Soil enzyme activity of an intensively grown rice field as influenced by long term fertilization and manuring under subtropical climatic conditions

A long term fertilizer experiment conducted on rice-rice system since 2005 on acidic sandy loam inceptisol of Bhubaneswar in Odisha was used for studying the effect of continuous application of NPK fertilizers alone or with Zn, B and S fertilizers and FYM on activities of three important soil enzymes viz dehydrogenase, urease and phosphatase. Dehydrogenase activity was greater in control and 100 per cent NPK + FYM treatment than soils that received only NPK plus other nutrients. Lower dehydrogenase in NPK fertilizers alone or with Zn, B and S fertilizers can be attributed to negative effect of K on fungi and algae and Zn on algae. Dehydrogenase poorly correlated with SOC (r= 0.19). On the other hand urease and phosphatase strongly correlated with SOC content and increased substantially with fertilizer application. Addition of FYM along with NPK fertilizers recorded highest urease and phosphatase activity due to increase in SOC and microbial population. Continuous application of Zn alone or with B or S for 18 cropping cycles did not have any significant effect on urease activity. The effect was positive with Zn and S and negative with Zn and B combination. But in case of phosphatase activity all the three nutrients had positive effect which was significant with combined application of Zn and S.