Hiroto Toda

Comparative Evaluation on Nitrogen Saturation of Forest Catchments in Japan and Northeastern United States

To analyze the differences in the status and processes of nitrogen saturation in Japan and northeastern United States, we examined the hydrobiogeochemistry of nitrogen of forested watersheds in these regions. Two distinct differences were found between watersheds in Japan compared with those in US. 1) In Japanese watersheds, marked decreases of NO3− concentration in surface waters during the summer growing season were not found and NO3− concentrations sometimes increased especially in the summer at nitrogen saturated sites. This contrast with watersheds in US where decreases in NO3− concentration during the summer are commonly observed except those watersheds in advanced stages of nitrogen saturation. These differences in NO3− concentration relationships can be attributed to climatic differences, with Japan having high precipitation and high discharge during the summer, while in many regions of North America lowest discharges are found in the summer. The climatic regime in Japan leads to high rates of mineralization and the rapid transport of NO3− to streams in summer. 2) Japanese watersheds, even those with high NO3− concentrations in surface waters, show little evidence of acidification. This is in contrast to sites in US where increased NO3− concentrations, especially during episodic events, result in surface water acidification.

Microbial denitrification dominates nitrate losses from forest ecosystems

Significance Nitrogen (N) losses from terrestrial ecosystems can occur as inert forms or heat-trapping greenhouse gases, and via nitrate (NO3−) leaching to drainage waters, which can contribute to eutrophication and anoxia in downstream ecosystems. Here, we use natural isotopes to demonstrate that microbial gaseous N production via denitrification is the dominant pathway of NO3− removal from forest ecosystems, with gaseous N losses that are up to ∼60-fold higher than those based on traditional techniques. Denitrification becomes less efficient compared with NO3− leaching in more N-polluted ecosystems, which has important implications for assessing the connections between terrestrial soils and downstream ecosystems under rising anthropogenic N deposition. Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3−) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3− leaching, pointing to widespread dominance of denitrification in removing NO3− from forest ecosystems across a range of conditions. Further, we report that the loss of NO3− to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Regional Comparison of Nitrogen Export to Japanese Forest Streams

Nitrogen (N) emissions in Asian countries are predicted to increase over the next several decades. An understanding of the mechanisms that control temporal and spatial fluctuation of N export to forest streams is important not only to quantify critical loads of N, N saturation status, and soil acidification N dynamics and budgets in Japanese forested watersheds is not clear due to the lack of regional comparative studies on stream N chemistry. To address the lack of comparative studies, we measured inorganic N (nitrate and ammonium) concentrations from June 2000 to May 2001 in streams in 18 experimental forests located throughout the Japanese archipelago and belonging to the Japanese Union of University Forests. N concentrations in stream water during base flow and high flow periods were monitored, and N mineralization potential in soil was measured using batch incubation experiments. Higher nitrate concentrations in stream water were present in central Japan, an area that receives high rates of atmospheric N deposition. In northern Japan, snowmelt resulted in increased nitrate concentrations in stream water. The potential net N mineralization rate was higher in surface soil than in subsurface soil, and the high potential for N mineralization in the surface soil partly contributed to the increase in nitrate concentration in stream water during a storm event. Regional differences in the atmospheric N deposition and seasonality of precipitation and high discharge are principal controls on the concentrations and variations of nitrates in stream water in forested watersheds of Japan.

Changes in nitrogen transformation in forest soil representing the climate gradient of the Japanese archipelago

Net nitrogen transformation was investigated under different climate conditions by soil transplantation and in situ incubation of forest surface soils using the resin-core method. Selected conditions were considered to reflect those of the natural climate gradient in the Japanese archipelago. Study sites were established in natural forests in northern Hokkaido (Uryu), northern Kanto (Kusaki), central Kinki (Kamigamo), and southern Kyushu (Takakuma), representing the northernmost to the southernmost island regions of Japan. Field experiments comparing soils incubated at “native” and “transplanted” sites were conducted from June 2008 to May 2009. Net production, accumulation, and leaching of soil ammonium (NH4+) and nitrate (NO3−) were measured at each of the sites during the growing season (June–October), the dormant season (November–May), and throughout the year. Net nitrate production was highest in Kusaki soil, especially during the growing season, whereas net ammonium production was highest in Uryu soil, the coldest site, especially during the dormant season. Net nitrate production increased significantly in soils transplanted to a warmer climate during the growing season. However, net ammonium production increased in soils transplanted to colder climates during the dormant season. These findings indicate that, with the exception of the infertile soil samples from Kamigamo, the range of natural climates in Japan has a significant effect on nitrogen availability in surface soil. In addition, the original characteristics of the nitrogen cycle of the surface soil from each native site were retained, even when marked changes in soil temperature (approximately 8°C) occurred after transplantation.

Microbial biomass and activity along a natural pH gradient in forest soils in a karst region of the upper Yangtze River, China

Mountain closure, considered an effective and economic measure for natural restoration of degraded forest ecosystems, has been widely carried out in the karst region of southwest China. The aim of this study was to evaluate microbial aspects of soil quality after mountain closure by analyzing soil microbial biomass, basal respiration, metabolic quotient, and relationships with basic chemical properties in Guizhou Province, a karst region of the upper Yangtze River. Soil quality was considered poor from the low levels of microbial biomass carbon (MBC), nitrogen (MBN), and microbial quotient (MBC/total C and MBN/total N), but high metabolic quotient (qCO2). Soil pH, showing marked variation from 4.1 to 7.9 in this karst region, was proved to significantly affect soil microbial biomass and activity. Soil microbial biomass, microbial quotient, and soil basal respiration declined significantly with decreasing soil pH, while qCO2 showed an apparently increasing, but not statistically significant, trend. The changes in microbial biomass and activity following the change in soil pH could possibly be because of a change in soil microbial composition, and more detailed research is necessary. Compared with soil pH, soil organic matter content was another, more important, factor that directly restricted microbial growth because of the serious loss as a result of disturbance. As a practical application based on microbial aspects, introduction of some N-fixing tree species may be an active and effective measure to improve soil fertility and thus to accelerate restoration of the forest ecosystem in the karst region.

Sustainable site productivity and nutrient management in a short rotation plantation of Gmelina arborea in East Kalimantan, Indonesia

AbstractSustainable Forest Management (SFM) requires information on plant growth and nutrient dynamics in forest ecosystems. To obtain fundamental information for SFM in short-rotation plantations in tropical regions, a serial study was conducted on: (i) land use changes and effects on soil chemical properties in tropical forestland, (ii) site index and nutrient dynamics in Gmelina arborea Roxb. (yemane), (iii) stand age and nutrient cycles in the first rotation of a yemane plantation on a moderately productive site, and (iv) biomass and N2-fixation of legume cover crops (LCC) for soil amelioration. Land use change from natural stands to plantations was linked to a decrease in electric conductivity (EC), cation exchange capacity (CEC), total carbon (T-C), carbon to nitrogen ratio (C/N ratio) and exchangeable ca. Three sites classified by productivity (poor, moderate and good) were examined using annual inventory data from 3 to 4% sampling intensity of yemane plantations in the Sebulu site, PT Sumalindo Lestari Jaya. The growth of yemane was very rapid during the first 6 years and was strongly influenced by site quality. The differences in average stand height between the site classes at 6 years were 3 m, representing stand volume differences of approximately 40 m3 ha−1. Whole-tree harvesting removed 50% of the aboveground biomass and nutrients. The good site had greater amounts of total phosphorus (T-P), total calcium (T-Ca), total magnesium (T-Mg), available P, and exchangeable Ca and Mg in the soil than moderate and poor sites. There were no significant differences in nutrient amounts in the soil at different stand ages (4, 6 and 8 years of age). The effects of site class on the topsoil characteristics were greater than those for land use change and stand age. Yemane plantations in tropical regions are supported more by nutrient cycling rate than by the amount of nutrient availability in soil. Nitrogen and phosphorus were the limiting nutrients for yemane growth and regeneration in East Kalimantan. Legumes can promote sustainable site productivities in short-rotation plantations in tropical forests by supplying N to the soil through N2-fixation and organic matter. Annual N2-fixation from the atmosphere by LCC was 10–60 kg N ha−1 yr−1. Application of fertilizer needs to be considered for development of yemane on very poor soils. Palabras clave: Cambios del use de la tierra, Cultivos de leguminosas, Plantación de corta rotación, Calidad de sitio, Manejo de nutrientes en bosques sustentables, Bosque tropical Resumen. Para un Manejo Forestal Sostenido (MFS) se necesita información acerca del crecimiento de las plantas y la dinámica de los nutrientes en los ecosistemas forestales. Para obtener información fundamental para el MFS en regiones tropicales de plantaciones de corta rotación, se condujeron estudios en serie acerca de (i) los cambios en la utilización de los terrenos y sus efectos en las propiedades químicas del suelo en terrenos forestales tropicales (ii) el índice de sitios y la dinámica de los nutrientes en la Gmelina arborea Roxb. (yemane) (iii) la edad de los rodales y el ciclo de los nutrientes en la primera rotación de una plantación de yemane en un sitio moderadamente productivo (iv) y estudios sobre la biomasa y fijación del N2 en cultivos con leguminosas para mejorar el suelo. El cambio en el uso de sitio bosque natural a plantaciones forestales, se relacionó con una disminución en la conductividad eléctrica, capacidad de intercambio de cationes, carbón total, tasa de carbón a nitrógeno (C/N) y el Ca intercambiable. Se examinaron tres sitios clasificados por su productividad (baja, moderada y buena), utilizando datos de un inventario anual con una intensidad de muestreo del 3-4 % de las plantaciones de yemane en el sitio Sebulu, de PT Sumalindo Lestari Jaya. El crecimiento del yemane durante los seis primeros años fue muy rápido y estuvo fuertemente influenciado por la calidad del suelo. Las diferencias en la altura promedio del rodal entre las distintas clases de sitios fue de 3 m, lo cual representa diferencias en el volumen del rodal de aproximadamente 40 m3 ha−1. La cosecha total de los árboles removió el 50% de la biomasa y de los nutrientes en la capa superior del suelo. En el sitio de buena calidad, el suelo tenía mayores cantidades de fósforo total (T-P), calcio total (T-Ca), magnesio total (T-Mg), fósforo disponible y calcio y magnesio intercambiables, que en los sitios de calidad baja o moderada. No se encontraron diferencias significativas en la cantidad de nutrientes en el suelo a las diferentes edades de los rodales (4, 6 y 8 años de edad). Los efectos de la clase de sitio en las características de la capa superficial del suelo fueron mayores que los efectos debido a los cambios de uso del terreno y por la edad del rodal. Las plantaciones de yemane en regiones tropicales, utilizan más los nutrientes reciclados que la cantidad de nutrientes disponibles en el suelo. El nitrógeno y el fósforo fueron los nutrientes limitantes en las plantaciones del oriente de Kalimantan. En plantaciones de corta rotación en bosques tropicales, las leguminosas pueden promover una productividad sostenida del sitio, al suministrarle N al suelo por medio de la fijación del N2 y de la materia orgánica. La fijación anual de N2 de la atmósfera en los cultivos con una cobertura de leguminosas fue del 10 a 60 kg N/ha/año. Para el desarrollo de plantaciones de yemane en suelos de muy baja calidad, se debe considerar la aplicación de fertilizantes.

Chemical Characteristics in Stemflow of Japanese Cedar in Japan

To clarify the characteristics in stemflow of Japanese cedar (Cryptomeria japonica), we conducted the annual and extensive observation. We examined the chemistry of bulk and wet deposition, throughfall and stemflow at 26 forested sites in June and September 1998. The each sampling site was broadly distributed in all over the Japanese archipelago. The stemflow pH of Japanese cedar was significantly lower (p<0.01) than precipitation and stemflow of broad-leaved species in both months. There were significant anion deficits in stemflow of Japanese cedar, suggesting that organic anions derived from plant sources play an important role in the stemflow acidity. Our results suggested that the strong stemflow acidity in Japanese cedar was derived from an internal biological characteristic rather than influences of external acidic deposition.

Seasonal changes and controlling factors of gross N transformation in an evergreen plantation forest in central Japan

We conducted a year-round measurement of gross N transformation rates using the 15N dilution method, and analyzed seasonal changes and the mechanisms regulating gross N transformation in the Kiryu Experimental Forest in central Japan. While soil microbial biomass C (SMB-C) decreased from the dormant to growing seasons at the organic (O) horizon, no significant trend was observed in SMB-N. This resulted in SMB-C/N being high in the dormant season and low in the growing season, and suggests that the microbial composition changed seasonally. No clear seasonal trend was found in gross NH4+ production rates at either the O or surface mineral soil horizons. In contrast, the NH4+ consumption rate varied seasonally, with high values in January and April during the dormant season and low values in July and October during the growing season. There was no clear trend in seasonal fluctuation of net NH4+ production rates. Gross NH4+ production and gross NH4+ consumption rates were 10 times greater than the gross nitrification rate. Almost all of the produced NH4+ was immobilized, indicating that N tightly cycles at this study site. Considered together with results of the gross N transformation rates, the dominance of high SMB-C/N microbes might stimulate immobilization in the dormant season. At this study site, the change in microbial composition likely influences gross N transformation through immobilization efficiency.

pH and substrate regulation of nitrogen and carbon dynamics in forest soils in a karst region of the upper Yangtze River basin, China

Nitrogen (N) is the most limiting nutrient for forest development. In this study, using a natural pH gradient, N and carbon (C) mineralization was investigated and the effects of soil pH as well as the total C and N contents and the soil C/N ratio were evaluated in forest soils after mountain closure in a karst region. The N availability was poor based on the low N mineralization potential (N0) and the low active fraction of soil total N (N0/total N), while high microbial decomposition activity was indicated by a high mineralization rate constant (kN). N0 was positively correlated with soil pH as well as the total C and N contents. Additionally, multiple regression analysis revealed that total CN (the product of the soil total C and total N contents) and the C/N ratio had more significant effects on N0 than soil pH. In contrast, the mineralization rate constants kN and kC were positively affected by soil pH. The results indicated that N availability was regulated by soil organic matter (SOM), while microbial activity was restricted by soil pH. Also, the lack of nitrification and the high C0/N0 ratio observed at soil pH <5.5 may be a strong indicator of alterations to the microbial composition prompted by severe soil acidification. Further research is required to determine the changes in soil microbial composition with the drop in soil pH and their effects on SOM decomposition and nutrient availabilities.

Changes in Soil Chemical and Physical Characteristics in Japanese Cypress (Chamaecyparis obtusa Endl.) Stands by Mixture of Deciduous Broad-leaved Trees in the Northern Kanto Region of Japan

In order to clarify the effects of a mixture of deciduous broad-leaved trees on soil fertility, we investigated litter biomass accumulation, mineral soil chemical and physical characteristics, characteristics of nitrogen mineralization, and the mutual relationships between them in Japanese cypress (Chamaecyparis obtusa) stands mixed with deciduous broad-leaved trees at different ratios (mixture ratio; MR = 0, 16, 33, 43, 100% by basal area) in the northern Kanto region of Japan. Litter biomass in the forest floor and mineral soil was 19.1 Mg ha−1 in MR 0% and decreased approximately 60 % in MR 33%, MR 43% and MR100%. The permeability at 0–5 cm soil depth in MR100% was twice as much as that in MR 0%. Increases in soil permeability were likely due to larger soil pores in the higher MR with much accumulated deciduous broad-leaves. At 0–5 cm soil depth, the differences in carbon concentration among the plots were not clear. On the other hand, carbon concentrations at 5–10 cm depth increased from 90 g kg−1 to 147 g kg−1 with increases in MR from 0% to 100%. Concentrations of exchangeable bases increased two to four times with increases in MR from 0 to 100% at 0–10 cm depth. Soil pH (H2O) generally increased with increases in MR at each depth. The rates of net nitrogen mineralization at 0–5 cm depthin vitro increased from 25 to 87 mg kg−1 2 weeks−1 with increases in MR from 0 to 100%. However, increases in nitrification with increases in MR were not clear compared with nitrogen mineralization. These results indicated that a mixture of deciduous broad-leaved trees in a Japanese cypress stand was effective in preventing soil fertility decline.