Effects of pyrolysis temperature on soil-plant-microbe responses to Solidago canadensis L.-derived biochar in coastal saline-alkali soil

Highlights

• Solidago canadensis L.-derived biochar ameliorate exchangeable sodium stress in soil.

• Biochar pyrolysis temperature affected Brassica chinensis L. and microorganism responses obviously.

• Biochar possessed higher toxic carboxyl acids, phenols and amines at 400–550 °C.

• High-temperature (≥550 °C) biochar was recommended as coastal soil amendment.

Abstract

Because salinity of coastal soils is drastically increasing, the application of biochars to saline-alkali soil amendments has attracted considerable attention. Various Solidago-canadensis-L.-derived biochars prepared through pyrolysis from 400 to 600 °C were applied to coastal saline-alkali soil samples to optimise the biochar pyrolysis temperature and investigate its actual ecological responses. All biochars reduced the soil bulk density and exchangeable sodium stress and increased soil water-holding capacity, cation exchange capacity, and organic matter content. Principal-component-analysis results showed that pyrolysis temperature played an important role in the potential application of biochars to improve the coastal saline-alkali soil, mainly contributed to ameliorating exchangeable sodium stress and decreasing biochar-soluble toxic compounds. Furthermore, soil bulk density and organic matter, as well as carboxylic acids, phenolic acids and amines of biochar were major driving factors for bacterial community composition. Compared to low-temperature biochar (pyrolyzed below 550 °C), which showed higher toxicity for Brassica chinensis L. growth due to the higher content of carboxylic acids, phenols and amines, high-temperature biochar (pyrolyzed at or above 550 °C) possessed less amounts of these toxic functional groups, more beneficial soil bacteria and healthier for plant growth. Therefore, high-temperature biochar could be applied as an effective soil amendment to ameliorate the coastal saline-alkali soil with acceptable environmental risk.

Introduction

Soil salinization and alkalization are typical factors restraining the improvement of land-use efficiency and development of agricultural production worldwide (Cho et al., 2018; Ivushkin et al., 2019; Sun et al., 2017a). It has been reported that over 25% of the total arable land worldwide has been adversely affected by salinization and alkalization in >100 countries (Xia et al., 2019). Coastal soils, in particular, exhibit high concentrations of exchangeable sodium, which consolidates soil, thereby resulting in poor soil structure and limiting water permeability. In addition, soil salinity reduces plant nutrient acquisition and transport, leading to nutrient imbalance (Cheng et al., 2019). Once coastal saline-alkali soils have been improved and treated, they could become important land resources for food production and afforestation. Therefore, improvement and utilization of coastal saline-alkali soils have attracted considerable attention.

Recently, biochar also has attracted considerable attention as a promising economical soil amendment. Compared with other soil amendments (El hasini et al., 2019; Nouri et al., 2017; Oo et al., 2015; Wu et al., 2018b; Xia et al., 2019), biochar could retain more nutrients, provide more carbon sources, and maintain more soil moisture owing to its large surface area and recalcitrant organic carbon content. It has been reported that biochar could enhance soil properties by balancing water content, improving air porosity, and retaining polyvalent cations, thereby replacing Na⁺ from exchange sites by providing Ca²⁺ in salt-affected soils (Saifullah et al., 2018; Zheng et al., 2018). Moreover, biochar could function as habitat for many soil microorganisms, which could help improve soil health (Amini et al., 2016).

Pyrolysis temperature is one of the most important conditions affecting biochar properties and composition. Zhang et al. (2020b) found that pyrolysis temperature negatively affected biochar hydrogen and oxygen contents and H/C and O/C ratios while positively affecting aromaticity, pH, and electrical conductivity. Pariyar et al. (2020) also found that pyrolysis temperature highly influenced biochar physicochemical properties, surface morphology, and mineral composition. Chandra and Bhattacharya (2019) advised that optimal pyrolysis temperatures for rice-straw-derived biochars were in the range 500–600 °C to achieve high soil nutrient contents, ideal pH, and low volatile-organic-compound (VOC) contents. Meanwhile, biochars could carry contaminants consisting of hazardous elements (e.g., heavy metals (HMs) and metalloids) in feedstocks or unexpected carcinogenic by-products (e.g., polycyclic aromatic hydrocarbons (PAHs) and dioxins) generated during pyrolysis; therefore, biochar toxicity is related to pyrolysis temperature (Hilber et al., 2017). Some studies concluded that biochar addition into soil posed risks of heavy metals or PAHs (Khan et al., 2015; Zhao et al., 2019). Consequently, impact of intrinsic toxicity on living organisms must be considered when applying biochars to soils (Yang et al., 2019). According to properties of raw materials and biochar-treated soils, pyrolysis temperature must be selected carefully to ensure safety of soils to which biochars are applied.

However, few studies comprehensively have explored effects of pyrolysis temperature on actual ecological responses to biochar in coastal saline-alkali soils. The Lingang New Area, located on the coast of the East Sea, was selected as a sampling site. Most of the Lingang New Area was formed by land reclamation; therefore, soil there shows very high salinity, making it very difficult for plants to grow. Furthermore, much of the uncultivated land in the Lingang New Area has been overgrown with invasive species such as Solidago canadensis L., thereby greatly reducing abundance and diversity of native plant communities. Therefore, we prepared Solidago-canadensis-L.-derived biochar to improve salt-affected soil, to optimise biochar pyrolysis temperature and assess its impact on plant growth in coastal saline-alkali soil, to elucidate the mechanism by which pyrolysis temperature affects biochar soil application and determine the relationship between pyrolysis temperature and biochar soil eco-toxicity, and to improve resource utilization of invasive species so that they might be used as an effective, economical amendment for application to coastal saline-alkali soil.