MICROBIAL-GROWTH AND SULFUR IMMOBILIZATION FOLLOWING THE INCORPORATION OF PLANT RESIDUES INTO SOIL

The interaction between microbial growth and S immobilization was inve stigated in an arable soil amended with oil-seed rape (young leaves) a nd barley straw (1% w/w). Initially, the rape decomposed more rapidly (40 vs 10% by day 5) and produced a larger microbial biomass (990 mug C g-1 soil) than the straw (710 mug C g-1 soil). The biomass in both o f the amended soils then decreased to amounts 30-50% higher than those in the unamended soil by day 35 and was maintained at these levels th roughout the 195 day incubation. Most of the rape-S (>80%) and straw-S (>60%) added to the soil was released as SO42--S or converted to biom ass-S in 5 days. By this time, the amount of S assimilated by the biom ass in the rape-amended soil was three times that found using straw. B iomass-S in both soils then decreased but remained twice as high in th e rape-amended soil over the period of 15 195 days. The biomass in the straw amended soil had a similar C:S (85-120:1) to that of the unamen ded soil but was narrower (40-50:1) in the rape-amended soil. By day 5 , SO4-S in both of the amended soils had increased significantly. The increase in SO42--S in the rape-amended soil was maintained over the 1 95 day incubation, suggesting that this S was available for plant upta ke. However, by day 15, a net immobilization of soil S by the biomass (25% of soil inorganic S) was found using straw. This immobilized S wa s retained by the biomass throughout the 195 day incubation and was, t herefore, unavailable for plant growth. This suggests that the incorpo ration of plant residues such as straw which contain low amounts of S may decrease the plant availability of soil S.