Nitrification, acidification, and nitrogen leaching from subtropical cropland soils as affected by rice straw-based biochar: laboratory incubation and column leaching studies

Few studies have examined the effects of biochar on nitrification of ammonium-based fertilizer in acidic arable soils, which contributes to NO3 (-) leaching and soil acidification. We conducted a 42-day aerobic incubation and a 119-day weekly leaching experiment to investigate nitrification, N leaching, and soil acidification in two subtropical soils to which 300 mg N kg(-1) ammonium sulfate or urea and 1 or 5 wt% rice straw biochar were applied. During aerobic incubation, NO3 (-) accumulation was enhanced by applying biochar in increasing amounts from 1 to 5 wt%. As a result, pH decreased in the two soils from the original levels. Under leaching conditions, biochar did not increase NO3 (-), but 5 wt% biochar addition did reduce N leaching compared to that in soils treated with only N. Consistently, lower amounts of added N were recovered from the incubation (KCl-extractable N) and leaching (leaching plus KCl-extractable N) experiments following 5 wt% biochar application compared to soils treated with only N. Incorporating biochar into acidic arable soils accelerates nitrification and thus weakens the liming effects of biochar. The enhanced nitrification does not necessarily increase NO3 (-) leaching. Rather, biochar reduces overall N leaching due to both improved N adsorption and increased unaccounted-for N (immobilization and possible gaseous losses). Further studies are necessary to assess the effects of biochar (when used as an addition to soil) on N.