Higher Biochar Rate Can Be Efficient in Reducing Nitrogen Mineralization and Nitrification in the Excessive Compost-Fertilized Soils

The effects of a high biochar rate on soil carbon mineralization, when co-applied with excessive compost, have been reported in previous studies, but there is a dearth of studies focusing on soil nitrogen. In order to ascertain the positive or snegative effects of a higher biochar rate on excessive compost, compost (5 wt. %) and three slow pyrolysis (>700 °C) biochars (formosan ash (<i>Fraxinus formosana</i> Hayata), ash biochar; makino bamboo (<i>Phyllostachys makino</i> Hayata), bamboo biochar; and lead tree (<i>Leucaena leucocephala</i> (Lam.) de. Wit), lead tree biochar) were applied (0, 2 and 5 wt. %) to three soils (one Oxisols and two Inceptisols). Destructive sampling occurred at 1, 3, 7, 28, 56, 84, 140, 196, 294, and 400 days to monitor for changes in soil chemistry. The overall results showed that, compared to the other rates, the 5% biochar application rate significantly reduced the concentrations of inorganic N (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mo>−</mo></msubsup><mo>-</mo><mi mathvariant="normal">N</mi></mrow></semantics></math></inline-formula> + <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NH</mi></mrow><mn>4</mn><mo>+</mo></msubsup><mo>-</mo><mi mathvariant="normal">N</mi></mrow></semantics></math></inline-formula>) in the following, decreasing order: lead tree biochar > bamboo biochar > ash biochar. The soil response in terms of ammonium and nitrate followed a similar declining trend in the three soils throughout the incubation periods, with this effect increasing in tandem with the biochar application rate. Over time, the soil <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mo>−</mo></msubsup><mo>-</mo><mi mathvariant="normal">N</mi></mrow></semantics></math></inline-formula> increased, probably due to the excessive compost N mineralization; however, the levels of soil <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mo>−</mo></msubsup><mo>-</mo><mi mathvariant="normal">N</mi></mrow></semantics></math></inline-formula> in the sample undergoing the 5% biochar application rate remained the lowest, to a significant degree. The soils’ original properties determined the degree of ammonium and nitrate reduction after biochar addition. To reduce soil <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mo>−</mo></msubsup><mo>-</mo><mi mathvariant="normal">N</mi></mrow></semantics></math></inline-formula> pollution and increase the efficiency of compost fertilizer use, a high rate of biochar application (especially with that pyrolyzed at high temperatures (>700 °C)) to excessively compost-fertilized soils is highly recommended.