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

Biochar Co-Applied with Lime Enhances Soil Phosphorus Availability via Microbial and Enzymatic Modulation of Paddy Soil

Soil microorganisms play a crucial role in improving soil phosphorus (P) availability. However, few studies have explored the changes in microbial community structure and their underlying mechanisms for improving soil P availability with the application of biochar and lime. Three kinds of biochar, made from rice straw (SB), Chinese fir wood sawdust (WB), and pig manure (MB), alone and with lime (SBL, WBL, and MBL), were applied to paddy soil to reveal the biochemical mechanisms for enhancing soil P availability. High-throughput sequencing and real-time PCR were used to investigate soil microbial communities and P functional genes. The three biochars increased the soil's available P in the order of MB > SB > WB. Biochar co-applied with lime increased the available P (Olsen-P by 169-209%) and inorganic P (Al-P by 53.4-161%, Fe-P by 96.3-198%, and Ca-P by 59.0-154%) more than biochar alone, compared to the control (CK). Both biochar alone and co-applied with lime increased the activities of alkaline phosphomonoesterase (ALP), phosphodiesterase (PD), and inorganic pyrophosphatase (IPP) by 369-806%, 28.4-67.3%, and 37.9-181%, respectively, while it decreased the activity of acidic phosphomonoesterase (ACP) by 15.1-44.0%, compared to CK. Biochar, both alone and co-applied with lime, reduced the copy number of phoC gene by 5.37-88.7%, while it increased the phoD, gcd, and pqqC genes by 51.3-533%, 62.1-275%, and 25.2-158%, respectively, compared to CK. A correlation analysis and partial least squares path modeling (PLS-PM) indicated that Olsen-P, Bray-1 P, and inorganic P were significantly positively correlated with the activities of ALP, PD, IPP, and the phoD gene. Biochar co-applied with lime increased the relative abundances of the phoD-harboring bacteria Proteobacteria, Firmicutes, and Acidobacteria, which promoted the transformation of P to the effective state. Meanwhile, the dominant species Anaerolinea, Ascomycota, Mucoromycota, and Chaetomium provided rich effective nutrients for the soil microorganisms by accelerating the decomposition of soil organic matter, thus promoting phosphatase activity. It could be inferred that the optimized microbial community structure improved phosphatase activity by increasing the phoD gene and available nutrients, thus promoting the soil P availability. Biochar co-applied with lime had a better effect on increasing the P availability and rice yields than biochar alone.

Effects of Mg-Palygorskite Modified Biochar on the Growth of Sedum alfredii Hance in Heavy Metal Contaminated Soil

Heavy metal contamination of soil poses a serious threat to agricultural production and human health. Biochar modified with Mg and palygorskite can reduce the content of available heavy metals in soil; however, its passivation effect is affected by the modification method, and there is a lack of research on its impact on plant growth in heavily polluted soil. In this study, four types of modified biochar were prepared using MgCl2, palygorskite, and wood as raw materials, including MBC and MPB prepared by pre-modification and BCM and BPM prepared by post-modification. Sedum alfredii Hance was selected as the test plant, and a pot experiment was conducted to explore the effects of unmodified and modified biochar on the growth of Sedum alfredii Hance in heavily polluted soil with Cu, Pb, Zn, and Cd. Compared with the original biochar, the modified biochar, especially the pre-modified biochar, significantly increased the ash content, pH, O/C ratio, surface functional group count, and mineral content. The adsorption capacity for heavy metals was also significantly enhanced, with the main adsorption mechanisms being precipitation, complexation, and ion exchange. The four types of modified biochar promoted the growth and biomass of Sedum alfredii Hance to varying degrees, with the promotion effect in the order of MPB > MBC > BPM > BCM, and the effect was more significant with a 3% addition. The modified biochar significantly reduced the content of available heavy metals in the rhizosphere soil, with a passivation effect in the order of MPB > MBC > BPM > BCM, and the 3% addition had the greatest effect. Further analysis via the Mantel test and structural equation modeling confirmed that modified biochar promoted the growth of Sedum alfredii Hance by reducing the available heavy metal content in the rhizosphere soil and increasing the NO3--N and AP contents. This study provides data support for the development of functionalized biochar for the remediation of heavy metal pollution in soil.

Co-composting of tail vegetable with flue-cured tobacco leaves: analysis of nitrogen transformation and estimation as a seed germination agent for halophyte

Resource utilization of tail vegetables has raised increasing concerns in the modern agriculture. However, the effect and related mechanisms of flue-cured tobacco leaves on the product quality, phytotoxicity and bacterially-mediated nitrogen (N) transformation process of tail vegetable composting were poorly understood. Amendments of high-dosed (5% and 10% w/w) tobacco leaves into the compost accelerated the heating process, prolonged the time of thermophilic stage, increased the peak temperature, thereby improving maturity and shortening composting duration. The tobacco leaf amendments at the 10% (w/w) increased the N conservation (TN and NH4-N content) of compost, due to the supply of N-containing nutrient and promotion of organic matter degradation by tobacco leaves. Besides, tobacco leaf amendments promoted the seed germination and root development of wild soybean, exhibiting the feasibility of composting product for promoting the growth of salt-tolerant plants, but no dose-dependent effect was found for tobacco leaf amendments. Addition of high dosed (5% and 10% w/w) tobacco leaves shifted the bacterial community towards lignocellulosic and N-fixing bacteria, contributing to increasing the compost maturity and N retention. PICRUSt 2 functional prediction revealed that N-related bacterial metabolism (i.e., hydroxylamine oxidation and denitrifying process) was enhanced in the tobacco leaf treatments, which contributed to N retention and elevated nutrient quality of composting. To the best knowledge, this was the first study to explore the effect of tobacco waste additives on the nutrient transformation and halophyte growth promotion of organic waste composting. These findings will deepen the understanding of microbially-mediated N transformation and composting processes involving flue-cured tobacco leaves.

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.