Sediment induced soil spatial variation in paddy fields of Northwest Vietnam

The aim of this study was to assess the impact of various sedimentation pathways (flooding, irrigation and runoff) on the spatial variability of soil fertility in rice paddy terraces in tropical mountainous regions of Northwest Vietnam. Topsoil samples were taken during two subsequent rice cropping seasons and analyzed using a combination of diffuse reflectance mid infrared spectroscopy and conventional lab analysis. A mixed model was used (i) to evaluate the spatial variability among and within paddy cascades before planting in function of field position to the main irrigation channel, and (ii) to assess the impact of various sediment deposition pathways on soil nutrients and textural changes. The topsoil taken before planting contained on average 1.75 ± 0.57 g 100 g? 1 soil organic carbon (SOC), 0.18 ± 0.06 g 100 g? 1 total nitrogen (TN) with silt being the dominating soil fraction (0.68 ± 0.11 g g? 1). Moderate sediment delivery of high quality through the irrigation system resulted in a significant enrichment in lower lying paddies following a linear trend for SOC (SOC (g 100 g? 1) = 1.4 + 0.02 Distance (m), R2 = 0.31 ? 0.62), total nitrogen (TN (g 100 g? 1) = 0.11 + 0.004 Distance (m), R2 = 0.33 ? 0.61) and a significant linear decrease in the sand fraction (sand (g g?1) = 0.3 ? 8 E?04 Distance (m), R2 = 0.28 ? 0.48) with increasing distance from the irrigation channel along the cascade. Comparison of the samples taken before planting and after harvesting proved that the spatial variability in the topsoil was induced by sediment deposition resulting in a decrease of 0.11 g 100 g?1 of SOC and 0.01 g 100 g?1 of total N and an increase of 0.02 g g?1 of the sand fraction in paddies close to the irrigation channel which received less nutrient rich sediment deposition. However, besides the effect of sediment rich irrigation water, direct sediment depositions originating from the highly eroded and unfertile uplands or deposited during flooding events (typhoons) strongly decreased soil fertility in the rice fields due to their low nutrient and high sand content. In conclusion, the alterations and maintenance of soil fertility of rice fields depended on the balance of the various sediment sources, i.e. quality and quantity, and is thus, strongly related to both upland management and extreme weather events and irrigation practices. These findings are relevant in the framework of site-specific fertilizer management by taking advantage of spatial variability in soil fertility along cascades of rice paddy terraces in tropical mountainous regions.

P Schmitter et al., 'Sediment induced soil spatial variation in paddy fields of Northwest Vietnam', Geoderma, vol. 155(3-4), pp.298-307, 2010