Stoichiometry of the soil microbial biomass in response to amendments with varying C/N/P/S ratios

The interacting effects of N, P, and S limitation were investigated by applying four different organic components, i.e., cysteine, chitosan, glucose-6-phosphate, and glucose, to a soil at two different clearly defined N, P, and S levels in a 5-fold range, one with sufficient and one with limited nutrient supply. Initially, MB-CN, MB-CP, and MB-CS ratios were lower after organic substrate amendments with the higher concentration of N, P, and S. The close relationship between the nutrient supply and elemental MB ratios was strongly modified within the next 14days for the MB-CN ratio, probably due to a strong shift in the microbial community composition towards fungi, determined by the ergosterol content of soil. This shift was promoted by high N and low P and S availability, contrasting the view that S is important for the formation of fungal biomass. However, the negative interactions between P limitation and MB-CS ratio suggest that the microbial S metabolism has specific importance under P-limiting conditions. Low substrate CN ratio increased carbon use efficiency (CUE) by 20% in comparison with high substrate CN ratio, calculated at day 5, solely due to an increased formation of microbial residues, as the formation of MBC was not affected by differences in substrate CN ratio. In contrast, high substrate CP and CS ratios reduced MBC formation but did not affect CUE values.