Effects of glucose and rhizodeposits (with or without cysteine‐S) on immobilized‐ ³⁵ S, microbial biomass‐ ³⁵ S and arylsulphatase activity in a calcareous and an acid brown soil

Abstract

Summary Our aim was to study the effects of C (as glucose and artificial rhizodeposits) on S immobilization, in relation to microbial biomass‐S and soil arylsulphatase (ARS) activity, in contrasting soils (a calcareous and an acid brown soil). The glucose‐C and artificial rhizodeposit‐C with or without cysteine were added at six rates (0, 100, 200, 400, 600 and 800 mg kg −1 soil) to the two soils and then incubated with Na 2 35 SO 4 for 1 week prior to analysis. The percentages of 35 S immobilized increased when C as glucose and rhizodeposit (without cysteine) were added to both soils. With cysteine‐containing rhizodeposit, the percentages of 35 S immobilized remained relatively stable (23.5% to 29.9%) in the calcareous soil, but decreased in the acid brown soil (52.7% to 31.5%). For both soils, cysteine‐containing rhizodeposit additions showed no significant correlation between immobilized‐ 35 S and microbial biomass‐ 35 S, suggesting that microorganisms immobilized cysteine‐S preferentially instead of 35 S from the tracer (Na 2 35 SO 4 ). In the calcareous soil, a positive and significant correlation was found between ARS activity and microbial biomass‐ 35 S ( r = 0.85 , P < 0.05) when glucose was added. We also saw this correlation in the acid brown soil when rhizodeposit‐C without cysteine was added ( r = 0.90, P < 0.05). Accordingly, the results showed the presence of extracellular arylsulphatase activity of 48.7 mg p ‐nitrophenol kg −1 soil hour −1 in the calcareous soil and of 27.0 mg p ‐nitrophenol kg −1 soil hour −1 in the acid brown soil.