Desorption Kinetics of Legacy Soil Phosphorus: Implications for Non-Point Transport and Plant Uptake

Soil phosphorus (P) solubility and kinetics partly control dissolved P losses to surface water and uptake by plants. While previous studies have focused on batch techniques for measuring soil P desorption kinetics, flow-through techniques are more realistic because they simulate P removal from the system, akin to runoff, leaching, and plant uptake. The objectives were to measure soil P desorption by a flow-through technique at two flow rates and several batch methods, and utilize both for understanding how flow rate impacts the thermodynamics and kinetics of soil P desorption. Desorption obeyed first-order kinetics in two different phases: an initial rapid desorption phase followed by a gradual release. Desorption was limited by equilibrium and the kinetics of physical processes as demonstrated by an interruption test. Dilution-promoted desorption occurred with increasing cumulative volume, which increased desorption rate via first-order kinetics. The batch tests that simulated cumulative solution volume and time of flow-through were similar to the flow-through results; however, the batch methods overestimated the desorption rates due to less limitations to diffusion. Fast flow rates desorbed less P, but at a greater speed than slow flow rates. The differences were due to contact time, cumulative time, and solution volume, which ultimately controlled the potential for chemical reactions to be realized through physical processes. The interaction between these processes will control the quantity and rate of desorption that buffer P in non-point drainage losses and plant uptake.

Single and Binary Fe- and Al-hydroxides Affect Potential Phosphorus Mobilization and Transfer from Pools of Different Availability

Phosphorus (P) fixation is a global problem for soil fertility and negatively impacts agricultural productivity. This study characterizes P desorption of already fixed P by using KCl, KNO3, histidine, and malic acid as inorganic and organic compounds, which are quite common in soil. Goethite, gibbsite, and ferrihydrite, as well as hydroxide mixtures with varying Fe- and Al-ratio were selected as model substances of crystalline and amorphous Fe- and Al-hydroxides. Especially two- and multi-component hydroxide systems are common in soils, but they have barely been included in desorption studies. Goethite showed the highest desorption in the range from 70.4 to 81.0%, followed by gibbsite with values in the range from 50.7 to 42.6%. Ferrihydrite had distinctive lower desorption in the range from 11.8 to 1.9%. Within the group of the amorphous Fe-Al-hydroxide mixtures, P desorption was lowest at the balanced mixture ratio for 1 Fe: 1 Al, increased either with increasing Fe or Al amount. Precipitation and steric effects were concluded to be important influencing factors. More P was released by crystalline Fe-hydroxides, and Al-hydroxides of varying crystallinity, but desorption using histidine and malic acid did not substantially influence P desorption compared to inorganic constituents.

Novel Application of Hybrid Anion Exchange Resin for Phosphate Desorption Kinetics in Soils: Minimizing Re-Adsorption of Desorbed Ions

The process of phosphate desorption from soils is difficult to measure using stirred batch techniques because of the accumulation of desorbed ions in a bathing solution. To accurately measure the apparent rate coefficient of phosphate desorption from soils, it is necessary to remove the desorbed ions. In this study, a novel hybrid (i.e., iron oxide coated) anion exchange resin was used as a sink to study long-term (seven days) P desorption kinetics in intensively managed agricultural soils in the Midwestern U.S. (total phosphorus (TP): 196–419 mg/kg). The phosphate desorption kinetics in the hybrid anion exchange resin method were compared with those in the other conventional batch desorption method with pure anion exchange resins or without any sink. The extent of P desorption in the hybrid resin methods was >50% of total desorbed phosphate in the other methods. The initial kinetic rate estimated in the pseudo-second-order kinetic model was also highest (3.03–31.35 mg/(g·hr)) in the hybrid resin method when the same soil system was compared. This is because adsorbed P in the hybrid resins was nearly irreversible. The hybrid anion exchange resin might be a new and ideal sink in measuring the P desorption process in soils and sediments.

Autogenous Eutrophication, Anthropogenic Eutrophication, and Climate Change: Insights from the Antrift Reservoir (Hesse, Germany)

Climate change is projected to aggravate water quality impairment and to endanger drinking water supply. The effects of global warming on water quality must be understood better to develop targeted mitigation strategies. We conducted water and sediment analyses in the eutrophicated Antrift catchment (Hesse, Germany) in the uncommonly warm years 2018/2019 to take an empirical look into the future under climate change conditions. In our study, algae blooms persisted long into autumn 2018 (November), and started early in spring 2019 (April). We found excessive phosphorus (P) concentrations throughout the year. At high flow in winter, P desorption from sediments fostered high P concentrations in the surface waters. We lead this back to the natural catchment-specific geochemical constraints of sediment P reactions (dilution- and pH-driven). Under natural conditions, the temporal dynamics of these constraints most likely led to high P concentrations, but probably did not cause algae blooms. Since the construction of a dammed reservoir, frequent algae blooms with sporadic fish kills have been occurring. Thus, management should focus less on reducing catchment P concentrations, but on counteracting summerly dissolved oxygen (DO) depletion in the reservoir. Particular attention should be paid to the monitoring and control of sediment P concentrations, especially under climate change.

Mobilization of P from crystalline and amorphous Fe- and Al-hydroxides

<p>In neutral to acidic soils, the availability of phosphorus (P) is affected by its strong affinity for mineral surfaces. Especially the interaction between P and iron- and aluminum-(oxy)hydroxides (Fe- and Al-hydroxides) plays a crucial role in the immobilization and hence, availability of P for plants. In this context, the fixation of P is mainly determined by processes of adsorption, desorption, and precipitation. In that sense, the kinetics and mechanisms of P desorption from synthetic well crystalline goethite (α-FeO(OH)) and gibbsite (γ-Al(OH)<sub>3</sub>) as well as from amorphous ferrihydrite (Fe<sub>2</sub>O<sub>3</sub>·H<sub>2</sub>O) and Al-hydroxide (Al(OH)<sub>3</sub>) were characterized.</p><p>Different inorganic and organic desorption solutions were selected for these experiments. On the one hand, substance conversion processes take place in the soil system. High-molecular-weight organic compounds formed during humification and mineralization play an important role in soil environment and P mobilization. On the other hand, plants had developed a range of adaptive strategies in case of P demand. Plant roots excrete complex mixtures of organic compounds such as organic acids, amino acids, and sugars. Additionally, there are equilibrium reactions, which are determined by the respective ionic strength of the soil solution itself. For a comparison regarding the efficiency of P mobilization from synthetic Fe- and Al-hydroxides, the desorption solutions CaCl<sub>2</sub>, and CaSO<sub>4</sub> were chosen as main components of the soil solution, and humic and citric acid were selected as organic ligands following humification or produced by organisms in the rhizosphere.</p><p>Previous P adsorption experiments revealed the formation of adsorbed P surface complexes on crystalline hydroxides by using Fourier-Transform Infrared spectroscopy. Amorphous Al-hydroxides, characterized by a less rigid crystal structure, revealed higher accessibility of P binding sites within the particle structure. The higher accessibility of binding sites was also observed for ferrihydrite. The amorphous character enabled the diffusion of P into the mineral particle, where stable surface complexes and precipitates were formed. Hence, the grade of crystallinity affects the extent of precipitated and low-soluble P complexes.</p><p>After 8 weeks of desorption time, the cumulative P desorption increased following the order CaCl<sub>2</sub> < CaSO<sub>4</sub> < humic acid < citric acid. Amorphous ferrihydrite exhibited much less desorption when exposed to inorganic solutions than goethite, gibbsite, or Al-hydroxide. Modeling of the desorption data suggested a diffusion-controlled desorption step for ferrihydrite with citric acid as sorptive. The determination of C<sub>Total</sub> also indicated various release mechanisms of the organic acids: while the use of humic acid led to the accumulation of metal-organic complexes in the solution, citric acid dissolved the mineral phase and hence, also low-soluble precipitated P-complexes. The results suggest organic compounds, especially citric acid, are more important for the mobilization of P from both crystalline and amorphous Fe- and Al-hydroxides than inorganic ions present in the soil solution.</p>

Linking phosphorus sorption and magnetic susceptibility in clays and tropical soils

Maghemite (Mh) and magnetic susceptibility have been little studied in relation to phosphorus (P) sorption, despite the fact that tropical soils – particularly those derived from mafic rocks – may contain substantial amounts of this iron oxide. In this work, we investigated the relationship between P adsorption and magnetic susceptibility in tropical soils, and determined the maximum adsorption capacity of P (MACP) and P desorption in seven pedogenic clays from magnetic soils with contrasting parent materials and three synthetic Mh samples. Considering the heterogeneity of the soil dataset in this study, the exclusive adoption of magnetic susceptibility as an indicator of P adsorption potential in soil remains uncertain. The relationship between magnetic susceptibility and adsorbed P was more evident in the B horizon of red soils from basic igneous rocks. In this group, soils with magnetic susceptibility above 20 × 10−6 m3 kg−1 had high adsorbed P. Although the pedogenic clays exhibited lower MACP values (1353–2570 mg kg–1) than the synthetic Mh samples (3786–4321 mg kg–1), P desorption exhibited the opposite trend (~14% vs ~8%). The substantial P adsorption capacity of synthetic Mh confirmed the adsorption data for pedogenic clays, which were strongly influenced by magnetic susceptibility, Mh and gibbsite contents, and specific surface area.

Quantifying citrate-enhanced phosphate root uptake using microdialysis

Aims Organic acid exudation by plant roots is thought to promote phosphate (P) solubilisation and bioavailability in soils with poorly available nutrients. Here we describe a new combined experimental (microdialysis) and modelling approach to quantify citrate-enhanced P desorption and its importance for root P uptake. Methods To mimic the rhizosphere, microdialysis probes were placed in soil and perfused with citrate solutions (0.1, 1.0 and 10 mM) and the amount of P recovered from soil used to quantify rhizosphere P availability. Parameters in a mathematical model describing probe P uptake, citrate exudation, P movement and citrate-enhanced desorption were fit to the experimental data. These parameters were used in a model of a root which exuded citrate and absorbed P. The importance of soil citrate-P mobilisation for root P uptake was then quantified using this model. Results A plant needs to exude citrate at a rate of 0.73 μmol cm−1 of root h−1 to see a significant increase in P absorption. Microdialysis probes with citrate in the perfusate were shown to absorb similar quantities of P to an exuding root. Conclusion A single root exuding citrate at a typical rate (4.3 × 10−5 μmol m−1 of root h−1) did not contribute significantly to P uptake. Microdialysis probes show promise for measuring rhizosphere processes when calibration experiments and mathematical modelling are used to decouple microdialysis and rhizosphere mechanisms.

Rapid detection of nicotine from breath using desorption ionisation on porous silicon

Desorption ionisation on porous silicon mass spectrometry was used for the detection of nicotine from exhaled breath.

Electrons, excitons and hydrogen bonding: electron-promoted desorption from molecular ice surfaces

Desorption of benzene from methanol and diethyl ether ices during irradiation with 250 eV electrons is reported and compared with our previous work on benzene/water ices to highlight the role of hydrogen bonding in excitation transport.