Soil carbon and nitrogen pools, their depth distribution and stocks following plantation establishment in south east Queensland, Australia

Abstract

Changes in land use affect the soil organic carbon (SOC) stocks and pools. This is currently of great interest as soils are a major pool of carbon in the biosphere and can sequester atmospheric carbon. This study investigated the effect of pasture conversions into reforestation or cropping land on SOC, humic organic C (HOC), particulate organic C (POC) and resistant organic C (ROC) stocks, and soil nitrogen, to 0.5 m depth using mid infrared spectroscopy and combustion analyses. The ten study sites in south east Queensland, Australia were cleared of native vegetation to pasture decades earlier and were more recently (6–19 years ago) converted to either forest plantations (eucalypts) or cropping land. The SOC was significantly affected by land use and site location. Conversion of pasture to forest plantation had no significant effect on SOC compared to pasture up to 19 years since reforestation, but SOC increased slightly with plantation age at a rate of 3.03 t ha−1 per year. Conversion of pasture to cropping land significantly decreased SOC. The stocks of SOC ranged from 47.0 to 107.6 t ha−1. The HOC pool contributed the most towards the total SOC, and ranged from 17.7 to 67.8 t ha−1 across all sites. The POC pool ranged from 2.1 to 17.8 t ha−1 and the ROC pool varied between 5.3 and 22.3 t ha−1. The total SOC stock determined by MIR was closely related to SOC determined by dry combustion analysis. Soil N was significantly affected by land use and location, but trends were not consistent. On pasture sites, total N was higher likely due to fertiliser applications and total N ranged between 3.7 and 15.0 t ha−1 across sites. In summary, conversion of pasture to forest plantation had no significant effect on SOC stocks and different soil carbon pools over 19 years in the subtropical climate of south east Queensland.