Chemical Fractions and Availability of Zn in a Calcareous Soil in Response to Biochar Amendments

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.