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 (r2=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 SO42−-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 (r2=0.904, P<0.02) and roots (r2=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.