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

The turnover times of soil microbial biomass carbon (biomass C) and phosphorus (biomass P) were estimated from the declines in biomass C-14 and P-32 following the addition to soil of C-14-labelled glucose with added (KH2PO4)-P-32 or with the separate addition of ryegrass which had been doubly labelled with Co-14(2) and (KH2PO4)-P-32 (both at 1000 mug C and about 10 mug 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 degreesC and 40% water holding capacity. Addition of P-32 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 P-32, 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. (C) 2002 Elsevier Science Ltd. All rights reserved.