Flow processes and their impacts on soil chemistry (Chapter 3)

Pattern analysis based on the risk index for vertical solute propagation revealed the occurrence of preferential flow at the German study site. The second part of this thesis is devoted to statistical analysis of mechanisms of preferential flow (Chapter 3.1) and its impacts on soil chemistry (Chapter 3.2). We did three tracer experiments and qualitatively identified the dominant flow regime based on stained patterns. We sampled soil material and analysed soil texture, fine root density and soil bulk density from preferential flow paths and the soil matrix and tested whether these parameters differed significantly between regions of preferential flow and soil matrix and could give insight into mechanisms of water flow.

The data were sampled hierarchically on three plots, in several profiles and different horizons. Since some horizons were bypassed by the flow we had to deal with missing values. Hierarchical sampling might induce dependencies in data so that classical statistical techniques like the analysis of variance are not applicable.

Most studies that investigate differences between preferential flow paths and soil

matrix ignore the hierarchical nature of data sampled from different plots and use the paired t-test of mean values or its non-parametric equivalent and test different depths separately. We propose to employ mixed-effects models and to consider all plots and all depths in one single analysis. Mixed-effects models can account for fixed-effects representing parameters of the entire population or certain repeatable levels of experimental factors (like horizon) and for random-effects associated with individual experimental units drew at random from a population (like plots or profiles). Furthermore, they are robust against missing values (Pinheiro & Bates, 2000).

Our data showed that at this study site roots constituted main preferential flow paths and induced macropore flow, especially in the topsoil. In the subsoil, root density decreased and inhomogeneous infiltration from preferential flow paths into the soil matrix caused unstable flow. Due to the large sand content (i.e. high permeability) the dye spread from preferential flow paths into the soil matrix creating large stained objects. We found no significant differences in soil texture between preferential flow paths and soil matrix. In contrast, fine root density was higher in preferential flow paths indicating the importance of roots channels as macropores. Soil bulk density was lower in preferential flow paths probably because of higher organic matter content. Root turnover is an important source of soil carbon and decomposition of dead roots is a major input to soil organic matter (Tate et al., 1993; Guo et al., 2005). Soil bulk density is known to decrease with increasing content of organic matter (e.g. Balland et al., 2008).

Root macropores promote preferential transport of solutes from the organic horizons to the subsoil. Furthermore, roots are known to strongly influence their immediate environment, the rhizosphere, by exudation of organic compounds. As a consequence distinct chemical compartments might develop with gradients in the transition zone between the soil matrix and preferential flow paths. For that reason, we analysed exchangeable cations, pH, and total C and N contents in the same soil samples to elucidate eventual impacts of preferential flow on soil chemistry by means of mixed-effects modelling.

Brilliant Blue (C37H34N2Na2O9S3, molar mass 792.9 g mol^-1) is an organic molecule consisting of 56% C and 4% N. Sorption of the dye on soil particles affects the C and N contents of soil and should be corrected. Usually, Brilliant

Blue concentrations are determined by extracting the dye with a water acetone solution or a 0.5 M K2SO4 (e.g. Bundt et al., 2001). This is a laborious procedure with changing accuracy due to varying mass recovery (Forrer et al., 2000). We developed a method to measure the content of Brilliant Blue by visible diffuse reflectance spectroscopy (VIS-DRS) directly on soil samples without extraction (Appendix A). We corrected the content of total C and N for presence of Brilliant Blue prior to mixed-effects modelling.

We found smaller pH values more Ca, more Mg, more C and more N in preferential flow paths. Compared to the adjacent soil matrix, more Al and more Fe (but small absolute amounts) were found in the subsoil where macropore flow along root channels decreases and heterogeneous matrix flow dominates. These distinct chemical properties can be explained by root activity and translocation of solutes via preferential flow paths. Higher Ca and Mg concentration in preferential flow paths are probably due to transport from the soil surface after liming. Smaller pH values could be explained by transport of acid soil solution from organic horizons along preferential flow paths. Higher Al and Fe concentration in the subsoil probably results from release and translocation of these solutes during podzolisation. Rhizodeposition of organic compounds, decomposition of dead roots and transport of DOC from organic to mineral horizons are major sources of organic C input to the soil (e.g. Kuzyakov &

Komansky (2000)). Higher root densities in preferential flow paths lead to a higher C input through roots, but also facilitate preferential transport of DOC.

Indeed, there is strong experimental indication of transport of DOC via preferential flow paths at our study site (Schulze et al. 2009). DOC is strongly adsorbed in soils by Al and Fe oxides/hydroxides and clay minerals (Kalbitz et al., 2000). During transport along preferential flow paths contact time between DOC and soil is reduced so that DOC is transported to greater depth where it potentially form organo-mineral associations. If this holds true, preferential flow is a mechanism that promotes C sequestration in subsoil. We conclude that preferential flow does not only influence its immediate environment around paths, but also underlying subsoil horizons.