Solid-Phase Speciation and Solubility of Phosphorus in an Acid Sulfate Paddy Soil during Soil Reduction and Reoxidation as Affected by Oil Palm Ash and Biochar

Corn straw biochar addition elevated phosphorus availability in a coastal salt-affected soil under the conditions of different halophyte litter input and moisture contents

Improving salt-affected soil health using different strategies is of great significance for Sustainable Development Goals. The effects of biochar as a sustainable carbon negative soil amendment on phosphorous (P) pools in the degraded salt-affected soils of the of coastal wetlands (as one of the primary blue carbon ecosystems) with halophyte litter input under different water conditions (the two intrinsic characteristics of coastal wetlands) are poorly understood. Thus, a corn straw derived biochar (CBC) was added into a coastal salt-affected soil collected from the Yellow River Delta to investigate its effect on P fractions and availability under the input of three different local halophyte litters (i.e., Suaeda salsa, Imperata cylindrica and Phragmites australis) and under the unflooded and flooded water conditions. The results showed that the individual input of Suaeda salsa increased soil P availability by 28.2-40.9 %, but Imperata cylindrica and Phragmites australis had little effect on P availability. CBC individual amendment more efficiently enhanced P availability in the unflooded soil than the flooded soil. However, the co-amendment of CBC with litters showed little synergistic effect on P availability. CBC sharply increased the proportion of Ca-bound labile P fraction, but moderately lifted the proportion of Al/Fe-bound mediumly labile P fraction. CBC-enhanced P availability and altered inorganic P fractions were mainly resulted from the provision of labile inherent P by biochar, improved soil properties (i.e., increased CEC), and altered bacterial community composition (i.e., elevated abundance of P-solubilizing and phosphate-accumulating bacteria). These findings give new insights into understanding P biogeochemical cycling in the coastal salt-affected soils amended with biochars, and will be helpful to develop biochar-based technologies for enhancing P pools and improving soil health of the blue carbon ecosystems.

Phosphorus Footprint in the Whole Biowaste-Biochar-Soil-Plant System: Reservation, Replenishment, and Reception

Two phosphorus (P)-rich biowastes, sewage sludge (SS) and bone dreg (BD), were selected to clarify P footprints among biowaste, biochar, soil, and plants by introducing a novel "3R" concept model. Results showed that pyrolysis resulted in P transformation from an unstable-organic amorphous phase to a stable-inorganic crystalline phase with a P retention rate of 70-90% in biochar (P reservation). In soil, SSBC released more P in acid red soil and alkaline yellow soil than BDBC, while the opposite result appeared in neutral paddy soil. The P released from SSBC formed AlPO4 by combining with Al in soil, whereas P from BDBC transformed into Ca5(PO4)3F(or Cl) in conjunction with Ca in the soil (P replenishment). Various plants exhibited an uptake of approximately 2-6 times more P from biochar-amended soil than from the original soil (P reception). This study can guide the application of biochar in various soil-plant systems for effective nutrient reclamation.

Correlation between phosphorus removal technologies and phosphorus speciation in sewage sludge: focus on iron-based P removal technologies

Phosphorus recovery from sewage sludge as secondary raw materials or as a direct P-rich fertiliser is one of the top frontrunner solutions to tackle Phosphorus (P) scarcity and depletion. However, the efficiency of this P recovery process greatly depends on its phosphorus dissolution potential, which in return relies on the phosphorus speciation in the sewage sludge. This article investigates the potential correlation between P speciation in sewage sludge and the iron-based P removal technologies used in sewage treatment plants (STP) through an innovative sequential extraction method based on the SEDEX method that distinguishes quantitatively between ferrous bound phosphate and ferric bound phosphate. XRD and SEM-EDX were also used to characterise P and Fe species in the studied sludge qualitatively. Principal component analysis showed that the sludge characterised by P bound to ferric iron (as the dominant P fraction) are mostly correlated with sludge produced from the CPR process (chemical phosphorus removal) and primary sludge. Moreover, sludge with a non-negligible amount of P bound to ferrous iron were correlated with sludge from the mixed EBPR-CPR process (Enhanced Biological P Removal assisted with CPR). However, Vivianite was only found in CPR sludge with Fe/P molar ratio higher than 0.6.

Increasing plant availability of legacy phosphorus in calcareous soils using some phosphorus activators

Legacy phosphorus (P) in soil, accumulated over several years of fertilizer application in excess of crop demand, represents a huge and largely untapped resource. P activators can increase the availability of this P to plants by accelerating its transformation into soluble P fractions. In this study, we evaluated the potentials of four "P activators" (oxalic acid, lignin, phytase and ascorbic acid) to increase plant available P in a laboratory incubation experiment with two P-deficient calcareous soils used for wheat production. Samples were analysed for Olsen P, phosphomonoesterase and with Hedley sequential P fractionation. All four treatments had significant effects on different soil P fractions. Oxalic acid mainly enhanced inorganic P (P_i) solubility from the HCl-extractable P pool. Lignin enhanced P lability from the NaOH-, HCl- and residual-P pools. Phytase and ascorbic acid principally affected the organic P fractions (P_o). Oxalic acid and lignin showed most potential to improve P (H₂O-P, NaHCO₃-P_i and NaHCO₃-P_o) availability, which increased by 110-419% and 4.1-122%, respectively. These findings illustrated the potential mechanisms responsible for P release associated with different P activators and reinforced the case for their use in increasing legacy P availability for agriculture in calcareous soils.

Phosphorus Release from Unamended and Gypsum- or Biochar-Amended Soils under Simulated Snowmelt and Summer Flooding Conditions

Prolonged flooding changes the oxidation-reduction status of soils, often enhancing P release to overlying floodwater. We studied P release from unamended, gypsum-amended, and biochar-amended soils under simulated snowmelt flooding (previously frozen, cold flooding at +4°C) and summer flooding (unfrozen, warm flooding at +22°C) using two soils, Fyala clay (FYL-Cl) and Neuenberg sandy loam (NBG-SL), from Manitoba, Canada. Amended and unamended soils were packed into vessels and flooded under cold and warm temperatures in the laboratory. Pore water and floodwater samples were taken weekly for 6 wk after flooding (WAF) and thereafter biweekly for 10 WAF and analyzed for dissolved reactive P (DRP), pH, and cation concentrations. The NBG-SL showed a significantly higher DRP concentration in pore water and floodwater despite its low Olsen P content. Redox potential (Eh) decreased slowly under cold versus warm flooding; hence, redox-induced P release was substantially lower under cold flooding. Gypsum amendment significantly decreased the floodwater DRP concentrations in NBG-SL by 38 and 35% under cold and warm flooding, respectively, but had no significant effect in FYL-Cl, which had low DRP concentrations (<1.2 mg L) throughout the flooding period. Biochar amendment significantly increased floodwater DRP concentrations by 27 to 68% in FYL-Cl under cold and warm flooding, respectively, but had no significant effect in NBG-SL. The results indicate substantially less P release under cold than under warm flooding. Gypsum was effective in reducing floodwater DRP concentrations only at high DRP concentrations; thus, the effectiveness was greater under warm than under cold flooding conditions.

Transformation of uranium species in soil during redox oscillations

Redox oscillation is commonly found in near-surface environment, where soils are often polluted with many redox active contaminants, including uranium (U). In order to investigate the transformation of U species in near-surface soil under redox oscillations conditions, redox oscillations and reduction experiments were performed, biogeochemical parameters and native microbial community composition were monitored, main elements on the surface of solid-phase were analyzed by XPS, and labile U(IV) species and stable U(IV) species in solid-phase were provisionally defined using an anoxic 1 M sodium bicarbonate extraction. It was found that redox oscillations slightly increased the water-soluble U but significantly increased the stable U(IV) species (P < 0.05) in soil. In reduction experiment, there was upper limit value for percentage of stable U(IV) species, and the labile U(IV) species could not transform to stable U(IV) species in a short period of time under reduction conditions. The redox transition of Fe enriched on the surface of soil and the conversion of microbial community composition played a major role in speciation transformation of U under redox oscillations conditions. In addition, sequential extraction revealed that the increase of stable U(IV) species content reflected the U speciation transition from acetate extract to more recalcitrant hydroxylamine extract. The finding provides a potential method for improving the stability of U when bio-reduction is used to remediate the U-contaminated soils.

Phosphorus speciation in sewage sludge and the sludge-derived biochar by a combination of experimental methods and theoretical simulation

Sewage sludge is an inevitable byproduct from municipal wastewater treatment processes and conversion of sewage sludge into high value-added biochar is an effective strategy for resource utilization of sewage sludge. The characterization on P speciation is important to control the practical applications and environmental behavior of the sewage sludge and its derived biochar. The mobility and bioavailability of P is closely related to its speciation, but knowledge gap in P speciation is still existed which needs to be narrowed by a systematic study including existing experimental methods and techniques as well as theoretical prediction. To achieve a comprehensive understanding of P speciation, this study conducted a series of research combining conventional fractionation of P with consideration of major concomitant elements (Al, Ca, Fe), advanced analytical techniques (solution and solid-state ³¹P nuclear magnetic resonance (NMR), X-ray diffraction (XRD)), and theoretical simulation with respect to solubility of major elements in different acid-base extractants. Results showed that Ca and Fe were dominantly extracted by HCl whereas P and Al were both NaOH- and HCl-soluble. Inorganic orthophosphates were detected as predominant molecular configurations of P from solution ³¹P NMR spectra, while the existence of calcium and aluminum orthophosphates was revealed by solid-state ³¹P NMR. Moreover, the poor-crystalline AlPO₄ mineral was further evidenced by XRD patterns of the sewage sludge and biochar. Finally, a theoretical simulation of P-containing minerals was performed on the basis of results above, which suggested the existence of variscite/berlinite and amorphous Ca₃(PO₄)₂ in the sludge and biochar. This study does not only explore the P speciation in the sewage sludge and its derived biochar, but also provides a methodology for further research on P speciation. Through the intensive study on the environmental behavior of P, this work might further contribute to the fundamental knowledge basis for the recycle of P during integrate waste management.

Applications of biochar in redox-mediated reactions

Biochar is chemically more reduced and reactive than the original feedstock biomass. Graphite regions, functional groups, and redox-active metals in biochar contribute to its redox characteristics. While the functional groups such as phenolic species in biochar are the main electron donating moieties (i.e., reducers), the quinones and polycondensed aromatic functional groups are the components accepting electrons (oxidants). The redox capacity of biochar depends on feedstock properties and pyrolysis conditions. This paper aims to review and summarize the various synthesis techniques for biochars and the methods for probing their redox characteristics. We review the abiotic and microbial applications of biochars as electron donors, electron acceptors, or electron shuttles for pollutant degradation, metal(loid)s (im)mobilization, nutrient transformation, and discuss the underlying mechanisms. Furthermore, knowledge gaps that exist in the exploration and differentiation of the electron transfer mechanisms involving biochars are also identified.