Seasonal variation in extracted proteases and relationship to overall soil protease and exchangeable ammonia in paddy soils

Abstract

In paddy fields treated with organic manure or chemical fertilizer or not treated, seasonal changes in soil protease [hydrolysis to benzyloxycarbonyl-l−phenyl-alanyl-l−leucine (z-FLase) or casein (caseinase)] extracted with phosphate buffer were investigated during the cultivation of rice. Both activities reached a maximum on July 21 (after irrigation), and gradually decreased, which was correlated with the soil water content. Increases in z-FLase and caseinase activity extracted coincided with increases in exchangeable NH ⁺₄ and the ninhydrin-reactive N content. The overall soil caseinase activity on August 16 (after midsummer drainage) was inhibited by metal chelators and p−chloromercuribenzoic acid and was similar to that extracted with 1.0 mol l⁻¹ phosphate buffer on June 9 (before irrigation), but different from that extracted by 0.1 mol l⁻¹ phosphate buffer on July 22. The overall soil z-FLase activity on August 16 was inhibited by p−chloromercuribenzoic acid and 1,10-phenanthroline, and was similar to that extracted with 0.1 mol l⁻¹ phosphate buffer on June 9, but differed from that extracted on July 22. The results indicate that the soil proteases extracted on July 22 had a high potential for ammonification and were short lived.