EFfect of phosphorus on N2-fixation as measured by the field acetylene reduction technique in Thailand long-term fertility plots

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Abstract
An in situ acetylene reduction technique was applied to wetland fields in Thailand to assess the relative contributions of N2-fixation associated with rice, and algal N2-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-luang, 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 N2-fixation associated with rice at Supanburi and Klong-luang, but not those at Chainat. The PK fertilizer affected the algal N2-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 CA C2H4·m-1 per day) and highest in PK-fertilized plots at Chainat and Supanburi (1.7-1.5 mmol C2H4·m-1 per day). The N2-fixation associated with rice was lowest in non-fertilized plots at Klong-luang (0.07mmol C2H4·m-1 per day) and highest in PK-fertilized plots at Supanburi (0.4 mmol C2H4·m-1 per day). The reported experiments provide field evidence for enhancement of N2-fixation by phosphorus fertilizer in plots where rice yields have increased without additions of nitrogen fertilizer.