Is soil carbon a useful indicator of sustainable forest soil management?—a case study from native eucalypt forests of south-eastern Australia

Abstract

The area and percent of forest land with significantly diminished soil organic matter (SOM) and/or changes in other soil chemical properties has been proposed as a regional sustainability indicator for Australia’s forests. This study examined for native eucalypt forest in south-eastern Australia how representative soil organic carbon (SOC) is of important ecosystem properties and processes. In addition, it was investigated how sensitive the proposed indicator is to management-induced disturbance of these forests. Three sites in Victoria with a known disturbance history and a mapped distribution of soil disturbance (different intensities of fire, snig tracks, and soil perturbation) were included. These sites (2× Eucalyptus regnans, 1× Eucalyptus sieberi) had been clearfelled 9–10 years prior to sampling. The fine-soil fraction (<2 mm) was analysed for SOC, microbial C and N, mineralisable C and N, and available P. In addition, SOC was determined in rocks, plant residues, and charcoal in the fraction greater than 2 mm. Non-labile SOC in the fine-soil fraction contributed to 47–66% of the total in soil profiles (litter layer and 0–30 cm mineral soil). Charcoal in the >2 mm fraction contributed to 13–27% of total SOC. The relationships between SOC and measures of organic matter quality and N and P availability were inconsistent across sites and management practices. The effects of disturbance were most pronounced in the surface soil (0–7.5 cm). Including deeper soil layers in the statistical analysis diluted disturbance effects. Significant differences in the SOC content between soil disturbance intensities in the fine-soil fraction disappeared when size fractions>2 mm were considered. In addition, soil compaction masked changes in SOC concentration in some cases, when SOC was expressed on a volume basis. When all size fractions were considered, SOC stocks increased with fire intensity as a result of charcoal inputs. Physical soil disturbance appeared to be more critical for regrowth productivity than the effects of slash fires. Our results showed that the proposed indicator, changes in SOC, does not meet several of the attributes of what is commonly regarded as a good ecological indicator. Of particular concern are the changes in SOC resulting from charcoal inputs, which are difficult to interpret with regard to soil fertility. Without further qualifications, the changes in SOC cannot be recommended for implementation as an indicator of sustainable soil management in native eucalypt forests. Alternative approaches for the monitoring of changes in soil properties are discussed.