Seasonal variation of soil protease activities and their relation to proteolytic bacteria and Bacillus spp in paddy field soil

Hydrolytic activities toward benzyloxycarbonyl-l-phenylalanyl-l-leucine (z-FLase) and casein (caseinase) in soil were measured during rice cultivation, before irrigation, before and after midsummer drainage, and before and after the final drainage. The activities were maintained at a high level in three paddy fields, an organic-manure field (z-FLase, 191–684 pKat g−1 dry wt soil; caseinase, 220–652 pKat g−1 dry wt soil), a chemical-fertilizer field (z-FLase, 368–472 pKat g−1 dry wt soil; caseinase, 319–773 pKat g−1 dry wt soil) and a no-fertilizer field (z-FLase, 358–702 pKat g−1 dry wt soil; caseinase, 424–758 pKat g−1 dry wt soil). The total numbers of bacteria did not correlate with soil protease activities. Numbers of proteolytic bacterial groups, counted on azocoll agar plates and with a MPN method, correlated well with the soil z-FLase activity (r=0.625 to 0.767). Numbers of vegetative cells and total cells of Bacillus spp decreased after irrigation and continued to decrease until after the final drainage, with only a weak negative correlation between their number and soil water content (r=−0.488 and −0.555). The ratio of spores-to-total cell numbers of Bacillus spp increased after irrigation and reached a maximum after 1–2 months, when 43–100% of Bacillus cells existed as spores. The number of spores of Bacillus spp was weakly correlated with the soil caseinase activity (r=0.459).