Effects of nitrate and labile carbon on denitrification of southern temperate forest soils

The pressure for anthropogenic land use changes and logging of temperate forests in southern Chile is rapidly increasing, with its potentially high impacts on the capacity of soils to retain important limiting elements. We tested the hypotheses that logging increases the denitrification rates and nitrate and C limitation of denitrifiers activity would be higher in soils of unlogged, old-growth forests than in soils of logged forests. Potential denitrification rates were estimated by the acetylene inhibition assay in intact soil cores in laboratory short-term aerobic incubations using the following treatments: 0.7 mmol NO 3 -N addition, the same nitrate addition plus 23.3 mmol C-glucose, and controls (no additions) with and without 10% v/v acetylene. Forest logging did not significantly change soil nitrate content and C lability (e.g. soil C/N ratio). A nested two-factor ANOVA for repeated measures showed that denitrification was enhanced by nitrate plus labile C additions in both forests, suggesting that in both logged and unlogged forests labile C and nitrate limit denitrifiers activity. Increases were up to one order of magnitude when glucose was added to nitrate treated soils; from 373 ± 113 to 3 353 ± 451μg N 2 0-N m -2 d -1 in the unlogged, old-growth forest and from 1 369 ± 941 to 12 192 ± 7 474 μg N 2 0-N m 2 d -1 in the logged forest. We conclude that, denitrification would be enhanced in logged forests in the longer term due to a greater nitrate and labile C availability of both in disturbed soils. La presión por el cambio en el uso del suelo y la tala de los bosques templados de Chile está aumentando rápidamente, con altos impactos potenciales sobre la capacidad de retención en el suelo de elementos limitantes del crecimiento y regeneración del bosque. Evaluamos las hipótesis que la tala selectiva del bosque incrementaría las tasas de desnitrificación y que la limitación de N y C para la actividad de los desnitrificadores sería más elevada en los bosques testigos que en los manejados. Las tasas de desnitrificación potencial se estimaron mediante el método de inhibición con acetileno en suelos intactos, a través de incubaciones de laboratorio a corto plazo, con los siguientes tratamientos: adición de 0.7 mmol N-NO3, nitrato más la adición de 23.3 mmol C-glucosa, y controles con y sin 10% v/v acetileno. La tala selectiva no cambió significativamente la labilidad del C de la materia orgánica del suelo (e.g. C/N) ni el contenido de nitrato en el suel