Internal nitrogen cycling in two humic-rich acidic soils

Surface samples (0–10 cm) of two equally-acidic soils (pH 4.5) exhibited very different net N mineralization rates. In an andic soil, it was negligible despite a high (46%) organic matter content, whereas it was appreciable in a colluvial soil of lower (14%) organic matter content. During incubation experiments no NO^?₃ was observed in the andic soil, whereas nitrification occurred in the colluvial soil. Incorporation of added ¹⁵NH₄ is much higher in the andic soil, despite no greater biological immobilization than in the colluvial soil.Added ¹⁵NO₂^? and nitrapyrin experiments showed that incorporation of inorganic-N into the organic fraction of the andic soil can also proceed via a chemical pathway, NO₂^? self-decomposition and fixation on organic matter. This can be a limitation to NO₃^? appearance in this soil. The protective effect of amorphous aluminium is also considered to lower mineralization of organic-N. These two mechanisms could be responsible for low concentration of inorganic-N in many aluminous humic-rich acidic soils.