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Estimating dinitrogen fixation of hedgerow vegetation using the Nitrogen-15 natural abundance method.

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Leguminous trees play a major role in alley farming or hedgerow by providing or recycling N and organic matter to annual crops. Little is known however about their capacity to fix N2 under field conditions because of methodological difficulties. This study evaluated whether the 15N natural abundance (δ15N) method could be applied successfully to an alley-cropping system to estimate N2 fixation by Gliricidia sepium (Jacq.) Walp. The study also assessed the suitability of the non-nodulating legume Cassia spectabilis (L.) DC [syn. Senna spectabilis (DC.) Irwin and Barneby] as a suitable reference for investigations with N2-fixing trees. The hedgerow species were planted in double rows 3 m long with 5-m-wide alleyways at an acid upland site in the Philippines. The δ15N of total N of Cassia and Gliricidia prunings was determined from six samplings between January 1990 (17 mo after establishment) and July 1991. The δ15N of the total N of samples from nonfixing Cassia ranged from 4.47 to 7.28‰ with an average and standard error of 6.16 ± 0.41‰. These values were similar to those of extractable N of soil from different soil depths ranging from 4.66 to 7.33% suggesting that Cassia is a suitable nonfixing reference species. The changes in δ15N of total N of prunings were similar in both tree species; therefore the observed variation was considered not to have interferred with estimation of N2 fixation (%Ndfa). At four of the six sampling times Gliricidia had an Ndfa close to 50% whereas at other two sampling dates the Ndfa dropped to 30 and 35%. This study also provides the first quantitative data demonstrating that Cassia is a non-N2-fixing legume.

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