Distribution of 14C between particle size fractions and carbohydrates separated from a peat incubated with 14C-glycine

Samples of fresh Sphagnum peat from a raised bog were amended with ¹⁴C-labelled glycine. The distribution of ¹⁴C between particle size fractions obtained by wet sieving (sieve sizes 1, 0.5, 0.25, 0.15 and 0.05mm) was determined immediately on control (unincubated) samples and after 1, 6 and 12 months incubation at 10°C. The recovery of glycine in solution was almost 100%. During the incubation with ¹⁴C-glycine, ¹⁴CO₂ was released within the first 20 weeks, equivalent to 51.5% of the added ¹⁴C, but thereafter very little ¹⁴CO₂ was evolved. After 26 weeks a substantial amount of ¹⁴C was distributed amongst all the fractions, but the greatest incorporation (4.47%) occurred in the finest fraction (0.005–0.05 mm). Labelling of the other particle size fractions was <2.3% of added ¹⁴C. Carbohydrate accounted for 23% of ¹⁴C in the finest fraction and the sugars, rhamnose, arabinose, xylose, mannose, galactose and glucose all became labelled. Rhamnose showed the greatest, and arabinose, galactose and xylose the least, increase in specific activity; glucose and mannose had intermediate values. It was concluded that the finest fraction in peat contains a significant proportion of the microbially-synthesized material.     

Mean residence time of soil organic matter associated with kaolinite and smectite

To gain insight into the effect of clay mineralogy on the turnover of organic matter, we analysed the ¹⁴C activity of soil organic matter associated with clay in soils dominated by kaolinite and smectite in natural savanna systems in seven countries. Assuming that carbon inputs and outputs are in equilibrium in such soils, we took the ¹⁴C age as mean residence time of the organic matter. We corrected the ¹⁴C activity for the Suess effect, Bomb effect and difference between date of sampling and date of ¹⁴C measurement. Organic matter associated with kaolinite turned over fast (360 years on average). Organic matter associated with smectite turned over relatively slowly, with an average mean residence time for the whole clay-size fraction of 1100 years. Multiple linear regression indicates that clay mineralogy is the main factor explaining differences in the mean residence time of the organic matter extracted.     

Changes in the N Recovery Process from 15N-Labeled Swine Manure Compost and Rice Bran in Direct-Seeded Rice by Simultaneous Application of Cattle Manure Compost

The N recovery from ¹⁵N-labeled swine manure compost and rice bran with or without simultaneous application of unlabeled cattle manure compost was examined in a paddy field with direct-seeded rice during a 1-year period (1 crop season). In all the ¹⁵N-labeled materials including (¹⁵NH₄)₂SO₄, the processes of N recovery from the ¹⁵N materials by rice plants were different between the plots with and without application of cattle manure compost. At the tillering stage, the N recovery rates from the ¹⁵N materials in the plots with application of cattle manure compost were significantly lower than those in the plots without application of cattle manure compost. These recovery rates, however, became close and no significant differences were observed at the maturity stage. Thus, simultaneous application of cattle manure compost could impede the N recovery from swine manure compost, rice bran as well as (NH₄)₂SO₄.     

The influence of earthworms and cranefly larvae on the decomposition of uniformly 14C labelled plant material in soil

Amounts of the individual carbohydrate components derived from uniformly ¹⁴Clabelled grass added to soil were monitored in incubations lasting for up to 2 years. Decomposition was enhanced by the presence of the earthworms. Allolobophora caliginosa and Lumbricus rubellus and the cranefly larvae Tipula paludosa. After 28 d larvae had increased the loss of ¹⁴C-arabinose by 8.8% (compared to soil without larvae), -xylose by 15%, -non cellulose glucose by 5.3% and -cellulose glucose by 12.5%. Earthworms had increased the loss of ¹⁴C-xylose by 5%, -galactose by 21% and -total glucose by 11% after 6 months, and by another 29%, 14% and 8% respectively after 12 months. The losses of ¹⁴C, initially about 60% from soil with tipulid larvae after 1 month, and 50% from soil with earthworms after 3 months, became progressively less. Total ¹⁴C losses in the presence of earthworms were 66% after 12 months and 60% after 2 years, respectively. Increased decomposition is considered to be the result of the mixing of soil and substrate by the invertebrates, rather than an effect of their digestive capabilities.     

Use of agricultural by-products to study the pH effects in an acid tea garden soil

The amelioration of an acid Alfisol from a tea garden was studied by incorporating various plant materials: canola straw, wheat straw, rice straw, corn straw, soybean straw, peanut straw, faba bean straw, Chinese milk vetch shoot and pea straw prior to incubation for a maximum of 65 days. Soil pH increased after incubation with all the incorporated materials with the legumes causing the largest increases. The final soil pH was correlated with ash alkalinity ( r ² = 0.73), base cations ( r ² = 0.74) and N content ( r ² = 0.93) of the applied materials. It was assumed that the incubation released the base cations in plant materials as they decomposed which ultimately increased the base cation saturation of the soil. Similarly, soil exchangeable Al was also decreased with the incorporation of the legume plant materials and corn straw and rice straw. Our investigation demonstrated that legumes are the preferred choice for controlling the soil acidity and also for reducing the toxicity of Al in acid soils.     

Short-term kinetics of residual wheat straw C and N under field conditions: characterization by 13C15N tracing and soil particle size fractionation

Summary A clear understanding of the short-term decomposition and fate of crop residues is necessary to predict the availability of mineral N in soil. The fate of ¹³ C¹⁵N-labelled wheat straw in a silty soil (Typic Hapludalf) was studied using particle size fractionation and in situ incubation in which the equivalent of 8 t dry matter per ha of straw was incorporated into the soil over 574 days. Soil samples were separated into five particle-size fractions by wet sieving after disruption of aggregates. The weight, C and N contents, and ¹³C and ¹⁵N atom excess of each fraction were determined. Straw-derived C disappeared rapidly from the > 2000-μm fraction with an estimated half-life of 53 'normalized' days (equivalent of 10°C and −0−01 MPA water potential). Straw-derived C appeared to be only temporarily stored in the intermediate fractions (1000–2000 and 200–1000 pm). The maximum net ¹³C accumulation in the 50–200-μm fraction was 4·4% of added ¹³C. Straw-derived C accumulated most rapidly and preferentially in the 50-μm fraction, which stabilized after 265 days and accounted for 70% of the residual ¹³C on day 574. Although there was more residual ¹⁵N than ¹³C, the distributions and kinetics of the two isotopes in the fractions were similar.     

Real-time imaging of nitrogen fixation in an intact soybean plant with nodules using 13N-labeled nitrogen gas

Real-time images of nitrogen fixation in an intact nodule of hydroponically cultured soybean ( Glycine max [L] Merr.) were obtained. In the present study, we developed a rapid method to produce and purify ¹³N-labeled radioactive nitrogen gas (half life: 9.97 min). ¹³N was produced from a ¹⁶O (p, α) ¹³N nuclear reaction. The target chamber was filled with CO₂ and irradiated for 10 min with protons at an energy of 18.3 MeV and an electric current of 5 μA, which was delivered from a cyclotron. All CO₂ in the collected gas was absorbed and removed with powdered soda-lime in a syringe and replaced with helium gas. The resulting gas was injected into gas chromatography and separated and a 35 mL fraction, including the peak of [¹³N]-nitrogen gas, was collected by monitoring the chromatogram. The obtained gas was mixed with 10 mL of O₂ and 5 mL of N₂ and used in the tracer experiment. The tracer gas was fed into the underground part of intact nodulated soybean plants and serial images of the distribution of ¹³N were obtained non-invasively using a positron-emitting tracer imaging system (PETIS). The rates of nitrogen fixation of the six test plants were estimated to be 0.17 ± 0.10 μmol N₂ h⁻¹ from the PETIS image data. The decreasing rates of assimilated nitrogen were also estimated to be 0.012 ± 0.011 μmol N₂ h⁻¹. In conclusion, we successfully observed nitrogen fixation in soybean plants with nodules non-invasively and quantitatively using [¹³N]N₂ and PETIS.     

Analysis of Potassium Uptake by Rice Roots Treated with Aluminum Using a Positron Emitting Nuclide, 38K

We investigate the effect of Al on K⁺ uptake by rice roots. Potassium-38 (³⁸K), a positron emitting nuclide (the half-life: 7.61 min), was used to trace K⁺ behavior. When a rice root was treated with 10μM Al for 24 h, the uptake of ³⁸K in the root was increased in the range of 1 to 2 cm from the root tip compared with that of the control sample. Because the root continued to grow without showing any damage of plasma membrane during the Al treatment, it was suggested that the ³⁸K uptake was not occurred through diffusion into the cells. The uptake of ³⁸K in both treatments, with/without Al, was decreased by VO₄^3- (inhibitor of H⁺-ATPase on plasma membrane) and DNP (H⁺ ionophore) treatment, which suggested that the K⁺ uptake was performed through an active transport, such as H⁺:K⁺ transport or H⁺ gradient promoted by an Al treatment.     

Kinetic Parameters of Gross N Mineralization of Peat Soils as Related to the Composition of Soil Organic Matter

Peat land has been considered as an alternative type of land for agricultural development especially in the tropics. In the present study, the N-supplying capacity, one of the most important soil properties in terms of crop production, of peat soils was examined. Ten peat soil samples were collected from Indonesia, Malaysia, and Japan. Gross N mineralization in the soil samples was estimated using a zero-order model, and kinetic parameters of mineralization were determined using a simple type model. Soil organic matter composition was investigated using ¹³C CPMAS NMR. Mineralization potential ( N ₀), apparent activation energy ( E _a), and mineralization rate constant ( k ) ranged between 571–2,445 mg kg⁻¹, 281–8,181 J mol⁻¹, and 0.009–0.020 d⁻¹, respectively. Although none of the parameters showed a significant correlation with the soil C/N ratio, a negative correlation was observed between the k value and the ratio of the proportion of alkyl C in total C to that of O -alkyl C estimated by ¹³C CPMAS NMR. The latter suggested that the k values were higher in the peat soils relatively rich in readily decomposable organic matter including carbohydrates.     

The fate at several time intervals of 15N-labelled ammonium nitrate applied to an established grass sward

¹⁵N-labelled ammonium nitrate solution was applied in late April to circular, enclosed micro-plots prepared by pressing open-ended polypropylene cylinders into an established sward of perennial ryegrass. Cylinders were removed from the ground at intervals between 2 and 370 days after the application and assessments made of the distribution of ¹⁵N in plant and soil components. Of the added labelled N, 54.7% was recovered in the herbage which was cut four times during the growing season and again at the final sampling date. After two days, 37% of the labelled N was recovered in the soil microbial biomass. Large fluctuations occurred in the amount of ¹⁵N recovered in the soil microbial biomass over the next 14 days suggesting that rapid cycling of ¹⁵N occurred between this fraction and the mineral N fraction. After the first cut in late May, translocation of¹⁵N occurred more slowly from the roots into the stubble than from stubble into new herbage, so that the amount in the stubble declined more rapidly than did that in the roots. During the winter, there was no net transfer of N from the roots to above-ground components of the sward. By the end of the growing season, half the ¹⁵N remaining in the sward was immobilized in the humified fraction of the soil organic matter; some of this was mineralized in the following spring.     

Recovery of 15N-labelled fertilizer in crops, drainage water and soil using monolith lysimeters in south-east Nigeria

Labelled urea was applied to monolith lysimeters in the 1st year of a 2-year experiment at Onne in south-east Nigeria. On eight lysimeters maize and rice were grown in each of the 2 years. Four lysimeters were similarly cropped in the 2nd year after being uncropped in the 1st year. Measurements were made over the 2-year period of labelled and unlabelled mineral nitrogen in the drainage water, and labelled and unlabelled nitrogen in the crops. At the end of the experiment, weeds and the soil were also analysed for labelled and unlabelled nitrogen.
The total recovery of ¹⁵N in crop, soil and leachate varied between 70 and 93%. It was lowest when applied to the second season rice crop, which recovered only 15%, and highest when it was leached in the 1st year or was taken up by the maize crop. The highest crop uptake was 31%. The results indicate that, depending on the treatment, between 10 and 30% of the ¹⁵N was immobilized in the soil, lysimeters cropped in the 1st year lost between 22 and 29% of the ¹⁵N in drainage water, and between 7 and 30% was lost by denitrification. The accuracy of these figures is discussed.     

Decomposition in a peaty soil improved for pastoral agriculture

Abstract. The rates of CO₂ production and decomposition of ¹³C-enriched Lolium perenne leaves and roots in soil from the surface five cm of two upland stagnohumic gley soils were measured in laboratory experiments. One of the soils had been limed (pH 6.8) 13 years earlier. The other was unlimed (pH 3.7). Liming increased the rate of CO₂ release from soil to which no L. perenne had been added. About 30% of the ¹³C in L. perenne leaves remained in both limed and unlimed soil after 224 days. By contrast, less ¹³C-remained in the limed soil amended with L. perenne roots (44%) than in the limed soils (55%). Although the daily rate of CO₂ from the plant material-amended soils was initially greater in the improved than in the unimproved soil, it subsequently declined more rapidly.     

Studies on the rate of decomposition of plant residues in soil by following the changes in sugar components

Uniformly ¹⁴C labelled straw and ryegrass leaf were incubated in soil in the proportions 0.5 and 1% w/w, respectively at 12°C for various periods up to 5 years.
Samples were hydrolysed with 2.5 m H₂SO₄ under reflux for 20 min to release non-cellulosic sugars and subsequently with 12 M/0.5 m H₂SO₄ to release glucose from cellulose-like polymers. Total radioactivity was determined in whole soil and hydrolysates and in individual sugars.
Fitting the data to simple exponential equations, total and 2.5 m H₂SO₄ hydrolysate radioactivity were best expressed by three part exponential functions, whereas radioactivity in 12M/0.5M H₂SO₄ hydrolysates and sugars were best expressed by two part functions, except for arabinose from the straw, for which a three-part function was better.
A comparison of the fit was made with that using the single-term exponential expressions devised by Janssen, 1984 and Weibull, 1951 and with a modification of the two part exponential expression by the substitution of one term by the Weibull expression. Apart from the Weibull expression on its own, there was no significant difference between the expressions.
It is suggested that, even for well defined carbohydrate components of the substrate, the decomposability changes with time because of their increasing inaccessibility to microorganisms, rather than that several components are present each with a different constant rate of decomposition.     

Microbial Immobilization and Plant Uptake of Different N Forms in Three Forest Types in Shikoku District, Southern Japan

Soil microbial immobilization and plant uptake of N were evaluated for three forest types in Kochi, Shikoku district. During 196-d laboratory incubation, soil NO₃-N production in the Hinoki cypress forest was negligible for the initial 40 d and then rapidly increased, whereas NO₃-N production was rapid from the beginning in Japanese cedar and deciduous hardwood forests. Microbial immobilization of the labeled ¹⁵N decreased in the order of NH₄-N>glycine-N>NO₃-N. The ¹⁵N immobilization was higher for soil in the Hinoki cypress forest than other two soils. The delayed NO₃-N production in the Hinoki cypress forest was likely related with low availability of NH₄-N due to NH₄-N immobilization and substantial NO₃-N immobilization. In the field experiment, ¹⁵N uptake by roots decreased in the order of NH₄-N>NO₃-N>glycine-N. The absorption of the labeled ¹³C suggested direct uptake of organic N. The preference of N forms by root uptake was not different among forest types. Trees in three forest types can absorb inorganic and organic forms of N, suggesting trees absorb the N form that is the most abundant in the soil.     

Effect of no-tillage on turnover of organic matter in a Rhodic Ferralsol

Abstract. Soil organic matter (SOM) is considered to be key to sustainability of agriculture in the tropics. In southern Brazil, no-tillage has been adopted widely to control soil erosion, but its impact on the dynamics of SOM is not well established. We measured soil carbon (C) and δ¹³C in two crop rotations, one of which contained C4 maize ( Zea mays L.), after 21 years of contrasting tillage (conventional tillage versus no-tillage). Adjacent sites that reflected historic land-uses were also sampled. In the tillage experiment there was no effect of tillage on the total amount of C in the 0–40 cm profile (even when contrasting bulk density was accounted for), and the concentration of C differed only in the 0–5 cm and 5–10 cm layers. However, the occasional input of C4 material in the maize rotation resulted in a significant effect of rotation on δ¹³C ( P <0.001). Using ¹³C as a tracer for the SOM formed since the start of the experiment, we estimated the abundance of 'recent' and 'old' C within each depth interval. We found the main effect of tillage was to increase the medium-term turnover of SOM, particularly in the subsoil (i.e. below 20 cm depth). Compared with no-tillage, there was almost five times more recent C in the subsoil, and 20% more recent C in the 0–40 cm profile as a whole.     

Fixation and transfer of nitrogen by white clover to ryegrass

Abstract. ¹⁵N₂ was used in a sealed controlled environment chamber to investigate the transfer of fixed nitrogen from white clover to perennial ryegrass growing in soil in pots. There was no difference in the ¹⁵N content of roots and shoots of clover plants after exposure to ¹⁵N. No labelled fixed nitrogen was detected in ryegrass plants growing with the clover plants for a period of 129 days. There was therefore no evidence of rapid direct transfer (excretion) of fixed nitrogen from clover to ryegrass.     

Quantitative Modeling of Photoassimilate Flow in an Intact Plant Using the Positron Emitting Tracer Imaging System (PETIS)

The photoassimilate flow in an intact plant stem was imaged in real-time and its dynamics was quantitatively described using the Positron Emitting Tracer Imaging System (PETIS). Radioactive ¹¹CO₂ was fed to a leaf of an intact broad bean ( Vicia faba L.) plant, together with air containing an ambient concentration of non-radioactive carrier CO₂ gas. Movies of flow of the ¹¹C-labeled photoassimilates in the plant body were captured with PETIS. Here we demonstrate that the average flow speeds and the distribution ratios of photoassimilates in the respective nodes and internodes of the observed stem can be estimated by the transfer function analysis, one of the mathematical modeling methods. We also estimated the changes in the spatial distribution of the average flow speeds in the same stem when the fed leaf was exposed to enriched carrier CO₂ gas.     

A new method for evaluating symplastic cadmium absorption in the roots of Solanum melongena using enriched isotopes 113Cd and 114Cd

Radioisotope techniques are well known as methods for evaluating symplastic ion absorption in roots. In the present study, a new method for evaluating symplastic cadmium (Cd) absorption in plant roots was developed using the enriched isotopes ¹¹³Cd and ¹¹⁴Cd. Seedlings of Solanum melongena were exposed to an enriched isotope solution of ¹¹³Cd at 25°C for 30 min. The roots were excised from each seedling and were then immersed in a cold buffer solution without Cd at 2°C for 120 min to suppress the metabolic activity of the roots. Finally, the roots were treated with a cold buffer solution containing enriched stable isotope ¹¹⁴Cd at 2°C for 120 min, whereby the apoplastically bound ¹¹³Cd was desorbed. We tested the validity of our method for evaluating symplastic Cd in roots compared with the conventional method based on differences in the amount of Cd absorbed at 2°C and 25°C using unlabeled Cd. There was no difference in the symplastic Cd content of the roots between the two methods. These results indicate that it is possible to evaluate the symplastic Cd content in roots using the enriched isotopes ¹¹³Cd and ¹¹⁴Cd.     

Effect of Oxidizing Power of Roots on Iodine Uptake by Rice Plants

Although iodine is harmful to plants, rice plants ( Oryza sativa L.) absorbed iodine more selectively than bromine. To explain this selective absorption, the authors proposed the following hypothesis based on the fact that the standard redox potential for (I₂+ 2e = 2I⁻) is lower than that for (Br₂+ 2e = 2Br⁻) and (Fe³⁺+ e = Fe²⁺), and the roots of rice plants are able to oxidize ferrous ion (Fe²⁺) into ferric ion (Fe³⁺), namely rice plants oxidize iodide ion (I⁻) to form molecular iodine (I₂) via the oxidizing power of their roots, and absorb the molecular iodine formed more selectively than iodide ion. Bromine, by contrast, is absorbed by rice plants only in the form of ion (Br⁻). According to this hypothesis, there should be a significant correlation between the oxidizing power of the rice roots and the amount of iodine absorbed. Therefore, the relationship between the oxidizing power of the roots and the concentration of iodine absorbed was studied in a water culture using 8 varieties of rice plants. Rice seedlings, 14 d after germination, were cultured in a solution containing 1 mg L⁻¹ each of iodide and bromide ions for 3 d. The oxidizing power of the rice roots was evaluated based on the amount of 1-naphthylamine oxidized by the roots. A significant correlation (0.78, n = 16, 0.1% significant level) was found between the oxidizing power and the concentration of iodine absorbed by the roots. However, no relationship was found between the oxidizing power of the roots and the amount of bromine absorbed.     

Role of soil redox in growth and fertilizer nitrogen uptake by rice in Thailand paddy soils

Abstract. In a laboratory study, ¹⁵N ammonium fertilizer uptake and rice growth was determined in a non-acid sulphate marine soil (Typic Tropaquept) and an acid sulphate soil (Sulfic Tropaquept). Acid sulphate sensitive (IR 26) and acid sulphate tolerant (IR 46) rice varieties were grown in soil suspensions incubated at four Eh levels (+500, +250, +50, and -150 mV) in microcosms for three weeks. The results showed that rice grown in non-acid sulphate marine soils gave slightly better dry matter weight of 1.8g/pot, greater ¹⁵N uptake of 12.8 mg N/pot, and higher total N uptake of 38.4 mg N/pot than under acid sulphate soil conditions indicating the non-acid marine soil is more favourable to rice culture. Growth as measured by weight of dry matter was significantly reduced from 2.1g/pot under oxidized condition (+500 mV) to 0.8g/pot under highly reduced condition (-150 mV). N uptake by rice was significantly reduced from 16.9 mg/pot at + 500 mV to 4.5 mg N/pot at -150 mV Total N uptake also decreased with decreasing Eh. Growth, ¹⁵N uptake and total N uptake by acid sulphate tolerant rice, IR 46 were significantly higher than the acid sulphate sensitive rice variety, IR 26. Under highly reduced soil conditions (-150 mV), growing rice in acid sulphate soil would require additions of lime, intermittent irrigation and/or mid season drainage in order to increase soil redox potential and remove toxic substances.