Heterotrophic nitrogen fixation (C2H2 reduction) as influenced by phosphorus application in paddy soils

Abstract

The influence of phosphorus application on soil nitrogenase and N2-fixers in tropical paddy soils differing in their properties was investigated in a laboratory incubation study. Application of P stimulated the soil N2-ase in an alluvial soil and in a P-deficient soil under both flooded and nonflooded conditions. The stimulation of N2-ase by P was more pronounced under nonflooded conditions. A corresponding increase in N2-ase occurred with an increase in the P level at least up to 80 ppm level. A depressive effect of P on N2-ase occurred after 16 days under nonflooded conditions when the level of P was increased to 100 ppm. But under flooded conditions the stimulation was almost continuous. Results indicate that the effect of P on N2-ase depended on the water regime, level of P and soil type. Addition of P had a little effect on the population of N2-fixing micro-organisms in alluvial soil. On the contrary, addition of P stimulated the population of Azospirillum and Azotobacter in a P-deficient soil. Data suggested that the alteration in the N2-fixing microbial populations and the levels of available P might be responsible for changes in the N2-ase activity in these soils. Application of superphosphate and dicalcium phosphate stimulated N2-ase activity; while the rock phosphate exhibited an innocuous effect in alluvial and P deficient soils. In Sukinda soil, however, super phosphate slightly stimulated N2-ase at early stages, while other P sources had innocuous effect. Results indicate that the level and source of applied P exhibited differential influence on N2-ase and N2-fixers in tropical paddy soils.