Contribution of Biogas Slurry-Derived Colloids to Plant P Uptake and Phosphatase Activities: Spatiotemporal Response.

The bioavailability for varied-size phosphorus (P)-binding colloids (Pcoll) especially from external P sources in soil terrestrial ecosystems remains unclear. This study evaluated the differential contribution of various-sized biogas slurry (BS)-derived colloids to plant available P uptake in the rhizosphere and the corresponding patterns of phosphatase response. Keeping the same content of total P input (15 mg kg-1), we applied different size-fractioned BS-derived colloids including nanosized colloids (NCs, 1-20 nm), fine-sized colloids (FCs, 20-220 nm), and medium-sized colloids (MCs, 220-450 nm) respectively to conduct a 45-day rice (Oryza sativa L.) rhizotron experiment. During the whole cultivation period, the dynamics of chemical characteristics and P fractions in each experimental rhizosphere soil solution were analyzed. The spatial and temporal dynamics examination of P-transforming enzymes (acid phosphatases) in the rice rhizosphere was visualized by a soil zymography technique after 5, 25, and 45 days of rice transplantation. The results indicated that the acid phosphatase activities and its hot spot areas were significantly 1) correlated with the relative bioavailability of colloidal P (RBAcoll), 2) increased with the colloid-free (truly dissolved P) and BS-derived NC addition, and 3) affected by the plant growth stage. With the nanosized BS colloid addition, the RBAcoll and plant biomass were respectively found to be the highest (64% and 1.22 g plant-1), in which the acid phosphatase-catalyzed hydrolysis of organic Pcoll played an important role. All of the above suggested that nanosized BS-derived colloids are an effective alternative to conventional phosphorus fertilizer for promoting plant P uptake and P bioavailability.

Prediction of nano, fine, and medium colloidal phosphorus in agricultural soils with machine learning.

Soil colloids have been shown to play a critical role in soil phosphorus (P) mobility and transport. However, identifying the potential mechanisms behind colloidal P (Pcoll) release and the key influencing factors remains a blind spot. Herein, a machine learning approach (random forest (RF) coupled with partial dependence plot analyses) was applied to determine the effects of different soil physicochemical parameters on Pcoll content in three colloidal subfractions (i.e., nano- (NC): 1-20 nm, fine- (FC): 20-220 nm and medium-sized colloids (MC): 220-450 nm) based on a regional dataset of 12 farmlands in Zhejiang Province, China. RF successfully predicted Pcoll content (R2 = 0.98). Results showed that colloidal- organic carbon (OCcoll) and minerals were the major determinants of total Pcoll content (1-450 nm); their critical values for increasing Pcoll release were 87.0 mg L-1 for OCcoll, 11.0 mg L-1 for iron (Fecoll) or aluminium (Alcoll), 2.6 mg L-1 for calcium (Cacoll), 9.0 mg L-1 for magnesium (Mgcoll), 2.5 mg L-1 for silicon (Sicoll), and 1.4 mg L-1 for manganese (Mncoll). Among three colloidal subfractions, the major factors determining Pcoll were soil Olsen-P (POlsen; 125.0 mg kg-1), Cacoll (2.5 mg L-1), and colloidal P saturation (21.0%) in NC; Mncoll (1.5 mg L-1), Mgcoll (6.8 mg L-1), and POlsen (135.0 mg kg-1) in FC; while Mncoll (1.5 mg L-1), Alcoll (2.5 mg L-1), and Fecoll (3.8 mg L-1) in MC, respectively. OCcoll had a considerable effect in the three fractions, with critical values of 80.0 mg L-1 in NC or FC, and 50.0 mg L-1 in MC. Our study concluded that the information gleaned using the RF model can be used as crucial evidence to identify the key determinants of different size fractionated Pcoll contents. However, we still need to discover one or more easy-to-measure parameters that can help us better predict Pcoll.

Biochar reduces colloidal phosphorus in soil aggregates: The role of microbial communities.

Colloidal phosphorus (Pcoll) in paddy soils can pose a serious threat to the water environment. Biochar amendment not only directly absorb Pcoll to reduce the runoff loss, but also create hotspots for microbial communities which simultaneously affects soil Pcoll. However, despite the crucial role of microorganisms, it remains elusive regarding how biochar and its feedstock types affect the relationships of soil microbial communities and Pcoll in soil matrix (such as at soil aggregate level). To address the knowledge gap, we explored the (in)direct effects of biochar on the soil Pcoll in physically separated fractions including micro- (53-250 µm) and macroaggregates (250-2000 µm). Results showed that straw and manure biochars decreased the soil Pcoll content by 55.2-56.7% in microaggregates and 41.2-48.4% in macroaggregates after 120 days of incubation, compared to the respective control. The fungal communities showed a significantly correlation (0.34, p < 0.05) with Pcoll content in the macroaggregates, whereas the bacterial communities were extremely significantly correlated (0.66, p < 0.001) with Pcoll content in the microaggregates. Furthermore, the partial least squares path model analysis indicated that biochar amendments directly increased Pcoll content (0.76 and 0.61) in micro- and macroaggregates, but the reduced Pcoll content by biochar was mainly derived from indirect effects, such as changed soil biological characteristics carbon (C)/P (-0.69), microbial biomass C (-0.63), microbial biomass P (-0.68), keystone taxa Proteobacteria (-0.63), and Ascomycota (-0.59), particularly for the macroaggregates. This study highlights that to some extent, biochar addition can reduce soil Pcoll content by affecting microbial communities (some keystone taxa), and soil biological characteristics at soil aggregate level.

Pteris vittata plantation decrease colloidal phosphorus contents by reducing degree of phosphorus saturation in manure amended soils.

The agricultural use of manure fertilizer increases the phosphorus (P) saturation of soils and the risk of colloidal P (Pcoll) release to aquatic ecosystems. Two experiments were conducted to identify whether Pteris vittata plantation can decrease Pcoll contents in two soils (Cambisol and Anthrosol) amended with various manure P rates (0, 10, 25, and 50 mg P kg-1 of soil). The total Pcoll contents in manured soil without P. vittata were 1.14-3.37 mg kg-1 (Cambisol), and 0.01-2.83 mg kg-1 (Anthrosol) across manure-P rates. The corresponding values with P. vittata were 0.97-2.33 mg kg-1 (Cambisol) and 0.005-1.6 mg kg-1 (Anthrosol). Experimentally determined colloidal minerals (Fe, Al, Ca), colloidal total organic carbon, Mehlich-3 nutrients (Fe, Al, and Ca), and the degree of P saturation were good predictors of Pcoll concentrations in both soils with and without P. vittata plantation. In unplanted soils, P adsorption decreased and the degree of P saturation increased which released more Pcoll. However, P. vittata plantation decreased the Pcoll release and P loss risk due to the increase of P adsorption and reduced DPS in both soils. The P fractions (NaOH, NH4F, and HCl-P) contributed to increase the P pool in planted soils which enhanced the bioavailability of Pcoll and increased the P. vittata biomass. It suggested that P. vittata plantation was an effective approach to reduce Pcoll release from manure amended soils.

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.

Predicting the governing factors for the release of colloidal phosphorus using machine learning.

Predicting the parameters that influence colloidal phosphorus (CP) release from soils under different land uses is critical for managing the impact on water quality. Traditional modeling approaches, such as linear regression, may fail to represent the intricate relationships that exist between soil qualities and environmental influences. Therefore, in this study, we investigated the major determinants of CP release from different land use/types such as farmland, desert, forest soils, and rivers. The study utilizes the structural equation model (SEM), multiple linear regression (MLR), and three machine learning (ML) models (Random Forest (RF), Support Vector Regression (SVR), and eXtreme Gradient Boosting (XGBoost)) to predict the release of CP from different soils by using soil iron (Fe), aluminum (Al), calcium (Ca), pH, total organic carbon (TOC) and precipitation as independent variables. Results show that colloidal-cations (Fe, Al, Ca) and colloidal-TOC strongly influence CP release, while bioclimatic variables (precipitation) and pH have weaker effects. XGBoost outperforms the other models with an R2 of 0.94 and RMSE of 0.09. SHapley Additive Explanations described the outcomes since XGBoost is accurate. The relative relevance ranking indicated that colloidal TOC had the highest ranking in predicting CP. This was supported by the analysis of partial dependence plots, which showed that an increase in colloidal TOC increased soil CP release. According to our research, the SHAP XGBoost model provides significant information that can help determine the variables that considerably influence CP contents as compared to RF, SVM, and MLR.

Biochar-coupled organic fertilizer reduced soil water-dispersible colloidal phosphorus contents in agricultural fields.

Soil water-dispersible colloidal phosphorus (WCP) presents high mobility, however, the regulatory effect of biochar-coupled organic fertilizer is rarely known, especially under different cropping patterns. This study investigated the P adsorption, soil aggregate stability, and WCP in three paddy and three vegetable fields. These soils were amended with different fertilizers (chemical fertilizer, CF; substitution of solid-sheep manure or liquid-biogas slurry organic fertilizer, SOF/LOF; substitution of biochar-coupled organic fertilizers, BSOF/BLOF). Results presented that the LOF averagely increased the WCP contents by 50.2% across the sites, but the SOF and BSOF/BLOF averagely decreased their contents by 38.5% and 50.7% in comparison with the CF. The WCP decline in the BSOF/BLOF-amended soils was mainly attributed to the intensive P adsorption capacity and soil aggregate stability. The BSOF/BLOF increased the amorphous Fe and Al contents in the fields in comparison with the CF, which improved the adsorption capacity of soil particles, further improving the maximum absorbed P (Qmax) and reducing the dissolved organic matter (DOC), leading to the improvement of > 2 mm water-stable aggregate (WSA>2mm) and subsequent WCP decrease. This was proved by the remarkable negative associations between the WCP and Qmax (R2 = 0.78, p < 0.01) and WSA>2mm (R2 = 0.74, p < 0.01). This study manifests that biochar-coupled organic fertilizer could effectively reduce soil WCP content via the improvement of P adsorption and aggregate stability.

Improved phosphorus availability and reduced degree of phosphorus saturation by biochar-blended organic fertilizer addition to agricultural field soils.

Phosphorus (P) availability and loss risk are linked to P species; however, their alternations in the soil amended with biochar-blended organic fertilizer is not well known, particularly under contrasting soil properties and land management. In this study, the variance of soil P species extracted by sequential chemical extraction (SCE) and 31P NMR techniques, as well as the degree of P saturation (DPS), were investigated throughout three paddy and three vegetable fields. These fields were amended with three different fertilizers at the same P application rate: chemical fertilizer (CF), organic fertilizer substitution (sheep manure/biogas slurry, SM/BS), and biochar-blended organic fertilizer substitution (BSM/BBS). Results showed that the BSM/BBS and SM increased the total P contents by 7.5% and 5.9% (TP) and available P contents by 30.1% and 19.2% (AP), but decreased the DPS values by 19.4% and 11.7%, compared to the CF treatment. Yet, the BS decreased the TP and AP contents but increased the DPS values across the experimental sites. In the BSM/BBS amended soils, high AP contents were due to the increased inorganic P (NaHCO3-Pi), while the increased organic P (monoester and DNA) induced low DPS values and reduced soil P loss risk. Our study highlights that biochar-blended organic fertilizer is an effective agronomic way for improving P availability and decreasing P loss risk via the alteration of soil P species.

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.

Reduced colloidal phosphorus release from paddy soils: A synergistic effect of micro-/nano-sized biochars and intermittent anoxic condition.

Colloidal phosphorus (CP) has high mobility and great loss risk; their biogeochemical processes are influenced by agricultural management such as redox oscillation and biochar-amendment application. This study monitored CP concentration in pore-water, soil P species and P adsorption capacity, to investigate CP release from paddy soils as affected by the interactive effects of oxygen status (continuous anoxic/oxic for 12 days, CA/CO; intermittent anoxic for 2, 4, 6, 8, 10 days during the 12-day cycle, IA2-10) and management (soil only, CK; bulk/micro/nano-sized biochar with various properties: SBBulk, SBMicro, and SBNano). Compared to the control (0.25-0.84 mg L-1, CK-CA), the single intermittent anoxic treatment (CK-IA) reduced CP concentrations by 45 %, due to the rise of Eh and pH and the decline of the degree of P saturation along with the increased soil Fe/Al-P and organic-P. Longer anoxic duration under the CK-IA reduced CP release, probably donated from massive production of redox-stable amorphous Fe/Al-bound P. The single biochar treatment (SB-CA: SBBulk-CA > SBMicro-CA > SBNano-CA) decreased CP release by 37 % as compared to the CK-CA, ascribed to the increased soil pH, Eh, and P adsorption capacity. The combined treatment (SB-IA: SBBulk-IA2 > SBNano-IA10) synergistically reduced CP release by 68 % in comparison with the CK-CA, due to the increase of adsorption through interactions of soil Fe/Al/Ca- and organic-P. Therefore, nano-sized biochar and long intermittent anoxic duration are recommended for reducing CP release from paddy soils.

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 Pcoll were positively correlated with P-lability parameters from diffusive gradient in thin films (DGTA-labile P concentration, r ≥ 0.88; and DGTA-effective P concentration, r ≥ 0.87). The corresponding relations with MC-bound Pcoll are weaker (r values of 0.50 and 0.51). NC- and FC-bound Pcoll were also strongly correlated with soil P-resupply (r ≥ 0.64) and desorption (r ≥ 0.79) parameters during DGTA deployment, and the mobility of these colloids was corroborated by electron microscopy of DGTA gels. MC-bound Pcoll 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 DGTA 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.

Phytate exudation by the roots of Pteris vittata can dissolve colloidal FePO4.

Phosphorus (P) is limiting nutrient in many soils, and P availability may often depend on iron (Fe) speciation. Colloidal iron phosphate (FePO4coll) is potentially present in soils, and we tested the hypothesis that phytate exudation by Pteris vittata might dissolve FePO4coll by growing the plant in nutrient solution to which FePO4coll was added. The omission of P and Fe increased phytate exudation by P. vittata from 434 to 2136 mg kg-1 as the FePO4coll concentration increased from 0 to 300 mM. The total P in P. vittata tissue increased from 2880 to 8280 mg kg-1, and the corresponding increases in the trichloroacetic acid (TCA) extractable P fractions were inorganic P (860-5100 mg kg-1), soluble organic P (250-870 mg kg-1), and insoluble organic P (160-2030 mg kg-1). That is, FePO4-solubilizing activity was positive correlated with TP, TCA P fractions in P. vittata, TP in growth media, and root exudates. This study shows that phytate exudation dissolved FePO4coll due to the chelation effect of phytic acid on Fe; however, the wider question of whether phytic acid excretion was prompted by deprivation of P, Fe, or both remains to be answered.

Qualitative and quantitative investigation on adsorption mechanisms of Cd(II) on modified biochar derived from co-pyrolysis of straw and sodium phytate.

Developing effective modification methods and obtaining a comprehensive understanding of adsorption mechanisms are essential for the practical application of biochars for the removal of heavy metals from solutions. In this study, rice straw was impregnated with sodium phytate and pyrolyzed at 350 °C, 450 °C, and 550 °C to synthesize modified biochars (i.e., MBC350, MBC450, and MBC550). The Cd(II) adsorption capacities and contributions of different mechanisms, including the effects of biochar-derived dissolved organic matter (BDOM), were investigated using batch sorption experiments and characterization analyses. The modification of sodium phytate promoted the pyrolysis of biomass, thereby increasing the BDOM content and aromatic structures at low and high pyrolysis temperatures, respectively. Moreover, the modification also increased the exchangeable Na+ and carbonate contents in the modified biochars. Compared with the raw biochars, the Cd(II) adsorption capacities of modified biochars increased by 3.3-4.3 times, and MBC550 had the highest Cd(II) adsorption capacity (126.5 mg/g), of which precipitation with minerals and interaction with π-electrons contributed 41.7% and 45.8%, respectively. However, at a lower pyrolysis temperature, the Cd(II) adsorption attributed to ion exchange and co-deposition with BDOM significantly increased, especially on MBC350 (33.9 and 12.6 mg/g, respectively). These results indicate that modification by sodium phytate effectively enhanced various adsorption mechanisms, thereby increasing the Cd(II) adsorption capacity. In addition, the contribution of co-deposition with BDOM to adsorption was unneglectable for the biochars pyrolyzed at low temperatures.

Nano and micro manure amendments decrease degree of phosphorus saturation and colloidal phosphorus release from agriculture soils.

The manure fertilizer increases the phosphorus (P) saturation of soils and the colloidal P release to water bodies. Manure of different particle-sizes may have different effects on colloidal P release by soil, and to date there is limited knowledge on colloidal P release from soils amended with different size manures. We produced sheep micro- (SMicro) and nano-manure (SNano), and poultry micro- (PMicro), nano-manure (PNano) from bulk samples by wet fractionation method. The fractionation reduced the P contents of micro- and nano-manures, and enriched them in ash and calcium, iron (Fe), magnesium, and aluminum (Al) phosphate minerals compared with the bulk manures. The degree of P saturation (DPS) in Anthorsol and Cambisol was decreased (SMicro, 17.6 and 17.2 %; SNano, 14.5 and 13.3 % and PMicro, 19.0 and 19.7 mg kg-1; PNano, 17.0 and 14.3 mg kg-1) and released less colloidal P (SMicro, 3.12 and 3.78 mg kg-1; SNano, 3.01 and 3.56 mg kg-1 and PMicro, 3.34 and 3.92 mg kg-1; PNano, 3.21 and 3.65 mg kg-1) than the soils receiving the bulk manures. The decrease in colloidal P was correlated with less DPS in both soils amended with micro and nano manures. That is, the only measurable effect of manure particle size on colloidal P release from the amended soils was due to chemical fractionation during separation of the size fractions. It was suggested that nano and micro manures were the effective approach to reduce colloidal P release from manure amended soils.

Effect of sheep manure-derived biochar on colloidal phosphorus release in soils from various land uses.

Colloidal phosphorus (CP) as an additional route of P mobilization in soil solution has gained much attention. A batch experiment was conducted to investigate the effect of sheep manure-derived biochar (SMB) on CP release from various land uses (paddy, vegetable, tea, and citrus) at a rate of 0% as a control treatment (CK), 1% as a low (L) level, 2% as a middle (M) level, and 4% as a high (H) level of SMB application. The CP and MRPcoll in the solution increased from 30.58 to 88.97% and from 2.45 to 55.54% of total P (TP), respectively. The SMB enhanced CP release in all the soils and all the treatments (except CK and L levels in tea soil; CK, L, and M levels in vegetable soil; and L and M levels in citrus soil). Multiple linear regression revealed a significant correlation between CP and MRPcoll and between colloidal iron, aluminum, calcium, and total organic carbon (Fecoll, Alcoll, Cacoll, and TOCcoll) and pH, which may play an important role as CP carriers that could depend on the pH. This study suggests that the application of SMB in the soil at an appropriate rate of 1 and 2% for tea and vegetable soils, respectively, could be beneficial to avoid the risk of CP release in water bodies.