Linking transport pathways and phosphorus distribution in a loamy soil: a case study from a North-Eastern German Stagnosol.

Heterogeneous flow pathways through the soil determine the transport of dissolved and particle-bound nutritional elements like phosphorus (P) to ground and surface waters. This study was designed to understand the spatial patterns of P in agriculturally used soils and the mechanisms causing P accumulation and depletion at the centimetre scale. We conducted dye tracer experiments using Brilliant Blue on a loamy Stagnosol in North-Eastern-Germany. The plant-available P was analysed using double lactate extraction (DL-P). The plant-available P content of the topsoil was significantly higher than that of the subsoil in all three replicates (p < 0.001). The topsoil's stained areas showed significantly higher P contents than unstained areas (p < 0.05), while the opposite was found for the subsoil. The P content varied enormously across all observed soil profiles (4 to 112 mg P kg-1 soil) and different categories of flow patterns (matrix flow, flow fingers, macropore flow, and no visible transport pathways). The P contents of these transport pathways differed significantly and followed the order: Pmatrix flow > Pfinger flow > Pno visible transport pathways > Pmacropore flow. We conclude that P tends to accumulate along flow pathways in the topsoil in the observed fertilized and tilled mineral soil. In contrast, in the subsoil at a generally lower P level, P is depleted from the prominent macroporous flow domains.

Visualization of Colloid Transport Pathways in Mineral Soils Using Titanium(IV) Oxide as a Tracer.

In soils, colloidal transport has been identified as the most important pathway for strong adsorbing, environmental contaminants like pesticides, heavy metals, and phosphorus. We conducted a comparative dye tracer experiment using a Brilliant Blue (BB) solution and a Titanium(IV) oxide (TiO) colloid suspension (average particle size 0.3 µm), aiming to visualize and quantify colloid pathways in soils. Both dye tracers showed comparable general flow patterns with preferred transport over the deepest part of the soil profile, independent of clay content. The stained area was generally smaller for TiO than for BB by a factor of ten, however, and there was no TiO to be found at all in the low clay content soil. The travel distance was almost identical for the solution and the suspension (0.7 m) giving evidence that environmentally critical compounds bound to microparticles may be vertically transported over longer distances in soils, even within single rainfall events. The spatial variability of the dye patterns was large on a small scale with a range of 0.35 m for TiO in the horizontal plane, which was taken as a general proof for a pronounced preferential transport situation. The study indicates that TiO is transported exclusively through singular macropores of biogenetic nature, while BB passes also through the soil matrix of coarse-bedded soils, the secondary pore system or interaggregate pore space. The results emphasize the general suitability of TiO for the visualization of colloid transport pathways in soils, opening up new research opportunities for contaminant transport in soils.

Water regime of mechanical-biological pretreated waste materials under fast-growing trees.

In this study mechanical-biological pre-treated waste material (MBP) was tested for suitability to serve as an alternative surface layer in combination with fast-growing and water-consumptive trees for final covers at landfill sites. The aim was to quantify evapotranspiration and seepage losses by numerical model simulations for two sites in Germany. In addition, the leaf area index (LAI) of six tree species over the growing season as the driving parameter for transpiration calculations was determined experimentally. The maximum LAI varied between 3.8 and 6.1 m2 m(-2) for poplar and willow clones, respectively. The evapotranspiration calculations revealed that the use of MBP waste material for re-cultivation enhanced evapotranspiration by 40 mm year(-1) (10%) over an 11 year calculation period compared to a standard mineral soil. Between 82% (for LAI(max) = 3.8) and 87% (for LAI(max) = 6.1) of the average annual precipitation (506 mm) could be retained from the surface layer assuming eastern German climate conditions, compared with a retention efficiency between 79 and 82% for a mineral soil. Although a MBP layer in conjunction with water-consumptive trees can reduce vertical water losses as compared to mineral substrates, the effect is not sufficient to meet legal regulations.