长江中游水稻土潜育作用的特点

The gleyization of representative paddy soils in the middle reaches of the Yangtze River was characterized, taking oxidation-reduction potential(Eh), the amount of active reducing substances and the forms of iron and manganese as the parameters. The Eh value was linearly related with the logarithm of the amount of active reducing substances, which was contributed by ferrous iron by 83% on an average. The degree of gleyization of different horizons was graded as ungleyed, slightly gleyed, mildly gleyed and gleyed. The Eh of the four grades was > 500, 300-500, 100-300 and < 100 mV, respectively, and the corresponding amount of active reducing substances was < 1, 1-7, 7-30 and > 30 mmol_c kg^-1, respectively. The amount of ferrous iron of the four grades was < 0.5, 0.5-5, 5-25 and > 25 mmol kg^-1, respectively. The extent of gleyization of a soil was classified as upper-gleyed, middle-gleyed and lower-gleyed, depending on whether the depth of the gley horizon was less than 30 cm, 30-60 cm or more than 60 cm.     

Iodine desorption from rice paddy soil

Laboratory experiments on the desorption phenomena of iodine from rice paddy soil under waterlogged conditions, with a special reference to soil redox potential (Eh) and pH, have been conducted. Radioiodine tracer (1251), added to the soil, was readily sorbed on it. At the beginning of the waterlogging, the iodine desorption was low. However, iodine was desorbed into soil solution with time. The iodine desorption was enhanced markedly by the addition of organic substances such as straw pieces and glucose to the soil. Cultivation of rice plants in soil also affected the iodine desorption, suggesting root exudates and/or root autolysis might be participating in the desorption process. Eh dropped considerably after soil was waterlogged due to microbial metabolisms. Particularly low Eh values were observed in soils with plants and also with added organic substances. A negative correlation was seen between the desorption and soil Eh. High desorption was frequently observed when the Eh dropped to about -100 mV or below. Due to the reducing conditions (low Eh) by waterlogging, iodine in soil was leached into the soil solution; consequently total iodine concentration in paddy soil was considerably lower than forest and upland field soils. These iodine desorption phenomena under anaerobic conditions should be considered in assessing transfer of the long-lived radioiodine (129I) in the environment, especially in rice fields and marshland.     

Growth, Net Photosynthetic Rate, Nutrient Status and Secondary Xylem Anatomical Characteristics of Fagus Crenata Seedlings Grown in Brown Forest Soil Acidified with H2SO4 Solution

Dry matter production, net photosynthetic rate, leaf nutrient status and trunk anatomical characteristics of Fagus crenata seedlings grown in brown forest soil acidified by adding H₂SO₄ solution were investigated. The soil acidification leaded to decreased (Ca+Mg+K)/Al molar ratio in the soil solution. Dry mass per plant of the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1 was significantly reduced compared with the control value at 0 mg·L^?1. When net photosynthetic rate was reduced in the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1, the carboxylation efficiency and maximum net photosynthetic rate at saturated CO₂-concentration were lower than the control values. The addition of H⁺ to the soil at 120 mg·L^?1iinduced a reduction in the concentration of Ca in the leaf. By contrast, the concentration of Al in the leaf was increased with increasing the amount of H⁺ added to the soil. The annual ring formed in the seedlings grown in the soil treated with H⁺ at 120 mg·L^?1 was significantly narrower than that at 0 (control), 10, 30, 60 or 90 mg·L^?1. Based on the results obtained in the present study, we conclude that Fagus crenata is relatively sensitive to a reduction in the (Ca+Mg+K)/Al molar ratio of soil solution compared with Picea abies.     

Hydrological controls on soil redox dynamics in a peat-based,restored wetland

Increasing areas of altered wetland are being restored by re-flooding the soil. Evidence in the literature indicates that this practice can induce the redox-mediated release of soil nutrients, thereby increasing the risk of diffuse water pollution. However, for the sake of improving wetland management decisions, there is a need for more detailed studies of the underlying relationship between the hydrological and redox dynamics that explain this risk; this is particularly the case in agricultural peatlands that are commonly targeted for the creation of lowland wet grassland. A 12-month field study was conducted to evaluate the relationship between hydrological fluctuations and soil redox potential (Eh) in a nutrient-rich peat field (32 g N kg^− 1 and 1100 mg P kg^− 1 in the surface 0–30 cm soil) that had been restored as lowland wet grassland from intensive arable production. Field tensiometers were installed at the 30-, 60- and 90-cm soil depths, and Pt electrodes at the 10-, 30-, 60- and 90-cm depths, for daily logging of soil water tension and Eh, respectively. The values for soil water tension displayed a strong negative relationship (P < 0.001) with monthly dip well observations of water table height. Calculations of soil water potential from the logged tension values were used, therefore, to provide a detailed profile of field water level and, together with precipitation data, explained some of the variation in Eh. For example, during the summer, alternating periods of aerobism (Eh > 330 mV) in the surface, 0–10 cm layer of peat coincided with intense precipitation events. Redox potential throughout the 30–100 cm profile also fluctuated seasonally; indeed, at all depths Eh displayed a strong, negative relationship (P < 0.001) with water table height over the 12-month study period. However, Eh throughout the 30–100 cm profile remained relatively low (< 230 mV), indicating permanently reduced conditions that are associated with denitrification and reductive dissolution of Fe-bound P. The implications of these processes in the N- and P-rich peat for wetland plant diversity and water quality are discussed.     

Behavior of Iodine in a Forest Plot,an Upland Field and a Paddy Field in the Upland Area of Tsukuba,Japan. Iodine Concentration in Precipitation,Irrigation Water,Ponding Water and Soil Water to a Depth of 2.5 m

In the course of a series of studies conducted to investigate the long-term behavior of ¹²⁹I (which has a half-life of 16 million years) in the environment, the concentration of stable iodine (¹²⁷I) in precipitation, irrigation water and soil water to a depth of 2.5 m in a forest plot, an upland field and a paddy field in the upland area of Tsukuba, Japan, was determined. In the forest plot, the mean iodine concentrations in soil water at all the depths ranged from 0.13 to 0.21 μg L^?1, about one-tenth of the values recorded in precipitation (weighted mean 2.1 μg L^?1). This finding suggests that the major part of iodine in precipitation was sorbed onto the surface soil horizon under oxidative conditions. In the upland field, the mean iodine concentration in soil water was 2.2 μg L^?1 at a depth of 0.2 m and it decreased to 0.34–0.44 μg L^?1 at a depth of 0.5 m or more; these concentrations were about one-fifth of that in precipitation. This suggested that the major part of the iodine derived from precipitation was sorbed onto the subsurface soil horizon (at depths between 0.2 and 0.5 m). In the paddy field, during the non-irrigation period, the mean iodine concentrations in soil water at all the depths ranged from 1.8 to 4.8 μg L^?1, almost the same values as those recorded in precipitation. During the irrigation period, the mean iodine concentrations at depths of 0.2 and 0.5 m were 18.8 and 16.7 μg L^?1, values higher than the 10.9 μg L^?1 value recorded in irrigation water and the 11.8 μg L^?1 value recorded in ponding water. However, at a depth of 1.0 m or more, the mean iodine concentrations in soil water rapidly decreased from 7.3 to 1.8 μg L^?1. These data suggested that a significant amount of iodine flowed out from the paddy field by surface runoff and a considerable amount of iodine that leached to a depth of 0.5 m was retained onto the mildly oxidative soil horizon (2Bw) that lay at depths between 0.5 and 1.0 m. At a depth of 2.5 m in the paddy field, the mean iodine concentration in soil water decreased to 1.8 μg L^?1, but this level was much higher than those in the forest plot and upland field at the same depth, which suggested that a significant amount of iodine had leached into the groundwater-bearing layer. There was a negative correlation (r=-0.889) between the E_h of soil and the iodine concentration in soil water (0.2 m depth) of the paddy field. Particularly, when the E_h of soil fell below approximately 150 mV, the iodine concentration rapidly increased to above 10μg L^?1. As for the chemical forms of iodine in precipitation, irrigation water, ponding water and soil water during the winter irrigation period in the paddy field with oxidative conditions, 58–82% of iodine consisted of IO^? ₃ and 17–42% of iodine consisted of I^?. In the soil water during the summer irrigation period in the paddy field under reductive conditions, 52–58% of iodine consisted of I^?, and 42–47% consisted of IO^? ₃.     

Comparison of the influence of tannic acid and selected low-molecular-weight organic acids on precipitation products of aluminum

The crystallization and surface properties of hydrolytic reaction products of Al precipitated in the presence of 1.0 · 10^?6M and 1.0 · 10^?4M tannic acid from systems at the initial Al concentrations of 1.1 · 10^?4M and 1.1 · 10^?3M and OH/Al molar ratios of 2.0 or 3.0 were examined.X-ray diffraction analyses show that the structural distortion within the hydrolytic precipitation products of Al increases with the molar ratio of tannic acid to Al. The non-crystalline to poorly ordered hydrolytic precipitation products of Al, including pseudoboehmite, are formed in the presence of tannic acid and are shown by transmission electron micrographs to be fine, shapeless and hollow colloids which are deformed and aggregated. In contrast to low molecular weight organic acids (p-hydroxybenzoic, aspartic, malic, and citric acids), the influence of tannic acid on the surface reactivities of the precipitation products of Al cannot be solely explained on the basis of its chemical affinity towards Al. In addition to complexation, the physical adsorption of the tannic acid initially present at 1.0 · 10^?6M apparently promotes structural distortion in the precipitation products and thereby increases the specific surface and the formation of active sites for surface charge development and for phosphate and Ca retention to an extent even greater than that observed with the more strongly complexing malic acid. On the other hand, when the concentration of tannic acid is increased to 1.0 · 10^?4M, the physical adsorption of the tannic acid, while enhancing Ca retention through its ability to ionize and provide negative charges, hampers phosphate retention by masking some of the active sites.     

Biodegradation kinetics of monosaccharides and their contribution to basal respiration in tropical forest soils

Low molecular weight (LMW) organic compounds in soil solution are easily biodegradable and could fuel respiration by soil microorganisms. Our main aim was to study the mineralization kinetics of monosaccharides using ¹⁴C-radiolabelled glucose. Based on these data and the soil solution concentrations of monosaccharides, we evaluated the contribution of monosaccharides to basal respiration for a variety of tropical forest soils. Further, the factors controlling the mineralization kinetics of monosaccharides were examined by comparing tropical and temperate forest soils. Monosaccharides comprised on average 5.2 to 47.7% of dissolved organic carbon in soil solution. Their kinetic parameters (V _max and K_M ), which were described by a single Michaelis-Menten equation, varied widely from 11 to 152?nmol?g^?1?h^?1 and 198 to 1294?µmol?L^?1 for tropical soils, and from 182 to 400?nmol?g^?1?h^?1 and 1277 to 3150?µmol?L^?1 for temperate soils, respectively. The values of V _max increased with increasing microbial biomass-C in tropical and temperate soils, while the K_M values had no correlations with soil biological or physicochemical properties. The positive correlation between V _max values and microbial biomass-C indicates that microbial biomass-C is an essential factor to regulate the V _max values in tropical and temperate forest soils. The biodegradation kinetics of monosaccharides indicate that the microbial capacity of monosaccharide mineralization far exceeds its rate at soil solution concentration. Monosaccharides in soil solution are rapidly mineralized, and their mean residence times in this study were very short (0.4–1.9?h) in tropical forests. The rates of monosaccharide mineralization at actual soil solution concentrations made up 22–118% of basal respiration. Probably because of the rapid and continuous production and consumption of monosaccharides, monosaccharide mineralization is shown to be a dominant fraction of basal respiration in tropical forest soils, as well as in temperate and boreal forest soils.     

Mercury concentrations in forest trees from Slovakia

The concentration of mercury has been determined in assimilation organs of forest trees from 10 main industrial regions of Slovakia, four mountain forests and one military area and compared with concentration of mercury from 1356 permanent monitoring plots of Slovakia. The mercury concentration ranges for 2 yr old needles of Picea abies Karst. were (in mg kg^?1): 1.249–4.402 (Rudnany iron ore mines), 0.013–0.749 (nine other industrial regions), 0.021–0.737 (four mountain forests) and 0.053–0.538 (military area). The mercury content in the soil (0–5 cm) from a mercury smelting plant ranged from 9.9 to 130 mg kg^?1, and the moss Pleurozium schreberi contained 3.8–9.1 mg kg^?1. The values obtained were compared with the available literature data.     

Behavior of iodine in a forest plot,an upland field and a paddy field in the upland area of Tsukuba,Japan: Seasonal variations in iodine concentration in precipitation and soil water and estimation of the annual iodine accumulative amount in soil horizons

Abstract

In the course of a series of studies conducted to investigate the long-term behavior of ¹²⁹I (which has a half-life of 16 million years) in the environment, seasonal variation in the concentration of stable iodine (¹²⁷I) in precipitation and soil water to a depth of 2.5 m in a forest plot, an upland field and a paddy field in the upland area of Tsukuba, Japan, were determined. Iodine concentration in precipitation tended to increase during the summer (high air temperature) season and low-rainfall period, and a positive high correlation was observed between annual rainfall and the annual amount of iodine supplied by precipitation. No seasonal variations in iodine concentration in soil water were observed at any depth in the forest plot and upland field unlike at shallow depths (0.2 and 0.5 m) in the paddy field. In the paddy field, from the beginning of summer irrigation, under flooding conditions, iodine concentration in soil water at shallow depths (0.2 and 0.5 m) continuously increased, and immediately before mid-summer (intermittent) drainage and drainage, the maximum iodine concentration (approximately 50 µg L^?1) and lowest E_h values (approximately ?150 to ?200 mV) were recorded. These high iodine concentration levels and low E_h values were ascribed to high air temperature (approximately > 25°C on average every 10 days) and the continuation of the groundwater level above the ground surface. As for the temporary winter irrigation period (mean daily air temperature 2?4°C), the iodine concentration was low (1.7–3.7 µg L^?1) at all depths, as was the case in the non-irrigation period. After mid-summer drainage, and drainage, the iodine concentration in soil water at depths of 0.2 and 0.5 m decreased drastically as the groundwater level decreased. The mean annual amount of iodine accumulated in the surface soil horizons (0–0.67 m) in the forest plot was estimated to be approximately 2.9 mg m^?2 (7.5 µg kg^?1 dry soil), which coincided with the mean annual amount of iodine supplied to the earth surface by precipitation. A mildly oxidative subsurface 2Bw horizon (0.60–0.89 m) in the paddy field was estimated to illuviate approximately 3.1 mg m^?2 (20 µg kg^?1 dry soil) of iodine annually by retaining iodine in the soil water percolated to this horizon.     

Nitrification mineralisation and inorganic-N uptakein evergreen forests of the central Himalayas

Nitrogen mineralisation and available nitrogen (NO₃^– + NH₄⁺) in two evergreen forests species, viz. Quercus leucotrichophora and Pinus roxburghii, were examined. The plant available N ranged from 7.7–35.8 μg·g^–1·m^–1 with maximum values in March and minimum in November. The trend for N-mineralisation was opposite to that of the size of the available N-pool. N-Mineralisation rates ranged from 1.7–30.3 μg·g^–1·m^–1 within an annual cycle. Inorganic-N uptake was calculated for each incubated period, and for an entire year showed that in an oak forest site, nitrate-N was the dominant form of mineral nitrogen taken up by plants from soil. However, in a chir pine forest, nitrate-N and ammonium-N are equally taken up by plants from the soil. In both oak and pine forest sites, the nitrate-N uptake was maximum in the month of July and ranged between 2.4–11 μg·g^–1·m^–1 in the pine forest site and from 0–25 μg·g^–1·m^–1 in the oak forest site. In addition, ammonium-N varied from 0–12 μg·g^–1·m^–1 in the pine forest site and from 1–20 μg·g^–1·m^–1 in the oak forest site. N-Mineralisation was greater in N-rich forests and was moisture (soil) dependent and inversely related to bulk density.     

Kinetics of the reaction between the hydroxyl radical and organic matter standards from the International Humic Substance Society

Purpose

The objective of this study was to evaluate the effect of the physicochemical properties of five dissolved organic matter (DOM) isolates on their reactivity with the hydroxyl radical (HO·) in water.

Materials and methods

Five DOM isolates were purchased from the International Humic Substance Society (IHSS). Weight average molecular weight (M _W) of these samples was quantified using size exclusion chromatography based on polyethylene glycols as reference standards. Functional group and elemental composition of the DOM samples were available from the IHSS website. Room temperature rate constants were measured using electron pulse radiolysis.

Results and discussion

Five IHSS standards were examined in this study: two soil organic and three aquatic organic matters. The composition varied from samples that had primarily aliphatic carbon (Pony Lake fulvic acid) to mostly aromatic carbon moieties (Elliot Soil humic acid). The M _W values of the five samples ranged from 2,400 to 4,100 Da, with an average value of 3,060 Da. Second-order reaction rate constants between DOM and HO· (k _DOM-HO·) were measured using thiocyanate competition kinetics, giving values ranging from 1.21 to 10.36?×?10⁸ M_C ^?1?s^?1. The k _DOM-HO· values were not found to correlate with either M _W or the aliphatic-aromatic carbon ratio, which is consistent with previous reports looking at natural organic matter (NOM), but is different to reports on size-fractionated (ultrafiltration through 15–1 kDa membranes) effluent organic matter (EfOM). We attribute this difference to the larger molecular weight distributions in size-fractionated EfOM compared to NOM.

Conclusions

The k _DOM-HO· values in this study ranged over a factor of 10, suggesting that hydroxyl radical reactivity does depend on the sample composition; however, no major correlation was found between the measured reactivity and bulk physicochemical properties of DOM.     

Factors controlling carbon dioxide and methane production in acid sulfate soils

Methane and C02 production in flooded acid sulfate soils of Thailand were governed primarily by soil oxidation-reduction potential (Eh) and pH. The critical Eh and pH levels at which CH₄ emission began was Eh-150 mV, and pH 6.1. Low soil pH limited soil reduction and subsequently CH₄ production. Soil respiration (C02 production) was influenced by Eh-pH levels and organic matter content. Soils with higher C02 production rates produced greater amounts of CH₄. Soil pH, however, was the dominant variable which influenced organic matter decomposition, low soil Eh conditions and subsequent CH₄ and CO₂ production. Curvilinear or log transformations of pH, Eh and organic matter content (OM) were used in explaining variables controlling CH₄ and CO₂ production; CH₄ = ?2.359 ? 0.0001 Eh + 2.047 pH ? 3.019 (In pH)² CO₂ = ?5210 ? 1.6 Eh + 3144 (In pH) + 1011 (In OM).     

Effect of lead on soil respiration and dehydrogenase activity

The effect of added Pb on the respiration and dehydrogenase activity of two sandy soils, a clay soil and a peat soil, (all with different physico-chemical properties), was studied.A concentration of 375 μg Pb· g^? inhibited the respiration of the sandy soil by ca. 15%, 1500 μg Pb· g^?ca. 50%. In the clay soil 1500 μg Pb· g^? caused a 15% reduction in respiration. The inhibition of respiration in the sandy soil was still ca. 30% 40 months after the addition of Pb. Respiration of the peat soil was not affected by even 7500 μg Pb· g^?.Dehydrogenase activity was affected by Pb in a similar way to soil respiration. In the sandy soil a considerable reduction occurred, while in the clay and peat soils dehydrogenase activity was not reduced.It was concluded, that a relationship exists between the inhibitory effects of Pb and the buffering capacity of the soil as expressed by its cation-exchange capacity. Because of these different effects of the same Pb concentration on the various soil types, no single value for the permitted concentration of lead pollution in soil could be established.     

Profile carbon and nutrient levels and management effect on soil quality indicators in the Mardi watershed of Nepal

Abstract

Soil organic carbon (SOC) and nutrient stocks in the soil profile (0–80 cm) in four dominant land uses [forest, upland maize and millet (Bari), irrigated rice (Khet), and grazed systems)] and 0–15 cm depth along elevation gradient 1000 to 3000 m, and aspects in the Mardi watershed were measured. Soil properties at 0–15 cm depth were also measured in undisturbed forest, forest with free grazed system, managed forest, and grassland to compare the soil quality index (SQI) of topsoils. The SOC and nutrient concentration decreased with increasing profile depth. The SOC and N contents in the 0–15 cm depth of forest soils were significantly greater than the corresponding depth in upland maize and millet, irrigated rice, and grazed systems. On the other hand, available P and K concentrations at the same depth were significantly greater in upland maize and millet compared to irrigated rice, grazed system, and forest land uses. The SOC and N stocks (0–15 cm) increased from agricultural land at the valley bottom at about 1000 m above mean sea level (a.s.l.) (24 and 3 Mg ha^?1) compared to undisturbed forest (74 and 5.9 Mg ha^?1) at 2600 m a.s.l, demonstrating the effects of cover and elevation. Both SOC and N stocks decreased sharply in grassland (54 and 4.5 Mg ha^?1) at elevations of 2600 to 2800 m a.s.l. compared with undisturbed forest. Above 2800 m a.s.l. the cover type changed from grass to coniferous forest, and the SOC and N stocks steadily increased at the summit level (3200 m a.s.l.) to 65 and 6.9 Mg ha^?1, respectively. Slope and aspect significantly affected SOC with the northwest aspect having significantly higher concentrations (46 g kg^?1) than other aspects. Similarly, SOC concentration at the lowest slope position (39 g kg^?1) was significantly higher than the middle or upper positions (25 and 13 g kg^?1). Integrated soil quality index (SQI) values varied from 0.17 to 0.69 for different land uses, being highest for undisturbed forest and lowest for irrigated rice. The SQI demonstrated the degradation status of land uses in the following ascending order: irrigated rice?>?grazed system?>?forest with free grazing?>?upland maize and millet?>?managed forest?>?grass land?>?undisturbed forest. The irrigated rice, grazed system, upland maize and millet, and freely grazed forestlands need immediate attention to minimize further deterioration of soil quality in these land uses.     

Proton Budget for a Japanese Cedar Forest Ecosystem

The proton budget for a Japanese cedar (Cryptomeria japonica) forest in Gunma Prefecture, Japan, was studied by estimating biogeochemical fluxes. The proton budgets were estimated for three individual compartments of the ecosystem: vegetation canopy, and the upper (O horizon + 0–10 cm) and lower (10–100 cm) soil layers. The dominant proton sources in the compartments were atmospheric deposition (1.2 kmol ha^?1 yr^?1), nitrification (5.1 kmol, ha^?1 yr^?1) and base-cation uptake by vegetation (8.0 kmol, ha^?1 yr^?1) respectively. These proton sources were neutralized almost completely within the individual compartments mainly by base-cation release from the canopy or the soil. The sum of internal proton sources was five times as large as that of external ones. Nitrogen input from the atmosphere was 2.2 kmol ha^?1 yr^?1, whereas its output from the lower soil layer was 3.9 kmol ha^?1 yr^?1, indicating that a net loss of nitrogen occurred in the ecosystem. However, this did not cause the acidification of soil leachates because of a sufficient release rate of base cations from the soil.     

Soil and tree response to P fertilization in a secondary tropical forest supported by an Oxisol

Mature tropical forests are considered to be P limited and to cycle P efficiently. Whether P limitations are significant in younger secondary tropical forests, however, remains largely unexplored. This study evaluated P limitation by observing the P fertilizer response of a naturally regenerated 24-year-old forest and its soil. In February 1999, six 20?×?20-m plots were established in secondary forest in the Brazilian Amazon. After 1?year of pre-treatment tree measurements, 50?kg?P?ha^?1 was applied in January 2000 and again in January 2001. Soil sorption of P was relatively low (～100?μg?g^?1) in the surface 0–20?cm while sorption increased to ～180?μg?g^?1 at 20–50?cm and approached ～500?μg?g^?1 for the 50- to 200-cm layers. Soil P in 0–10?cm, measured as sequentially extractable fractions (resin, HCO₃-Pi, NaOH-Pi, NaOH-Po, and 1?M HCl), increased shortly after fertilization and could account for nearly all the 50?kg?P?ha^?1 added at each date. During the following 6?years, soil P in fertilized plots declined in all pools other than resin P, and by June 2006, concentrations returned to pre-fertilization levels. Despite the increase in extractable P with fertilization, increased tree growth was not detected from stand age 25 to 31?years. It appears that during secondary forest succession at this site, the forest P cycle was conservative so as to maintain available P at a sufficient concentration to meet forest P demands.     

Methane and water soluble iron production under controlled soil pH and redox conditions

Abstract

When a soil is flooded, iron (Fe) reduction and methane (CH₄) production occurred in sequence as predicted by thermodynamics. The dissolution and precipitation of Fe reflected both soil pH and soil redox potential (Eh). The objective of our experiment was to determine both CH₄ production and Fe reduction as measured by Fe in solution in a flooded paddy soil over a wide range of closely controlled pH and Eh conditions. The greatest release of CH₄ gas occurred at neutral soil pH in combination with low soil redox potential (‐250 mV). Production of CH₄ decreased when soil pH was lowered in combination with an increase in the soil redox potential above ‐250 mV. Highest concentration of ferrous‐iron (Fe²⁺) under reducing conditions occurred when soil pH was lowered. Thus Fe reduction influenced CH4 formation in the flooded paddy soil. Results indicated that CH₄ production was inhibited by the process of ferric‐iron (Fe³⁺) reduction.     

Changes in soil organic carbon and nitrogen in an area of Andisol following afforestation with Japanese cedar and Hinoki cypress

Abstract

To determine the rates of increase in C and N stocks in the soil and organic layers following afforestation in Andisols, we measured C and N densities in the organic and soil layers at depths of 0–5, 5–15 and 15–30?cm, together with a chronosequence analysis of 4-year-old, 14-year-old and 23-year-old Japanese cedar (Cryptomeria japonica) and 4-year-old, 12-year-old and 25-year-old Hinoki cypress (Chamaecyparis obtusa) plantations. The short-term changes in C and N were confirmed by repeated sampling 5?years after the first sampling. Tree growth, biomass accumulation and organic layers were much greater in Japanese cedar than in Hinoki cypress plantations. Soil C density (kg?m^?3) increased and bulk density decreased with stand age in the surface layer (0–5?cm). The average soil C accumulation rate was 22.9?g?C?m^?2?year^?1 for Japanese cedar and 21.1?g?C?m^?2?year^?1 for Hinoki cypress. Repeated sampling showed that the rate of increase in C in the surface soil was relatively slow in young stands and that soil C density (kg?m^?3) in the subsurface soil did not change over a 5-year period. Although N accumulated in the tree biomass and organic layers, the soil N density (kg?m^?3) did not change after afforestation. Although the andic properties of the soil and differences in the planted species did not influence the rate of increase in soil C, soil C density was expected to increase to a concentration greater than 80?g?kg^?1, possibly because of the large C accumulation capacity of Andisols.     

Effects of Ozone and/or Excess Soil Nitrogen on Growth, Needle Gas Exchange Rates and Rubisco Contents of Pinus Densiflora Seedlings

The effects of ozone (O₃) and excess soil nitrogen (N), singly and in combination, on growth, needle gas exchange rates and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents of Pinus densiflora seedlings were investigated. One-year-old seedlings were grown in 1.5-L pots filled with brown forest soil with 3 levels of N supply (0, 100 or 300 mg N·L^?1 fresh soil volume). The seedlings were exposed to charcoal-filtered air or 60±5 nL·L^?1 O₃ (8 hours a day) in naturally-lit phytotrons for 173 days from 22 May to 11 November. The exposure to O₃ or high N supply to the soil caused a significant reduction in the dry weights of the seedlings. Although no significant interactive effects of O₃ and excess soil N were detected on the dry weight growth of the seedlings, the whole-plant dry weight of the O₃-exposed seedlings grown in the soil treated with 300 mg N·L^?1 was greatly reduced compared with the control value. Ozone reduced net photosynthetic rate at 350 µmol·mol^?1 CO₂ (A ₃₅₀ ), carboxylation efficiency (CE) of photosynthesis and Rubisco content without a significant change in the gaseous phase diffusive conductance to CO₂ (gs) of the needles. The excess soil N reduced the A ₃₅₀ , CE, gs and Rubisco content of the needles. These results suggest that the reduction in the dry weight growth of Pinus densiflora seedlings induced by the exposure to O₃ and/or excess soil N was caused by reduction in the net photosynthetic rate mainly due to the decrease of Rubisco quantity in the chloroplasts.