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

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 twofactorial 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 N 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), (ii) low-P (45 kg P ha), 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, N2 fixed with high-P increased from an initial value of 64 kg ha to 807 kg ha after three years in E. angustifolia, and from 9 kg ha to 155 kg ha 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 Nenrichment technique (NET) and the A-value (AV). The non-N-fixers G. triacanthos and Ulmus pumila L. served as a reference. Twenty kg N ha of 5 atom % N excess ammonium nitrate was applied to one-year-old trees in 2007 and to two-year-olds in 2008. This rate was suspected insufficient for the growth of older reference trees hence a treatment with 60 kg N ha was included in 2008. With NET, 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 year when compared against both reference species. The findings of the N2 fixation rates of E. angustifolia measured with the NET and AV methods were compared also with the N natural abundance (NA) 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. Tree plantation management would benefit from tools to support harvest scheduling of E. angustifolia and R. pseudoacacia foliage that have high N contents. During 2006-2008, the chlorophyll meter SPAD-502 was tested, calibrated and validated in the core experiment for the N2fixers and the non-N-fixer G. triacanthos. The temporally and spatially based validation of the species showed very high correlations with the empirically monitored values. The SPAD-502 is therefore helpful for livestock rearers who intend to include tree foliage in feed diets of their animals. Leaf crude protein (CP), an important indicator for feed quality, can now be determined for the three species given the established relationships between N/CP and the SPAD-502 readings. This determination, however, is only valid within the SPAD-502 range of readings determined during the calibration process for each species. Based on a least-cost-ratio model, the time of inclusion of foliage in the feed was simulated, which predicted that the leaves of non-fixer G. triacanthos would be best harvested in May, whereas the N2-fixer E. angustifolia should be harvested in July and September. Therefore, P fertilization at planting and the use of optical-based sensors during the vegetation season are two potential means for improving tree plantation management on marginal croplands that benefit the environment and farmers in the dryland regions of Uzbekistan. Auswirkungen von Phosphorzugaben auf die Stickstoffbindung und das Wachstum von Bäumen auf Agrarflächen mit hoher Bodensalinität im Unterlauf des Amu Darya in Usbekistan