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Effets de litières sur l'offre en azote d'origine organique dans des systèmes de culture de maïs à couvertures végétales Etude de cas dans la zone à forêt semi-décidue de Côte d'Ivoire

Abstract : An agronomic device was conduced between 1995 and 1999 with maize cropping systems in the ecological zone of the semi-deciduous forest of Ivory Coast. We compared the supply of organic origin N to the maize with two fallow periods, 6 months (SC6MJ) and 18 months (SC18MJ). The fallows are occupied either by Chromolaena odorata (subspontaneous) or Pueraria phaseoloides (introduced). These four systems are compared to two controls, a non-Chromolaena fallow (artificially) for SC6MJ and with burning of Chromolaena for SC18MJ. Systems cover crops are managed as live-mulch with herbicides. Systems SC6MJ received moderate phosphorus fertilization. The study poses as assumptions that (i) determinants of short-term supply (crop cycle) and medium term (cumulative effects) mainly depend on quantities and litter quality original surface fallow (ii) these effects will concentrate on the surface of the soil. Mineralization potential litter is correlated with the laboratory / N, lower their report Pueraria C (C / N between 20 to 23) for Chromolaena (C / N of 25 to 43). In 1998 the rate of disappearance of litter followed with litterbags is much faster than in 1999 in relation the rainfall pattern. Two years the amount of litter present after cutting determines the rate of decomposition of litter in the medium term. These dynamics, modeled for two years, used to distinguish a litter compartment that disappeared rapidly and another one that disappeared slowly. Comparisons of losses in C in the litterbags with different mesh sizes showed that the activity of mesofauna and soil macrofauna could be responsible for the disappearance of 50% of the litter at the beginning of the decomposition process and 35% at the end of decomposition process. Their action of fragmentation and soil incorporation of litter (fungus growing termites) allowed a comparatively rapid loss of both litters, Chromolaena and Pueraria. The application of models to the data obtained on the mobilization of N fallow in relation to the needs of culture N indicates (i) a risk of "asynchrony" early cycle (ii) insufficient supply at end of cycle for SC6MJ (iii) little differentiation between the supply of Chromolaena and Pueraria. The supply of in situ mineral N in situ at soil surface (0-10 cm) in 1998 and 1999, estimated for periods of 90 days during the maize cycle was evaluated by measurement and incubations. With SC6MJ the mobilization of N in corn is greater with Pueraria the two years and this N offer is comparable to SC18MJ. The supply of N varies little SC18MJ treatments. The rainfall occurs (i) a multi-level, by determining the peak intensity of N mineralization soil mineral N and accumulation in the aerial parts of fallow (ii) the scale of the campaign interaction with the surface litter and activity of root systems of vegetation cover. Litter Pueraria seemed to conserve better the soil moisture. In limiting water situations, the net mineralization is correlated with soil moistures. Litters increased microbial biomass of the soil surface thus creating a potential source of temporary immobilization of N. In 1998 the supply of estimated net N mineralization indicates a better synchrony with the needs of the crop despite a more abundant rainfall cycle start over in 1999. A study using the 15N natural abundance to estimate the litter Pueraria can contribute significantly to the nitrogen nutrition culture, 30 to 41% in 1998 and 64 to 87% in 1999 in the culture system fallow 6 months. Differentiations on soil organic matter (SOM) indicate early effect on stocks valued at 0-10 cm averaged 2.34 t ha-1 N for SC6MJ and averaged 2.83 t ha-1 N for SC18MJ. For difference SC6MJ obtained between inputs and losses over four years suggests that Chromolaena best preserved stocks of C and N that Pueraria. The comparison with forest fallow located outside the device indicates that the availability of materials easily mineralizable in our agroecosystems (i) is reduced at the end of the rainy season (ii) reduced by the practice of burning (iii) slightly increased by fallowing. Studies on long-term incubations (106 and 85 days) show significant effects on the models made by fitting the experimental points on the kinetics of respiration and cumulative net mineralization (expressed in proportion to the total C and N soil). They identify important factors of differentiation in compartment MOS rate high turnover. For SC6JM this compartment represents a larger part with Pueraria certainly directly related to a better biochemical quality (ratio lignin / N). The duration of the fallow does not improve the availability of easily mineralizable N indicating a block of C and N in the soil physical and / or biochemical nature
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Submitted on : Thursday, May 23, 2013 - 11:16:38 AM
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Patrice Autfray. Effets de litières sur l'offre en azote d'origine organique dans des systèmes de culture de maïs à couvertures végétales Etude de cas dans la zone à forêt semi-décidue de Côte d'Ivoire. Agronomie. Université Montpellier II - Sciences et Techniques du Languedoc, 2002. Français. ⟨tel-00825211⟩

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