Elevated chromium (Cr) level is challenging agricultural production and affecting soil biochemical process. This study evaluated the effect of amendments including surface-modified biochars (HBC: acid washing, Fe(III)-HBC: ferric iron loading, nZVI-HBC: nanoscale zero-valent iron loading) and activated carbon on hexavalent chromium (Cr(VI)) removal in soil and on N cycling enzyme activities, transformation of soil inorganic nitrogen, and growth of maize under Cr stress. The results showed that amendments increased Cr(VI) removal by 72.9%-96.34% at three levels of spiked Cr(VI) (low: 125 mg kg-1, moderate: 250 mg kg-1, high: 500 mg kg-1). Under low Cr stress, amendments generally significantly decreased urease and nitrite reductase activities but increased nitrate reductase activity (p < 0.05). The NH4+-N content had a significant positive correlation with urease activity (p < 0.01), while both NO2--N and NO3--N were absent correlations with N cycling enzyme studied. Amendments decreased NH4+-N/NO3--N ratio under low Cr stress but increased it under moderate Cr stress, although the difference was not significant. Under high Cr stress, only Fe(III)-HBC significantly increased NH4+-N/NO3--N ratio (p < 0.05). The decrease and increase of NH4+-N/NO3--N ratios indicate the enhancement of nitrification and denitrification, respectively. The increase in Cr(VI) removal by amendments contributed to the increase in the migration of NO3--N from roots to shoots. Amendments (except for nZVI-HBC in soil under low Cr stress) increased maize height by 20%-59%. Under low Cr stress, however, nZVI-HBC significantly decreased maize height by 65% (p < 0.05), indicating the toxic effect of nZVI on maize growth overwhelmed low Cr stress.
Keywords: Amendment; Chromium stress; Maize; Nitrogen cycling enzyme; Soil inorganic nitrogen.
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