Long-term impacts of soil compaction and cultivation on soil carbon and nitrogen pools, foliar δ13C and δ15N as well as tree growth in a hoop pine plantation of subtropical Australia

Abstract

The aim of this study was to examine the long-term impacts of mechanical compaction and cultivation on soil C and N pools, δ13C and δ15N of soil and foliage as well as tree growth. In a hoop pine plantation of subtropical Australia, the diameter at breast height over bark (DBHoB) and tree height; δ13C and δ15N of soil and foliage; and soil C and N pools were determined after 17 years following forwarder compaction (control, light and heavy) and cultivation (control and disc plough) treatments. Light compaction was associated with the significantly higher soil total carbon (TC) and total nitrogen (TN) in the 20–30 cm soil and higher TN in the 30–60 cm soil (p < 0.01). Cultivation was linked to the significantly higher soil TC in the 0–10 cm soil, and TC, TN and hot water-extractable total nitrogen (HWETN) in the 10–20 cm soil (p < 0.01). Higher δ15N in the 0–10 cm soil and in the foliage in the heavily compacted plots was due to previous soil denitrification, while light compaction and cultivation were associated with the lower foliar δ13C due to decreased foliar N concentration and reduced photosynthesis rate with increasing foliar biomass under higher N supply. The δ13C and δ15N of foliage and soil were useful indicators of the C and N processes in plant-soil system in response of management impacts in the plantation. Light compaction and cultivation generally had beneficial impacts on soil C and N pools over long term, whereas there was no difference in the impacts of different compaction and cultivation treatments on tree growth.