The five typical halophyte feedstocks had a similar biochar yield, ranging from 32.75 to 39.20%, under the slow pyrolysis process at 500 °C for 1 h (Table 2). The biochar yields of the halophyte feedstocks were close to those of the fibrous materials such as gramineous crop straws, while much lower than those of woody, animal manure, and sludge feedstocks (Hassan et al. 2020; Hopkins and Hawboldt, 2020). It is known that the large molecules in the feedstocks are broken and converted into small molecules during pyrolysis (Chen and Chen 2009). The yield of biochar is therefore dependent on the characteristics of the feedstock and the pyrolysis process. In a specific pyrolysis process, especially at a certain pyrolysis temperature, the yield of biochar is mainly decided by the characteristics of the feedstock. Zhao et al. (2013a, b) reported that the feedstocks with high ash and lignin contents often had a high yield in biochar. However, a weak relationship between the ash content in the feedstocks and biochar yield was found in this study (r = 0.55, p > 0.05). The carbon content of the halophytes was closely related to the biochar yield (r = 0.83, p < 0.05), which might imply that lignin content in the halophytes rather than the ash determined the biochar production.
The obtained biochars from the five typical halophytes had similar contents of structural elements, compared to those of glycophyte-derived biochars (Sun et al. 2013). They had higher C and N contents, 58.50–71.73% and 0.66–1.49% respectively, compared to the feedstocks (Table 2), which showed the relative enrichment of both C and N during pyrolysis. The significant positive correlation between the C and N in the feedstocks and their biochars (r = 0.90, p < 0.01) might imply that enrichments of both C and N in the biochars were proportional to those in the feedstocks. In contrast, both O and H contents in the biochars decreased from 34.58–42.26% to 6.03–11.81% and 6.20–6.60% to 2.90–4.25%, respectively, which indicated remarkable losses of both O and H during pyrolysis. The chemical reactions such as dehydration, dehydrogenation, deoxygenation, and decarboxylation occuring during pyrolysis can cause large changes in these structural elements, in which C is relatively enriched, while O reduced, with small changes in both H and N (Yue et al. 2017). In addition, the significant relationships of C vs lignin (r = 0.82, p < 0.01) and hemicellulose (r = − 0.75, p < 0.01), O and H vs hemicellulose (r = − 0.71, − 0.65, respectively; p < 0.05), H vs cellulose (r = 0.70, p < 0.01), and N vs cellulose (r = − 0.92, p < 0.01) might reveal the complicated reactions and changes during pyrolysis.
The atomic ratios of H/Ca, O/Ca and (O + N)/Ca in the biochars decreased by 63%, 86%, and 84%, respectively, while the mass ratio of C/Nm increased by 9%, compared with those of the feedstocks. The values of H/Ca and C/Nm in the biochars were independent of those of feedstocks, while both O/Ca and (O + N)/Ca ratios in the feedstocks negatively decided those in the biochars (r = − 0.83, − 0.84, respectively, p < 0.01). It was evident that O in the feedstocks might undergo deoxygenation and decarboxylation and thus result in a high loss of O (Calvelo Pereira et al. 2011). The means of H/Ca, O/Ca and (O + N)/Ca and C/Nm in the halophyte-derived biochars were 0.65, 0.09, 0.11, and 80.44, respectively, which fell within the ranges of those from glycophytes (Wang et al. 2015). However, the low ratios of H/Ca, O/Ca and C/Nm of both BKF and BPA might indicate a high degree of aromatization and therefore a stable structure (Yan et al. 2012).
It is widely known that biochar usually contains a certain amount of ash, showing alkalinity and high EC value since the feedstock contains a certain number of mineral elements. Those minerals can be mostly remained in the biochar during the pyrolysis process (Shi et al. 2019). The halophyte-derived biochars in this study had variable contents in ash, EC, and pH values, ranging from 7.26–23.64%, 1.76–23.45 mS cm-1 and PH 7.86–10.47, respectively. The ash content of the biochars was linearly related to that in the halophyte feedstocks (r = 0.97, p < 0.01). The biochars derived from euhalophytes, such as BSA and BSS, had much higher ash since they developed a compartmentalized vacuole structure for salt storage (Table 2). The glycophyte-derived biochar was reported to have ash content ranging from 2.28 to 37.3% (Zhang et al. 2017; Han et al. 2017), while that of biochars from manure and sludge was relatively high, 14.41–57.45% (Singh et al. 2010; Xiao et al. 2019). Generally, the ash content of the halophyte-derived biochars was higher than that of most glycophyte-derived biochars, but lower than that of manure and sludge biochars.
The halophyte-derived biochars had a diverse composition of minerals, dominated in Na (1.06–33.93 g kg−1), followed by K (1.68–6.17 g kg−1) (Table 3). The euhalophyte-derived biochars such as BSA, BSS, BKF had relatively high contents of both Na and Mg, while the recretohalophyte biochar of BPA was high in K, and pseudohalophyte biochar of BTC high in Ca. It was obvious that halophyte-derived biochars had a comparable Na content with manure biochar (Subedi et al. 2015), but much higher than glycophyte-derived biochar (0.17–2.00 mg kg−1) (Suliman et al. 2015).
The minerals as the main portion of the ash in biochar often exist as oxides, hydroxides, carbonates, phosphates and what like, which are assumed to be related to the pH and EC values in biochar (Hung et al. 2017; Piash et al. 2021). The ash percentage of the halophyte biochars was strongly related to the pH value (r = 0.84, p < 0.05) and K content (r = 0.94, p < 0.01), but weakly to EC value (r = 0.61, p > 0.05). The EC value was strongly dependent on Na content in the biochars (r = 0.99, p < 0.01), while the pH value was somewhat linked with K content (r = 0.69, n = 4) rather than Na content (r = 0.42, n = 4) in the biochars. Cantrell et al. (2011) measured that less than one-third of the ash could be dissolved in water, which might explain the inconsistency of ash with EC value. Current data have shown that the ash composition in biochar is attributed to its liming effect in acid soils (Masud et al. 2020). Halophyte-derived biochars may therefore have a remarkable liming effect for their high ash content. However, the biochars should be checked for their high Na content which may cause a risk for some Na+-sensitive plants.
Remarkable amounts of dissolved organic carbon, nitrogen, and phosphorus (DOC, DON, DOP) were detected in the halophyte-derived biochars (Table 3), which fell within the ranges of those in glycophyte biochars (Wu et al. 2012). The biochars derived from euhalophytes, such as BSA, BSS, and BKF, had higher dissolved organic carbon than the biochar from either recretohalophyte (BPA) or pseudohalophyte (BTC). The BSS even had high DOP, 94.04 mg kg−1. It was not certain whether more dissolved organic matter might be released during the pyrolysis process of euhalophyte. It is believed that the dissolved organic matter in biochar can be used as a substrate for microorganisms and thus influence the microbial activity and community structure as well (Luo et al. 2012). Some of the small molecules in biochar were reported to stimulate or inhibit plant growth (Yuan et al. 2021). It would be worthy of understanding the unique composition of the dissolved organic matter in halophyte-derived biochar and its impacts on soil microbes.
Biochar usually has a porous structure since the organic matter in the feedstock is gradually volatilized during the pyrolysis process. The porosity of biochar is therefore dependent on both feedstock and pyrolysis conditions such as temperature. However, the anatomical structure of the feedstock may largely decide the biochar porosity under-designed pyrolysis conditions (Lehmann et al. 2011). The obtained biochars in this study had distinct pore structures depending on the halophyte feedstocks (Fig. 1). Both BSA and BSS possessed mainly mesopores of 2–50 nm, and thus had higher values of porosity, mode pore diameter (MPD), and average pore size (APS). The biochars of BKF, BPA, and BTC were dominant in 1–3 nm pores, and had low values of MPD and APS, while both the specific surface area (SSA) and pore volume were relatively large. Among the five biochars, BPA had the largest SSA and pore volume (PV), 344.02 m2 g−1, and 0.24 cm3 g−1, respectively (Table 3). Both feedstocks of Suaeda altissima and Suaeda salsa had an anatomical structure of salt compartmentalization, and might easily form columnar and cavity-like large pores. The feedstocks of Phragmites australis and Tamarix chinensis Lour. could readily develop fine and small pores for their uniform and dense anatomical structure (Figs. 2, 3). In particular, the biomass of Phragmites australis might be likely to form more pores for its well-developed vascular structure (Fig. 2).
A large number of data has shown that the biochar with a well-developed pore structure and the large SSA often has strong adsorption capacity for ions, polar and nonpolar substances as well (Yue et al. 2016). It can be thereafter used as the absorbent for removing heavy metals such as Pb2+, Cd2+, and Cu2+ ions (Kalinke et al. 2019). A few studies have reported that biochar amendments can significantly enhance soil water holding capacity (Liu et al. 2015) and nutrient ion retention capacity as well (Rajkovich et al. 2012). In addition, the micropores in biochar can provide habitats for bacteria (0.3–3 μm) and even fungi (2.0–80 μm) and thus help them escape from predators (Luo et al. 2012). Further work may be required for exploring adsorption behaviors of the halophyte-derived biochars with unique pore structures in soil and water as well.
The obtained biochars were all rich in pH-dependent charges (Fig. 4). The positive charge gradually decreased with enhancing medium pH value. The positive charge of BSA decreased from 260.92 to 133.78 cmol kg−1 when the pH value increased from 1 to 12 (Fig. 4a). The positive charge in BPA also increased to 149.780 cmol kg−1 though it was the lowest among the tested biochars (Fig. 4c). The negative charge, in contrast, increased with lifting pH value of the medium. The BSS showed higher negative charge, 0.46–172.92 cmol kg−1 (Fig. 4b), while BPA lower, 0.52–17.55 cmol kg−1. Our results fell within the reported ranges of the surface charge (Yue et al. 2017; Chen et al. 2019).
The five halophyte-derived biochars had similar values of point of zero net charge (PZNC), pH 4.3–5.6 (Table 4), which might suggest the similarity in their charged functional groups. The low PZNC of the biochars probably implied a high capacity to enhance negative charge under an alkaline environment.
Compared with other biochars, BSA had much higher AECpH3.5, 16.99 cmol kg−1, and might be suitable to remove the positively charged pollutants under acidic conditions (Table 5). In contrast, except for BSA, other four biochars might have better adsorption capacity for negatively charged materials under alkaline conditions since they tended to have a high value of CECpH7.0 19.36 cmol kg−1.
It is widely known that biochar usually contains a large number of pH-dependent charges regardless of the feedstock type. The negative charge is assumed to mainly come from functional groups such as hydroxyl and carboxyl groups. The positive charge may be stemmed from nitrogen-containing functional groups, surface oxygen-containing alkaline substances, such as amino, pyrone, or benzopyran, and from the electron-rich centers between graphite layers that attract protons in solution (Chen et al. 2019). It is not certain if the halophyte-derived biochars have similar characteristics as those from glycophyte feedstock. A few studies have shown that biochar amendment can significantly increase soil CEC, and thus improve soil nutrient retention capacity (Chen et al. 2018; Shi et al. 2019). However, we have little understanding in the roles of halophyte biochar as a soil amendment. It is thus meaningful to investigate the impacts of the halophyte-derived biochars on soil physicochemical properties.