水稻光合碳在植物-土壤系统中的分配及其对CO2升高和施氮的响应

采用¹³C-CO₂进行连续标记,研究水稻分蘖期和孕穗期光合碳在植株-土壤系统中的分配及其对大气CO₂浓度升高(800 μL·L^-1)和施氮(100 mg·kg^-1)的响应.结果表明: CO₂浓度升高显著提高分蘖期根系生物量和孕穗期地上部生物量,并使生物量根冠比在分蘖期增加,而在孕穗期减小.CO₂浓度升高条件下,施氮使水稻地上部分生物量增加,却显著降低了孕穗期水稻根系生物量.CO₂浓度升高使光合¹³C在孕穗期向土壤的输入显著增加,然而施肥并没有促进由CO₂浓度升高驱动的光合¹³C在土壤中的积累,而且还降低了土壤中的光合¹³C的分配比例.综上,CO₂浓度升高显著提高了稻田土壤光合碳输入,促进稻田有机碳周转;施氮促进了水稻地上部的生长,却降低了光合碳向地下的分配比例.     

水稻光合碳在植物-土壤系统中的分配及其对CO2升高和施氮的响应

采用¹³C-CO₂进行连续标记,研究水稻分蘖期和孕穗期光合碳在植株-土壤系统中的分配及其对大气CO₂浓度升高(800 μL·L^-1)和施氮(100 mg·kg^-1)的响应.结果表明: CO₂浓度升高显著提高分蘖期根系生物量和孕穗期地上部生物量,并使生物量根冠比在分蘖期增加,而在孕穗期减小.CO₂浓度升高条件下,施氮使水稻地上部分生物量增加,却显著降低了孕穗期水稻根系生物量.CO₂浓度升高使光合¹³C在孕穗期向土壤的输入显著增加,然而施肥并没有促进由CO₂浓度升高驱动的光合¹³C在土壤中的积累,而且还降低了土壤中的光合¹³C的分配比例.综上,CO₂浓度升高显著提高了稻田土壤光合碳输入,促进稻田有机碳周转;施氮促进了水稻地上部的生长,却降低了光合碳向地下的分配比例.     

Contribution of plant photosynthates to dissolved organic carbon in a flooded rice soil

Dissolved organic C (DOC) plays important roles in nutrient cycling and methane production in flooded rice ecosystem. The microcosm experiment was carried out to measure directly the contribution of photosynthates to DOC by using a ¹³C pulse-chase labeling technique. DOC was operationally divided into water-extractable organic C (WEOC) and salt-extractable organic C (SEOC) by successive extraction firstly with deionized water and then with 0.25?M K₂SO₄. Total WEOC increased with plant growth, whereas SEOC concentration did not change significantly over the growing season. About 0.037–0.36% (mean 0.16%) of the assimilated ¹³C was incorporated into WEOC immediately after ¹³CO₂ assimilation (Day 0), but only 0–0.025% (mean 0.01%) was incorporated into SEOC. At the end of the growing season, the ¹³C amounts of WEOC substantially decreased, while those of SEOC slightly increased. The estimated net plant C contribution was 21?mg?C?plant^?1 to WEOC and 6?mg?C?plant^?1 to SEOC, corresponding to 33.8% of total WEOC and 20.2% of total SEOC at the end of the growing season, respectively. The results suggest that the incorporation and decomposition of the photosynthesized C occurred rapidly in rice soil which significantly affected the WEOC dynamics, but SEOC appeared to be in equilibrium with the native soil organic matter, receiving less effect from the plant growth.     

Contribution of plant photosynthates to dissolved organic carbon in a flooded rice soil

Dissolved organic C (DOC) plays important roles in nutrient cycling and methane production in flooded rice ecosystem. The microcosm experiment was carried out to measure directly the contribution of photosynthates to DOC by using a ¹³C pulse-chase labeling technique. DOC was operationally divided into water-extractable organic C (WEOC) and salt-extractable organic C (SEOC) by successive extraction firstly with deionized water and then with 0.25 M K₂SO₄. Total WEOC increased with plant growth, whereas SEOC concentration did not change significantly over the growing season. About 0.037–0.36% (mean 0.16%) of the assimilated ¹³C was incorporated into WEOC immediately after ¹³CO₂ assimilation (Day 0), but only 0–0.025% (mean 0.01%) was incorporated into SEOC. At the end of the growing season, the ¹³C amounts of WEOC substantially decreased, while those of SEOC slightly increased. The estimated net plant C contribution was 21 mg C plant^–1 to WEOC and 6 mg C plant^–1 to SEOC, corresponding to 33.8% of total WEOC and 20.2% of total SEOC at the end of the growing season, respectively. The results suggest that the incorporation and decomposition of the photosynthesized C occurred rapidly in rice soil which significantly affected the WEOC dynamics, but SEOC appeared to be in equilibrium with the native soil organic matter, receiving less effect from the plant growth.     

Distribution of assimilated carbon in plants and rhizosphere soil of basket willow (Salix viminalis L.)

Willow is often used in bio-energy plantations for its potential to function as a renewable energy source, but knowledge about its effect on soil carbon dynamics is limited. Therefore, we investigated the temporal variation in carbon dynamics in willow, focusing on below-ground allocation and sequestration to soil carbon pools. Basket willow plants (Salix viminalis L.) in their second year of growth were grown in pots in a greenhouse. At five times during the plants growth, namely 0, 1, 2, 3 and 4 months after breaking winter dormancy, a subset of the plants were continuously labelled with ¹⁴CO₂ in an ESPAS growth chamber for 28 days. After the labelling, the plants were harvested and separated into leaves, first and second year stems and roots. The soil was analysed for total C and ¹⁴C content as well as soil microbial biomass. Immediately after breaking dormancy, carbon stored in the first year stems was relocated to developing roots and leaves. Almost half the newly assimilated C was used for leaf development the first month of growth, dropping to below 15% in the older plants. Within the second month of growth, secondary growth of the stem became the largest carbon sink in the system, and remained so for the older age classes. Between 31 and 41% of the recovered ¹⁴C was allocated to below-ground pools. While the fraction of assimilated ¹⁴C in roots and root+soil respiration did not vary with plant age, the amount allocated to soil and soil microbial biomass increased in the older plants, indicating an increasing rhizodeposition. The total amount of soil microbial biomass was 30% larger in the oldest age class than in an unplanted control soil. The results demonstrate a close linkage between photosynthesis and below-ground carbon dynamics. Up to 13% of the microbial biomass consisted of carbon assimilated by the willows within the past 4 weeks, up to 11% of the recovered ¹⁴C was found as soil organic matter.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon distribution in grass (Festuca pratensis L.) during regrowth after cutting—utilization of stored and newly assimilated carbon

Distribution of net assimilated C in meadow fescue (Fectuca pratensi L.) was followed before and after cutting of the shoots. Plants were continuously labelled in a growth chamber with ¹⁴C-labelled CO₂ in the atmosphere from seedling to cutting and with ¹³C-labelled CO₂ in the atmosphere during regrowth after the cutting. Labelled C, both ¹⁴C and ¹³C, was determined at the end of the two growth periods in shoots, crowns, roots, soil and rhizosphere respiration. Distribution of net assimilated C followed almost the same pattern at the end of the two growth periods, i.e. at the end of the ¹⁴C- and the ¹³C-labelling periods. Shoots retained 71–73% of net assimilated C while 9% was detected in the roots and 11–14% was released from the roots, determined as labelled C in soil and as rhizosphere respiration. At the end of the 2nd growth period, after cutting and regrowth, 21% of the residual plant ¹⁴C at cutting (¹⁴C in crowns and roots) was found in the new shoot biomass. A minor part of the residual plant ¹⁴C, 12%, was lost from the plants. The decreases in ¹⁴C in crowns and roots during the regrowth period suggest that ¹⁴C in both crowns and roots was translocated to new shoot tissue. Approximately half of the total root C at the end of the regrowth period after cutting was ¹³C-labelled C and thus represents new root growth. Root death after cutting could not be determined in this experiment, since the decline in root ¹⁴C during the regrowth period may also be assigned to root respiration, root exudation and translocation to the shoots. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Rice biomass production and carbon cycling in 13CO2 pulse-labeled microcosms with different soils under submerged conditions

Aims

Rice fields are an important source for the greenhouse gas methane. Plants play an essential role in carbon supply for soil microbiota, but the influence of the microbial community on carbon cycling is not well understood.

Methods

Microcosms were prepared using sand-vermiculite amended with different soils and sediments, and planted with rice. The microcosms at different growth stages were pulse-labeled with ¹³CO₂ followed by tracing ¹³C in plant, soil and atmospheric carbon pools and quantifying the abundance of methanogenic archaea in rhizosphere soil.

Results

Overall,?>85 % of the freshly assimilated carbon was allocated in aboveground plant biomass, approximately 10 % was translocated into the roots and?4, but emission of ¹³C-labeled CH₄ started immediately and ¹³C enrichment revealed that plant-derived carbon was an important source for methanogenesis. The results further demonstrated that carbon assimilation and translocation processes, microbial abundance and gas emission were not only affected by the plant growth stage, but also by the content and type of soil in which the rice plants grew.

Conclusions

The study illustrates the close ties between plant physiology, soil properties and microbial communities for carbon turnover and ecosystem functioning.     

Long‐term biochar application promotes rice productivity by regulating root dynamic development and reducing nitrogen leaching

Biochar is beneficial for improving soil quality and crop productivity. However, the long‐term effects of biochar addition on temporal dynamics of plant shoot and root growth, and the changes in soil properties and nitrogen (N) leaching are still obscure. Here, based on a long‐term (7 years) biochar field experiment with rice in northwest China, we investigated the effects of two biochar rates (0 and 9 t ha^?1 year^?1) and two N fertilizer rates (0 and 300 kg N ha^?1 year^?1) on shoot and root growth, root morphology, N leaching, and soil physicochemical properties. The results showed that both biochar and N fertilizer significantly promoted rice growth, with their interaction significant only in some cases. Both fertilizers enhanced rice shoot biomass and N accumulation in various growth stages as well as increased grain yield. Nitrogen fertilizer significantly promoted root growth regardless of biochar application. However, biochar application without N fertilizer increased root biomass and length during the whole growth period, except in the booting stage; biochar with N application promoted root growth at tillering, reduced root biomass but maintained root length with low root diameter and high specific root length during the jointing and booting stages, and then delayed root senescence in the grain filling stage. Long‐term applications of biochar and N fertilizer reduced 10%–12% bulk density of topsoil compared to the control treatment with no N fertilizer and no biochar. Long‐term biochar application also improved soil total organic carbon and concentrations of available N, phosphorus, and potassium. In addition, biochar and N fertilizer applied together significantly reduced nitrate and ammonium concentration in leachate at different soil depths. In conclusion, biochar could regulate root growth, root morphology, soil properties, and N leaching to increase rice N fertilizer‐use efficiency.     

Behaviour of photosynthetic products associated with growth and grain production in the rice plant

Summary A study of the behaviour of the photosynthetic products assimilated at different growth stages was conducted in the field and in the greenhouse using C¹⁴ tracer.In general, the assimilated carbon is translocated to and accumulates in the growing organs. The carbon assimilated at the maximum tiller number stage is distributed mostly to the lower leaves. The carbon assimilated at the booting stage is distributed mostly to the spikelet, certain leaf sheaths and culms. The carbon accumulated in the form of carbohydrates in the leaf sheaths and the culm before flowering is retranslocated to the panicle after flowering. However, because of the consumption by respiration, the efficiency of this type of carbohydrate in grain production is not very high. The carbon assimilated after flowering accumulated mostly and efficiently in the brown rice.The release of the assimilated carbon as CO₂ is most intense immediately after assimilation. Thirty-five to 60 per cent of the assimilated carbon is consumed through respiration under the conditions of this experiment. As the carbon, which is in the form of sugars, rapidly changes to other forms, and also is consumed by respiration, the consumption declines rapidly. The retention percentage of assimilated carbon decreases as mutual shading increases.The large proportion of carbon released through respiration indicates the importance of studies on the significance of respiration in relation to growth.A portion of the thesis for the Master of Science degree submitted by Mr. Shen Lian to the Graduate School, University of the Philippines, College of Agriculture.     

水稻-褐飞虱-拟水狼蛛食物链的定量研究

应用荧光物质稀土元素铕示踪法对水稻褐飞虱拟水狼蛛食物链进行了定量分析研究。所施荧光物质的三种浓度20 mg/L、50 mg/L和100 mg/L 均为有效浓度,对水稻生长无可见的影响,孕穗期、抽穗期、乳熟期和黄熟期四个生育期水稻、褐飞虱和拟水狼蛛的荧光物含量均随所施荧光物质浓度的增高而增高。褐飞虱对不同生育期水稻的取食量依次为抽穗期>孕穗期>乳熟期>黄熟期,分别为每克褐飞虱生物量在24 h内摄食水稻17.5910 g, 17.4510 g, 13.8290 g和8.7070 g。拟水狼蛛对褐飞虱的捕食量为乳熟期>孕穗期>抽穗期>黄熟期,分别为每克拟水狼蛛生物量24 h捕食褐飞虱3.6380 g, 3.0830 g, 3.0770和2.8000 g。根据田间调查数据换算为每头拟水狼蛛捕食褐飞虱在孕穗期、抽穗期、乳熟期和黄熟期分别为11头、11头、13头和10头。     

Methane and soil CO2 production from current‐season photosynthates in a rice paddy exposed to elevated CO2 concentration and soil temperature

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above‐ground assimilation and below‐ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free‐air CO₂ enrichment (FACE) fumigating depleted ¹³CO₂ in rice paddy resulted in a smaller ¹³C/¹²C ratio in plant‐assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO₂ and CH₄) into current‐season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil‐warming treatment nested within the CO₂ treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO₂ treatment, the FACE treatment decreased the ¹³C/¹²C ratio not only in the rice‐plant carbon but also in the soil CO₂ and CH₄. The estimated new C contribution to dissolved CO₂ was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle‐formation stage onwards. For CH₄, the contribution of new C was greater than for heterotrophic CO₂ production; ca. 40–60% of season‐total CH₄ production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH₄ production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH₄ emission through greater rhizodeposition.     

Fate and dynamics of recently fixed C in pasture plant–soil system under field conditions

The flow of photosynthetically fixed C from plants to selected soil C pools was studied after ¹³CO₂ pulse labeling of pasture plants under field conditions, dynamics of root-derived C in soil was assessed and turnover times of the soil C pools were estimated. The transport of the fixed C from shoots to the roots and into the soil was very fast. During 27 h, net C belowground allocation reached more than 10% of the fixed C and most of the C was already found in soil. Soil microbial biomass (C_MIC) was the major sink of the fixed C within soil C pools (ca 40–70% of soil ¹³C depending on sampling time). Significant amounts of ¹³C were also found in other labile soil C pools connected with microbial activity, in soluble organic C and C associated with microbial biomass (hot-water extract from the soil residue after chloroform fumigation-extraction) and the ¹³C dynamics of all these pools followed that of the shoots. When the labelling (2 h) finished, the fixed ¹³C was exponentially lost from the plant–soil system. The loss had two phases; the first rapid phase corresponded to the immediate respiration of ¹³C during the first 24 h and the second slower loss was attributable to the turnover of ¹³C assimilated in C_MIC. The corresponding turnover times for C_MIC were 1.1 days and 3.4 days respectively. Such short turnover times are comparable to those measured by growth kinetics after the substrate amendment in other studies, which indicates that microbial growth in the rhizosphere is probably not limited by substrate availability. Our results further confirmed the main role of the soil microbial community in the transformation of recently fixed C, short turnover time of the easily degradable C in the rhizosphere, and its negligible contribution to more stable soil C storage.     

Partitioning of CH4 and CO2 Production Originating from Rice Straw,Soil and Root Organic Carbon in Rice Microcosms

Flooded rice fields are an important source of the greenhouse gas CH₄. Possible carbon sources for CH₄ and CO₂ production in rice fields are soil organic matter (SOM), root organic carbon (ROC) and rice straw (RS), but partitioning of the flux between the different carbon sources is difficult. We conducted greenhouse experiments using soil microcosms planted with rice. The soil was amended with and without ¹³C-labeled RS, using two ¹³C-labeled RS treatments with equal RS (5 g kg⁻¹ soil) but different δ¹³C of RS. This procedure allowed to determine the carbon flux from each of the three sources (SOM, ROC, RS) by determining the δ¹³C of CH₄ and CO₂ in the different incubations and from the δ¹³C of RS. Partitioning of carbon flux indicated that the contribution of ROC to CH₄ production was 41% at tillering stage, increased with rice growth and was about 60% from the booting stage onwards. The contribution of ROC to CO₂ was 43% at tillering stage, increased to around 70% at booting stage and stayed relatively constant afterwards. The contribution of RS was determined to be in a range of 12–24% for CH₄ production and 11–31% for CO₂ production; while the contribution of SOM was calculated to be 23–35% for CH₄ production and 13–26% for CO₂ production. The results indicate that ROC was the major source of CH₄ though RS application greatly enhanced production and emission of CH₄ in rice field soil. Our results also suggest that data of CH₄ dissolved in rice field could be used as a proxy for the produced CH₄ after tillering stage.     

Dynamics and allocation of recently photo-assimilated carbon in an Inner Mongolia temperate steppe

Information on carbon (C) dynamics and allocation in plant–soil system is essential for understanding the terrestrial C cycle. Using a ¹³C pulse-labeling chamber (1 m × 1 m) technique, we carried out three separate experiments in an Inner Mongolia temperate steppe (Leymus chinensis–Stipa grandis–Cleistogenes squarrosa). The first experiment determined mainly the temporal variation of δ¹³C (‰) signatures over the chase period of 6–27 July in a fenced site. The second experiment compared the dynamics and allocation of recently assimilated C over 10–20 August between a fenced site and a grazed site. The third experiment measured the effect of N application on assimilated C fluxes over 26 August–4 September in a fenced site. The above- and below-ground partitionings of labeled ¹³C were found to vary with site, growth stage and management state. The labeled ¹³C in shoots was maximal during the first day after labeling and then declined, whereas it roughly increased in roots. There was the absence of significant variation in soil δ¹³C. In the fenced site, the labeled ¹³C partitioning to the shoots accounted for 24.4, 16.8 and 11.1% of initial additions by 10 days after the labelings on 6 July, 10 August and 26 August 2003, respectively. However, the percentage of recently assimilated C partitioning to the roots, about 22–23%, was almost unchanged throughout growing stages. In the grazed site, the labeled ¹³C of about 50% was respired, 13% was remained in the shoots, and 37% was translated to the roots; the corresponding percentages, for the fenced site with N, were approximately 60, 20 and 18%, respectively. This study suggests that carbon was rapidly and substantially cycled in the Inner Mongolia temperate steppe by means of photosynthesis and respirations. It appears that the grazing and the N application had significant effects on the dynamics and allocation of recently photo-assimilated C in the plant–soil system.     

Effect of rice plants on methane production and rhizospheric metabolism in paddy soil

In order to elucidate the effects of rice plants on CH₄ production, we conducted experiments with soil slurries and planted rice microcosms. Methane production in anoxic paddy soil slurries was stimulated by the addition of rice straw, of unsterile or autoclaved rice roots, and of the culture fluid in which rice plants had axenically been cultivated. The addition of these compounds also increased the concentrations of acetate and H₂, precursors of CH₄ production, in the soil. Planted compared to unplanted paddy soil microcosms exhibited lower porewater CH₄ concentrations but higher CH₄ emission rates. They also exhibited higher sulfate concentrations but similar nitrate concentrations. Concentrations of acetate, lactate and H₂ were not much different between planted and unplanted microcosms. Pulse labeling of rice plants with¹⁴CO₂ resulted during the next 5 days in transient accumulation of radioactive lactate, propionate and acetate, and after the second day of incubation in the emission of¹⁴CH₄. Most of the radioactivity (40–70%) was incorporated into the above-ground biomass of rice plants. However, during a total incubation of 16 days about 3–6% of the applied radioactivity was emitted as¹⁴CH₄, demonstrating that plant-derived carbon was metabolized and significantly contributed to CH₄ production. The sequence of the appearance of radioactive products and their specific radioactivities indicate that CH₄ was produced from root exudates by a microbial community consisting of fermenting and methanogenic bacteria.     

Fate of nitrogen applied as Azolla and blue-green algae (Cyanobacteria) in waterlogged rice soils—a15N tracer study

Summary ¹⁵N tracer was used to detect the extent to which nitrogen of appliedAzolla caroliniana, Anabaena variabilis andNostoc muscorum was available for assimilation by the growing rice plants in pots under 4 cm flood water for 60 days. The rate of release of nitrogen from the above biofertilizers, the amount of nitrogen remaining in the soils and the amount that was lost from the soils during this period were also examined. Previously¹⁵N-labelled biomass of Azolla, Anabaena and Nostoc to provide 40 mg N was mixed thoroughly with 0.5 kg silt loam Bangladesh soil (Sonatola series) in each of three pots used for a single treatment. Each pot received four 16 days old IR8 rice seedlings. A parallet set of experiments was conducted without rice plants.It was found that nitrogen uptake in the rice plants was increased by 91, 176 and 215% on using Azolla, Anabaena and Nostoc which resulted in increased total dry matter yields (shoot plus root) of 74, 105 and 125%, respectively. Of the total¹⁵N applied at the start, 26, 49 and 53% was released from Azolla, Anabaena and Nostoc; about 7, 14 and 13% was lost by denitrification and 74, 51 and 47% remained in the soils as the undecomposed part of the biofertilizers, respeciively, after 60 days. Of 15.76, 22.72 and 25.92 mg N assimilated by the rice plants, 48, 61 and 62% was supplied by Azolla, Anabaena and Nostoc, respectively. The rest was obtained from the soil used.In the absence of the rice plants 30, 43 and 45% of applied¹⁵N of Azolla, Anabaena and Nostoc was released, respectively, in 60 days of which 93–96% was lost as N₂ through denitrification.     

Allantoin involved in species interactions with rice and other organisms in paddy soil

Allantoin (5-ureidohydantoin) plays an essential role in the assimilation, metabolism, transport, and storage of nitrogen in numerous higher plants, but its ecological implications are largely unknown. In this study allantoin was found in tissues of 11 rice (Oryza sativa) varieties tested, and its structure was characterised by X-ray diffraction analysis to confirm the fact that allantoin was actually obtained from the rice plants. Furthermore, the endogenous allantoin was exuded from the rice roots into the rhizosphere soils and had a great diversity of biological effects on associated weeds and microbes by soil interactions once released. However, allantoin levels in tissues or soils could not be distinguished between the allelopathic and non-allelopathic rice varieties. Field experiments showed that levels of allantoin released from rice varieties varied with their growth stages and reached the maximal levels at the stem elongation or panicle initiation to booting stages and then decreased dramatically. Allantoin could significantly stimulate the germination and growth of Echinochloa crus-galli and populations of soil bacteria and actinomycetes at selected test concentrations (30–500 μg/g), but had no effect on soil fungi. The half-life (t _1/2 ) of allantoin in autoclaved soil (20.2 ± 2.5 h, r ² = 0.95) was almost three-times longer than in non-autoclaved soil (7.3 ± 1.9 h, r ² = 0.92), indicating that rapid biodegradation or transformation of allantoin occurs in paddy soil. The results suggest that not only may allantoin play a role in the transport and storage of nitrogen in rice tissues but it may also participate in species interactions between rice and other organisms in paddy soil.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers