Graduate Thesis Or Dissertation
 

Soil changes after afforestation in Yellow River loess : a case study in Gansu Province, People's Republic of China

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  • Widespread erosion of the Yellow River loess plateau has led to serious soil and water conservation problems for north central China. In the DingXi District of Gansu Province, with less than 400 mm annual precipitation, terraced plantations for timber, fuel, or fodder form part of a large scale afforestation project to meet local needs as well as conservation goals. To determine changes in the calcareous (pH 8.5-9.5) loess soil due to plantations, 14 and 24 year old stands of Pinus tabulaeformis Carr. were compared to an adjacent 26 year old stand of the shrubby legume Caraana microphylla (Pall.) Lam. and to unterraced pasture and new unplanted terraces. The 24 year old pine plot had significantly less soil nitrogen and organic carbon than the pasture or legume plots, but had higher pH than the legume plot. The nitrogen-fixing legume maintained soil nitrogen and organic carbon levels equivalent to the pasture, and had lower pH and bulk density in the upper horizons. Other nutrients showed no significant differences due to plantations. Nutrient depletion under pure P. tabulaeformis plantations may limit future growth under these conditions. The shrubby legume can improve soil fertility while providing fuel and fodder in pure or mixed stands. Results from this case study suggest that the site loses significant amounts of nitrogen from terraced plantations. Hypotheses are developed linking the alkaline and calcareous properties of the soil to loss of nitrogen made available by enhanced decomposition and mineralization of native organic matter. Weeding and cultivation (to conserve soil moisture) and gleaning for fuel and fodder which removes ground vegetation and litter from the site may also affect the nitrogen cycle by lowering uptake, retention, and inputs.
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