Successional changes in soil nitrogen availability, non-symbiotic nitrogen fixation and carbonitrogen ratios in southern Chilean forest ecosystems

摘要

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 (N-min) in surface soil and N availability (N-a; N-a=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 N-a (r(2)=0.282, P<0.001) and total N (N-tot; r(2)=0.192, P<0.01), and negatively related to soil C/N ratios (r(2)=0.187, P<0.01) in surface soils. A weak linear increase in soil N-min (upper plus deep soil horizons) was found across the chronosequence (r(2)=0.124, P<0.022). N-min occurred at modest rates in early successional stands, suggesting that soil disturbance did not impair microbial processes. The relationship between N fixation (N-fix) in the litter layer and stand age best fitted a quadratic model (r(2)=0.228, P<0.01). In contrast to documented successional trends for most temperate, tropical and Mediterranean forests, non-symbiotic N-fix 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.