The significance of microbial biomass sulphur in soil

The soil microbial biomass S fraction of total organic S in soil is considered to be relatively labile and the most active S pool for S turnover in soil. Its significance has been demonstrated in studies of S deficiency in agronomic situations and in those of S pollution from high atmospheric inputs. The utility of the CHCl₃ fumigation-extraction technique for the measurement of microbial S has been proved for a range of soils and conditions. The various methodologies currently available are discussed, including the need for determination of the conversion (K _s) factor. Microbial S values, summarized from the available literature, ranged from 3 to 300 g S g^-1 dry weight soil. They were generally greater in grassland than in arable systems, though the greatest values were obtained in the few examples from forest and peatland soil systems. Microbial S values showed direct relationships with both microbial C and with total soil organic S. Again, there were significant differences between arable and grassland systems. The effect of factors such as organic and inorganic inputs as well as soil physical conditions on microbial S are described. Microbial S turnover rates were estimated from seasonal, ³⁵S-labelling and modelling studies. These rates varied between an approximately annual turnover rate in undisturbed soils up to 80 year^-1 following the addition of readily available substrates. Prospective future research areas are also outlined.