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 N 2 fixation, (ii) determined gross ammonification rates to identify the onset of mineralization, (iii) quantified the retention of 15 NH 4 + and 15 NO 3 − in various ecosystem compartments to evaluate the accumulation of deposited N and (iv) followed the 15 NH 4 + label into the soil NO 3 − pool to explore the development of nitrification along the subtropical alpine Hailuogou glacial retreat chronosequence, SW China. Methods We measured N stocks and δ 15 N values in the dominant tree species, organic layer and 0–10 cm of the mineral soil and quantified N turnover rates and accumulation via 15 N 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 δ 15 N 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 15 NH 4 + label hardly appeared in the NO 3 − pool reflecting little nitrification. Conclusions Strong initial biological N 2 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.

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