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

Abstract

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.