Ecohydrological modelling- Linking earthworms and preferential flow

EARTHWORM POPULATIONS

Preferential flow often takes place along macropores of biological origin, such as earthworm burrows and root channels. Earthworms are key soil ecosystem engineers that significantly influence soil organic matter dynamics, hydraulic properties, pedogenetic processes, and plant performance by modulating soil structure and forming long-lasting microstructures. They exert significant influence on process thresholds for preferential flow and overland flow and the initiation of rapid solute transport. Their burrows strongly influence infiltration, water movement, aeration, nitrogen leaching, phosphorous distribution and the fate of pesticides. Furthermore, the earthworm burrow walls, with their accumulation of organic carbon content, favourable aerobic conditions and enrichment in clay content, sustain a high microbial biomass and provide many more sorption sites than bulk soil. This filter function in the earthworm burrow walls diminishes strongly with increasing distance from the burrows, even over the first centimetres. Therefore, earthworms play a pivotal role in determining the ecosystem filter function and the provisioning of additional ecosystem services.
By their engineering activity, they are involved in several feedbacks regarding the coupled water-vegetation system determining the distribution of water regarding plant availability (and thus plant performance) as well as the effectivity of bypasses by preferential and overland flow, activity of microorganisms, and nutrient cycling. These feedbacks might be affected directly and indirectly by climate change due to alterations in temperatures and precipitation patterns (higher variability, more extreme events regarding heavy rainfalls and droughts), which will also affect vegetation and land use. To understand how these feedbacks affect the ecosystem filter function, we need to understand the interactions between soil structure, water flow, and earthworm activity. Analysing the feedbacks requires the development of an integrated eco-hydrological model (which may even be coupled to an eco-hydrological model of the vegetation-water system). Therefore the influence of the spatial and temporal variability of effective earthworm burrows from plot scale to catchment scale on the filter function of a catchment and the response to climate change should be studied.

At field scales earthworm abundance is known to depend on temperature, soil moisture, texture and management influence. Within this project the presence/absence of earthworms is modelled well at catchment scale, but the abundance modelling results are relatively poor. This is probably due to the high spatial variability in both earthworms as well as predictor variables at small scales. Therefore single measurements of earthworm abundance and predictor variables are not representative of the average field scale values.

Therefore a total number of 225 sampling plots divided over two agricultural fields are used to compare the short range variability of Lumbricus terrestris abundance between a field with high and another with low earthworm abundance. This leads to a recommendation on an earthworm sampling design which may result in a field scale representative value, taking into account short range variability.

PREFERENTIAL FLOW PATTERNS

Infiltration variability in the topsoil strongly determines the distribution of precipitation water to surface runoff, soil moisture storage and rapid percolation to groundwater. Slow matrix flow and rapid by-pass, i.e. preferential flow, result in a large variability of flow in the topsoil. As preferential flow may strongly influence the residence time in the top soil filter system, it is generally recognized as one of the most important processes which affect solute transport and contamination – and thus the catchment filter function. Preferential flow rate is found to depend mainly on the infiltration to preferential flow paths as the flow capacity within macropores is not likely to be the limiting factor. Different numbers and sizes of pores may be hydrologically effective under different conditions. The occurrence of preferential flow and its effectiveness at different scales is dependent on the antecedent moisture contents, rainfall forcing and distribution and connectivity of the preferential flow paths. In view of climate change with increasing rainfall intensities, the ability of the macropore system to increase or limit infiltration will determine water distribution to the different filter systems. The residence time in macropores and their filter function depends on their depth distribution and connectivity.

At a tile-drained field site the mobility of two different herbicides was studied with a controlled irrigation experiment (approx. 400m²). The less “sorptive” herbicide is Isoproturon (IPU) (N,N-dimethyl-N′-[4-(1-methylethyl)phenyl]urea) that is frequently used in this area. IPU has been previously shown to be susceptible for fast transport through structured soils (e.g., Zehe and Flühler, 2001a). Application of IPU at tile-drained sites in Germany is therefore forbidden. Flufenacet (FLU) (N-(4-fluorophenyl)-N-(1-methylethyl)-2-[[5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl]oxy]acetamide) is proposed as a successor of IPU. FLU is highly sorptive, has a low solubility in water, and is thus deemed to be less mobile in soils.

With this study we try to answer the following questions:

• How is the transport behavior of the two herbicides compared to the conservative tracer bromide?
• Are the concentration patterns of pesticides in soil profiles sufficient for characterising leaching depths in particular regarding the question whether transport into tile-drains might occurs or not?
• Is the stronger sorptivity of FLU sufficient to avoid rapid transport into the tile drains and surface waters body during a precipitation event?
• How does transport behavior of the tracer and pesticides compare to isotope data that are usually employed to separate different runoff components (Sklash and Farvolden, 1979)?

LINK EARTHWORMS - MACROPORES - FLOW PATTERNS

There are three different earthworm types which have different burrowing patterns. These result in varying infiltration patterns: from rapid deep vertical infiltration to a stronger diffuse distribution of water and solutes in the upper soil layers. Thus the spatial distribution of different ecological earthworm types can help us to understand the spatial variability in preferential infiltration patterns. However varying numbers and sizes of macropores may be hydrologically active under different conditions.

Therefore in this study the relationships between the abundance of different earthworm species, the resulting macropore numbers and sizes in different soil depths and their effectivity under high intensity rainfall are studied. This will lead to information for quantifying the spatial distribution of potential preferential flow paths using earthworm distribution models.