Phuy-Chhoy Vong

Effect of NO3− on the fate of 99TcO4− in the soil-plant system

Abstract This work was undertaken to study the effect of NO3− fertilization on the uptake of 99TcO4− by plants. Rye-grass (Lolium perenne) was grown in a growth chamber on a silty loam soil to which were added increasing quantities of ammonium nitrate (21, 50 and 100 mg N kg−1). Soil samples were then amended with 99TcO4− at a constant level of 29.6 kBq kg−1 dry weight. Total initial NO3− content was 43.3, 57.8 and 82.8 mg N kg−1, and total NH4+ content was 24.7, 39.2 and 64.2 mg N kg−1. Three cuts were made at 4-week intervals, and aerial biomass was analysed for total 99Tc and N. Results showed that total uptake of 99Tc was decreased by increasing mineral N additions at the first cut, as mineral nitrogen was not a limiting factor for plant growth at this stage. Technetium-99 content in plants varied from 64% of the total applied 99Tc with the lowest NO3− content to 31% with the highest. However, at the third cut, cumulative 99Tc uptake reached the same level for the three treatments (80–83%) with no further effect of initial N applications, as NO3− was depleted in the soil. Therefore, 99TcO4− was diluted in the pool of soil NO3− and was absorbed proportionally to nitrate by rye-grass.

Mobilization of labelled organic sulfur in rhizosphere of rape and barley and in non-rhizosphere soil

ABSTRACT Rape is known to require more sulfur (S) than is necessary for the secondary metabolite synthesis. It is hypothesized that its coarse root system harbors more homogenous microbial biomass specific for sulfur immobilization/remobilization. In a growth chamber experiment, this work examined the ability of rape and barley to take up the labile 35S (including 35S–SO4 2− and 35S-organic extracted by hot water). For that, the endogenous compounds of soil organic matter were previously labeled with 35S–SO4 2− for three months prior to plant growth. The use of “rhizobag”, a polyamide bag, easily allows the separation of rhizosphere from non-rhizosphere soil. In order to simulate the high input agrosystems, the soil received just before sowing, a unique dose of 76.9 mg N kg−1 soil as ammonium nitrate and three levels of S (20.4, 30.8, and 61.5 mg kg−1 soil) as MgSO4. Despite the dilutions made by levels of additional S–SO4 2−, the results showed an increase in 35S uptake by rape in contrary to barley, which showed a progressive decrease with increasing S–SO4 2− dilutions. The mean percentage values of total 35S taken up by rape were higher than barley (8.9, 22.2, and 28.2% at day 20, 42, and 56 after sowing vs. 3.7, 17.1, and 21.8% respectively). Correlated to 35S-uptake, significant coefficient was found with the 35S–SO4 2− (0.78, p<0.01), and 35S extracted by hot water at 70°C for 18 h in rape rhizosphere (r 2=0.75, p<0.05) but not in barley. Similarly, significant correlation coefficient was observed between soil arylsulfatase and root 35S uptake of rape (r 2=0.79, p<0.01) but not of barley. These results clearly showed the higher capacity of rape rhizosphere in regulating via the arylsulfatase activity the uptake of 35S-endogenous compounds.

Use of labeled sulphur‐35 for tracing sulphur transfers in developing pods of field‐grown oilseed rape

Abstract This study was conducted to better understand the dynamics of sulphur (S) transfer between pod walls and seeds of field‐grown oilseed rape by using sulphur (35S) as an investigative tool. Labeling experiments with 35S were carried out to determine the effects of nitrogen (N) and sulphur fertilization on these transfer mechanisms. Sixty‐four plants from field trials fertilized with 200 kg N ha‐1 in the forms of ammonium nitrate (AN) or urea (U), with or without 75 kg S ha‐1 in the forms of ammonium thiosulphate and MgSO4 were sampled. At 30, 43, 56, and 77 days after flowering (DAF), terminal racemes were cut and labeled with 35S‐SO4 2‐. After labeling, pods and seeds were separated into 3 groups according to their position on stem, and measurements of 35S levels were performed accordingly. This short‐term labeling experiment showed that the pod walls retained from 39 to 61% of labeled 35S, according to the different treatments, whereas seeds accumulated from only 1 to 16% of applied 35S. On avera...

Fate of nitrogen and sulphur as affected by the rhizosphere of oilseed rape and barley

Abstract Within plants, sulphur (S), and nitrogen (N) equilibrium is a requisite for their normal development. Pot experiments with oilseed rape and barley fertilized at different N to S ratios were carried out under glasshouse conditions by using the “rhizobag”; technique. The objective was to compare the induced‐influence of rhizosphere and non‐rhizosphere soil on N and S nutrition of the studied plants. Thus, SO4 2‐S, NC3 ‐‐N and NH4 +‐N concentrations, and total N and S taken up by the plants were examined. Barley increased the pH of rhizosphere soil whereas no real change of pH was observed with oilseed rape. Both plants took up all the NO3 ‐ present in the soil solution, but rapeseed took up greater quantities of NH4 +‐N and SO4 2‐ ‐S than barley. Moreover, the ratio values of N to S of the aerial parts of the rapeseed were significantly and positively correlated to those of soil available‐N to ‐S ratios while this correlation was significant but negative with barley. This indicated a clear‐cut different influence between the two rhizospheres which oppositely induce the N and S nutrition of the two plant species.

Incorporation of a nitrogen fertilizer in the humified compounds of a typic hapludalf

Abstract We have evaluated in this study the distribution of 15 N of ammonium sulfate in the humified compounds of a typic hapludalf. After 1 month of evolution under controlled conditions with and without plant, the average of 15 N amounts distributed in the following order : humins (51%), alkali-soluble fractions (29%), fractions extracted by ultra-sound (19%). The amount of 15 N was important in the fulvic acids. The rhizospheric effect induced by the presence of plant led to an increase of 15 N in the humin fraction μm . These results allowed us to outline the mechanisms of transfer of 15 N in different fractions of organic matter.

Evolution of soluble organic carbon in the rhizosphere and in corresponding non-rhizosphere soil in field-grown oilseed rape and barley. Immobilisation and turn-over of sulphur-35 in the rhizosphere soil

The water soluble carbon (WSC) is a key substrate of nutrient cycling in the plant-soil system. In this work, determinations of WSC amounts in the rhizosphere and non-rhizosphere soil of rape and barley grown on a calcareous soil were carried out from vegetative growth to maturity. Then, the same sampled soils were incubated under controlled conditions to compare the immobilisation and remineralisation of 35S in both compartments. The results show the same patterns of evolution of WSC in the rhizosphere as well as in the non-rhizosphere soil. The amounts of WSC observed at 45, 59 and 73 days after the first sampling were found to be significantly higher in rape than in the barley rhizosphere (444, 588 and 571 vs. 385, 481 and 462 mg C kg−1 soil respectively); whereas in the non-rhizosphere soil, no significant differences were recorded. After 36 days of soil incubation, barley rhizosphere soil immobilised significantly larger 35S than rape (77.5% vs. 40.5% of 35S added respectively). But, in contrast, the percentage value of 35S released by 3N H2SO4 hydrolysis expressed as percentage of 35S immobilised were about 1.5 times higher in rape than in the barley rhizosphere. The overall results underline the specific microbial biomass in the rape rhizosphere which is able to turnover larger freshly immobilised 35S when compared with barley.

Uptake of organic labelled sulphur by oilseed rape and barley in the rhizosphere and in corresponding non-rhizosphere soil

Spring rape and barley were grown in “rhizobag”, in a calcareous soil previously labelled with 35 S. Before sowing, the soil had received two levels of N and five levels of S. Three pot destructive samplings were made at 20, 42 and 56 days after sowing. Despite the dilutions made by levels of added S-SO 4 2, the results averaged over the three sampling dates showed a significant increase (p<0.05) in 35 S uptake by rape, in contrary to barley that showed a trend to decrease with increasing 35 S dilutions. Comparatively, rape rhizosphere was able to mobilise more 35 S organic extracted by hot water at 70°C as well as 35 S-labile] (35 S extracted by 0.01 M CaCl 2 + 35 S-hot water) than barley rhizosphere, especially at high levels of S addition. Significant linear correlation coefficients between 35 S-uptake and 35 S-hot water ( r2=0,72, p<0.01), and 35 S-labile (r 2 =0.75, p<0.01) were found in rape but not in barley rhizosphere. It was concluded that rape rhizosphere was endowed with specific microbial biomass that could regulate the uptake of organic 35 S when compared with barley. Thus, a high S demanding plant, such as rape, needs S adding to maintain its normal development