Immobilization of 15N-ammonium and nitrate in forest soils

Human activities have led to elevated N deposition in many parts of the world. We studied the fates of deposited N in forest soils using ¹⁵N-labelled NH₄⁺ and NO₃^-. Soils were collected from a 60-year tree plantation in monoculture sites of Norway spruce, red pine, sugar maple, and red oak; and further divided into surface 0-5 cm organic soil and 5-15 cm mineral soil. 15N-labelling added to the soil was either immediately extracted with cold K₂SO₄ solution (T0), or incubated for 7-days and then extracted (T7). The ¹⁵N immobilization into residue soil at T0 was limited. There was significantly more ¹⁵NH₄⁺ being immobilized into organic soils than into mineral soils (3.67±0.64% vs. 1.03±0.09%), while ¹⁵NO₃^- immobilization averaged at 1.12 and 0.43% only. Most ¹⁵N recovered in the K₂SO₄ extraction remained in the form that was originally added. After 7-day incubation, between 20-70% of the ¹⁵NH₄⁺ added was found in soil, with average values of 48.2±4.1% and 32.2±2.1% for organic and mineral soils respectively. On average 50% of unimmobilized ¹⁵NH₄⁺ was nitrified to ¹⁵NO₃^- although the range of nitrification was highly variable and soil specific. Immobilization of ¹⁵NO₃^- at T7 remained low (7.40±1.08%). Unimmobilized ¹⁵NO₃^- was recovered mostly as extractable NO₃^- (68.2±1.39%), with tiny amount as NH₄⁺ (1.33±0.20%). Immobilization of ¹⁵NH₄⁺ into soil residue at T7 was positively correlated to the level of soil organic matter (r²=0.3225). Our results suggested that plant-soil interactions play strong roles in the immobilization of newly deposited inorganic N.