Successional changes in soil nitrogen availability, non-symbiotic nitrogen fixation and carbon/nitrogen ratios in southern chilean forest ecosystems

Abstract

Vast areas of southern Chile are now covered by second-growth forests because of fire and logging. To study successional patterns after moderate-intensity, anthropogenic fire disturbance, we assessed differences in soil properties and N fluxes across a chronosequence of seven successional stands (2-130 years old). We examined current predictions of successional theory concerning changes in the N cycle in forest ecosystems. Seasonal fluctuations of net N mineralization (Nmin) in surface soil and N availability (Na; Na=NH4+-N+NO3 --N) in upper and deep soil horizons were positively correlated with monthly precipitation. In accordance with theoretical predictions, stand age was positively, but weakly related to both Na (r2=0.282, P&lt;0.001) and total N (Ntot; r2=0.192, P&lt;0.01), and negatively related to soil C/N ratios (r2=0.187, P&lt;0.01) in surface soils. A weak linear increase in soil Nmin (upper plus deep soil horizons) was found across the chronosequence (r2=0.124, P&lt;0.022). Nmin n occurred at modest rates in early successional stands, suggesting that soil disturbance did not impair microbial processes. The relationship between N fixation (Nfix) in the litter layer and stand age best fitted a quadratic model (r2=0.228, P<0.01). In contrast to documented successional trends for most temperate, tropical and Mediterranean forests, non-symbiotic Nfix in the litter layer is a steady N input to unpolluted southern temperate forests during mid and late succession, which may compensate for hydrological losses of organic N from old-growth ecosystems