Micro and nano sized particles in leachates from agricultural soils: Phosphorus and sulfur speciation by X-ray micro-spectroscopy

Humic acid enhances the co-transport of colloids and phosphorus in saturated porous media

Phosphorus (P) has been widely recognized as a substance that is difficult to transport due to its tendency to become easily fixed in the soil. However, many reports demonstrate that groundwater P pollution is rising in humus-rich areas. Research is urgently needed to confirm (or reject) the hypothesis that increased P pollution is related to humus, as there is currently limited quantitative research on this topic. In this study, we conducted a series of batch equilibrium adsorption-desorption experiments and column experiments to quantify the effects of montmorillonite colloids (MCs) and humic acids (HCs, the main components of humus) on the P transport behavior. The results indicate that P's adsorption and desorption behavior on MCs can be well simulated using the Langmuir and Temkin models (R2 > 0.91). Compared to the non-HC treatments, HCs significantly increased MCs' P adsorption and desorption capacity 5.18 and 7.21 times, respectively. Moreover, HCs facilitated the transport ability of the MC-P mixture through the saturated quartz sand column. In a 0.1 M NaCl solution, the MC-P mixture is nearly completely adsorbed on the surface of quartz sand, with a penetration rate of only 0.5%. In contrast, the HC-MC-P mixture can evidently penetrate further at a rate of 26.1%. The transport parameters fitted using HYDRUS-1D further indicated that the presence of humic acids significantly decreased the deposition coefficients of colloids, thereby enhancing the co-transport of colloids and P through the quartz sand porous medium. The potential mechanism of P pollution in humus-rich areas is likely enhanced by the formation of an HC-colloid-P mixture, which greatly increases the adsorption amount of P on colloids and enhances the electrostatic and spatial repulsion between colloids as well as between colloids and quartz sand. It reduces the aggregation and adsorption of colloids, ultimately transferring P into groundwater through colloid-facilitated co-transport. The findings of this study clarified the relationship between the transport of P, colloids, and HCs, which provides a theoretical basis for explaining the P pollution mechanism in humus-rich areas.

Performance of an integrated sediment interceptor in removing phosphorus from agricultural drainage water

Reducing phosphorus (P) loss from agricultural drainage water is challenging. In this study, we aimed to remove P from agricultural drainage water by developing an integrated sediment interceptor with adsorbent modules filled with Zr/Zn nanocomposite-modified ceramsite (ZMC-interceptor). The results of sequential chemical extraction and 31P NMR showed that the contents of H2O-P (1.15 % of total P), NaHCO3-Pi (10.48 % of total P), and ortho-P (orthophosphate, 90.6 % of total P) in the sediments of the ZMC-interceptors were higher than those in nearby field soils. The average enrichment ratios of particulate P (PP, >450 nm), medium-colloidal P (MCP, 220-450 nm), fine-colloidal P (FCP, 1-220 nm), and truly dissolved P (Truly DP, <1 nm) in the sediment over the field soil were 1.37, 1.21, 1.70, and 3.01, respectively. No significant differences were found in the sediment P-trapping function with and without ZMC integrated sediment interceptors. However, the ZMC-interceptors remarkably reduced total P (39.7 % for influent concentrations of 0.19-0.68 mg L-1) from agricultural drainage water compared to those unmodified ceramsite-interceptors (21.7 % for influent concentrations of 0.17-0.66 mg L-1) during the drainage 'window period' (June-August 2022). This was mainly due to the higher removal efficacies of MCP (19.7 %), FCP (23.3 %), and Truly DP (34.8 %) of the ZMC-interceptors. This study highlighted that the ZMC-interceptor not only trapped P in the sediment but also facilitated the removal of different-sized P fractionated from agricultural drainage water.

Restricted colloidal-bound phosphorus release controlled by alternating flooding and drying cycles in an alkaline calcareous soil

Colloid-facilitated phosphorus (P) migration plays an important role in P loss from farmland to adjacent water bodies. However, the dynamics of colloidal P (Pcoll) release as influenced by irrigation in alkaline calcareous soil remains a knowledge gap. The present study, monitored the dynamic change of Pcoll under different water management strategies: 1) control, 2) flooding, and 3) alternating flooding and drying cycles. Soil water-dispersible colloids (0.6 nm-1 μm) were extracted by combining filtration and ultrafiltration methods. The contents of P, cation and organic carbon in the water-dispersible colloids were determined and the stability and mineral composition of colloidal fractions were characterized. The results showed that Pcoll ranged from 16.5 to 25.5 mg kg-1 and represented 42.8%-64.9% of the water-extracted P in the control. Flooding significantly decreased the Pcoll content by 16.0%-62.1% (mean 32.7%) and it may be attributed to the dissolution of colloidal iron (Fe) bound P. The alternating flooding and drying treatment significantly reduced the Pcoll content by 11.6%-88.0% (mean 67.6%). The Pcoll content of the flooding event was always greater than the Pcoll content of the drying event during flooding and drying cycles. Redundancy analysis and random forest modeling showed that the colloidal calcium (Ca) and ionic strength in soil solutions had negative correlations with the Pcoll content, and pH, ionic strength and truly dissolved P were the critical factors affecting Pcoll. Drying of the flooded soil led to the decrease of pH and the increase of ionic strength, colloidal Ca content and positive charges of colloid surfaces, which promoted colloid aggregation and enhanced soil P sorption capacity. This restricted the loss potential of Pcoll. In summary, controlled flooding and drainage when managed correctly have a role to play in mitigating Pcoll loss from P-enriched calcareous soils.

UV irradiation enhanced removal of colloidal phosphorus in agricultural runoff

Colloidal phosphorus (P) is an important P form in agricultural runoff and can threaten water quality. However, up to date, there are few effective approaches to mitigate colloidal P pollution. This study investigated the effect of ultraviolet (UV) irradiation on medium-colloidal (MC; 220 nm-450 nm) and fine-colloidal (FC; 3 kDa-220 nm) P in agricultural runoff. Under 24 h of UV irradiation, as the most abundant colloidal P fraction, concentration of total P (TP) in FC consistently decreased by 81.0%, while TP concentration in MC first increased by 74.4% after 3 h and then decreased with irradiation time. At the same time, particulate TP (>450 nm) concentration was found to be increased from 0 to 14.7 μM. However, there were no obvious variations in TP concentrations in FC and MC fractions under dark conditions. In FC fraction, with the decrease of TP, the corresponding concentrations of iron (Fe), aluminum (Al), silicon (Si) declined synchronously, and ferric iron/ferrous iron (Fe(III)/Fe(II)) ratio and organic matter (OM) concentration were reduced as well. These results suggested that P in FC fraction was gradually transformed into particulate P during photoreduction of Fe(III) and photodegradation of OM under UV irradiation. Our study helps to understand the mechanism of the phototransformation of colloidal P, and propose an UV irradiation-based approach to remove colloidal P in agricultural runoff.

Alternate wetting and drying water management can reduce phosphorus availability under lowland rice cultivation irrespective of nitrogen level

The limited availability of phosphorus (P) in the soil, which is affected by soil moisture, has a significant impact on crop production. However, we still do not fully understand how water management and nitrogen (N) addition affect the availability of P in paddy soil. An evaluation of the effects of two water management strategies that is continuous flooding (CF) and alternate wetting and drying (AWD) irrigation along with various nitrogenous fertilizer addition rates (equivalent to 0, 100%, 133%, and 166% recommended dose of N addition) on P availability in paddy soil took place over the course of a 2-year field experiment. The results showed that water management had a significant influence on ferrous iron, microbial biomass P, and soil-available P. However, the addition of N did not affect the availability of P in the soil. When N was added at various rates, AWD consistently reduced the amount of soil-available P compared to CF. This was primarily because AWD increased microbial biomass, which immobilized P and decreased the content of ferrous iron. As a result, the soil's ability to absorb P increased, leading to a decrease in the amount of P available. In conclusion, AWD decreases the amount of available P in paddy soil compared to CF.

Zr/Zn nanocomposites modified ceramsite enhances phosphorus removal from agricultural drainage water

Developing metal-based nanocomposites as adsorbent for phosphorus (P) removal is a simple and effective strategy, while the separation of nanoscale adsorbents from water after adsorption is a tedious job. In this work, a novel Zr/Zn nanocomposite (Zr/Zn NCs) modified ceramsite (ZZMC) was synthesized to enhance P removal from agricultural drainage water. Characterization results showed that Zr/Zn NCs with fusiform nanostructures were uniformly loaded on the ceramsite, hence depending on the high mechanical strength and large size of ceramsite, the Zr/Zn NCs can be conveniently handled and separated after adsorption with P. The common issues of weak adsorption capacity and short using life related to ceramsite for P removal in wastewater were also significantly improved in complementarity combination with Zr/Zn NCs. The ZZMC exhibited higher P removal efficiency (>90%) at 5 mg-P L-1 in a wide pH range (5-9) than bulk ceramsite (<10%) and performed well when other ions were co-existed. For two real agricultural drainage water samples with total phosphorus (TP) of 0.526 mg-P L-1 and 0.865 mg-P L-1, the ZZMC demonstrated desirable adsorption performance not only for truly dissolved P (<3 kDa; >85%), but also for fine colloidal P (3 kDa-220 nm; 76.1%-79.1%) and medium colloidal P (220-450 nm; 80.7%-82.2%) within 30 adsorption cycles that included two-time regeneration treatments towards this material. Moreover, the adsorption capacity of TP by ZZMC after two regenerated treatments was more than 90% of that of fresh ZZMC. The results revealed the feasibility to remove different-sized P at low concentration for agricultural drainage water by ZZMC.

Size and composition of colloidal phosphorus across agricultural soils amended with biochar, manure and biogas slurry

The long-term application of organic amendments like manure, biochar and biogas slurry can increase phosphorus (P) levels in agricultural soils; however, at present, it's not clear how this affects the P association with different mobile water-dispersible colloidal particles (Pcoll). Thus, this study aimed to assess the effects of the long-term application of different organic amendments on the abundance, size and compositional characteristics of Pcoll. For this purpose, a total of 12 soils amended with the above three organic amendments were sampled from the Zhejiang Province, China, and Pcoll were fractionated into nano-sized (NC; 1–20 nm), fine-sized (FC; 20–220 nm), and medium-sized (MC; 220–450 nm) by a combination of differential centrifugation and ultrafiltration steps. These three Pcoll forms together accounted for 74 ± 14% of the total soil solution dissolved P content, indicating that Pcoll release was a key process in the overland P transport from these soils. Soils treated with biochar showed lower Pcoll contents than those treated with manure or slurry alone; this effect should be further explored in a controlled inductive research approach. Compositional analysis showed that inorganic P was the predominant Pcoll form in the NC (54 ± 20%) and FC (63 ± 28%) fractions, but not in the MC (42 ± 26%) fraction. Among the three fractions, the organic carbon (OC)–calcium (Ca) complex was the major carrier of NC-bound Pcoll, MC-bound Pcoll was better correlated with OC–manganese/iron/aluminium colloids than with OC–Ca colloids, and both of these phenomena co-occurred in the FC fraction. The current study provides novel insights into the impact of various carbon amendments on the propensity for P loss associated with different soil mobile colloidal fractions, and will therefore, inform future agronomic and environmental-related policies and studies.

Nano and fine colloids suspended in the soil solution regulate phosphorus desorption and lability in organic fertiliser-amended soils

Mobile colloids impact phosphorus (P) binding and transport in agroecosystems. However, their relationship to P-lability and their relative importance to P-bioavailability is unclear. In soils amended with organic fertilisers, we investigated the effects of nano (NC; 1-20 nm), fine (FC; 20-220 nm), and medium (MC; 220-450 nm) colloids suspended in soil solution on soil P-desorption and lability. The underlying hypothesis is that mobile colloids of different sizes, i.e., NC, FC, and MC, may contribute differently to P-lability in soils enriched with organic fertiliser. NC- and FC-bound P_coll were positively correlated with P-lability parameters from diffusive gradient in thin films (DGT_A-labile P concentration, r ≥ 0.88; and DGT_A-effective P concentration, r ≥ 0.87). The corresponding relations with MC-bound P_coll are weaker (r values of 0.50 and 0.51). NC- and FC-bound P_coll were also strongly correlated with soil P-resupply (r ≥ 0.64) and desorption (r ≥ 0.79) parameters during DGT_A deployment, and the mobility of these colloids was corroborated by electron microscopy of DGT_A gels. MC-bound P_coll was negatively correlated with the solid-to-solution distribution coefficient (r = -0.42), indicating this fraction is unlikely to be the source of P-release from the solid phase after P-depletion from the soil solution. We conclude that NC and FC mainly contribute to regulating soil desorbable-P supply to the soil solution in the DGT_A depletion zone (in vitro proxy for plant rhizosphere), and consequently may act as critical conditioners of P-bioavailability, whereas MC tends to form complexes that lead to P-occlusion rather than lability.