Effects of co-addition of ammonium, nitrite, and glucose with methionine on soil nitrogen

Methionine is one of the many amino acids in the soil. In order to study the role of methionine in acidic forest soil, the effect of methionine (Met) was compared with control together with addition of ammonium (Met + A), nitrite (Met + N), and glucose (Met + C) under 60% or 90% water holding capacity (WHC), because ammonium and nitrite are important factors in nitrification, and glucose affect the heterotrophic nitrification and nitrogen immobilization. We found that methionine addition significantly reduced NO₃^- concentration in acidic forest soil. Compared to Met, Met + A and Met + N treatments non-significantly enhanced nitrification; however, Met + C treatment decreased NH₄⁺ concentration which suggested that soil autotrophic and heterotrophic nitrification were limited in the presence of methionine at 60% WHC. Further, our findings of ¹⁵N-labeled treatment showed the impact and priming effect of methionine was negative for NO₃^- concentration and positive for N₂O emission, which were observed mainly from the soil N source rather than methionine. At 90% WHC, Met + C treatment significantly lessened concentrations of NH₄⁺ and NO₃^-, nonetheless improved N₂O compared to Met treatment. Therefore, besides the denitrification and dissimilatory NO₃^- reduction to ammonia, the immobilization might be the key factor to explain this decrease in NO₃^- concentration at 90% WHC, while these processes were induced with the C addition. This study indicated that the positive role of amino acids in soil N cycling might be overrated.