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Accueil du site → Doctorat → Allemagne → 2011 → The effect of phosphorus amendments on nitrogen fixation and growth of trees on salt-affected croplands in the lower reaches of Amu Darya, Uzbekistan

Rheinischen Friedrich-Wilhelms-Universität Bonn (2011)

The effect of phosphorus amendments on nitrogen fixation and growth of trees on salt-affected croplands in the lower reaches of Amu Darya, Uzbekistan

Djumaeva, Dilfuza

Titre : The effect of phosphorus amendments on nitrogen fixation and growth of trees on salt-affected croplands in the lower reaches of Amu Darya, Uzbekistan

Auteur : Djumaeva, Dilfuza

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität Bonn

Grade : Doktor der Agrarwissenschaften (Dr. agr.) 2011

Résumé partiel
Afforestation, particularly with the use of N2-fixing trees (NFTs), is an option for ecological restoration of salinized, irrigated croplands in the lower reaches of the Amu Darya River. But current knowledge of enhanced juvenile tree growth and their N2 fixation rates is sparse for the marginal irrigated croplands of the Khorezm Region of Uzbekistan. A superior understanding would increase productivity of such lands, improve soil fertility status and increase profits for farmers. A two-factorial field experiment was therefore conducted during 2006-2008 to compare the effect of three phosphorus (P) amendments on N2 fixation, biomass and growth rates of actinorhizal Elaeagnus angustifolia L. and leguminous Robinia pseudoacacia L. N2 fixation was quantified through 15N natural abundance, based on foliar and whole-tree sampling against non-N-fixing Gleditsia triacanthos L. The P rates included : (i) high-P (90 kg P ha-1), (ii) low-P (45 kg P ha-1), and (iii) no P applied (0-P). With high-P, N2 fixation by E. angustifolia increased by 81% and almost doubled for R. pseudoacacia when compared to 0-P. At a tree density of 5,714 trees ha-1, N2 fixed with high-P increased from an initial value of 64 kg ha-1 to 807 kg ha-1 after three years in E. angustifolia, and from 9 kg ha-1 to 155 kg ha-1 in R. pseudoacacia stands. The P-effect was inconsistent when analyzing absolute growth and biomass increase. Compared to 0-P, high-P increased total biomass, total above ground biomass and biomass of different tree fractions, but the increments in absolute growth were statistically insignificant. In contrast, high-P significantly increased relative growth rates of height for E. angustifolia, and the unit production rate and nitrogen productivity for R. pseudoacacia. Hence tree growth analyses should combine absolute and relative growth to gain a full insight. N2 fixation of E. angustifolia, the species with the highest potential in the field trial, was also quantified in lysimeters to eliminate inaccuracies that can occur when harvesting large trees in open fields. Here we used more than one reference species and two assessment methods, the 15N-enrichment technique (15NET) and the A-value (AV). The non-N-fixers G. triacanthos and Ulmus pumila L. served as a reference. Twenty kg N ha-1 of 5 atom % 15N excess ammonium nitrate was applied to one-year-old trees in 2007 and to two-year-olds in 2008. This rate was suspected to be insufficient for the growth of elder reference trees, hence a treatment with 60 kg N ha-1 was included in 2008. With 15NET, the proportion of atmospheric N2 (%Ndfa) of E. angustifolia in 2007 was 79% when referenced against U. pumila and 68% against G. triacanthos. The results of the AV method showed that the %Ndfa of two-year-old E. angustifolia was 80% and 68% when referenced against the same two species, respectively. Over two years, E. angustifolia fixed an average of 16 kg N2 ha-1 year-1 when compared against both reference species. The findings of the N2 fixation rates of E. angustifolia measured with the 15NET and AV methods were compared also with the 15N natural abundance (15NA) and total N difference (ND) methods. The highest accuracy was obtained with the AV method, but financial and material considerations may favor the total ND method, especially when used in lysimeter trials when facilities are accessible for accurate dry matter and total N determination.

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