Turnover of biomass C and P in soil following incorporation of glucose or ryegrass

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The turnover times of soil microbial biomass carbon (biomass C) and phosphorus (biomass P) were estimated from the declines in biomass 14C and 32P following the addition to soil of 14C-labelled glucose with added KH 2 32PO 4 or with the separate addition of ryegrass which had been doubly labelled with 14CO 2 and KH 2 32PO 4 (both at 1000 μg C and about 10 μg added P g −1 soil). The labelled substrates were added separately to an UK grassland soil which was then incubated for 60 days at 25 °C and 40% water holding capacity. Addition of 32P caused a considerable displacement (up to 60%) of non-labelled P in the biomass of soils also given glucose. There was also significant displacement of non-labelled biomass P in soils given 32P only. With ryegrass, the labelled P addition increased the total biomass P pool size, rather than displacing native biomass P. However, with biomass C the increase in total biomass C pool size was larger than the displacement of original biomass C at this relatively low addition of substrate C. Under the above incubation conditions biomass C had a turnover time of about 82 days for glucose and 95 days for ryegrass. The turnover times of biomass P for glucose was about 37 and 42 days for ryegrass. The much shorter turnover time of biomass P than C may be because P is much more labile within the microbial cell than C. For example, unlike C, little P is generally located in the cell walls of micro-organisms, which turnover more slowly than the cytoplasmic components. These results illustrate the large potential of biomass P turnover as a source of P for plants, particularly in ‘low input’ systems.