Use of iron oxide nanoparticles for immobilizing phosphorus in-situ: Increase in soil reactive surface area and effect on soluble phosphorus

Phosphorus (P) immobilization has potential for reducing diffuse P losses from legacy P soils to surface waters and for regenerating low-nutrient ecosystems with a high plant species richness. Here, P immobilization with iron oxide sludge application was investigated in a field trial on a noncalcareous sandy soil. The sludge applied is a water treatment residual produced from raw groundwater by Fe(II) oxidation. Siliceous ferrihydrite (Fh) is the major Fe oxide type in the sludge. The reactive surface area assessed with an adapted probe ion method is 211-304 m² g^-1 for the Fe oxides in the sludge, equivalent to a spherical particle diameter of ~6-8 nm. This size is much larger than the primary Fh particle size (~2 nm) observed with transmission electron microscopy. This can be attributed to aggregation initiated by silicate adsorption. The surface area of the indigenous metal oxide particles in the field trial soils is much higher (~1100 m² g^-1), pointing to the presence of ultra-small oxide particles (2.3 ± 0.4 nm). The initial soil surface area was 5.4 m² g^-1 and increased linearly with sludge application up to a maximum of 12.9 m² g^-1 when 27 g Fe oxides per kg soil was added. In case of a lower addition (~10-15 g Fe oxides per kg soil), a 10-fold reduction in the phosphate (P-PO₄) concentration in 0.01 M CaCl₂ soil extracts to 0.3 µM was possible. The adapted probe ion method is a valuable tool for quantifying changes in the soil surface area when amending soil with Fe oxide-containing materials. This information is important for mechanistically predicting the reduction in the P-PO₄ solubility when such materials are used for immobilizing P in legacy P soils with a low P-PO₄ adsorption capacity but with a high surface loading.

Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review

The EU Water Framework Directive (WFD) obliges Member States to improve the quality of surface water and groundwater. The measures implemented to date have reduced the contribution of point sources of pollution, and hence diffuse pollution from agriculture has become more important. In many catchments the water quality remains poor. COST Action 869 was an EU initiative to improve surface water quality that ran from 2006 to 2011, in which 30 countries participated. Its main aim was a scientific evaluation of the suitability and cost-effectiveness of options for reducing nutrient loss from rural areas to surface waters at catchment scale, including the feasibility of the options under different climatic and geographical conditions. This paper gives an overview of various categories of mitigation options in relation to phosphorus (P). The individual measures are described in terms of their mode of action, applicability, effectiveness, time frame, environmental side-effects (N cycling) and cost. In total, 83 measures were evaluated in COST Action 869.

Phosphorus movement and speciation in a sandy soil profile after long-term animal manure applications

Long-term application of phosphorus (P) with animal manure in amounts exceeding removal with crops leads to buildup of P in soil and to increasing risk of P loss to surface water and eutrophication. In most manures, the majority of P is held within inorganic forms, but in soil leachates organic P forms often dominate. We investigated the mobility of both inorganic and organic P in profile samples from a noncalcareous sandy soil treated for 11 yr with excessive amounts of pig slurry, poultry manure, or poultry manure mixed with litter. Solution 31P nuclear magnetic resonance spectroscopy was used to characterize NaOH-EDTA-extractable forms of P, corresponding to 64 to 93% of the total P concentration in soil. Orthophosphate and orthophosphate monoesters were the main P forms detected in the NaOH-EDTA extracts. A strong accumulation of orthophosphate monoesters was found in the upper layers of the manure-treated soils. For orthophosphate, however, increased concentrations were found down to the 40- to 50-cm soil layers, indicating a strong downward movement of this P form. This was ascribed to the strong retention of orthophosphate monoesters by the solid phase of the soil, preventing orthophosphate sorption and facilitating downward movement of orthophosphate. Alternatively, mineralization of organic P in the upper layers of the manure-treated soils may have generated orthophosphate, which could have contributed to the downward movement of the latter. Leaching of inorganic P should thus be considered for the assessment and the future management of the long-term risk of P loss from soils receiving large amounts of manure.

Phosphorus leaching from cow manure patches on soil columns

The loss of P in overland flow or leachate from manure patches can impair surface water quality. We studied leaching of P from 10-cm-high lysimeters filled with intact grassland soil or with acid-washed sand. A manure patch was created on two grassland and two sand-filled lysimeters, and an additional two grass lysimeters served as blanks. Lysimeters were leached in the laboratory during 234 d with a diluted salt solution, and column effluent was passed through a 0.45-microm filter, analyzed for pH, dissolved reactive P (DRP), and total dissolved P (TDP). At the end of the experiment lysimeter soil was sampled and analyzed for pH, available P, and oxalate-extractable P, Fe, and Al. The concentration of TDP in the effluent from the sand column increased to 25 mg L-1 during the first weeks and remained above 10 mg L-1 during the rest of the percolation. In effluent from grass + patch lysimeters TDP gradually increased to 4 mg L-1. Both in the manure and in the effluent of the sand lysimeter P was found mainly in the form of DRP, but in the effluent from the grass lysimeters was found mainly as dissolved unreactive P (DUP=TDP-DRP). Earthworm activity was responsible for decomposition of the manure patch on the grass lysimeters. Manure patches and their remains were found to be a long-term source of high concentrations of P in leachates. Spreading of patches after a grazing period could reduce their possible negative impacts on the environment.

Disturbance of water-extractable phosphorus determination by colloidal particles in a heavy clay soil from the Netherlands

Water extraction methods are widely used to extract phosphorus (P) from soils for both agronomic and environmental purposes. Both the presence of soil colloids in soil water filtrates, and the contribution of colloidal P to the molybdate-reactive phosphorus (MRP) concentration measured in these filtrates, are well documented. However, relatively little attention has been given to the direct disturbance by colloids of MRP measurement. The objective of this paper is to show this influence found for water extracts with a soil to solution ratio of 1:60 (v/v) (P(w)), obtained from a heavy clay soil in the Netherlands. Colloidal particles, which passed a 0.45-mum filter, caused a large overestimation of MRP. The low ionic strength of the P(w) filtrates (on average 0.64 mmol(c) L(-1)) probably caused soil dispersion and increased detachment of colloids from soil during extraction. After NaCl addition, followed by 0.45-mum filtration, MRP was on average 93% lower. This can be ascribed to flocculation of colloids and removal by filtration. A low ionic strength can thus lead to the direct disturbance by colloidal particles of MRP measurement in waters from soils sensitive to release of colloids.

Phosphorus desorption dynamics in soil and the link to a dynamic concept of bioavailability

Soils under intensive livestock farming and heavily fertilized with animal manure may have elevated soil phosphorus (P) contents. We determined P desorption kinetics in batch experiments using soils from a pot experiment where grass was cropped on a P-rich noncalcareous sandy soil without P addition, to lower the soil P content. A diffusion model was used to describe P desorption kinetics from a spherical aggregate. The model was calibrated with data from the batch experiments. Simulation results show that in the pot experiment, P desorption from the solid phase of the inner layers was initially far from equilibrium with the rest of the aggregate, but desorption came closer to equilibrium as the soil P content decreased further. A simple tool is presented, referred to as the dynamic bioavailability index (DBI), to determine whether kinetics of P desorption limits plant uptake. This tool is the dimensionless ratio of the modeled maximal diffusive flux from soil aggregates to solution and the plant uptake rate measured in the pot experiment. The DBI was initially much larger than one; the maximal possible P desorption rate exceeded the uptake rate, so uptake was not limited by desorption. The DBI stabilized at a value somewhat larger than one after a while, due to soil transport limitations. This decrease coincided with a large decrease of the P content in the grass to a value (far) below what is considered as optimal; the supply rate of P from soil to the root cannot meet the demand needed for optimal P uptake. The DBI could be seen as a promising onset to a new dynamic approach of bioavailability.

Phosphorus Availability for Plant Uptake in a Phosphorus-Enriched Noncalcareous Sandy Soil

Mining soil phosphorus (i.e., harvesting P taken up from the soil by a crop grown without external P addition) has been proposed as a possible management strategy for P-enriched soils to decrease the risk of P leaching. We performed a pot experiment in a greenhouse where grass was cropped on a P-enriched noncalcareous sandy soil at zero P application over a period of 978 d. We determined the long-term availability of soil P and evaluated the effectiveness of mining soil P to decrease P in different pools. There were two treatments: soil layers in the pots of either 5 or 10 cm thickness. Soils were analyzed at various stages of the experiment. Phosphorus in soil solution and the total pool of sorbed P were estimated using 1:10 (w/v) 0.01 M CaCl2 extracts and acid ammonium oxalate (P(ox)), respectively. A desorption isotherm was constructed, which described the relationship between P in soil solution and the total pool of sorbed P for the soils of the 5- and 10-cm treatments. The Langmuir equation gave a very good description of the isotherm. In the long-term, 65% of P(ox) in the initial soil can be removed by plant uptake, as was calculated from the Langmuir equation and a critical P concentration in soil solution at which P uptake can just be maintained. Thus, P(ox) may be largely plant available. From the strong nonlinearity of the desorption isotherm, it can be understood why the relative decrease of the P concentration in the CaCl2 extracts was much larger than the relative decrease of P(ox). Mining soil P decreased the P concentration in soil solution effectively and, therefore, risk of P leaching from our P-enriched soil.

Wet chemical and phosphorus-31 nuclear magnetic resonance analysis of phosphorus speciation in a sandy soil receiving long-term fertilizer or animal manure applications

In areas under intensive livestock farming and with high application rates of animal manure, inorganic and organic phosphorus (P) may be leached from soils. Since the contribution of these P compounds to P leaching may differ, it is important to determine the speciation of P in these soils. We determined the effect of various fertilization regimes on the P speciation in NaOH-Na2EDTA (ethylenediaminetetraacetic acid) and water extracts of acidic sandy soil samples from the top 5 cm of grassland with wet chemical analysis and 31P nuclear magnetic resonance (NMR) spectroscopy. These soils had been treated for a period of 11 years with no fertilizer (control), N (no P application), N-P-K, or different animal manures. Inorganic P was highly elevated in the NaOH-Na2EDTA extracts of the soils amended with N-P-K or animal manures, while organic P increased only in the soil treated with pig slurry. Water-extractable P showed a similar trend. As indicated by 31P NMR, orthophosphate monoesters were the main organic P compounds in all soils. Our results suggest that long-term applications of large amounts of P fertilizer and animal manures caused an accumulation of inorganic P, resulting in an increase of the potential risk related to mobilization of inorganic P in the top 5 cm of these soils.

Soil phosphorus quantity-intensity relationships to predict increased soil phosphorus loss to overland and subsurface flow

Soil phosphorus (P) quantity-intensity (q-i) relationships, based on common extraction methods, may potentially be used to estimate the risk of P loss in overland flow and subsurface drainage water. Some workers have used nonlinear q-i relationships to derive thresholds in soil test P (STP; a quantity factor) above which the risk of P loss increases, while others find linear relationships and no threshold. We present here a simple modelling exercise (based on Langmuir adsorption theory) along with data from literature to explain the behaviour of q-i relationships, and to give an explanation for this apparent discrepancy. The data indicate that q-i relationships are dependent upon the soil to solution ratio of the P intensity parameter, adsorption capacity (Qmax) and strength (K) of the soil, and the total range in STP. In turn, this affects the calculation of a threshold in STP. The q-i relationship tends towards linearity under conditions of a narrow total range of STP and/or when using a wide soil to solution ratio for estimating the P intensity parameter. Under such conditions, a threshold is difficult to detect, and uncertain. We conclude that the sensitivity of thresholds to experimental conditions and soils needs to be considered if thresholds are to be successful in environmental management to decrease P loss to surface waters.