Short-term changes in soil labile carbon and nitrogen pools with biochar application in a suburban native forest in subtropical Australia

Soil labile carbon (C) and nitrogen (N) pools are vital indicators of soil fertility and soil organic matter (SOM) dynamics. We aimed to evaluate the short-term effects of biochar application on soil labile C and N pools in nutrient-deficient soil in subtropical Australia. A 9-month field study was conducted to investigate the short-term changes in soil water-extractable organic C (WEOC) and total N (WETN) as well as hot water-extractable organic C (HWEOC) and hot water-extractable total N (HWETN) in response to biochar application rates under two Acacia species (Acacia leiocalyx (A. leiocalyx) and Acacia disparrimma (A. disparrima)) and Eucalyptus psammitica (E. psammitica) in a native subtropical forest in southeast Queensland, Australia. Biochar was applied at three rates: 0 t ha−1 (B0), 5 t ha−1 (B5), and 10 t ha−1 (B10). We collected soil samples from 0–5 cm, 5–10 cm, and 10–20 cm depth after 6 months of biochar application, and soil samples at 0–5 cm depth were also collected shortly before biochar application and after 1, 2, 3, 6, and 9 months of biochar application. Soil WEOC and WETN significantly decreased with the biochar application rate during the study period. In the 10–20 cm layer, soil WEOC, WETN, HWEOC, and HWETN under E. psammitica were significantly higher than those under the Acacia species. Sampling time also significantly affected the soil labile C and N pools, and soil WEOC and HWEOC were significantly higher in the third month after biochar application than at other sampling times, whereas soil WETN and HWETN continuously increased from the third month and peaked in the ninth month after biochar application. Our results indicate that although biochar application consequently decreases labile pools on short timescales, it still highlights the potential to maintain stability of soil labile C and N pools in forest ecosystems in the short term. Our findings might further contribute to accelerating C sequestration and N cycles in low fertility soils and might ultimately be conducive to improving the adaptive capacity of vulnerable ecosystems in response to frequent prescribed burning and climate change.