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Podwojewski P.,University of KwaZulu - Natal | Podwojewski P.,IWMI International Water Management Institute | Poulenard J.,University of Savoy | Nguyet M.L.,IWMI International Water Management Institute | And 4 more authors.
Catena | Year: 2011

High mountain ecosystems are generally considered to be particularly sensitive to global climate change. Studies of pedogenesis associated with altitudinal variation, vegetation type and soil carbon content on the same type of parent rock are very limited in inter-tropical mountain areas. Therefore the altitudinal variation of soil pedogenesis through 9 selected profiles from the altitude of 1340m to 3143m asl, the summit of the Fan Si Pan Mountain, in the north of Vietnam was examined. Fan Si Pan Mountain is composed of a homogenous alkaline granite rock and is the highest point of the Inter-tropical Continental Asia. The Soil Organic Matter properties (C, N, δ13C and δ15N contents) of the different grain-size fractions of the topsoil of 4 selected profiles corresponding to different ecosystems were also examined. Three zones of different soil forming processes were present: Acrisols and Alisols at lower altitudes in sub montane forest, Podzols formed in montane and upper montane forest while Umbrisols formed at high altitude where the forest vegetation had given way to a shrubby vegetation or a steppe composed of dwarfed bamboo. With altitude, soils become sandier, have higher concentration of SiO2 and are lower in Al2O3. The selective Fe and Al oxalate (Feo and Alo) and pyrophosphate (Fep and Alp) extracts show a clear discrepancy between Acrisols or Umbrisols with no clear variation with soil depth and Podzols with high enrichment in their Bs horizon. The SOM status is highly dependent of the organic matter input by the vegetation. In Acrisols, the SOC is linked to the fine fraction within the soil profile with rapid turnover and low C/N values. Podzols are formed by the accumulation on soil surface of OM enriched in the coarse fraction with depleted δ15N and high C/N values. The organic matter input is exogenic and probably seasonal from leaves forming the surface litter while in Acrisols or Umbrisols the SOC is mainly linked to the fine fraction, and with constant δ15N values at depth. In Umbrisols, the SOC origin seems to be linked with endogenic inputs deep in the soil profile mostly produced by the decay of bamboo roots. In this tropical mountainous soil catena, the soil carbon mineralization depends not only on temperature and organo-metallic complexes that stabilize the non labile carbon pool but also is controlled by the pedogenetic process, which is linked with the vegetal ecosystem change with altitude. © 2011 Elsevier B.V.

Bui Y.T.,SFRI Soils and Fertilizers Research Institute | Orange D.,IRD Institute of Research for Development | Visser S.M.,Wageningen University | Hoanh C.T.,IWMI International Water Management Institute | And 4 more authors.
Hydrological Processes | Year: 2014

Developing models to predict on-site soil erosion and off-site sediment transport at the agricultural watershed scale represent an on-going challenge in research today. This study attempts to simulate the daily discharge and sediment loss using a distributed model that combines surface and sub-surface runoffs in a small hilly watershed (< 1km2). The semi-quantitative model, Predict and Localize Erosion and Runoff (PLER), integrates the Manning-Strickler equation to simulate runoff and the Griffith University Erosion System Template equation to simulate soil detachment, sediment storage and soil loss based on a map resolution of 30m × 30m and over a daily time interval. By using a basic input data set and only two calibration coefficients based, respectively, on water velocity and soil detachment, the PLER model is easily applicable to different agricultural scenarios. The results indicate appropriate model performance and a high correlation between measured and predicted data with both Nash-Sutcliffe efficiency (Ef) and correlation coefficient (r2) having values>0.9. With the simple input data needs, PLER model is a useful tool for daily runoff and soil erosion modeling in small hilly watersheds in humid tropical areas. © 2013 John Wiley & Sons, Ltd.

de Rouw A.,University Pierre and Marie Curie | Huon S.,University Pierre and Marie Curie | Soulileuth B.,MSEC programme | Jouquet P.,University Pierre and Marie Curie | And 7 more authors.
Agriculture, Ecosystems and Environment | Year: 2010

There is limited information, particularly in the tropics, of farming systems that loose or accumulate carbon in their soils. We compared no-till with a mulch-providing cover crop with conventional tillage without cover crop. Side effects were also investigated, weeds, surface crusting, soil macrofauna, infiltration, porosity and roots. The study site was a flat sandy clay loam. Treatments were maintained over five years; within this period, the time between the first and last soil sampling was exactly four years. Both times the same profile locations and exactly the same depths were sampled thereby greatly reducing inherent soil variability. Soil was sampled at five increments from 0 to 40 cm depth. The biomass contributions of maize, cover crop and weeds were measured. The main findings were: (1) The cover crop that was alleged to supply extra inputs to the no-till system failed to do so because the weeds in the tillage treatment became as efficient in accumulating biomass as the planted cover crop. (2) With equal organic inputs over four years (43.0 Mg dry weight ha-1 incorporated into the soil under conventional tillage, and 44.2 Mg dry weight ha-1 remaining on the soil surface as mulch under no-till), the tillage system stored (0-40 cm) significantly soil carbon (+590 g C m-2), whereas the no-till lost carbon (-133 g C m-2). The difference between the systems was significant. Carbon accumulated just below the plough layer. Nitrogen stocks remained unchanged. A very significant lowering of the C:N ratio occurred under no-till. The process of transforming the available biomass on the soil surface into organic matter is apparently too slow to avoid direct losses under no-till. Alternatively, ploughing plant residues into the soil enables to capture some of what would otherwise be lost as CO2 through decay, thereby increasing soil carbon. (3) In the last three years of the experiment, maize grain yields and crop residues stabilized at a lower level but were significantly higher under no-till, 16% and 34%, respectively. Higher yields were attributed to more soil water under no-till due to improved soil structure, though bulk density was not affected. The mulch layer protecting the soil surface favoured infiltration by keeping it crust-free. Water availability was further promoted by a better connectivity of pores and more macrofauna. However, the no-till system depended heavily on fertilizers and herbicides. The lack of effectiveness of herbicides against shifting weed communities threatens the continuation of the system. © 2009 Elsevier B.V. All rights reserved.

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