Immobilization and mobilization of labelled sulphur in relation to soil arylsulphatase activity in rhizosphere soil of field-grown rape,barley and fallow

During plant growth, rhizosphere soils from fallow, barley (Hordeum vulgare L. cv Esterel) and rape (Brassica napus L. cv Capitole) grown in a calcareous soil were sampled 5 times (every fortnight) from May to July 2001 at plant maturity. In order to estimate the impact of C derived from photosynthesis, the aerial parts of rape and barley in an area of 1 m² were cut off about 2 cm from the soil surface, and left a fortnight before each sampling. Both soil arylsulphatase activity and a 1-week immobilization of S fertilizer in the sampled soils were then measured. The immobilization of S fertilizer was higher in fallow, followed by barley and rape rhizosphere soil. A strong positive linear correlation (r ²=0.71, P<0.001) was found between soil arylsulphatase activity and S fertilizer immobilized. Conversely, the mobilization of endogenous organic ³⁵S (obtained after leaching free and adsorbed SO₄ ^2–-³⁵S by 0.009 M Ca(H₂PO₄)₂) in the rhizosphere soil of each plant cover pooled at the end of the 5 samplings and materialized by ryegrass (Lolium perenne L. cv Massa) ³⁵S uptake, was about 3 and 2 times higher, respectively, in rape and barley than in fallow rhizosphere soil. Accordingly, strong inverse polynomial relationships were observed between soil arylsulphatase activity and ³⁵S uptake by the whole plant (r ²=0.904, P<0.02) and roots (r ²=0.970, P<0.01) of ryegrass. Plant cuttings affected both the immobilization and mobilization of S. It is concluded that the turnover of S freshly immobilized in rape rhizosphere soil was relatively high. Therefore, rape as a preceding crop in the rotations may have a beneficial effect by increasing S availability on the succeeding crop.