How to Control the Lysimeter Bottom Boundary to Investigate the Effect of Climate Change on Soil Processes?

Abstract

Core Ideas The water flux in lysimeters with a tension‐controlled bottom boundary depends strongly on the applied pressure head. The surrounding subsurface conditions have a significant influence on the measured pressure heads that are used to control the bottom boundary of transferred lysimeters. Use of nonappropriate water table depths and soil textural properties which do not correspond to the conditions where the lysimeter originated can lead to large differences in soil water fluxes. The control of the bottom boundary of transferred lysimeters should be managed by measured pressure heads from the site where the lysimeter was taken from in order to enable a direct comparison of changes in soils and to investigate the effect of climate change on soil processes and soil functions. A dynamic tension‐controlled bottom boundary of lysimeters allows observing water and matter fluxes in lysimeters that are close to natural field conditions, as pressure heads at the lysimeter bottom are adjusted to measured pressure heads at the same depth in the surrounding field. However lysimeters are often transferred from their sampling location for practical reasons or to study, for example, the effect of climate change on soil functions. This transfer can be accompanied by a change aboveground but also in subsurface conditions that are used to control the bottom boundary and that may affect the soil water balance of lysimeters. This issue is also relevant for lysimeter stations which use a tension‐controlled bottom boundary and are not directly installed near the site of excavation. The potential impact of different bottom boundary conditions on the water balance of lysimeters that were transferred in a climate impact experiment (SOILCan) was investigated exemplarily by a numerical study. Results showed that by using nonappropriate pressure heads, which were measured in soil profiles with a different texture and water table depth than the profile where the lysimeter was taken from, had partially large impacts on soil water fluxes, especially when the water table was located within a specific critical range. Different climate conditions between sampling and installation site were buffered by the soil and did not show a strong influence on the bottom boundary control of lysimeters when the groundwater table depth was assumed to remain constant. Considering a change in groundwater table depths due to changing climate tempered the effects of climate change on the soil water balance terms. In general, results demonstrate the importance of a proper control of the lysimeters bottom boundary conditions in studies that investigate the influence of climate change on soil functions and ecosystem variables by transferring lysimeter along climate gradients.