Immobilized-S, microbial biomass-S and soil arylsulfatase activity in the rhizosphere soil of rape and barley as affected by labile substrate C and N additions

Bulk and rhizosphere soil of rape and barley grown in a calcareous soil were pre-incubated for 7 days at 20 °C with Na235SO4 to partially label soil organic S. The soils were then incubated for 7 days more with increasing levels of two C sources as organic acids (succinic and malic acids) and as glucose (from 0 to 640 mg C kg−1 soil) with or without increasing levels of N (from 0 to 15 mg N kg−1 soil) in the form of ammonium nitrate, in order to mimic rhizodeposition inputs into soil. A second incubation experiment with a single highest dose of the used substrates was undertaken and two destructive soil samplings on days 17 and 35 were carried out. Both incubation experiments showed the intensities of S immobilization in the order: barley rhizosphere>rape rhizosphere>bulk soil. Glucose addition generated positive S priming effects in all studied soils after one week of incubation. Significant correlation coefficients were observed between immobilized-S and microbial biomass-S (r=0.95, p<0.001), arylsulfatase activity (ARS) and microbial biomass-S (r=0.65, p<0.05) on day 17 but not on day 35, whereas significant correlation coefficients were found between arylsulfatase activity and immobilized-S at both days 17 (r=0.79, p<0.01) and 35 (r=0.75, p<0.01). A marked decline of biomass-S noted in substrate-amended treatments at day 35 suggests a quick turnover of this compartment followed by its incorporation into the organic S. Finally, with organic acids high values of ARS per unit of biomass-S were recorded over the two studied dates in the rhizosphere soil of rape. It is concluded that the rhizosphere microbial biomass under rape exhibited more efficient arylsulfatase activity and hence greater turnover of organic S than that under the barley rhizosphere soil.