Assay for microbial biomass based on ninhydrin-reactive nitrogen in extracts of fumigated soils

Soils held under chloroform vapour retained protease activities, but lost their dehydrogenase activities and their abilities to decompose glucose and immobilize NH 4 +-N. When the soils were held under chloroform vapour for up to 10 days the products of decomposition of the killed cells contained ninhydrin-rcactive compounds, most of which was organic (amino acids) but with increasing proportions as NH 4 +-N (up to 42% after 10 days fumigation). By contrast, under standard conditions of fumigation-incubation (1 day chloroform fumigation followed by 10 days aerobic incubation after removal of chloroform), all ninhydrin-reactive N was accounted for by NH 4 +-N. Ninhydrin-reactive N released from fumigated unamended soils accounted for up to 68% of the NH 4 +-N released in fumigated-incubated soils. But in soils amended with glucose or straw and held under chloroform vapour, ninhydrin-reactive N extracted exceeded the amounts of NH 4 +-N that were extracted from the same soils which had been fumigated-incubated and where immobilization reactions occurred and in some cases predominated. The flush of CO 2-C from 25 soils fumigated and incubated under conditions of assay for indigenous biomass C, was significantly related to the ninhydrin-reactive N extracted by 2M KC1 from the same soils held under chloroform vapour for 10 days. Significant ( P < 0.001) correlations were achieved with both unamended soils that had been incubated moist for 2 wk before assay and also with soils incubated with glucose for 44 wk or legume material for 66 wk. These results indicate a common source of substrate after fumigation. Measurements of ninhydrin-reactive N released and extracted from fumigated soils provide a useful sensitive assay of biomass C (and N). Based on assays of soils that had been incubated for 44 and 66 wk biomass C = 21 × the release of ninhydrin-reactive N from soils fumigated for 10 days at 25°C.