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Slaets J.I.F.,University of Hohenheim | Schmitter P.,National University of Singapore | Hilger T.,University of Hohenheim | Lamers M.,University of Hohenheim | And 3 more authors.
Journal of Hydrology

The aim of this study was to develop a method to continuously monitor sediment, carbon and nitrogen concentrations in streams using turbidity sensors. Field experiments were conducted in an irrigated and intensely cultivated watershed in Northwest Vietnam. Turbidity, discharge and rainfall were monitored during two successive rainy seasons from 2010 to 2011, and manual water samples were collected using a storm-based approach. Samples were analyzed for concentrations of suspended sediment (SSC), particulate organic carbon (POC) and particulate nitrogen (PN). A linear mixed model was developed to account for serial correlation, with turbidity, discharge and rainfall as predictor variables. Turbidity was the most important predictor variable in all models. Fivefold cross-validation showed best model performance for POC with a Pearson's correlation coefficient of 0.91, while predictions for SSC and PN achieved a satisfying correlation of 0.86 and 0.87, respectively. Laboratory testing of the turbidity sensors showed that the turbidity signal is sensitive to differences in organic matter content, and has the smallest variance for fine textures, both of which are correlated to POC and thus supporting the higher predictive accuracy for this variable. The developed methodology is widely applicable and can be used to simultaneously obtain reliable, cost-effective and continuous estimates of SSC, POC and PN with a single sensor. © 2014 Elsevier B.V. Source

Schmitter P.,University of Hohenheim | Schmitter P.,National University of Singapore | Frohlich H.L.,University of Hohenheim | Dercon G.,University of Hohenheim | And 6 more authors.

This study aimed at tracing and quantifying organic carbon and total nitrogen fluxes related to suspended material in irrigation water in the uplands of northwest Vietnam. In the study area, a reservoir acts as a sink for sediments from the surrounding mountains, feeding irrigation channels to irrigate lowland paddy systems. A flow separation identified the flow components of overland flow, water release from the reservoir to the irrigation channel, direct precipitation into the channel, irrigation discharge to paddy fields and discharge leaving the sub-watershed. A mixed effects model was used to assess the C and N loads of each flow component. Irrigation water had an average baseline concentration of 29 ± 4.4 mg l -1 inorganic C, 4.7 ± 1.2 mg l -1 organic C and 3.9 ± 1.6 mg l -1 total N. Once soils were rewetted and overland flow was induced, organic C and total N concentrations changed rapidly due to increasing sediment loads in the irrigation water. Summarizing all monitored events, overland flow was estimated to convey about 63 kg organic C ha -1 and 8.5 kg N ha -1 from surrounding upland fields to the irrigation channel. The drainage of various non-point sources towards the irrigation channel was supported by the variation of the estimated organic C/total N ratios of the overland flow which fluctuated between 2 and 7. Nevertheless, the majority of the nutrient loads (up to 93-99%) were derived from the reservoir, which served as a sediment-buffer trap. Due to the overall high nutrient and sediment content of the reservoir water used for irrigation, a significant proportion of nutrients was continuously reallocated to the paddy fields in the lowland throughout the rice cropping season. The cumulative amount of organic C and total N load entering paddies with the irrigation water between May and September was estimated at 0.8 and 0.7 Mg ha -1, respectively. Therefore deposition of C and N through irrigation is an important contributor in maintaining soil fertility, and a process to be taken into account in the soil fertility management in these paddy rice systems. © 2011 Springer Science+Business Media B.V. Source

Schmitter P.,University of Hohenheim | Dercon G.,International Atomic Energy Agency | Hilger T.,University of Hohenheim | Hertel M.,University of Hohenheim | And 4 more authors.
Agriculture, Ecosystems and Environment

In tropical mountainous regions of South East Asia, intensive cultivation of annual crops on steep slopes makes the area prone to erosion resulting in decreasing soil fertility. Sediment deposition in the valleys, however, can enhance soil fertility, depending on the quality of the sediments, and influence crop productivity. The aim of the study was to assess (i) the spatio-temporal variation in grain yield along two rice terrace cascades in the uplands of northern Viet Nam, (ii) possible linkage of sediment deposition with the observed variation in grain yield, and (iii) whether spatial variation in soil water or nitrogen availability influenced the obtained yields masking the effect of inherent soil fertility using carbon isotope (13C) discrimination and 15N natural abundance techniques. In order to evaluate the impact of seasonal conditions, fertilizer use and sediment quality on rice performance, 15N and 13C stable isotope compositions of rice leaves and grains taken after harvest were examined and combined with soil fertility information and rice performance using multivariate statistics. The observed grain yields for the non-fertilized fields, averaged over both cascades, accounted for 4.0±1.4Mgha-1 and 6.6±2.5Mgha-1 in the spring and summer crop, respectively, while for the fertilized fields, grain yields of 6.5±2.1Mgha-1 and 6.9±2.1Mgha-1 were obtained. In general, the spatial variation of rice grain yield was strongly and significantly linked to sediment induced soil fertility and textural changes, such as soil organic carbon (r 0.34/0.77 for Cascades 1 and 2, respectively) and sand fraction (r -0.88/-0.34). However, the observed seasonal alteration in topsoil quality, due to sediment deposition over two cropping cycles, was not sufficient to fully account for spatial variability in rice productivity. Spatial variability in soil water availability, assessed through 13C discrimination, was mainly present in the spring crop and was linearly related to the distance from the irrigation channel, and overshadowed in Cascade 2 the expected yield trends based on sediment deposition. Although δ15N signatures in plants indicated sufficient N uptake, grain yields were not found to be always significantly influenced by fertilizer application. These results showed the importance of integrating sediment enrichment in paddy fields within soil fertility analysis. Furthermore, where the effect of inherent soil fertility on rice productivity is masked by soil water or nitrogen availability, the use of 13C and 15N stable isotopes and its integration with conventional techniques showed potential to enhance the understanding of the influence of erosion - sedimentation and nutrient fluxes on crop productivity, at toposequence level. © 2010 Elsevier B.V. Source

Schmitter P.,University of Hohenheim | Dercon G.,University of Hohenheim | Hilger T.,University of Hohenheim | Thi Le Ha T.,Hanoi University | And 4 more authors.

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. © 2009 Elsevier B.V. All rights reserved. Source

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