Weyer, C; Peiffer, S; Schulze, K; Borken, W; Lischeid, G: Catchments as heterogeneous and multi-species reactors: An integral approach for identifying biogeochemical hot-spots at the catchment scale, J. Hydrology, 519, 1560-1571 (2014), doi:10.1016/j.jhydrol.2014.09.005
From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation or deposition as they pass through soils and aquifers towards draining streams. Understanding and modeling the variability of solute concentrations in catchment waters require the identification of the prevailing processes, determining their respective contribution to the observed transformation of substances, and the localization of "hot spots", that is, the most reactive areas of catchments.

For this study, we applied a non-linear variant of the Principle Component Analysis, the Isometric Feature Mapping (Isomap), to a dataset composed of 1686 soil solution, groundwater and stream water samples and 16 variables (Al, Ca, Cl, Fe, K, Mg, Mn, Na, NH4, NO3, SO4, total S, Si, DOC, electric conductivity and pH values) from the Lehstenbach catchment in Germany, (i) to assess the contribution of the prevailing biogeochemical processes to the variability of solute concentrations in water samples taken from soils, in groundwater and in stream water in a catchment and (ii) to identify hot spots at the catchment scale with respect to 16 solutes along different flow paths.

The first three dimensions of the Isomap analysis explained 48%, 30% and 11%, respectively, i.e. 89% of the variance in the data set. Scores of the first three dimensions could be ascribed to three predominating bundles of biogeochemical processes: (i) redox processes, (ii) acid-induced podzolization, and (iii) weathering processes, based on the relationship between SO4, NO3, Mn, K, Mg, Ca, Fe and Si concentrations, electric conductivity and pH values with respect to the first dimension, between DOC, Fe, Al, SO4, Na and pH values with respect to the second dimension, and between Cl, Si, NH4, K, Na, Al and SO4 concentrations and pH values with respect to the third dimension. Along the flow paths from upslope soils, the upper 90 cm layer could be considered as a hot spot both with respect to acid induced podzolization and weathering processes. Nearly 53% of the variance with respect to acid-induced podzolization could be traced back to the hot spot which represented only 2% of the total spatial volume of the catchment. Along the wetland flow paths, hot spots were found for all three bundles of biogeochemical processes, i.e. the upper 10 cm layer related both to redox processes and acid induced podzolization, and the upper 1 m layer with respect to weathering processes. Nearly 67% and 44% of the variance with respect to redox processes and acid-induced podzolization could be traced back to the respective hot spots, representing only 0.1% of the total spatial volume of the catchment. In contrast, biogeochemical processes in deep groundwater had only minor effects on the biogeochemical turnover in catchment waters. Hot spots with respect to weathering processes along upslope and wetland flow paths could not be quantified due to effects of preferential sampling in soil solution samples. Predominating flow paths and biogeochemical processes crucial for the variability of stream water chemistry differed substantially for three streams but were consistent with presumed mixing ratios.

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