Effect of phosphorus on n2-fixation as measured by the field acetylene reduction technique in thailand long-term fertility plots

Abstract

An in situ acetylene reduction technique was applied to wetland fields in Thailand to assess the relative contributions of Na-fixation associated with rice, and algal N,-fixation. Non-fertilized and PK-fertilized plots were used in long-term fertility trials at three sites with high (Chainat), medium (Supanburi), and low (Klong-Iuang, acid sulfate soil) soil fertilities. Assays were made at the maximum tillering, heading, and maturing stages during four continuous cropping seasons spanning 2 years. Since the regression coefficients for the logarithm of variance to the logarithm of the mean were close to 2.0, the log normal distribution of the acetylene reduction activity variables could be predicted, and the variables were subjected to logarithmic transformation for statistical calculation. Additions of PK fertilizer affected the rice yields and N,-fixation associated with rice at Supanburi and KJong-luang. but not those at Chainat. The PK fertilizer affected the algal N-fixation at Chainat and Klong-luang, but not that at Supanburi. The algal fixation was lowest in non-fertilized plots at Supanburi (0.36 mmol C2H4-m-2 per day) and highest in PK- fertilized plots at Chainat and Supanburi (1.7-1.5 mmol C2H4m-2 per day). The N,-fixation associated with rice was lowest in non-fertilized plots at Klong-luang (0.07 mmol C2H4-nr per day) and highest in PK-fertilized plots at Supanburi (0.4 mmol C2H4-nr per day). The reported experiments provide field evidence for enhancement of N,-fixation by phosphorus fertilizer in plots where rice yields have increased without additions of nitrogen fertilizer. © 1980 Taylor & Francis Group, LLC.