Shift of the ecosystem nitrogen cycle from open to closed within a century along a glacial retreat chronosequence at Mount Gongga, southwest China

Aims
To improve our understanding of N cycle development during primary succession after glacial retreat, we (i) assessed the role of biological N2 fixation, (ii) determined gross ammonification rates to identify the onset of mineralization, (iii) quantified the retention of 15NH4+ and 15NO3− in various ecosystem compartments to evaluate the accumulation of deposited N and (iv) followed the 15NH4+ label into the soil NO3− pool to explore the development of nitrification along the subtropical alpine Hailuogou glacial retreat chronosequence, SW China.

Methods
We measured N stocks and δ15N values in the dominant tree species, organic layer and 0–10 cm of the mineral soil and quantified N turnover rates and accumulation via 15N tracer experiments.

Results
N accumulated in the ecosystem at a fast mean rate of 4.5 ± 1.0 g m−2 yr−1 favored by an initially near-neutral soil pH value. The δ15N values of the vegetation started near 0‰ and decreased to a range of -2.7 to -4.4‰ in 127 years. Gross ammonification rates were initially low but increased with ecosystem age from 0.025 to 50.6 mg kg−1 d−1 N, matching those of mature (sub)tropical forests. The maximum accumulation of deposited N shifted from the bryophyte via the shrub layer to the soil organic layer. The 15NH4+ label hardly appeared in the NO3− pool reflecting little nitrification.

Conclusions
Strong initial biological N2 fixation and retention of deposited N was succeeded by a tight N cycling between soil and vegetation at the older sites within approximately 120 yr.