Muneto Hirobe

Spatial variability of soil nitrogen transformation patterns along a forest slope in a Cryptomeria japonica D. Don plantation

Abstract Spatial variability of soil nitrogen (N) transformation patterns was investigated along a slope of a Cryptomeria japonica plantation. The controlling factors of the soil N transformation patterns were examined by multivariate analysis. Net N mineralization showed no clear gradient along the slope, while net nitrification and percent nitrification were high in the lower part, and very low in the upper part of the slope. The middle part was the transition zone corresponding to the patchiness of the lower and upper soils. Soil properties (e.g. C/N ratio and pH) except total N significantly correlated with slope position. Principal component (PC) analysis showed soil properties to be divided into three groups which were PC1 (water content and extractable organic C and N), PC2 (C/N ratio and pH), and PC3 (total C and N). Regressions of soil N transformation pattern against PCs scores suggested that net N mineralization was mainly regulated by PC3, while net nitrification and percent nitrification were mainly regulated by PC1 and PC2. The scatter plots of percent nitrification and the first two PCs scores suggest that PC2 regulates the potentiality, and PC1 affects specially in the middle of the slope where the effect of PC2 was moderate.

Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (δ15N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ15N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ15N between soil N pools and plants, and by using an existing model. δ15N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO3– pool at two sites with different nitrification rates in a temperature forest in Japan. δ15N of plants was similar to that of soil NO3– in the high-nitrification site. Because of high foliar NRA and the large foliar NO3– pool at this site, we concluded that plant δ15N indicated a great reliance of plants on soil NO3– there. However, many δ15N of soil N overlapped each other at the other site, and δ15N could not provide definitive evidence of the N source. The existing model was verified by measured δ15N of soil inorganic N and it explained the variations of plant δ15N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ15N signatures from different ecosystems and to understand N dynamics.

Plant species effect on the spatial patterns of soil properties in the Mu-us desert ecosystem, Inner Mongolia, China

Although Artemisia ordosica Krasch. and Sabina vulgaris Ant. are the dominant shrub species in the Mu-us desert ecosystem, they differ in their botanical traits. We investigated the spatial patterns of soil properties using geostatistical analysis to examine the effect of plant species on these spatial patterns. Comparison among three microsite types (under A. ordosica, under S. vulgaris, and the opening between vegetation) showed that A. ordosica generally had less effect than S. vulgaris on local soil properties. The long life-span, prostrate life-form, and evergreen leaf-habit of S. vulgaris may lead to a higher accumulation of organic and fine materials under S. vulgaris. The range of spatial autocorrelation found in the mass of organic matter on the soil surface was smaller than that of the coverage of S. vulgaris (11.5 m) which corresponded to the canopy patch size of this species, and was longer than the canopy patch size of A. ordosica. The ranges of total C and N, and pH (11.7–15.6 m) were similar to that of S. vulgaris. The range of available P (106.3 m) was comparable to that of the coverage of A. ordosica (86.2 m) considered to be the scale of the distribution of this species. The ranges of silt+clay and exchangeable K, Ca, and Mg (31.0–46.7 m) were not related to plant presence, and were similar to that of topography (43.1 m).

Topographical differences in soil N transformation using 15N dilution method along a slope in a conifer plantation forest in Japan

Soil N transformation was investigated using15N dilution method along a slope on a conifer plantation forest. Although there was no significant difference in the net N mineralization rates by laboratory incubation, net nitrification rates increased downslope. Gross N transformation by15N dilution method showed a distinct difference not only on the rates, but also on the main process between the lower and the upper of the slope. Half of minelarized N was immobilized and the other half was left in NH4+ pool at the upper part of the slope, while all of mineralized N was used for immobilization or nitrification and NH4+ pool decreased at the lower of the slope. Soil N transformations were classified into two groups: one was shown below 773 m and the other was shown above 782 m. The incubation with nitrification inhibitor showed that nitrification was mainly conducted by autotrophs irrespective of the position of the slope. Microbial biomass and microbial C/N were similar among the sites. However, the gross mineralization rate was higher below 773 m than above 782 m under similar respiration rates. This suggests that the substrate quality may be one of the controlling factors for soil N transformation. Extractable organic C/N was similar to microbial C/N at the lower of the slope. It indicated that the substrate was more decomposable below 773 m. It is considered that soil N transformation is affected by topographical gradient of moisture and nutrient which makes plant growth and decomposition rate different.

Dynamics of the internal soil nitrogen cycles under moder and mull forest floor types on a slope in a Cryptomeria japonica D. Don plantation

To examine the influence of microbial carbon (C) availability on the internal soil nitrogen (N) cycles under moder and mull forest floor types within the same slope sequence, surface mineral soils (0–5 cm depth) taken at upper (moder-type forest floor) and lower (mull-type forest floor) positions on a slope in a Cryptomeria japonica D. Don plantation were incubated for 300 days. During the incubation, changes in net and gross N transformations, the organic C and N pools, and microbial respiration were monitored. Despite relatively small differences in net N mineralization in both soils, very rapid rates of gross N transformations were found in mull soil during the initial 15 days of the experiment. A rapid net nitrification occurred after days 150 and 100 in moder and mull soils, respectively, presumably because of decreased microbial C availability. However, a rapid net nitrification also occurred in the mull soil during the initial 15 days when microbial C availability was high, and gross nitrification was detected in both soils, except at day 0 in the moder soil. Changes in gross N transformations and in organic C and N pools over the experiment suggested that the start of rapid net nitrification might be influenced not only by microbial C availability, but also by the microbial availability of N relative to C.

Fire history influences on the spatial heterogeneity of soil nitrogen transformations in three adjacent stands in a dry tropical forest in Thailand

Spatial patterns of soil nitrogen (N) transformations were examined using geostatistical analysis in three adjacent stands with different fire history (0, 10 and 35 years since the latest fire, respectively) in a dry tropical forest in Thailand. A larger pool of total inorganic N and a faster rate of N mineralization were recorded in the stand with longer fire prevention. At the spatial scale analyzed, the proportion of spatially dependent variance to the total variance of N mineralization and nitrification increased from 0.39 to 0.73, and from 0.40 to 0.77, respectively, with the time since the latest fire. The spatial autocorrelation ranges of N mineralization and nitrification decreased from ≥9.0 to 3.28 m, and ≥9.0 to 2.77 m, respectively, with the time since the latest fire. These results suggested that fire history affected not only the level of available soil N, but also the spatial heterogeneity of soil N transformations, presumably due to the difference in plant influences on soil.

Fine root mass in relation to soil N supply in a cool temperate forest

Soil inorganic nitrogen supply and fine root mass in the top layers of mineral soil (0–5 and 5–10 cm) were investigated at upper and lower sites of a cool temperate forest where Fagus crenata and Quercus crispula dominate. At both sites, soil inorganic nitrogen supply was greatest in the 0–5 cm layer. The predominant forms of soil inorganic nitrogen supply were NH4 +-N at the upper site and NO3−-N at the lower site. Fine roots were concentrated in the 0–5 cm layer at the upper site, but not at the lower site. The form of supplied soil inorganic nitrogen supply can be important in determining the vertical distribution of fine roots.

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aimsPlant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.MethodsAt the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO3−-N) use traits, and compared the results with the soil traits.ResultsOur results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO3−-N. Pieris japonica lacks the capacity to use NO3−-N as a N source, but other species do use NO3−-N. Lindera triloba, whose distribution is unrelated to soil NO3−-N availability, changes the extent to which it uses NO3−-N in response to soil NO3−-N availability.ConclusionsOur results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO3−-N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.

The potential of NO3--N utilization by a woody shrub species Lindera triloba: a cultivation test to estimate the saturation point of soil NO3--N for plants.

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO3-N) supply were investigated to estimate the saturating point of available NO3-N for plant utilization. NO3-N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO3-N uptake and assimilation by L. triloba. Root NRA increased with NO3-N supply when concentrations were low and reached a plateau at high NO3-N concentrations. On the other hand, root NO3-N concentration increased linearly with NO3-N supply; therefore, it is suggested that NO3-N uptake did not limit NO3-N assimilation by L. triloba. In contrast, leaf NRA and leaf NO3-N concentration were low and were not influenced by NO3-N supply. This may be caused by the lack of transport of NO3-N from roots to leaves. The NO3-N retained in perlite was compared with NO3-N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO3-N availability to plants in the forest soil. The maximum NO3-N pool size in the forest soil was comparable to concentrations at which root NRA reached a plateau in perlite cultures. These results indicate that soil NO3-N availability is below the saturation point for NO3-N uptake by L. triloba, and it is the limiting factor of NO3-N utilization by L. trilobaunder field conditions in which this species naturally occurs.

Gross Soil N Transformations in a Coniferous Forest in Japan

Topographic influence on gross soil N transformation was investigated along a slope on a conifer plantation forest. The half of minelarized N was immobilized and the half was left in NH4+ pool at the upper part of the slope, while 93 % of minelarized N was used for immobilization or nitrification at the lower part of the slope. From the similarity of microbial biomass and microbial C/N, microbial flora was similar among the sites. However, the gross mineralization rate was four times faster below 15 m than above 30 m. It indicated that the substrate was decomposable below 15 m. At the upper part of the slope with Oa horizon, humified organic matter with high C/N would be resistible for microbes. It resulted in relatively slow N cycling.