P transport and accumulation along preferential flow pathways in forest soils

1. Roots > 2mm Tracer stainedarea Macropores C B A Soildepth [cm] P transport and accumulation along preferential flow pathways in forest soils Dorit Julich, Jianhong Liang, Karl-Heinz Feger Dyecoverage [%] Phosphorus (P) is an essential nutrient for living organisms. Whereas modern agriculture avoids P limitation of primary production through the continuous application of P fertilizers, forest ecosystems have developed highly efficient strategies to adapt to low P supply. A main hypothesis of the SPP 1685 is that P depletion of soils drives forest ecosystems from P acquiring system (efficient mobilization of P from the mineral phase) to P recycling systems (highly efficient cycling of P). Regarding P fluxes in soils and from soil to streamwater, this leads to the assumption that recycling systems may have developed strategies to minimize P losses. Further, not only the quantity but also the chemistry (P forms) of transported or accumulated P will differ between the ecosystems. In our project, we will therefore experimentally test the relevance of the two contrasting hypothetical nutritional strategies for P transport processes through the soil and into streamwater. As transport processes will occur especially during heavy rainfall events, when preferential flow pathways (PFPs) are connected, we will focus on identifying those subsurface transport paths. In a first step, we identified preferential flow pathways by dye tracer experiments and analyzed flow patterns by digital image analyzing tools. Further, soil samples were taken from stained (flow region) and unstained (non-flow region) compartments. The samples were analyzed for P fractions using a modified Hedley fractionation method. different P losses different P forms Tracking of P in preferentialflowpathways Water-Tracer-Solution Tracer Distribution Profile Cuts Digital Analysis Soilsampling P fractionation + Digital imageanalysis Hedleyfractionation (modifiedbyTiessen) • Original image • Geometriccorrection(spatialdistortion, lensaberration) • Color correction (chromaticaberra-tion, whitebalance) • Correctedimage • Adjustmentofpixel/cm relation • Transformation from RGB to CMYK colorspaceandextractionofcyanchannel • Binary image • Black/whitecoding • Statistical analysiswith R Dyecoverage - no. ofstainedpixels per depth: comparisonofprofilesfromdiffe-rentsites, rangeofpro-file cuts, depthofdyepenetration („tracerbreakthrough“), etc. P fractionsof different soilhorizonsasmeanof 4 profilecuts (A-organiclayer, M1-topsoil (Sw-Ah), M2-subsoil (Srw), M3-subsoil (IISrd) in flowpatrhways (PFP) andsoilmatrix (Ma) at thetestsiteRehefeld (Ore Mountain/NE Germany). First results Outlook • PFP’s can be identified and sampled by dye tracer experiments. • Visual distribution of PFP’s and dye coverage are highly variable in the different cuts of the test pro-file (Rehefeld/Ore Mountains). • The distribution of P fractions in PFP and soil matrix on the test site strongly depends on soil depth (horizon characteristics). In the topsoil, we found more labile (P-resin, P-NaHCO3) and moderately labile (P-NaOH) forms whereas apatite-bound P (P-HCldil) was higher concentrated in the subsoil horizons. The stabile residual fraction was slightly higher in the upper horizons compared to subsoil. • The comparison of P forms in PFP and matrix samples showed no significant differences in all P fractions and depths at this site. A reason may be the high variability of results for different profile cuts. Further, the fractionation method as to be optimized to gain more reliable results (e.g. problems with analysis of the concentrated HCl fraction). • Further statistical analysis of the flow pathways to identify flow mechanism and influences of soil parameters (Mixed Effect Modeling). • Tracer experiments on the SPP core sites with different soil and site characteristics with special focus on availability of mineral P (“nutrition strategy”). • Tracer experiments with different irrigation rates, pre-moisture conditions of the soil and along a hillslope (in process). • Improvement of the reliability of the P fractionation method. • Streamwater monitoring to assess water fluxes and P transport from soil into streams. • Model-based estimation of P losses and comparison of results for different sites of the SPP.