When biochars are produced, feedstock is a crucial factor that determines their physicochemical properties. However, the characteristics of tropical crop waste-derived biochar have not been described and limit its availability. In this study, pineapple leaf (PAL), banana stem (BAS), sugarcane bagasse (SCB) and horticultural substrate (HCS), were used to prepare biochar at 300, 500 and 700 °C. Properties of biochars and their applications were analysed. The results indicated that hydrophobicity, nonpolarity and aromaticity of SCB biochar (SCBB) were higher than other biochars due to the loss of H (hydrogen), O (oxygen), and N (nitrogen). The pH of PAL biochar (PALB) and BAS biochar (BASB) ranged from 9.69 to 10.30 higher than that of SCBB and HCS biochar (HCSB) with 7.17–9.77. In PALB and BASB, sylvite was the dominant crystal structure. With temperature rising, C–H stretching, C=C stretching and H–O in alcohol groups decreased, and Si–O stretching in HCSB and SCBB strengthened. Biochars obtained at 500 °C, especially SCBB and HCSB, significantly promoted the growth of maize. The PALB and BASB greatly increased the soil pH/EC to 6.90–7.35 and 0.67–0.95 ms/cm, while those of SCBB and HCSB were 5.97–6.74 and 0.23–0.45 ms/cm. The application of the biochars to the soil increased soil pH, reducing the acidic soil stress in maize growth, especially PAL and BAS biochars prepared at 300 °C. Biochar prepared at lower temperature will greatly reduce energy consumption and increase the utilization efficiency of tropical agricultural waste resources.

Biochar is a potential material for making slow-releasing phosphorus (P) fertilizers for the sake of increasing soil P-use efficiency. The adsorption of phosphorus by pineapple leaf biochar (PB) prepared at different pyrolysis temperatures and its mechanism remain unclear. In order to study the effect of preparation temperature on the structural characteristics of biochar from pineapple leaves and the adsorption of phosphorus by biochar, pineapple leaves were used as raw materials to prepare biochar by restricting oxygen supply at 300 °C, 500 °C, and 700 °C. The structural characteristics and adsorption of phosphorus by pineapple leaf biochar at different temperatures (PB300, PB500, and PB700) were analyzed. The results showed the following: (1) The pore structure of biochar pyrolysis at 300 °C (PB300) did not significantly change, while the surface structure of biochar pyrolysis at 700 °C (PB700) significantly changed, the specific surface area (S_BET) increased by 26.91~37.10 times that observed in PB300 and PB500, and the pore wall became thinner. (2) The number of functional groups (C=O) in PB700 decreased, and the relative content of C-H/-CHO in PB500 and PB700 increased by 4.38 times that observed in PB300. (3) The adsorption of phosphorus by biochar was a multi-molecular layer chemisorption, accompanied by single-molecular-layer physical adsorption and intramolecular diffusion. For PB300, both the physical and chemical processes of the adsorption of PO₄³⁻ by biochar were weakened, and the chemical process was dominated by cationic (Ca²⁺, Mg²⁺, Fe³⁺, and Al³⁺) adsorption at 500 °C. For PB700, the physical adsorption dominated by pore size structure was the main process, and the physicochemical adsorption at 700 °C was significantly stronger than that observed at 300 °C and 500 °C. These results indicate that biochar prepared at 500 °C can save energy in the preparation process and has excellent physical and chemical structure, which can be used as the basic material for further modification and preparation of biochar phosphate fertilizer.

Soil phosphorus (P) can be divided into inorganic P (Pi) and organic P (Po). Microorganisms play essential roles in soil P transformation. However, there are many ways to detect P transformation, and the relationship between P forms and microorganisms under long-term fertilization is largely unclear. In this study, soil P forms were analyzed by a chemical sequential fractionation method and solution ³¹P nuclear magnetic resonance (³¹P-NMR) technique. Phospholipid fatty acid (PLFA) contents were measured by gas chromatography as the characterization of soil microbial community structures. The objective was to determine the changes of soil P forms and associated microbial community composition in mollisol with long-term fertilization. We sampled soil from a field experiment with 26-year-old continuous maize (Zea mays L.) cropping in Northeastern China. Three fertilization treatments were selected as chemical fertilization (NPK), NPK with crop straw (NPKS), and NPK with manure (NPKM). As shown in ³¹P-NMR spectra, orthophosphate accounted for 62.8–85.8% of total extract P. Comparison to NPK and NPKS treatments, NPKM application notably increased the concentrations of Po, Olsen-P, orthophosphate, orthophosphate monoester, and total P. Soil P fractions including resin-Pi, NaHCO₃-P, NaOH-P, and HCl-P, especially Pi fractions, were enhanced by NPKM. The amounts of total PLFAs and PLFAs in bacteria, Gram-positive (G⁺) and Gram-negative (G⁻) bacteria, actinomycetes, and fungi were high in NPKM-treated soil. The percentages of PLFAs in bacteria and fungi in total soil PLFAs were 56.8% and 9.7%, respectively, which did not show any significant difference among the treatments. NPKM increased the proportions (%) of PLFAs in G⁺ bacteria, and NPKS increased the proportions (%) of G⁻ bacteria in total PLFAs. The composition of soil microbial community was found to be significantly affected by soil total carbon and pH. There was a close relationship between HCl-Pi, NaHCO₃-Po, orthophosphate, and pyrophosphate with anaerobe, aerobes, and G⁺. Manure addition directly increased soil available P concentrations, and indirectly acted through the alterations of anaerobe, aerobes, and G⁺. It is concluded that long-term NPKM application would lead to the accumulation of labile P and moderately labile P in mollisol through the activity of soil microbes.

Background: Returning pineapple leaves (PAL), banana stems (BAS), coconut husks (CCH), and organic fertilizer (OF) to the field is an important method for soil improvement. However, the effects of these materials on the soil remain unclear. Methods: This study employed a nylon-bag experiment filled with the above-mentioned materials to investigate the impacts on soil physicochemical properties and microbial community structure. Results: The short term acidification caused by PAL was due to the significant increase in isobutyric, isovaleric, and hexanoic acids. PAL and BAS promoted the formation of >0.25 mm aggregates in the short term (90 days). C and N were most abundant in 2 mm soil aggregates. The dominant biomarkers in the soil treated with PAL were Koribacteraceae, Chrysozymaceae, Trimorphomycetaceae, and Tremellales. The main biomarkers of soil treated with BAS were Caulobacteraceae, Aspergillaceae, Onygenales, and Ceratostomataceae. The dominant biomakers in the soil treated with CCH and OF were richer than those in soil treated with PAL and BAS. Conclusions: The long term return effects of CCH and OF are better than those of PAL and BAS.