Qimei Lin

Soil salinization is a significant obstacle to agricultural development in arid and semiarid regions. While short-term experiments have demonstrated the effective improvement of saline soils through biochar amendment, the long-term efficacy in sustainably ameliorating such soils remains uncertain. Addressing this knowledge gap, this study investigated the long-term effects of biochar amendment in a field setting by applying different rates of biochar to a salt-affected soil and cultivating silage maize for three consecutive years. The comprehensive assessment includes not only maize growth but also changes in soil physical and chemical properties over the study period. The results reveal a notable elevation in maize above-ground dry matter, directly correlated to the enhanced uptake of nitrogen, phosphorous, and potassium. Additionally, biochar application improves saline soil physical properties, including reduced bulk density (1–23%), increased soil large pores (0.7–12%), and macroaggregates (24–141%), and chemical properties, including a decrease in exchangeable sodium percentage (35–48%), and an increase in soil total organic carbon (112–857%), total nitrogen (9–198%), available nitrogen (12–49%), phosphorus (141–538%) and potassium (57–895%). These improvements ultimately resulted in better maize growth. However, the amelioration effect of biochar on these soil properties gradually diminished over the three-year study. Consequently, this study suggests that biochar is a promising soil amendment that can enhance maize growth in saline soil for at least three years in a field experiment, providing valuable insights for sustainable agricultural practices in salt-affected regions.

The literature has shown that biochar can serve as potential amendment to achieve sustainable agriculture and environment. The accessibility and availability of cheap feedstock are considered as important constraint factors for the widespread application of biochar in agriculture. Marginal lands are widely distributed globally, several times larger than arable land, and hold little value for food production due to poor soil conditions. However, these lands are suitable for growing plants, which can be used as feedstock for biochar production. The salt-affected lands, as one of the main marginal lands, are particularly suitable for cultivating diverse varieties of halophytes that can be pyrolyzed into biochar, bio-gas, and bio-oil. The halophyte-derived biochar is useful to produce a desirable acid soil conditioner due to its high ash and rich bases, and improves soil characteristics under extreme saline conditions. Additionally, syngas and bio-oil hold potential benefits as fuels and industrial raw materials. This study introduces an innovative management technique for marginal lands such as salt-affected land, which can provide all-round benefits in food production, land management, vegetation coverage, carbon sequestration, and climate change mitigation. Graphical Abstract

Glycophyte biomass-derived biochars are currently concerned in most studies. However, little attention is given to the characteristics of halophyte-derived biochars. In this study, five typical halophytes of euhalophytes (Suaeda altissima, Suaeda salsa, and Kalidium foliatum), recretohalophytes (Phragmites australis), and pseudohalophytes (Tamarix chinensis) which are widely distributed in the arid and semiarid regions of northwestern China were selected for producing biochars with a slow pyrolysis process at 500 °C for 1 h. The harvested biochars were characterized in elemental content, pores, surface area, and surface charges, and then their potential value as a soil conditioner was evaluated. The results showed that the halophyte-derived biochars had variable ash and Na⁺ contents, ranging from 7.26 to 23.64% and 1.06 to 33.93 g kg⁻¹ respectively. The EC value of the biochars ranged from 1.76 to 23.45 mS cm⁻¹. The biochar derived from Suaeda altissima had a very low specific surface area (SSA), 3.50 m² g⁻¹, while that derived from Phragmites australis (B_PA) had a very high SSA, 344.02 m² g⁻¹. All the biochars carried both positive and negative charges. Kalidium foliatum biochar (B_KF) possessed more negative charges, while Suaeda altissima biochar (B_SA) contained more positive charges. In general, the halophyte biochars had a higher ash content and lower point of zero net charge (PZNC) value, compared with the biochars derived from glycophytes, which would imply their higher potential value as an acidic soil conditioner.

In recent years, continuous efforts have been made to understand the impact of biochar on arable soil fertility. Little is known about whether the biochar derived from municipal sewage sludge has positive impacts on urban soil. In this study, we pyrolyzed spray-dried municipal sewage sludge at 200 °C, 300 °C, 500 °C, and 700 °C for 2 h in a muffle furnace and then amended it into an urban soil to grow turf grass in pots. The outcomes demonstrated that biochar incorporation caused remarkable increases in soil organic C, black C, total N, available P, and K by 3–8, 7–25, 2–9, 10–19, and 1.4–2 times, respectively. The dry matter of turf grass increased by 43–147%, probably due to the nutritional improvement after biochar addition. The turf grass grown in biochar-added soil had 4–70% lower heavy metals than that in the control, although the soils had much higher total heavy metals, which might imply that biochar amendment reduced the bioavailability of heavy metals. Considering the cost of biochar production and its impacts on both urban soil and grass, it would be alternative to convert the spray-dried municipal sewage sludge into biochar at 200 °C for 2 h and then used as an urban soil amendment.