ESTIMATING DINITROGEN FIXATION OF HEDGEROW VEGETATION USING THE N-15 NATURAL-ABUNDANCE METHOD

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 N-15 natural abundance (deltaN-15) 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 deltaN-15 of total N of Cassia and Gliricidia prunings was determined from six samplings between January 1990 (17 mo after establishment) and July 1991. The deltaN-15 of the total N of samples from nonfixing Cassia ranged from 4.47 to 7.28 parts per thousand with an average and standard error of 6.16 +/- 0.41 parts per thousand. 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 deltaN-15 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.