Patterns of biological nitrogen fixation during 60 000 years of forest development on volcanic soils from south-central Chile

Biological nitrogen fixation (BNF) is a key process for ecosystem development on new substrates. On young volcanic substrates, the near absence of nitrogen (N) and the presence of available phosphorus (P) in the soil should stimulate the activity of diazotrophic, N-fixing, bacteria. Our main hypothesis is that ecosystem N gain through BNF is tightly coupled to the development of progressive and maximum phases of ecosystem succession, as element contents build up. We investigated ecosystem development in a well-established 60 000-year-old chronosequence in Llaima volcano, south-central Chile. Tree basal areas and total carbon (C), N and P contents in soils showed declining trends over the millennial chronosequence following maximum values between 326 and 3470 years of succession. Maximum rates of BNF (3–6 kg N ha^–1 year^–1) were recorded in the interval from 776 to 3470 years of substrate age, partly associated with high soil C and P contents, and with the lowest denitrification rates in the chronosequence (0.17–0.42 kg N ha^–1 year^–1). Accordingly, over this time interval, there was a positive balance of gaseous N fluxes (5.7–2.8 kg N ha^–1 year^–1), which produced the lowest ¹⁵N signal in the surface soil (δ¹⁵N = −4.6 ‰). In turn, the first stage of the chronosequence was characterised by low symbiotic N fixation associated with Racomitrium moss carpets (0.07 kg N ha^–1 year^–1) that did not compensate for denitrification losses (2.6 kg N ha^–1 year^–1), yielding a negative balance of N gas exchanges. At 3470 years in the chronosequence, BNF rates declined (1.04–2.3 kg N ha^–1 year^–1) and denitrification increased (1.2–2.5 kg N ha^–1 year^–1), leading to N losses and enriching the δ¹⁵N signal in the surface soil (−2.4 ‰). Finally, the oldest sites of the chronosequence presented BNF rates that balanced denitrification rates (1.3 kg N ha^–1 year^–1), thus supporting the hypothesis that a net nitrogen budget of zero for N gaseous fluxes characterises the retrogressive phase in this chronosequence.