Effects of glucose on sulphur dynamics in calcareous arable and fallow soils

The magnitude of microbial sulphur (S) immobilization as well as the synthesis of inducible enzymes such as arylsulphatase (ARS) are basically controlled by carbon (C) additions. In order to assess soil fertility after a period of fallow a better knowledge of S dynamics including S immobilization and ARS activities of a natural fallow soil (3.9% organic C) in comparison with an arable soil (2.3% organic C) is required. Thus, the objectives of this work were to (i) examine the effects of glucose-C additions, a representative labile organic C in the rhizosphere, on the intensities of S immobilization in relation with ARS activities and (ii) distinguish extra- and intra-cellular ARS activities induced by glucose-C applied to the two soils. The amounts of glucose-C were added at six rates (0, 125, 250, 500, 750 and 1000 mg kg-1 soil) to the studied soils after a 1-week soil pre-incubation at 25°C and then incubated one week more (to promote microbial S immobilization over S remineralization) with a Na235S04 solution (518 kBq kg-1 dry soil and 20 mg S kg-1 soil) prior to analysis. All soils received the same amount of 80 mg N kg soil as NaN03. Soil S042 -S content was extracted with 0.009 M Ca (H2P04)2. The use of 35S makes it possible to calculate the amount of fertilizer S immobilized. The ARS method used determines the total ARS which corresponds to the ARS located either outside the cells (extra-cellular ARS) or inside the cells (intra-cellular ARS). Recovery tests using 0.009 M Ca(H2P04)2 showed a 100% recovery of S042- -35S introduced in the sample and extracted immediately after addition. Therefore, physical immobilization (adsorption of S042- -35S onto soil particles) was excluded and the 35S immobilized corresponded exclusively to microbial immobilization. Glucose-C additions increased both microbial S immobilization and ARS activities. Related to the rate 0 (control), largest increase in immobilized-S was observed in the arable soil (300.7%) compared with the fallow soil (153.1%). In contrast, the ARS activity increased by 16.4% in the arable soil versus 32.1% in the fallow soil. These results indicate that glucose proportionately affected more the intensities of immobilized-S than those of ARS. We found positive and significant correlation coefficients of ARS activities with immobilized-S in the arable soil (r = 0.86, P < 0.05) and in the fallow soil (r = 0.83, P < 0.05). Accordingly, by extrapolating to zero immobilized-S (or microbial biomass-S), the results showed a presence of extra-cellular ARS activity of 38.7 mg jc-nitrophenol kg-1 soil h-1 in the arable soil and of 63.5 mg jc-nitrophenol kg-1 soil h-1 in the fallow soil. Without glucose-C (rate 0), values of 3.7 and 20.0 mg jc-nitrophenol kg-1 soil h-1 of intra-cellular ARS activity (difference between total ARS and extra-cellular ARS activity) were found in the arable and fallow soil, respectively. It was concluded that fallowing generated greater ARS activities. Fallow regime re-established soil fertility through increasing soil organic matter content and thereby enhancing the activities of extra- and intra-cellular ARS enzyme compared with the arable soil that is regularly disturbed by tillage, pesticide treatment and fertilization