology of fine root segments with 1–3 branching orders in a 100-year-old coniferous forest of Chamaecyparis obtusa. Higher soil N fertility induced higher Rr rates, root N concentration, and specific root length (SRL), and lower root tissue density (RTD). In all fertility levels, the Rr rates were significantly correlated positively with root N and SRL and negatively with RTD. The regression slopes of respiration with root N and RTD were significantly higher along the soil N fertility gradient. Although no differences in the slopes of Rr and SRL relationship were found across the levels, there were significant shifts in the intercept along the common slope. These results suggest that a contrasting pattern in intraspecific relationships between specific Rr and N, RTD, and SRL exists among soils with different N fertility. Consequently, substantial increases in soil N fertility would exert positive effects on organ-scale root performance by covarying the Rr, root N, and morphology for their potential nutrient and water uptake.

Reply to: “Comment on root orientation can affect detection accuracy of ground-penetrating radar”

IntroductionWe showed that root orientation affected a parameter of ground penetrating radar (GPR), amplitude area (A) (Tanikawa et al. Plant Soil 373:317–327, 2013). The aims of this reply to Wu et al. (2014) are (i) to correct the two inaccuracies in Tanikawa et al. (2013) and (ii) to improve our method of estimating A(90°) using A(x) of root angle x.MethodsMeasured A values of Tanikawa et al. (2013) were analyzed with the modified equations.ResultsThe first inaccuracy was the use of incorrect units for the coefficient b (the phase shift) in the sinusoidal waveform of A(x). The units should have been radians instead of degrees. The second inaccuracy was the mis-derivation of A(x) into A(x + 90°). In the modified method, A(90°) was estimated by A(x) from two orthogonally intersecting transect lines and a transect line at a diagonal to them.ConclusionsThe two inaccuracies did not affect the previous main conclusions that the parameter T was suitable for estimating root diameter and that grid transects are likely to identify clear hyperbolas reflecting roots in radar profiles (Tanikawa et al. 2013). By the improved method, we could accurately estimate root diameter by scanning using three transect lines intersecting at angles of x, x + 45°, and x + 90°.

Extractable sulfate content in Japanese forest soils

The amounts of extractable sulfate in 12 forest soils were measured as a preliminary work to study sulfur dynamics in forest ecosystems. The sulfate content determined by the distillation method varied widely (10–880 × 10−6 gSg−1) depending on the soil type and the depth. Japanese forest soils were broadly divided into two groups in sulfate level: one retains a large amount of sulfate and the other does not. In general, the surface soils to the depth of 10 cm contained small amounts of sulfate (< 100 × 10−6 gSg−1), while subsoils contained more. The soil samples with low (< 4.6) or high (6 <) pH (H2O) retained small amounts of sulfate. Black soils derived from volcanic ash retained great quantities of sulfate. The two levels of sulfate contents in Japanese forest soils suggests that the sulfur dynamics are different in these soils.

Relationships between specific root length and respiration rate of fine roots across stands and seasons in Chamaecyparis obtusa

AimsFine root respiration (Rr) is closely linked with fine root morphology, especially with specific root length (SRL), in short-term measurements in some tree species. However, whether these relationships are also valid across different stands and seasons is not yet known. This study aimed to investigate these relationships in the fine roots of Chamaecyparis obtusa.MethodsThe Rr, mean root diameter, and SRL of fine root segments of two C. obtusa stands were determined every three months over two years.ResultsWe detected significant positive correlations between Rr and SRL of fine root segments across the stands over two years. The relationship of Rr with SRL was stronger than that with the mean diameter of fine roots. The slopes and intercepts for the Rr and SRL relationships did not differ among stands and measurement times. Further, we proposed a simple approach for estimating CO2 flux from fine roots at the stand level based on SRL and confirmed that the ranges of estimated CO2 values were comparable with those of values reported using the conventional approach.ConclusionsThe fine root morphology typified by SRL is a key variable in Rr of fine roots and CO2 flux at the stand level.

Ungulates decelerate litter decomposition by altering litter quality above and below ground

Ungulates can greatly affect forest ecosystems’ functional characteristics. However, limited information is available about their influence on litter decomposition, a major ecosystem process, despite disturbance of ungulates on vegetation through selective browsing and trampling. This study focused on effects of the presence/absence of deer herbivory on decomposition of leaves and roots of three major tree species in a Hokkaido, Japan forest. Our litterbag experiment showed that litter decomposition was significantly faster for both leaves and roots in a deer exclosure than in a control site with deer herbivory. Possible factors for this slowed decomposition because of deer presence include their physical disturbance on soil through trampling. In both sites, the remaining mass of litter was positively correlated with the C:N ratio and lignin content. When analyzed for leaf litter, species with lower C:N ratio and lignin content showed lower litter mass remaining in both sites. Deer generally prefer species with a low leaf C:N ratio and lignin content; the results suggest that leaves of palatable species were less resistant to decomposition. A similar interspecific difference in decomposition was not observed for roots, most likely resulting from the small difference in root litter quality among species. In this forest, tree species with unpalatable leaves, which are becoming predominant, likely decreases leaf litter decomposition, as leaves of palatable plants decompose more rapidly. Roots, however, are not exposed to browsing, regardless of aboveground palatability, and remain within soil as a recalcitrant slowly decomposing litter substrate. These synergetic influences could allow deer herbivory to reduce overall plant decomposition rates aboveground and belowground via changes in plant species composition.

Relating sulfate adsorption to soil properties in Japanese forest soils

The relationship of sulfate adsorption capacity to the presence of aluminum, iron oxides, and organic carbon in four Japanese forest soils was investigated. Sulfate adsorption capacities were higher in subsoils than in surface soils, and their profiles were the inverse of those of organic carbon content. The black soil derived from volcanic ash had high sulfate adsorption capacity and showed no release of sulfate, while some of the surface horizons of the other soils desorbed sulfate. Oxalate extractable aluminum (Al0) and iron (Fe0) and ditionite-citrate-bicarbonate (DCB) extractable iron (Fed) were positively correlated with the sulfate adsorption capacity, and their correlation coefficients were almost identical (0.58–0.61). The lack of significant correlation between Ald and the adsorption capacity suggests that DCB is unsuitable for extracting aluminum compounds which have the ability to adsorb sulfate. In contrast with oxides, organic carbon correlated with sulfate adsorption capacity negatively, and had a negative coefficient in a multiple regression equation, insignificantly. Furthermore, H2O2-treated samples containing decomposed organic matter adsorbed more sulfate than untreated samples. These results clearly indicated that organic matter hinders sulfate adsorption. A comparison of theβ value (standard regression coefficient) of organic carbon with that of Al0 showed that the negative contribution of organic matter to sulfate adsorption capacity was slightly less than the positive contribution of oxides.

Quantification of the contrasting root systems of Pinus thunbergii in soils with different groundwater levels in a coastal forest in Japan

AimsThe different root systems of Pinus thunbergii observed after the tsunami in 2011 were possibly influenced by different groundwater levels before the tsunami. The aims of this study were to quantify the tap and horizontal root structure and evaluate their relationship with the above-ground parts under different groundwater levels in a coastal P. thunbergii forest.MethodsTwo plots, sea- and land-side, with different groundwater levels, in a P. thunbergii stand, were established, and the entire root-systems of three select trees each were harvested to evaluate the biomass, lengths, and cross-sectional areas of the tap and horizontal roots.ResultsIn the sea-side plot, which had a shallower groundwater level, plate root systems with thicker and longer horizontal roots, but fewer tap roots were observed, whereas tap root systems were well developed in the land-side plots, where the groundwater level was deeper. The root-to- shoot ratio was significantly higher in the sea-side plot than in the land-side plot.ConclusionWe confirmed that quantitative contrasting root systems of P. thunbergii develop under different groundwater levels and higher biomass allocation to horizontal roots occur under shallower groundwater depths, emphasizing the need for management practices that promote the development of tap root systems to enhance resistance to tsunamis.

Calculation procedures to estimate fine root production rates in forests using two-dimensional fine root data obtained by the net sheet method

Several recent studies have used the net sheet method to estimate fine root production rates in forest ecosystems, wherein net sheets are inserted into the soil and fine roots growing through them are observed. Although this method has advantages in terms of its easy handling and low cost, there are uncertainties in the estimates per unit soil volume or unit stand area, because the net sheet is a two-dimensional material. Therefore, this study aimed to establish calculation procedures for estimating fine root production rates from two-dimensional fine root data on net sheets. This study was conducted in a hinoki cypress (Chamaecyparis obtusa (Sieb. & Zucc.) Endl.) stand in western Japan. We estimated fine root production rates in length and volume from the number (RN) and cross-sectional area (RCSA) densities, respectively, for fine roots crossing the net sheets, which were then converted to dry mass values. For these calculations, we used empirical regression equations or theoretical equations between the RN or RCSA densities on the vertical walls of soil pits and fine root densities in length or volume, respectively, in the soil, wherein the theoretical equations assumed random orientation of the growing fine roots. The estimates of mean fine root (diameter <1 mm) production rates were ∼80-100 g m-2 year-1 using the empirically obtained regression equations, whereas those from the theoretical equations were ∼40-50 g m-2 year-1. The difference in the estimates was attributed to larger slope values of the empirical regression equations than those of the theoretical equations, suggesting that fine root orientation was not random in our study site. In light of these results, we concluded that fine root production rates were successfully estimated from two-dimensional fine root data on the net sheets using these calculation procedures, with the empirical regression equations reflecting fine root orientation in the study site.

Leachate from fine root litter is more acidic than leaf litter leachate: A 2.5-year laboratory incubation

Some tree species increase fine root production under soil acidification, thus changing the balance of litter input from leaves and roots. Litter leaches a significant amount of acidic materials during its decomposition, which might facilitate soil acidification. In this context, we focused on dissolved organic matter (DOM) as the major component of acidic materials. We hypothesized that both the quality and quantity of DOM, which control its function (i.e., proton supply), differ between leaf and root litter. To test this hypothesis, we conducted a 2.5-year laboratory incubation experiment using fresh fine roots and fresh green leaves as litter of two coniferous species (Cryptomeria japonica and Chamaecyparis obtusa) and investigated the leachate pH and DOM composition based on the optical properties. After the early stage of decomposition when flash leaching of DOM converged, the amount of dissolved organic carbon (DOC) leached from roots increased again and leachate pH declined. In contrast, DOC concentrations continued to decrease in leaf leachates during the incubation period, and the pH decrease was not as striking as that of root leachates. Optical properties (ultraviolet visible absorption and fluorescence) of DOM revealed that humic-like substances in DOM played a central role in the acidic pH of root leachates. The total amount of protons released from roots of C. japonica and C. obtusa is about 13 and 18 times higher, respectively, than that from leaves. These results imply that the increase of fine root biomass may induce a positive plant-soil feedback in acidic soils, affecting soil biogeochemical functions of terrestrial ecosystems.

Reconstruction of root systems in Cryptomeria japonica using root point coordinates and diameters

Main conclusionWe developed simple algorithms for reconstructing tree root system architecture using only the root point coordinate and diameter, which can be systematically obtained without digging up the root systems.Root system architecture (RSA) is strongly related to various root functions of the tree. The aim of this study was to develop a three-dimensional (3D) RSA model using systematically obtained information on root locations and root diameters at the locations. We excavated root systems of Cryptomeria japonica and systematically obtained XYZ coordinates and root diameters using a 10-cm grid. We clarified the patterns of the root point connections and developed a reconstructed root system model. We found that the root diameters farther from the stump centre are smaller. Additionally, we found that the root lengths of the segments running between the base and the connected root point were smaller than those of other root segments, and the inner angle between the base and the stump and between the base and the connected root point was narrower than for the other pairs. The new RSA model developed according to these results had average accuracies of 0.64 and 0.80 for estimates of total volume and length, respectively. The developed model can estimate 3D RSA using only root point data, which can be obtained without digging up root systems. This suggests a wide applicability of this model in root function evaluation.