Tracing and modeling preferential flow in a forest soil — Potential impact on nutrient leaching

Abstract Accurate nutrient budgets in forest ecosystems are needed in order to plan sustainable forest management on poor soils. Such budgets require precise measurements of water and nutrient leaching through the soil. This study aims to characterize the hydrological processes and compute hydrological budgets occurring in a very poor and acidic soil under a 30-year-old beech stand in the Morvan Mountains (France). A forest plot was set up with rain collectors, lysimeters and TDR probes, and we used a deuterated water tracing experiment and two hydrological models (BILJOU and HYDRUS-1D) to estimate the proportion of preferential and slow convective water flow, and to compute the influence of preferential flow paths on nutrient leaching. Preferential flow paths were evidenced by the deuterium tracing experiment. Tracer dynamic through the soil and soil water content variations were successively modeled. This approach enabled us to define the main condition leading to preferential flow generation (rainfall above 3.5 mm/h) and quantify the proportion of preferential flow (54%). Finally, the computed nutrient leaching fluxes of major elements Ca, Mg, NO3 and Al were strongly increased when considering preferential flow paths. The experimental and modeling approach proved to be complementary and we recommend the use of tracing experiments for better model calibration, especially when their outputs are used to compute nutrient leaching fluxes.