ASHABUL ANHAR1, ROLF SOMMER2, MANFRED DENICH1, PAUL L. G. VLEK1
1Centre for Development Research (ZEF), University of Bonn, Ecology and Natural Resource Management, Germany
2International Maize and Wheat Improvement Center (CIMMYT), Tropical Ecosystems Program, Mexico
Nitrogen fixing trees play a major role in improving soil N fertility. However there are only a few studies available in evaluating their capacity to fix N2 especially under field condition due to methodological difficulties. Research was conducted at two sites (Kaduwaa and Makmur) in 7-8 years old cacao agroforestry system in Central Sulawesi Indonesia to evaluate whether 15N natural abundance method could be used to estimate N2 fixation by Gliricidia sepium (Jacq.) Walp. by i) measuring variability of 15N with soil depth and time, ii) measuring biological nitrogen fixation with different reference plants as well as at different times of the season.
The N value of total soil N ranges from 6.2 to 7.9 and from 7.4 to 8.8 in Kaduwaa and Makmur, respectively. The N value of total soil N at both sites shows little variation with soil depths (30-150cm) except that the top soil (0-10cm) is less enriched suggesting dilution by atmospheric N. Foliar N of fixing and reference plants depends upon plant species, time of sampling and site. The means and standard errors of foliar N for the five times of sampling are 3.50.23, 5.30.25 and 5.7 for Gliricidia sepium, Theobroma cacao and Coffea arabica in Kaduwaa and 2.30.23, 3.50.21 and 3.70.27 for Gliricidia sepium, Theobroma cacao and Sida retusa in Makmur, respectively. In Kaduwaa the proportion of nitrogen derived from atmosphere (%Ndfa) ranges over time between 11-52% (30.94.4) and 10-58% (33.83.9%) with cacao and coffee as reference plants, respectively. In Makmur the estimate ranges over time between 14-50% (31.89.5) and 26-58% (36.97.3) with cacao and sida as reference plants, respectively. Reference plants thus did not affect the estimate of %Ndfa of Gliricidia at all times of sampling. Total N of leaf litterfall and pruning of Gliricidia over 48 weeks is between 91-101kg N ha-1. Assuming %Ndfa of Gliricidia is between 30.9-36.9%, it means that Gliricidia could contribute 28-37kg N ha-1 or 50-60% of fertiliser N ha-1 yr-1. As farmers usually apply 60-70kg Urea-N ha-1 yr-1, they could save the amount of 12-15 ha-1 yr-1.
Keywords: Coffea arabica, fertiliser N, Sida retusa, Theobroma cacao