Saturday, 15 July 2006

Mineralization of C and N from Banana Crop Residues in Soil and Role of N from Residues in Banana Nutrition during a Crop Cycle.

Line Thieuleux1, Recous Sylvie1, Sierra Jorge1, Ozier-Lafontaine Harry1, André Lassoudière2, and Oliver Robert2. (1) Institut National de Recherche Agronomique, centre Antilles-Guyane, Unité Agropédoclimatique, Domaine de Duclos, Prise d'eau, Petit Bourg, 97170, France, (2) Centre de coopération International en Recherche Agronomique pour le Développement, BP 214, cedex 2, Lamentin, 97285, France

The study present experimental data allowing a better understanding of the fate of carbon and nitrogen from banana residues decomposition below highly fertilized banana cropping system on acid oxisol. Three experimental phases are proposed: (i) evaluating the potential decomposition of three banana organs in a laboratory experiments; (ii) quantify the potential decomposition of the entire plant in field conditions; (iii) measuring and interpreting the fate of 15N labelled residues, brought at the start of the cycle, in the plant-soil system at harvest. The incubations in laboratory were carried out with leaves, pseudostems and roots of banana crop (24g/kg) mixed with a rust-brown halloysitic soil (Colmet-Daage, 1965) with 65% clay, 13% silt and 17% sand, pH (water) 5.57. Control soil was also included. In the soil incubation in situ, 36g of mixed residues hand cut about 5 cm, have been brought to the surface of PVC tubes in proportion to the banana tree (45% pseudostems, 35% leaves, and 5% roots). The field study was carried out on two experimental plots of 200 m2, in which the banana trees were grubbed out at harvest and crop residues 15N labelled returned in agronomic proportion at the bottom of 18 banana trees. Measurement of total nitrogen and 15N enrichment of the plant and soil samples was made using a CHNS-O elemental analyser, and a stable isotope ratio mass spectrometer. Laboratory results shows that the ratio C to N of the leaves (composite mixture limb petioles) is weakest (C/N = 20) of the banana organs considered, follow-up by the roots (C/N = 26) which is relatively closer and by the pseudostems which represents the double of the precedents (C/N = 48). The three organs differed either in the biochemical composition. There was little soluble carbon in the roots (8%) and approximately 3 times more in the leaves (32%). The pseudostems had approximately 20%. Lignin is in small quantity in the pseudostems (6.7%) whereas its quantity is twice higher and equivalent in the leaves and the roots. Consequently, hemicellulose and cellulose pools are only about 52% in leaves, while it is about 75% for pseudostems and roots. Glucose decomposed about 70% in 84 days, indicate that the conditions of temperature and moisture of incubations favour the activity of the soil microflora. The mineralization of the crop residues is stabilized towards the 40th day of incubation. The percentage of carbon of the pseudostems which is broken up reaches a maximum to 35-40 % whereas for the leaves and the roots, this percentage is about 25 %. Depending on the residue, the apparent mineralization of residue N after 100 d varied from 100 to 250 g N kg-1 of dry matter. The incorporation of crop residues into soil led to various soil mineral N dynamics. Pseudostems caused net N mineralization from the time of their incorporation, whereas the leaves and roots induced N immobilization (up to 50 mg N kg-1) then mineralisation. After 100 d, all the residues induced a surplus of mineral N compared with the control soil. The %C mineralised were related mainly to the lignin content of the residues and to their C/N ratio. Most models that aim at simulating the decomposition of residues take into account the initial biochemical quality of residues, but the description is fairly variable. Models that simulate agronomic scenarios often describe the biochemical quality of crop residues only by their relative C to N contents (C/N ratio). C and N mineralisation kinetics obtained in laboratory incubations during decomposition of banana crop residues under non-limiting nitrogen conditions could be used to evaluate the ability of a simple dynamic model to predict leaf, pseudostems and root C and N mineralization. Relationships should confirm or not the relevance of expressions representing quality in existing decomposition models. To know the potential supply of mineral nitrogen by the banana tree according to measurements of incubation, calculations are carried out from the mineral nitrogen delta measured for each organ. A weighted average in proportion is applied to each organ of the banana tree and this calculation shows that the potential of nitrogen supply would be about 140 mg N per kg of dry soil. The nitrogen supply by decomposition of crop residues could be consequently provided an important quantity of the crop needs (150 kg.ha-1). In the field incubation, the decomposition of banana residues was completed about 150 days and 95% of N brought were remained to the soil-plant system. The constant of decomposition of the banana mulch measured were 0.0219. The percentage of 15N recovery in the plant from residues is about 68%. This approach therefore demonstrates that supply nitrogen to satisfy crop requirements can be possible from crop residues decomposition, and that use of fertilizer can be excessive. This consideration is important throughout crop growth so as to limit the risk of environmental pollution.

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