Goro Iwatsubo

Intermittent denitrification: The application of a 15N natural abundance method to a forested ecosystem

Abstract The 15N natural abundance method was used to assess the intermittent occurrence of denitrification in the Kiryu watershed in Shiga Prefecture, Japan. The concentration and isotopic composition (° 15N) of NO3− −N, as well as some physical and chemical variables that potentially affect denitrification, were measured along the flow path from precipitation to stream water via soil solution and groundwater. High maximum groundwater level promoted a decrease in N03−−N concentration that was associated with an increase in the δ 15N of N03−−N with soil depth; i.e., denitrification occurred in the soil due to increasing soil moisture. While the maximum level of groundwater fell, no such changes were observed. Dissolved oxygen (DO) and Mn2+ concentrations in soil solutions indicated that strong anaerobic condition did not occur during the study period. These results suggested that denitrification was occurring temporarily in anaerobic microsites such as waterlogged soil aggregates. Upward expansion of groundwater zone thus appeared to play an important role in promoting such microanaerobic sites, resulting in the intermittent occurrence of denitrification. Based on these data, a schematic model for assessing denitrification was proposed based on a N03−−N/Cl− − δ 15S N of NO3−−N map for soil-water systems. Our data showed that variations of NO3−-N concentration and δ 15N value are useful indicators for elucidating nitrogen dynamics as affected by water mixing, plant uptake, nitrification, and denitrification in forested ecosystems.

Nitrogen saturation in Japanese forests: an evaluation

Abstract Nitrogen biogeochemistry of 24 forested sites in Japan was used in evaluating the status of ‘nitrogen saturation’ for this region. Bulk deposition of inorganic N ranged from 3.5 to 10.5 kg N ha −1 yr −1 and losses in drainage waters ranged from 0.6 to 28 kg N ha −1 yr −1 . Concentrations of NO 3 − in drainage waters during the growing season either remained fairly constant or increased during periods of high precipitation inputs. This pattern is markedly different than that exhibited for most forested watersheds in Europe and North America where during the growing season nitrate concentrations often reach their lowest values. These Japanese sites had high rates of N mineralization as a function of abundant moisture and warm temperatures. Nevertheless, most sites, except those with elevated atmospheric inputs of N, had high levels of nitrogen retention. The general absence of seasonal patterns of NO 3 − concentrations also suggests that this condition cannot be used to evaluate nitrogen saturation in Japanese forests as has been done for some watersheds in North America and Europe.

Topographic variations of soil properties and stand structure in a Pinus thunbergii plantation

Soil properties and above-and belowground forest structure were studied across various topographies in a 20-year-oldPinus thunbergii Parl. plantation on Mt Tanakami, Japan. The soil properties and stand structure varied greatly with slope position fromridge top to valley floor. Soil thickness, fine soil content and soil moisture content were greater in lower slope positions. The amount of organic carbon in the forest floor was greater in upper slope positions. The organic carbon content in the mineral soil was slightly greater in lower slope positions. These changes in soil properties suggested an upslope decrease in decomposition rate and water and/or nutrient availability. The aboveground structure ofP. thunbergii was more developed at lower slope positions. The mean stem diameter, height and volume ofP. thunbergii increased downslope with decreasing tree density. However, fine root biomass increased greatly upslope. This inverse relationship between tree height and fine root biomass indicated morphological plasticity ofP. thunbergii in exploiting environmental heterogeneity. Variations in soil-plant interactions in the stand along various topographies caused spatial heterogeneity in the accumulation pattern of organic matter in plants and the soil.

Nutrient-uptake and nutrient-use efficiency ofPinus thunbergii Parl. along a topographical gradient of soil nutrient availability

To examine responses of a plant species to nutrient availability, we investigated changes in soil nutrient availability, litterfall production and nutrient content in litterfall along a topographic gradient in aPinus thunbergii Parl. plantation. Responses were evaluated in terms of three efficiency indices: (i) nutrient-uptake efficiency (the ratio of nutrient return in litterfall to soil nutrient availability); (ii) nutrient-use efficiency (the ratio of litterfall mass to nutrient return in litterfall); and (iii) nutrient-response efficiency (the ratio of litterfall mass to soil nutrient availability). These indices can distinguish the ability of a species to acquire nutrients and its ability to use them in litterfall production. Nitrogen and phosphorus availabilities in soil were lower in upper slope positions. The three efficiencies were higher in upper slope positions and negatively correlated with soil nutrient availability for both nitrogen and phosphorus. An increase in nutrient-response efficiency was achieved by both increases in nutrient-uptake and nutrient-use efficiencies.

Vertical changes in soil solution chemistry in soil profiles under coniferous forest

Abstract Soil solution samples from six layers in a brown forest soil were collected regularly over a four year period in a Cryptomeria japonica D. Don plantation, Shiga prefecture, Japan. Samples were analyzed for inorganic N, organic N, total P, Cl − , SO 2− 4 , Ca 2+ , Mg 2+ and Na + . Inorganic N mainly consisted of NO − 3 in the soil solutions. NO − 3 , Ca 2+ and Mg 2+ were predominant in the soil solutions. over the study period. NO − 3 , Ca 2+ and Mg 2+ concentrations and fluxes showed a clear seasonal pattern with peaks in late autumn to early winter in the Ah horizon throughout the study period. Nitrification potentials in the six soil layers were estimated. Nitrification occurred mainly in the Ah horizon. Nitrification in the Ah horizon accounted for about 90% of the total NO − 3 production in the soil. In the Ah horizon, the temporal variations in NO − 3 concentrations and fluxes were attributed to the seasonal nitrification processes. While in the Bw horizon, the patterns were variable, and were attributed to the percolation processes of NO − 3 produced in the Ah horizon. NO − 3 fluxes from the Bw horizon were estimated and ranged from 81 to 186 kg N ha −1 yr −1 . The estimated high fluxes of NO − 3 reflected the fertile and nitrogen saturated soil conditions in the Cryptomeria plantation.

Extensive study on forest runoff water chemistry over east Asia

The chemistry of runoff waters from 13 forested watersheds in six regions (four regions from Japan, one from southern China and one from northern Thailand) was evaluated. The Cl− concentrations in runoff waters were higher in those watersheds which had closed canopies and were nearer to the ocean. The NO3− concentrations were higher in those watersheds having the developed soils and high moisture conditions, but were lower in tropical and subtropical regions for those watersheds which had high rates of nitrogen uptake and for watersheds with large areas of saturated soils. The SO42− concentrations were affected by SO42− adsorption properties of the soils: at Shibecha, Jiulianshan, and Chiang Mai with high adsorption capacities SO42− concentrations in streams were low. High SO42− concentrations were found at Mt. Hiei and Kagawa due to the weathering of sulfur minerals and high levels of atmospheric sulfur deposition. Within regions SO42− concentrations were inversely related to NO3− concentrations. A comparison among the watersheds suggested H+ consumption in deeper soil that increased pH and HCO3− concentrations of the runoff waters of some watersheds. Anion concentrations increased with low H.I. (Humidity Index) values suggesting that dry conditions increased concentrations due to high rates of evapotranspiration.

Effects of sewage-water spraying on mineral cycling in a forest ecosystem

Abstract Sewage-water spraying into a forest was carried out for about 3 years to evaluate the ability of the forest to remove nitrogen and phosphorus and the fertilizing effects of the spraying on a deteriorated pine forest. An increase in Cl− and NO3− concentrations in the runoff from the forest was detected. It took about 9 years after spraying ceased to reach the previous concentration level. However, the amount of total nitrogen output in the runoff over a 12 year period was about 5% of the total nitrogen input of 724 kg ha−1. Considerable ammonia volatilization could have taken place. The increase in the soil nitrogen mineralization rate in the sewage-sprayed forest plot continued even after spraying ceased. The increment of tree biomass was significantly larger than in the control forest plot. In the experimental plot, the excess nitrogen accumulation (with reference to that in the control plot) in tree biomass was 66% of the total sewage nitrogen input, 5% was lost in runoff, and the rest might have been lost by ammonia volatilization.

An ecological study of soils in the Highlands of western Tibet I. Soils along an iso-altitudinal transect across western Tibet

A variety of soil studies were carried out along a 920-km transect running SE to NW from near the border with northern Nepal, through the Western Tibet Highlands (soils sampled at six locations at an elevation of about 4200 m), to the Takla Makan Desert. This transect, which parallels the China-India border, covers a range of aridities. Annual precipitation decreased exponentially (from 169 mm to ca. 50 mm) north-westwards along the transect. The humidity index also decreased exponentially to the desert zone in the Western Tibet Highlands, and decreased further in the Takla Makan Desert because of the high temperature in the latter area. Vegetation changed fromCaragana thorn-bush steppe, throughArtemisia sparse steppe, to desert. Soil properties were studied in relation to the humidity index. Organic C, total N, cation exchange capacity, and very low C/N ratio values (a maximum of 12.8) of soils decreased with the decrease in humidity index. In contrast, carbonate, pH (KCl), and sulfate increased exponentially. Available P and ammonia content were inversely related to the abundance of carbonate, whereas nitrate increased where carbonate was abundant. Illite was the most abundant of the clay minerals, followed in order by kaolinite and montmorillonite.

An ecological study of soils in the Highlands of western Tibet. II: Vertical change from 3900 m to 5450 m in elevation

AbstractsTwo kinds of soil study were carried out in the south-western Tibet region of China. Changes in soil properties with elevation were examined from the snout of a glacier at 5450 m above sea level down to a village (Burang) at 3900 m. The area close to the glacier was devoid of vegetation, probably because of the coldness. With a decrease in elevation, the landscape changed from desert to sparseArtemisia steppe, and below 4750 m, to a thorn-bush steppe dominated byCaragana spp. which continued down to the village (where the annual precipitation was 169 mm). Organic C, total N and cation exchange capacity (CEC) all increased with decreasing elevation down to 4700 m due to the increase in warmth, but the parameters decreased below 4700 m, reflecting the increase in aridity. Almost no carbonate was detected in areas above 4700 m, but below this elevation it increased exponentially, also reflecting the increase of aridity. Available P tended to decrease with both the decrease in elevation and the increase in carbonate.The vertical distribution of soil parameters was investigated in the soil profiles of dry and wet plots at 4700 m. Organic C, total N, and CEC decreased with increasing soil depth. Their values were higher in the dry plot than in the wet plot. In the dry plot, carbonate increased with depth. Total P was found to be similar in both the wet and dry plots, but there was less available P in the soil of the dry plot, reflecting the abundance of calcium carbonate.