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Influence of vegetation on chemical fluxes in a tropical watershed: Mule Hole, South India

Abstract : The experimental watershed of Mule Hole (4.3 km²) is located in the sub-humid zone of a steep climatic gradient induced by the Western Ghäts in South India. It is covered by a dry deciduous forest whose evapo-transpiration accounts for almost 90% of the 1100mm annual rainfall [1]. The forest induces low groundwater recharge and maintains the groundwater disconnected from the stream: for this reason, Mule Hole constitutes an ideal site for estimating the influence of vegetation (throughfalls, forest fire ashes) on the chemical flues exported by a stream. The chemical composition of rainfall, throughfalls, stream water and soil solutions are monitored since 2004. The composition of ash dissolution was characterized with batch experiments on ashes of the main tree leaves and grass species [2]. The contribution of vegetation components were calculated using End-Member Mixing Analysis (EMMA). The signature of the ash end-member varies during the experiment according to (1) the vegetal species, i.e. high Ca, Mg, carbonate, K in leaves and high Si, K, Cl, OH in grass, (2) the dissolution kinetics of minerals present in ashes, in decreasing order salts (KCl, KOH), carbonates and amorphous silica. This latter factor influences the nutrient cycles: for instance, K mobility is enhanced by the instantaneous dissolution of salts whereas the slow dissolution of silica may allow efficient recycling within the soil profile. The application of EMMA on the stream composition indicates that when forest fire occurs (in 2004) the chemical composition of the first storms of the humid season is >90% controlled by the vegetation component (mostly ashes) for K, Ca, Mg, Si, alkalinity and Cl. The relative contribution of the vegetation component decreases through the season as a result of (1) increase of soil solution contribution and (2) drop of ash contribution. The vegetation component still dominates during the first storms of 2005. [1] Marechal et al. (2009) J. Hydrol. 364, 272-284. [2] Akerman et al. (2008) Geochim. Cosmochim. Acta 72. A1-A40.
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J Riotte, J Marechal, A Akerman, S Audry, P Oliva, et al.. Influence of vegetation on chemical fluxes in a tropical watershed: Mule Hole, South India. Goldschmidt Conference, Jun 2009, Davos, Switzerland. ⟨hal-03172409⟩

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