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Accueil du site → Doctorat → Allemagne → 2014 → Exploring the potential of multipurpose tree species for revegetating abandoned cropping sites in the lower Amu Darya Basin

Rheinischen Friedrich-Wilhelms-Universität Bonn (2014)

Exploring the potential of multipurpose tree species for revegetating abandoned cropping sites in the lower Amu Darya Basin

Schachtsiek Tilman

Titre : Exploring the potential of multipurpose tree species for revegetating abandoned cropping sites in the lower Amu Darya Basin.

Auteur : Schachtsiek Tilman

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität Bonn

Grade : Doktor der Agrarwissenschaften (Dr. agr.) 2014

Résumé
The land degradation due to soil salinization that results from elevated groundwater table (GWT) is common in intensively irrigated and poorly drained cropping areas in the lower Amu Darya Basin. Given the decreasing availability of fresh water for soil reclamation via salt leach‐ ing, farmers in the region tend to abandon highly salinized areas from cropping. To assess the potential of afforestaton to rehabilitate the abandoned croplands, six tree species were exam‐ ined through field experiments during 2010 and 2011 at two sites. The main focus was on spe‐ cies’ survival, provision of ecosystem services (biomass production, replenishment of carbon (C) and nitrogen (N) soil stocks), as well as on the dynamics of soil salts and water. The empiri‐ cal data were used to parameterize the field‐scale, process‐based model EPIC (Environmental Policy Integrated Climate model), which was subsequently used to simulate biomass growth and salt and water dynamics in tree plantations under shallow GWT conditions. Both sites initially showed soil nutrient deficiency, high soil salinity (10 ‐ 12 dS m‐1 ) and moderately saline groundwater (2‐5 dS m‐1 ), but differed in texture (silty loam : G’oybu ; loamy sand : Beruniy) and GWT depth. The latter fluctuated between 0.6 and >2.8 m in G’oybu and between 0.2 and 1.4 m in Beruniy. The tree plantations received deficit irrigation of 154 mm yr‐1 . A survival analysis revealed differential response of tree species to plant‐available water and salt content in the root zone. Afforesting abandoned croplands was most feasible with Elaeagnus angustifolia L., Ulmus pumila L., Morus alba L., and Populus nivea x tremula L. given their survival rates of 75 ‐ 91% on both sites after two years. The major species of the native floodplain forest, P. euphratica Oliv. and Salix nigra Marsh., exhibited survival rates <19% and were therefore evaluated as not suitable for afforestation of the abandoned cropland. Overall, N2‐fixing E. angustifolia was assessed as most promising among all species tested since it combined highest survival rates with largest aboveground biomass increments (up to 904 kg ha‐1 yr‐1 ). Its N2‐fixation efficiency was estimated at 70 ‐ 74% (12 ‐  22 kg N ha‐1 yr‐1 ) with the 15N natural abundance and the total N‐difference method.  The afforestation impact on soil total N, C, and SOC was modest due to high inter‐ annual variability in soil characteristics, short observation period and modest addition of or‐ ganic matter by young trees. Subtle but statistically significant increases in SOC stocks in the heavier textured soil in plots with higher biomass yielding species are first positive indications for future trends in soil C status. The decline in soil salinity that occurred in both afforestation sites and the reference cropland fields were associated with the fluctuating GWT rather than the impact of afforestation. The EPIC model showed promise for simulating the processes in P. nivea x tremula plantation under deeper GWT conditions, given the percent bias ranging between ‐2.6 and 5.5%. The calibration results remain to be validated with with independent datasets. The pre‐ sent findings revealed a limited capability of EPIC for the site with shallow, fluctuating GWT, requiring that the latter is accounted in the model on the daily basis. Furthermore, considering plant survival and aboveground biomass partitioning would enhance the capability of EPIC for application in forestry studies.  The overall assessment of tree species potential for revegetating long‐term aban‐ doned croplands showed that the land’s productive capacity can be increased with selected species as opposed to a natural succession. However, afforestation remains associated with risks unless the availability of irrigation water during the initial establishment stage is warrant‐ ed.

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