Microbial utilization of [U14C]-labelled straw and [U13C]-labelled glucose in soils of contrasting pH and metal status

Long-term fertilization with ammonium sulphate [(NH4)2SO4] or amendment with sewage sludge have resulted in a reduction in the ratio of microbial biomass-C to soil-C in the Ultuna Long Term Soil Organic Matter Experiment. We explored whether a reduced substrate utilization efficiency and higher loss of substrate-derived biomass-C over an extended incubation period could account for the smaller biomass in these soils. Samples were taken from four soils of this field experiment, representing soils of contrasting pH, soil organic matter and heavy metal status. Soils were amended with either no substrate, [U13C]-labelled glucose, [U14C]-labelled straw or both substrates. The addition of glucose-C corresponded approximately to the amount of biomass-C in the soil, whilst the addition of straw-C was 5 times greater. The soils were incubated at 15°C and were sampled at intervals for CO2C and biomass-C for up to 134 days. There was initially a larger increase in biomass total-C than ninhydrin-N upon substrate addition, but by day 7 this ratio had fallen to that in the unamended soils. On day 7 less glucose-C was incorporated in the biomass of the (NH4)2SO4-fertilized soil with a pH of 4.4 and in the biomass of a sewage-sludge-amended soil, compared to a calcium-nitrate [Ca(NO3)2] fertilized and a farmyard-manure-amended soil. The proportion of glucose-C respired by day 7 was higher in the sewage-sludge-amended soil, but not the (NH4)2SO4-fertilized soil. The biomass of the (NH4)2SO4-fertilized soil had also incorporated less straw-C 7 days after addition, but this was not lower in the sewage-sludge-amended soil. Loss from the biomass of initially incorporated substrate-C over a period of more than 3 months was not higher in the (NH4)2SO4-fertilized and the sewage-sludge-amended soil. At the end of the incubation the sum of respired and biomass-incorporated glucose-C did not differ between soils, but less straw-C was metabolized in the (NH4)2SO4-fertilized and the sewage-sludge-amended soil compared to the Ca(NO3)2-fertilized and farmyard-manure-amended soil. The combined addition of glucose and straw resulted in an increased loss of glucose-derived C from the biomass in the first week, with most soils also showing an increased rate of 13CO2 production during this period. Microbial utilization of straw was reduced as a result of glucose addition, so that when considering both substrates combined the microbial utilization efficiency was markedly lower compared to when the substrates were added separately.