Fluxes and fates of nitrogen in soil of an unpolluted old-growth temperate forest southern Chile

Nitrogen retention and recycling are topics of enduring interest in ecosystem ecology, yet we lack a mechanistic field-tested model of how these processes work in unpolluted, old-growth temperate forests. Forests of the Cordillera Piuchué Ecosystem Study (CPES) in southern Chile provide an opportunity to examine nitrogen cycling and retention in a forest that is virtually free of human disturbance. We applied ¹⁵N pool dilution and pulse-chase tracer techniques as complementary approaches within small plots to understand flows of inorganic nitrogen in the surface soil of an evergreen mixed-angiosperm forest. We also followed separate pulses of ¹⁵NH₄⁺ and ¹⁵NO₃^- for two years to gain insights into how short-term mechanisms of inorganic nitrogen cycling translate into long-term patterns of ecosystem nitrogen retention. Strong consumption appears to limit losses of NH₄⁺ and NO₃^- from this forest, and predominantly by the same mechanisms for both forms of nitrogen. As a result, the extent of ¹⁵NH₄⁺ and ¹⁵NO₃^- retention were also similar, yet ∼44-fold higher rates of gross NH₄⁺ production leads to the dominance of NH₄⁺ over NO₃^- in soil and stream waters. Microbial biomass played a key role in the short-term assimilation of ¹⁵N tracers, but retention was only transient. Turnover of ¹⁵N through microbial biomass was rapid and appeared to be only weakly retained in soil exchangeable pools, fine roots, and soil organic matter, resulting in substantial losses of ¹⁵N from soils within weeks of tracer additions. Assimilation of ¹⁵N into fine roots was a much larger sink (13%) than has been reported for other forested ecosystems (1-3%), and the transport of ¹⁵N from microbial biomass to aboveground sinks in vegetation may explain the observed loss of ¹⁵N from surface soils over time. Losses of ¹⁵N from microbial biomass did not enter the extractable pool of dissolved organic nitrogen (DON), suggesting that DON losses do not originate directly from active microbial turnover, and also that microbial activity may not exert as much control over hydrologic losses of DON as compared to losses of NH₄⁺ and NO₃^-. Our results also suggest an additional rapid and extremely transient (1 d) mechanism of NO₃^- retention via incorporation into extractable-DON. The long-term retention of ¹⁵N at the whole-plot level did not differ significantly between ¹⁵NH₄⁺ and ¹⁵NO₃^- treatments, and averaged 65% after two years. The lack of an appreciable change in ¹⁵N recovery for ∼1.5 yr following the initial assimilation, redistribution, and loss of ¹⁵N suggests that the majority of ¹⁵N was not recycled over the long term through inorganic nitrogen pools and microbial biomass via mineralization/immobilization pathways. Instead, long-term retention of inorganic ¹⁵N appeared to be dominated by rapid and possibly direct assimilation into a slow-turnover pool of soil organic matter. Elevated ¹⁵N contents in fine-root and microbial pools for up to two years after ¹⁵N additions, however, also indicated sustained biotic retention of inorganic nitrogen. Our results suggest very similar retention of NH₄⁺ and NO₃^- that is dominated by rapid assimilation and turnover through microbial biomass in the short term (weeks), and transfer from microbial biomass into nitrogen-conserving plant (and to a lesser extent soil organic matter) pools in the long term (years). These processes result in efficient long-term retention of nitrogen in unpolluted old-growth temperate forests.