Biochar’s production and application in soils has been suggested as a means of abating climate change by sequestering carbon while simultaneously providing energy and increasing crop yields. However, little is known about biochar’s effect on nitrification in alkaline soil. This study focused on the effect of wheat straw-derived biochar (0%, 2%, 5%, and 10%, w/w) on nitrification in a calcareous clay soil with an incubation experiment. Moreover, the variations of ammonia-oxidizing bacteria (AOB) amount, urease activity, pH, and inorganic nitrogen contents during the incubation and their relationships with potential nitrification rates were also explored. The results indicated that nitrification was enhanced by wheat straw-derived biochar and showed an obvious dose-response to biochar application rate. Generally, the potential nitrification rate increased with incubation time elapsing for all four treatments, which were in the ranges of 21.0-33.9, 23.7-45.1, 21.4-57.5, and 31.8-66.1 nmol N/(g dry soil∙h), respectively. The potential nitrification rate increased by 1.36-2.40 times at 10% biochar application rate compared with the control (0%) at the same incubation stage. Except for NH₄⁺-N content of the soil-biochar mixture, AOB amount, urease activity, pH, and NO₃⁻-N content all showed increasing trends during incubation. Moreover, correlation analysis indicated that the potential nitrification rate was positively correlated with AOB amount, urease activity, pH, and NO₃⁻-N content (r≥0.713, P<0.01), but negatively correlated with NH₄⁺-N content (r = -0.408, P<0.01). Notably, though the biochar application in the Loess Plateau region has more benefits for soil condition improvement, the enhanced nitrification induced by biochar may pose a negative effect on fertilizer bioavailable efficiency in the agricultural system.
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