Short-term responses of soil carbon and nitrogen pools as well as their isotopic compositions to biochar applications in a suburban forest in subtropical Australia subjected to prescribed burning

PurposesBiochar has received widespread attention as a means for improving carbon (C) sequestration and soil fertility over the long term. However, information on its short-term effects on the soil C and nitrogen (N) pools is scarce, particularly in poor soils that are subjected to prescribed burning.Materials and methodsIn an effort to better understand the short-term effects of biochar on the soil C and N pools, a half-year field study was conducted in a suburban forest that is subjected to prescribed burning in subtropical Australia. In this experiment, biochar was applied to the soil once at rates of 0, 5 and 10 t ha(-1), and soil samples were collected for the top 20 cm soil profile under the canopies of leguminous Acacia leiocalyx (A. leiocalyx) and Acacia disparimma (A. disparimma) in the third month (August 2019) and the sixth month after biochar application (November 2019). At this site, we measured soil total C, total N, delta C-13 and delta N-15.ResultsWe observed that biochar generally impacted the soil C and N pools in the third month after biochar application: soil total C, particularly at the 0-5 cm depth, significantly decreased with increased biochar application rates. Soil C and N pools, particularly at the 10-20 cm depth, varied with sampling times, and soil total C and N under the A. leiocalyx canopy were significantly higher, while soil delta C-13 and delta N-15 were lower in the sixth month relative to the third month. Soil delta C-13 and delta N-15 were primarily linearly related to soil total C and N in the third month, while the linear relationship was closer than that between soil delta C-13 and delta N-15 and the labile C and N pools in the sixth month, which were regulated by biochar application rates.ConclusionsBiochar application significantly decreased the soil C and N pools at the 0-5 cm depth in the third month after biochar application. The soil C and N pools and soil labile C and N pools were responsible for the changes in the processes of soil organic matter (SOM) turnover and N cycling that were revealed by soil delta C-13 and delta N-15, and the changes were governed by the biochar application rates and the time elapsed after biochar application. The influence of understorey legume Acacia species, particularly A. leiocalyx, on N inputs and C sequestration in the poor forest soils was significantly enhanced in the sixth month after biochar application.