The continuous application of biochar in field: effects on P fraction, P sorption and release

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.

Manure-derived hydrochar superior to manure: Reducing non-point pollution risk by altering nitrogen and phosphorus fugacity in the soil-water system

Hydrothermal carbonization (HTC) technology is an emerging technology for the disposal of manure-based wet wastes. However, the effects of manure-derived hydrochar inputs to agricultural soils on nitrogen (N) and phosphorus (P) morphology and conversion in soil-water systems remain largely unexplored. In this study, pig and cattle manure (PM and CM), and their derived hydrochar (PCs and CCs) were applied to agricultural soils, with changes in nutrient morphology and enzyme activities related to N and P transformation in the soil-water systems observed through flooded incubation experiments. The results showed that floodwater ammonia N concentrations were reduced by 12.9-29.6% for PCs relative to PM, and 21.6-36.9% for CCs relative to CM, respectively. Moreover, floodwater total P concentrations of PCs and CCs were reduced by 11.7-20.7% relative to PM and CM. Soil enzyme activities closely related to N and P transformations in the soil-water system responded differently to manure and manure-derived hydrochar application. Compared to manure, the application of manure-derived hydrochar inhibited soil urease and acid phosphatase activity by up to 59.4% and 20.3%, respectively, whereas it had significant promotion effects on soil nitrate reductase (∼69.7%) and soil nitrite reductase (∼64.0%). The products of manure after HTC treatments have the characteristics of organic fertilizers, and the fertilization effects of PCs are more prominent than CCs, which are subject to further verification in field trials. Our findings improve the current understanding of manure-derived organic matter affecting N and P conversions in soil-water systems and the risk for non-point source pollution.

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-P_i), 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.

Phosphate Removal Mechanisms in Aqueous Solutions by Three Different Fe-Modified Biochars

Iron-modified biochar can be used as an environmentally friendly adsorbent to remove the phosphate in wastewater because of its low cost. In this study, Fe-containing materials, such as zero-valent iron (ZVI), goethite, and magnetite, were successfully loaded on biochar. The phosphate adsorption mechanisms of the three Fe-modified biochars were studied and compared. Different characterization methods, including scanning electron microscopy/energy-dispersive spectrometry (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), were used to study the physicochemical properties of the biochars. The dosage, adsorption time, pH, ionic strength, solution concentration of phosphate, and regeneration evaluations were carried out. Among the three Fe-modified biochars, biochar modified by goethite (GBC) is more suitable for phosphate removal in acidic conditions, especially when the pH = 2, while biochar modified by ZVI (ZBC) exhibits the fastest adsorption rate. The maximum phosphate adsorption capacities, calculated by the Langmuir-Freundlich isothermal model, are 19.66 mg g^-1, 12.33 mg g^-1, and 2.88 mg g^-1 for ZBC, GBC, and CSBC (biochar modified by magnetite), respectively. However, ZBC has a poor capacity for reuse. The dominant mechanism for ZBC is surface precipitation, while for GBC and CSBC, the major mechanisms are ligand exchange and electrostatic attraction. The results of our study can enhance the understanding of phosphate removal mechanisms by Fe-modified biochar and can contribute to the application of Fe-modified biochar for phosphate removal in water.

Biochar-based molybdenum slow-release fertilizer enhances nitrogen assimilation in Chinese flowering cabbage (Brassica parachinensis)

Low molybdenum (Mo) bioavailability in acidic soil obstructs vegetable nitrogen assimilation and thus increases the health risk of vegetable ingestion due to nitrate accumulation. Constantly providing available Mo in acidic soil is a challenge for decreasing nitrate accumulation in vegetables. In this study, three Mo application methods, including biochar-based Mo slow-release fertilizer (Mo-biochar), seed dressing, and basal application, were investigated to enhance Mo bioavailability in acidic soil and nitrogen assimilation in Chinese flowering cabbage (Brassica parachinensis). The results showed that Mo-biochar constantly and sufficiently supplied Mo nutrients throughout the growing period of Brassica parachinensis, as evidenced by the soil available Mo, plant Mo uptake, and Mo values. The improved Mo supply was attributed to the alleviation of acidic soil (pH from 5.10 to 6.99) and the slow release of Mo adsorbed on biochar. Mo-biochar increased the nitrate reductase (NR) activity by 238.6% and glutamate dehydrogenase activity by 27.5%, indicating an enhancement of the rate-limiting steps of nitrogen assimilation, especially for nitrate reduction and amino acid synthesis. The increase in Mo-containing NR could be directly ascribed to the high level of Mo in Brassica parachinensis. Compared with the control, the nitrate content of Brassica parachinensis decreased by 42.9% due to the nitrate reduction induced by increased NR. Additionally, Mo-biochar was beneficial to vegetable growth and quality. In contrast, the transformation from NO₃^- to NH₄⁺ was blocked with Mo seed dressing and basal application because of low Mo bioavailability in the soil, resulting in a high nitrate content in Brassica parachinensis. Conclusively, Mo-biochar can slowly release Mo and improve the neutral environment for Mo bioavailability, which is an effective strategy to mitigate the high nitrate accumulation of vegetables planted in acidic soil.

The Synergistic Effect of Biochar-Combined Activated Phosphate Rock Treatments in Typical Vegetables in Tropical Sandy Soil: Results from Nutrition Supply and the Immobilization of Toxic Metals

Sandy soils in tropical areas are more vulnerable to potential toxic elements as a result of their low nutrition. The composite addition of biochar and phosphate material is considered a promising method of immobilizing toxic metals in sandy soils, but the synergistic effects of this process still need to be further explored, especially in typical tropical vegetables. In this study, a pot experiment was conducted to evaluate the agronomic and toxic metal-immobilization effects of single amendments (phosphate rock, activated phosphate rock, and biochar) and combined amendments, including biochar mixed with phosphate rock (BCPR) and biochar mixed with activated phosphate rock (BCAPR), on vegetables grown in tropical sandy soil. Among these amendments, the composite amendment BCAPR was the most effective for increasing Ca, Mg, and P uptake based on water spinach (Ipomoea aquatica L.) and pepper (Capsicum annuum L.), showing increased ratios of 22.5%, 146.0%, and 136.0%, respectively. The SEM-EDS and FTIR analysis verified that the activation process induced by humic acid resulted in the complexation and chelation of the elements P, Ca, and Mg into bioavailable forms. Furthermore, the retention of available nutrition elements was enhanced due to the strong adsorption capacity of the biochar. In terms of cadmium (Cd) and lead (Pb) passivation, the formation of insoluble mineral precipitates reduced the mobility of these metals within the BCAPR treatments, with the maximum level of extractable Cd (86.6%) and Pb (39.2%) reduction being observed in the tropical sandy soil. These results explore the use of sustainable novel cost-effective and highly efficient bi-functional mineral-based soil amendments for metal passivation and plant protection.

Release characteristics of phosphate from ball-milled biochar and its potential effects on plant growth

Ball-milled biochar could potentially supply phosphorus, an essential element for plant growth. To realize resource reuse and phosphorus recovery, three feedstocks (rice straw, distillers grains, and Eupatorium adenophorum) were used to prepare ball-milled biochar to evaluate its release characteristics of phosphorus and potential effects on germination and growth. The results showed that the phosphate release performance of ball-milled distillers grains biochar (DM) at 300 and 600 °C was better than that of other biochars ball-milled for 12 h. The DM prepared at 600 °C and incubated for 12 (DM-12) or 24 h (DM-24) had the best phosphate release capacity. The solution with pH 3.0 was beneficial to the release of phosphate from DM-12. The pseudo-second-order model could better fit the phosphate release of DM-12. A germination and seedling growth experiment suggested that adding 2.5 wt% DM-12 was beneficial to the height of mung beans. This study shows that DM-12 can be used as a slow-release fertilizer for the growth of mung beans, which provides a new way for resource utilization of distillers grains and phosphorus-rich biochar.