Low nutrient availability and biological activity are the main challenges in calcareous soils with low organic matter (OM) content. The purpose of the present study was to evaluate the responses of soil nutrient status and biological traits to addition of corn residue biochar produced at different pyrolysis temperature in a calcareous soil. Biochars were made at 200 (BC200), 350 (BC350), and 500°C (BC500), added to a calcareous soil at 1 and 2% (w/w) and were incubated for 90 days. The application of biochars increased soil organic carbon (SOC), electrical conductivity (EC), cation exchange capacity (CEC), total N [1.21-to 1.41-fold], available P [1.71-to 2.65-fold], K [1.53-to 2.60-fold], Mn [1.14-to 1.21-fold], microbial respiration [1.21-to 2.23fold], substrate-induced respiration [1.22-to 2.63-fold], microbial biomass carbon [1.20-to 2.24-fold], the activity of catalase [1.80-to 2.93-fold], and dehydrogenase [1.47-to 2.30-fold], which varied with the pyrolysis temperature and application rate. Generally, all the measured biological attributes were higher in BC200 than the other treatments. The BC200 biochar increased soil inorganic nitrogen [1.14-to 1.21-fold] and available Fe [1.12-to 1.17-fold], Zn [1.32-to 1.42-fold], and Cu [1.06-to 1.10fold]. In contrast, the BC500 at 2% rate decreased available Fe, Zn, and Cu. The findings revealed that the application of corn biochar obtained at 200°C to calcareous soil was more efficient for improving the nutrient availability and microbial activity.

Availability of Zinc (Zn) is very low in calcareous soils and hence, an amendment must be used to increase Zn availability to plants. The main objective of this study was to assess the changes in chemical fractions and availability of Zn in a calcareous soil amended with corn residue biochar. Three corn residue biochars were produced at 200 (B200), 350 (B350), and 500°C (B500) and applied at 1 and 2% w/w to a calcareous soil with low organic C content (4.1 g kg −1) and high pH (7.7). The mixtures were incubated for 90 days in the laboratory (25 ± 2°C and 80% of soil field capacity). The application of biochar increased soil total organic carbon (TOC) (1.81-to 3.27-fold), cation exchange capacity (CEC) (1.03-to 1.14-fold) and Zn bound to organic matter (1.34-to 2.15-fold). Relative to untreated soil, the B200 biochar (1) decreased soil pH (0.22-0.30 unit); (2) increased dissolved organic carbon (DOC) (1.34-to 1.59-fold), microbial biomass carbon (MBC) (1.56-to 1.67-fold) and DTPA-extractable Zn (1.32-to 1.42-fold); and (3) maximized Zn content in 3 out of 5 soil pools, i.e., exchangeable Zn; organically bounded Zn; and Fe/ Mn-oxide-bounded Zn. In contrast, the B500 biochar (1) increased soil pH; (2) did not affect DOC or DTPA-extractable Zn quantities in soil extracts; and (3) maximized Zn content in carbonate-Zn and residual-Zn soil fractions. The B350 biochar (1) did not affect soil DOC and DTPA-extractable Zn and (2) slightly increased carbonate-Zn fractions. The effects of biochar addition on soil properties and chemical fractions of Zn were greater at 2% than 1% application rates. Results suggest that corn residue biochar produced at 200°C and applied to calcareous soils at a 2% rate may effectively increase Zn availability by increasing the amount of Zn held in the more labile Zn soil fractions.

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The immobilization of soil cadmium (Cd) by biochar and modified biochar is an eco-friendly and cost-effective strategy. In the current study, the effect of raw biochar (BC) and iron-modified biochar (Fe-BC) derived from common reed on the fractionation and mobility of Cd was evaluated, as was its effect on soil microbial activity in contaminated calcareous soil. Treatments involved a combination of two factors: type of biochar (CK: Control, BC, and Fe-BC) and soil Cd concentration (0, 15, and 30 mg kg −1). Treatments were applied to the soil and incubated for 90 days. The application of both biochars increased soil pH and soil organic carbon content (16.6-48.0%), microbial biomass carbon (40.5-75.1%), basal respiration (16.6-48.0%), substrate-induced respiration (12.4-41.9), and dehydrogenase activity (25.5-102.1%), while it reduced diethylene-triamine pentaacetic acid (DTPA)extractable Cd (22.1-39.5%). The addition biochars, particularly Fe-BC, prominently decreased the concentration of exchangeable and carbonate fractions and increased the concentration of Fe-MnOx, as well as the organic and residual fractions of Cd in the soil. Moreover, relative to the control treatment, the incorporation of raw and Fe-modified biochar into 30 mg kg −1 Cd-spiked soil significantly decreased the Cd mobility factor (MF) value by 14.5 and 21.8%, respectively. Fe-modified biochar had a more significant impact than raw biochar on the immobilization of Cd in the soil, and its improved soil microbial activity to a greater extent. Overall, the findings indicate that Fe-modified biochar derived from common reed can immobilize Cd and improve soil microbial attributes in contaminated calcareous soil. Therefore, it can be used as an eco-friendly amendment for restoring Cdcontaminated calcareous soil.

Microbe‐assisted phytoremediation is a promising technology for remediation of potentially toxic element contaminated soils. A greenhouse study was conducted to assess the effect of Pseudomonas spp. and Glomus spp. on phytoremediation of a Pb‐contaminated calcareous soil by Centaurea cyanus L. The study was carried out as a factorial experiment arranged in a randomized complete block design with three replications. Factors are microbial inoculation in three levels (mix inoculation with Glomus spp. or Pseudomonas spp. and non‐inoculated) and four Pb concentrations (0, 250, 500, and 1000 mg kg−1) in soil. The results revealed that microbial inoculation significantly increased the shoot dry weight and Pb accumulation in C. cyanus compared to non‐inoculated plants. Comparison of microbial treatments indicated that higher shoot Pb concentration, shoot modified bioconcentration factor (shoot mBCF, 5.33–5.63) and translocation factor (TF, 1.09–1.2) are obtained for Pseudomonas spp. inoculation, while higher plant biomass, plant Pb accumulation, root Pb concentration, root mBCF (5.91–6.46) and lower TF (0.77–0.99) are recorded for Glomus spp. inoculation. These results show that Pseudomonas spp. and Glomus spp. are more effective in phyotoextraction and phytostabilization of Pb by C. cyanus, respectively. It could be concluded that Pseudomonas spp. and Glomus spp. would be a promising strategy in bioremediation of Pb‐contaminated soils, especially at medium levels of soil Pb contamination (250 and 500 mg kg−1).