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Afforestation of degraded croplands by planting N-fixing trees in arid regions is highly recognized. However, fixation of atmospheric nitrogen gas (N) by woody perennials is often limited on phosphorus (P) poor soils, while any factor limiting N nutrition inhibits tree growth. In a two-factorial field experiment, the effect of three P amendments was examined during 2006-2008 on N fixation, biomass production, and foliage feed quality of actinorhizal Elaeagnus angustifolia L. and leguminous Robinia pseudoacacia L. With the N natural abundance method, N fixation was quantified based on foliar and whole-tree sampling against three non-N-fixing reference species: Gleditsia triacanthos L., Populus euphratica Oliv., and Ulmus pumila L. The P applications, in March 2006 and April 2007 only, included (i) high-P (90 kg P ha), (ii) low-P (45 kg P ha), and (iii) 0-P. After 3 years, the average proportion of N derived from atmosphere (Ndfa, %) increased from 78 % with 0-P to 87 % with high P when confounded over both N-fixing species. With the used density of 5,714 trees ha, the total amount of N fixed (Ndfa, kg N ha) with high-P increased from 64 kg N ha (year 1) to 807 kg N ha (year 3) in E. angustifolia and from 9 kg N ha (year 1) to 155 kg N ha (year 3) in R. pseudoacacia. Total above-ground biomass increases were too variable to be significant. Leaf N content and therewith also leaf crude protein content, which is an indicator for feed quality, increased significantly (24 %) with high-P when compared to 0-P for E. angustifolia. Overall findings indicated the suitability of the two N-fixing species for afforestating salt-affected croplands, low in soil P. With P-applications as low as 90 kg P ha, the production potential of E. angustifolia and R. pseudoacacia, including the supply of protein-rich feed, could be increased on salt-affected croplands. [ABSTRACT FROM AUTHOR]

Groundwater (GW) management is an essential element in irrigated agriculture. This paper analyzes the temporal dynamics of GW table and salinity in Khorezm, a region of Uzbekistan which is situated on the lower Amu Darya River in the Aral Sea Basin and suffering from severe soil salinization. We furthermore identify the critical areas for potential soil salinization by examining GW table and salinity measured during 1990–2000 in 1,972 wells, covering the entire region. Additionally, case studies were performed to assess the contribution of the GW to the soil salinization on a field scale. Over the entire area, GW was only moderately saline averaging 1.75 ± 0.99 g l−1 However, GW levels were generally very shallow averaging 148 ± 57 cm below the ground surface and thus likely to prompt secondary soil salinization. Three case studies where GW table, soil and GW salinity were closely monitored at the field scale, suggested that the elevated GW levels forced soil salinization by annually adding 3.5–14 t ha−1 of salts depending on the position and salinity of the GW table. Maps interpolated from the regional dataset revealed that GW was significantly shallower and more saline in the western and southern parts of Khorezm despite the presence of a drainage network which is rather uniformly distributed throughout the region. The results of the current study will assist the development of an improved drainage management in Khorezm. [ABSTRACT FROM AUTHOR]