Deutscher Tropentag, October 9 - 11, 2002 in Kassel-Witzenhausen "Challenges to Organic Farming and Sustainable Land Use in the Tropics and Subtropics"

N Dynamics of Leguminous Tree Residues as Indicator Plant under Greenhouse Conditions

¹Georg-August University Göttingen, Institute of Agriculture in the Tropics (IAT), Germany
²Georg-August University Göttingen, Centre for Tropical and Subtropical Agriculture and Forestry (CeTSAF), Germany
³University of Bonn, Center for Development Research, Germany

Abstract

In the Amazon, human activities such as slashing and burning activities converted large areas of primary forest to intermittently used agricultural land
The fallow vegetation plays an important role to maintain or restore soil productivity. However, intensification of land use has caused drastic reduction the fallow period. Therefore, the soil quality has to be restored in a shorter time period. The general objective of this work was to monitor the influence of leguminous tree species on soil N dynamics simulating a situation when burning is replaced by mulching in Amazon so-called slash-and-mulch systems.
The greenhouse experiment was divided in two parts: first, ¹⁵N-urea fertilizer (3.92 mg N pot^-1 with 5.34 atom% ¹⁵N) and leguminous leaf material (S. paniculatum, I. edulis and their mixture) with N natural abundance were combined to assess microbial immobilization and soil fixation. The second experiment, N-urea fertilizer (3.92 N mg pot^-1 with N at natural abundance) and ¹⁵N-labeled leguminous organic material from the same species with 0.392 atom% ¹⁵N, and 0.390 atom% ¹⁵N, respectively, were used to assess the amount of mineral-N that coming from organic matter decomposition and the extend of competition between microorganisms, soil + organic matter fixation, and rice absorption.
For all treatments, microbial net N-immobilization was higher than net N-mineralization after 50 days of rice plant growth. After 50 days of incubation, S. paniculatum and the mixture had C-to-N ratios of 16.5 and 14.7, respectively, which was significant higher than for I. edulis (13.0) and the control (12.5). Amounts of ¹⁵N recover by rice in this experiment were significant higher (p < 0.05) in the treatment with mixture of the two species in comparison with the same species separately. The same trend was found in the cumulative total N in the plant and the cumulative plant biomass.
The treatment with the mixture between two species had an intermediate cumulative immobilisation of ¹⁵N by soil microbes, higher cumulative rice biomass and total N, and higher recovery of ¹⁵N from urea fertilizer. This indicates that interaction of two contrasting leguminous species increase the nitrogen absorption by rice, apparently by an increase of mineral N and decrease of gross microbial N immobilization.

Keywords: Amazon, ¹⁵N, N microbial immobilization, N mineralization, slash-and-mulch systems

Contact Address: José Henrique Cattanio, Georg-August University Göttingen, Institute of Agriculture in the Tropics (IAT), Albrecht-Thaer-Weg 3, 37075 Göttingen, Germany, e-mail: jcattan1gwdg.de