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Phosphorus (P) is essential for sustainable forest growth, yet the impact of anthropogenic impacts on P leaching losses from forest soils is hardly known. We conducted an irrigation experiment with 128 mesocosms from three forest sites representing a gradient of resin extractable P of the A-horizon. On each site we selected a Fagus sylvatica and a Picea abies managed subsite. We simulated ambient rain (AR), anthropogenic nitrogen input (NI) of 100 kg (ha · a)−1 and forest liming (FL) with a dolomite input of 0.3 Mg (ha · a)−1. Soil solution was extracted from the organic layer, 10 cm depth and 20 cm depth of the mesocosms, and analyzed for molybdate reactive phosphorus (MRP) and molybdate unreactive phosphorus (MUP). Additionally, we separated colloids from the soil solution using Asymmetric Field Flow Fractionation for assessing the colloidal fraction of total element concentrations. NI increased MRP and MUP concentrations for all plots with one exception, while FL decreased MRP and MUP with the exception of another plot. While the irrigation treatments had little impact on the P-richest site, MRP and MUP concentrations changed strongly at the poorer sites. The colloidal fraction of P in the soil solution equaled 38–47% of the total P load. Nitrogen input and liming also affected the Fe, Al, Ca, and Corg contents of the colloidal fraction.

Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 ‘‘Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources’’ were added to provide up-to-date flux-based information. Nitrogen (N) additions increase the release of water-transportable P forms. Most P found in percolates and pore waters belongs to the so-called dissolved organic P (DOP) fractions, rich in orthophosphate- monoesters and also containing some orthophosphate-diesters. Total solution P concentrations range from ca. 1 to 400 mg P L–1, with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40–100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streamswas found to be less than 50mgPm–2 a–1, suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem-based assessments of dissolved and colloidalP fluxeswithin and fromtemperate forest systems. peerReviewed

Summary The effects of time and temperature on the release kinetics of water-dispersible colloids (WDCs) from three German silt loam topsoils in deionized water were investigated in batch experiments under low-energy rotating shaking conditions. The measured critical coagulation concentrations of Ca2+ and Na+ for extracted WDC were much larger than the experimental ionic conditions. This indicates a fast dispersion rate in the first detachment step of WDC mobilization from soil aggregates. The cumulative released WDC fraction F(t) (released WDC/clay content in bulk soil) was satisfactorily fitted to the square root of shaking time by a linear function in three soils with a similar clay content. This implies diffusion-controlled release kinetics in the second step of the WDC mobilization process. The mobilization kinetics were modelled by considering a diffusion-controlled transport through an immobile water layer in the macropores of soil aggregates formed by silt and sand particles. The effects of temperature on the mobilization kinetics and sedimentation volumes of saturated soils were compared at 7, 23 and 35°C. A linear correlation was found between immobile water layer thickness in soil macropores (lt) and the water volume (Vwater) in soil sediment, which indicates a strong dependence of lt on the soil texture. Temperature-sensitive lt and Vwater influenced the effect of temperature on WDC release, which counteracts the estimated effect of temperature on particle diffusion according to the Stokes-Einstein relation. A larger decrease in F(t) was found in grassland and forest soils than in an arable soil and can be related to greater stagnant water contents (larger lt and Vwater) in soil macropores, where particulate organic matter and polyvalent cations in their oxide forms at acidic pH will thus contribute to water immobilization.

Global change is increasingly affecting our terrestrial ecosystem, including the filtering and buffering capacity of soils, the supply of clean water, and the production of food, feed, and fiber. This article introduces a portfolio of papers in a special section of Vadose Zone Journal addressing flow, transport and biogeochemical processes of terrestrial ecosystems. The articles document research at the Agrosphere Institute, Forschungszentrum Julich, Germany. They are organized along three major themes of research at Agrosphere: functional analysis of soils and sediments, development of sensing methods to explore terrestrial systems, and modeling of flow and transport processes of terrestrial systems.

Column experiments were conducted to investigate the transport and deposition behavior of representative hydrophobic and hydrophilic bacteria strains in sand at different water saturations. These strains are similar in surface charge, shape, and size, and differ primarily in their surface hydrophobicity and tendency to form aggregates. The amount of bacteria that were retained in the sand increased with decreasing water saturation, especially for the more hydrophobic strain that formed larger cell aggregates. Most of the cells were retained close to the column inlet, and the rate of deposition rapidly decreased with depth. The experimental data were analyzed using a mathematical model that accounted for deposition on two kinetic sites. Consideration of depth-dependent deposition in the model formulation significantly improved the description of the data, and the amount of cell retention was typically dominated by this site. The depth-dependent deposition coefficient tended to increase with decreasing water content, especially for the hydrophobic bacteria. Straining is believed to account for these observations because it increases in magnitude with increasing cell and aggregate size and when a greater fraction of the water flows through a larger number of small pore spaces with decreasing water content. Cell retention on the other kinetic deposition site was well described using a conventional model for attachment and detachment. Consistent with interaction energy calculations for bacteria attachment, however, low amounts of cell retention occurred on this site. Attempts to separately determine the amounts of attachment to solid–water and air–water interfaces were confounded by the influence of straining.

Allein in Deutschland wurden im Jahre 1990 ca. 650 000 Tonnen Tenside produziert. Hinzu kommt noch eine deutlich grosere Menge an naturlichen Tensiden. Als Bestandteile von Abwassern, von landwirtschaftlich genutzten Klar-schlammen und als Formulierungshilfsmittel von Pflanzenschutzmitteln gelangen die Tenside in den Boden, wo sie neben ihrem biologischen Abbau physikalisch-chemischen Wechselwirkungen unterliegen, die im Gegensatz zum biologischen Abbau bislang nut wenig untersucht wurden. Aufgrund der besonderen grenzflachenaktiven Eigenschaften von Tensiden ist jedoch mit synergistischen Oder antagonistischen Effekten bei der Immobilisierung, Remobilisierung und Akkumulation der in den Boden eingetragenen organischen und anorganischen Substanzen zu rechnen. Diese Effekte mussen einerseits wegen der biologischen Verfugbarkeit dieser Stoffe beachtet werden. Andererseits beeinflussen sie den Transport von Schadstoffen in das Grundwasser, der wegen der moglichen Belastung des Trinkwassers verhindert werden mus1).

Natural fine colloids and nanoparticles have the potential to encapsulate and bind nutrients. Their size and composition is therefore relevant to understand the transport of essential nutrients like phosphorus in an aquatic ecosystem. The aim of this study was to characterize fine colloidal and nanoparticulate bound P of distinct hydromorphological areas in stream water from a forested test site in a small headwater catchment. Asymmetric flow field flow fractionation coupled online to inductively coupled plasma mass spectrometry was applied for size-resolved detection of P, Fe, and Al in the fractions. Online P detection was a challenge due to the low concentrations (in this study down to 0.1 μg/L) in many natural waters. Additionally, the “dissolved” organic matter (DOM) content was derived from the online UV signal. The colloidal P occurred in two size fractions (2–20 and 21–300 nm), which constituted up to 100% of the total river P discharge depending on hydromorphology. For the small size fraction, variations in P concentrations correlated with Al variations; in addition, a high Fe presence in both fractions was accompanied by high P concentrations. Moreover, DOM was detected with P in the presence of Fe and Al, suggesting that Fe and Al are carriers of P and associated with organic matter. The developed methodology enables the inputs and source regions of fine colloidal and nanoparticulate fractions within a small river of a headwater catchment to be traced and conceptually defined for the first time.