Fluxes and fates of nitrogen in soil of an unpolluted old-growth temperateforest, southern Chile

Nitrogen retention and recycling are topics of enduring interest in ecosyst em ecology, yet we lack a mechanistic field-tested model of how these proce sses work in unpolluted, old-growth temperate forests. Forests of the Cordi llera Piuchue Ecosystem Study (CPES) in southern Chile provide an opportuni ty to examine nitrogen cycling and retention in a forest that is virtually free of human disturbance. We applied N-15 pool dilution and pulse-chase tr acer techniques as complementary approaches within small plots to understan d flows of inorganic nitrogen in the surface soil of an evergreen mixed-ang io sperm forest. We also followed separate pulses of (NH4+)-N-15 and (NO3-) -N-15 for two years to gain insights into how short-term mechanisms of inor ganic nitrogen cycling translate into long-term patterns of ecosystem nitro gen retention. Strong consumption appears to limit losses of NH4+ and NO3- from this fores t, and predominantly by the same mechanisms for both forms of nitrogen. As a result, the extent of (NH4-)-N-15 and (NO3-)-N-15 retention were also sim ilar, yet similar to 44-fold higher rates of gross NH4+ production leads to the dominance of NH4+ over NO3- in soil and stream waters. Microbial bioma ss played a key role in the short-term assimilation of N-15 tracers, but re tention was only transient. Turnover of N-15 through microbial biomass was rapid and appeared to be only weakly retained in soil exchangeable pools, f ine roots, and soil organic matter, resulting in substantial losses of N-15 from soils within weeks of tracer additions. Assimilation of N-15 into fin e roots was a much larger sink (13%) than has been reported for other fores ted ecosystems (1-3%), and the transport of N-15 from microbial biomass to aboveground sinks in vegetation may explain the observed loss of N-15 from surface soils over time. Losses of N-15 from microbial biomass did not ente r the extractable pool of dissolved organic nitrogen (DON), suggesting that DON losses do not originate directly from active microbial turnover, and a lso that microbial activity may not exert as much control over hydrologic l osses of DON as compared to losses of NH4+ and NO3- Our results also sugges t an additional rapid and extremely transient (1 d) mechanism of NO3- reten tion via incorporation into extractable-DON. The long-term retention of N-15 at the whole-plot level did not differ sign ificantly between (NH4-)-N-15 and 15NO3- treatments, and averaged 65% after two years. The lack of an appreciable change in 15N recovery for similar t o1.5 yr following the initial assimilation, redistribution, and loss of N-1 5 suggests that the majority of N-15 was not recycled over the long term th rough inorganic. nitrogen pools and microbial biomass via mineralization/im mobilization pathways. Instead, long-term retention of inorganic N-15 appea red to be dominated by rapid and possibly direct assimilation into a slow-t urnover pool of soil organic matter. Elevated N-15 contents in fine-root an d microbial pools for up to two years after N-15 additions, however, also i ndicated sustained biotic retention of inorganic nitrogen. Our results sugg est very similar retention of NH4+ and NO3- that is dominated by rapid assi milation 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.