Carbon substrate mineralization and sulphur limitation

The addition of large quantities of carbon substrate to S-deficient soils will cause S immobilization and may bring about S limitation of the microbial biomass, thus depressing the rate of C mineralization. Glucose, cellulose or starch were added to eight arable soils from the northeast of Scotland having a range of phosphate extractable-S values (1–12 μg S g−1 dry weight soil). All nutrients were added in excess except C or S. Carbon dioxide emission was monitored over 3 weeks during incubation at 25°C. In the soil with the lowest extractable-S, the rate of C mineralization was depressed as compared to the same soil amended with excess S. This effect decreased with increasing extractable-S and was undetectable in the soil with the highest extractable-S. The effect also tended to decrease with increasing total soil carbon. S demand during cellulose decomposition was approximately twice that during glucose decomposition, with that for starch being intermediate. The critical C-to-S ratios, the ratios above which C mineralization would be limited by S deficiency, were estimated to be 1110, <720 and 490 for glucose, starch and cellulose, respectively. The relationship between the response of cellulose mineralization to S additions and a test to assess the S status of soils is discussed.