Radiorespirometry measurement of the soil microbial activity of trial fields treated with organic fertilizers

The soil microbial activity of trial fields treated with organic fertilizers was measured by incorporating into the fresh soil low amounts of D, L‐[l‐14C]‐glutamic acid (0.07 mg kgr1 fresh soil) and measuring after a short time (1 hr) of incubation the 14CO2 evolved, i.e. the initial rate of glutamic acid mineralization. One of the organic fertilizers: green manure, cow manure, pig slurry, or mushroom cultivation compost (at 50 or 100 tons ha‐1) had been incorporated into soil in the field plots either 1, 3 or 12 months before. On some plots, the same organic fertilizer treatment had been repeated once per year as far as 4 times in the past. Fresh soil sampling was made at the beginning of March 1994 in the tilled and naked fields made ready for sowing or planting of the 1994 spring crops. In all the organic fertilizer treated plots, the soil microbial activity was 1.2 to 2.4 times greater than in the control plots not treated with organic fertilizers. Repetition in the past of the organic fertilizer trea...