Toposequential variation in soil properties and crop yield from double-cropping paddy rice in Northwest Vietnam

To access the spatial differences in soil properties and crop yield at cascade level affected by either sediment induced or farmers’ fertility practice, field experiments were conducted in Cheing Khoi watershed area during spring and summer seasons with two different cascades. The cascades had fertilized and unfertilized parts and all the measurements were conducted at the top, middle and bottom field of each part.  In both rice cascades, middle and bottom fields showed high silt and clay content while sand was the dominant type in the top field. Total nitrogen (TN) and carbon (TC) contents were significantly higher in lower lying fields than top field. Seasonal average surface water NH4 concentrations for top, middle and bottom fields were 2.7, 3.4 and 3.2 mg L-1 in spring and 2.8, 3.5 and 3.1 mg L-1 in summer, respectively. Differences were substantial in yield components parameters and grain yield depending on toposequence position. Grain yields in the middle fields of both rice cascades were higher than other field positions in both fertilized and unfertilized fields. The grain yields were significantly related with surface water NH4 concentration, TN content, sand and silt content of soil. The larger toposequential differences in crop yield require different field specific management practices for each position in order to improve rice production in this watershed area.   Key words: Crop performance, double-cropping, paddy rice, spatial variability, toposequence.