Françoise Lasserre-Joulin

Emergent effects of ground beetles size diversity on the strength of prey suppression

Determining how multiple predators provide better prey suppression is a key step towards developing conservation biological control strategies. While numerous previous studies have demonstrated that diverse predator assemblages can be more effective in controlling pest populations, others have shown that it is the presence or absence of competitively superior species that is critical to pest biological control (i.e. selection effect). The present study investigated how increasing ground beetle body size diversity increases prey suppression. A mesocosm experiment was conducted to compare invertebrate prey suppression between nine created ground beetle assemblages. Size diversity of these assemblages was manipulated according to three diversity levels: low, medium, and high diversity. Partitioning of the diversity effects revealed that increasing the ground beetle size diversity had no effect on the strength of prey suppression. The absence of an effect of ground beetle size diversity may be because of the absence of resource partitioning among different‐sized ground beetles. The amount and range of prey consumed increased with increasing ground beetle body size. Thus, prey suppression was strongly strengthened by the presence of large ground beetles in the assemblages. The present results suggest that for biological pest control, Agri‐managers should emphasise practices that promote the presence of large carabids. This is not only because promoting the presence of large carabids could be at least as effective as conserving a diverse ground beetle community, but also because large ground beetles are more vulnerable to environmental disturbances and to predation than ground beetles of the other size classes.

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.

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