Effects of forest thinning on soil-plant carbon and nitrogen dynamics

Aims: Corymbia spp. (previously included in the genus Eucalyptus) are common species in sub/tropical Australia and produce high quality timber and round logs. Thinning of native forests helps to preserve native tree species and is more sustainable than replacing native forest stands with mono-species plantations to produce timber. This study aimed to explore the effects of native forest thinning on soil-plant carbon (C) and nitrogen (N) dynamics in two experimental sites, Esk (5 years post-thinning) and Herberton (7 years post-thinning), situated in Queensland, Australia. Methods: The two sites had different thinning regimes. The final stocking rates varied between 75 and 200 stems ha−1 at Esk and between 250 and 400 stems ha−1 at Herberton. The thinned plots were compared to un-thinned plots. Soil samples were collected to measure labile C and N. Leaf samples were collected from C. variegata and C. citriodora in Esk and Herberton respectively. Results: Thinning did not change soil total C, total N, δ15N and inorganic N at either Esk or Herberton. However, at Esk, intensive thinning resulted in decreases in water soluble total N (WSTN). Foliar δ13C did not vary with respect to thinning whereas foliar δ15N values were more enriched in thinned areas than those of un-thinned plots. The stepwise linear regression indicated that both foliar total N and δ15N were explained mainly by soil TN and WSTN. Conclusions: Thinning did not change soil C and N most likely due to the retention of thinned materials on site and their incorporation into soil. Foliar δ13C was not thinning-dependent due to homeostatic maintenance of the ratio of intercellular to ambient CO2 concentrations during photosynthesis. In our study, soil N was not a limiting factor for foliar N, however, foliar N was mainly driven by WSTN which may foreshadow a possible N limitation in severely thinned plots in the long term. We conclude that forest thinning does not decrease soil C and N availability in native Corymbia forests for several years post-thinning if the thinned materials are retained on site