Search

Soil organic carbon (SOC) stocks in Australia’s temperate forests have been overlooked in national soil databases and in global SOC analyses of natural ecosystems despite the importance of temperate forests to the global terrestrial carbon balance. This limits the potential to both predict change in SOC stocks in temperate Australia and to identify where and how SOC stocks can be managed to mitigate climate change. Based on data from 707 sites, we examine variations in SOC concentrations and stocks across a range of natural temperate broadleaf forests in south-eastern Australia. Comparisons with current Australia-wide databases highlight previous under-estimation of forest SOC concentrations, leading to substantial underprediction of SOC stocks in the most productive forests (e.g. this study’s mean of 207 Mg C ha−1 compared with a database mean estimate of 141 Mg C ha−1 for Tall open-forests to 30-cm soil depth). Random Forest models involving 27 environmental variables (representing climate, terrain, parent material, soil attributes, vegetation, and fire history) explained up to 79% of the variation in SOC concentrations and 77% of the variation in SOC stocks to 30-cm depth. Climate variables (precipitation, temperature) were of greatest importance to the prediction of both SOC concentrations and SOC stocks, tending to override the importance of terrain and fire-history variables at this study’s regional scale. While patterns in SOC concentrations and stocks were correlated, SOC concentrations were not a reliable proxy for SOC stocks to 10-cm depth, reiterating the importance of mass equivalent measures (i.e. soil bulk density) to assessing changes in soil carbon storage. Our study provides a timely check of the model-based estimates of SOC concentrations and stocks in Australia’s temperate forests that are currently available in nation-wide databases and improves the available information for defining benchmarks, and for identifying potential areas of SOC loss and gain, in programs that aim to mitigate climate change.

Plantings of diverse native tree species aim to reverse negative effects of widespread vegetation clearing in temperate Australia. Studies of the environmental outcomes of these biodiverse plantings currently provide little information on carbon sequestration, which could limit their integration into emerging carbon markets. This paper presents soil organic carbon (SOC) data from four biodiverse native plantings at age 9 to 17 years across north-central Victoria in south-eastern Australia. Effects of site and tree species on SOC (0–10 cm depth) were examined using soils from under four species common to all plantings ( Acacia implexa , Acacia mearnsii , Allocasuarina verticillata and Eucalyptus melliodora ) that were analysed for aggregate stability, and for SOC and δ 13 C in whole soil and in aggregate fractions. Tree establishment had no effect on soil aggregate stability despite significant effects on SOC. Weak associations between aggregation and SOC concentrations were presumably partly due to the clay mineralogy of the study soils, in which aggregate formation is more dependent on electrostatic bridging between 1:1 clay particles than on organic matter. This influence of clay mineralogy might also partly explain poor correlations between SOC and clay content across sites. Contrary to findings elsewhere of decreases in SOC to about 30 years after afforestation, effects of tree establishment on SOC in this study ranged from negligible to significant increases. Effects were species-specific and were largely limited to soils under A. mearnsii , which contained significantly more SOC in whole soil and in the macroaggregates (2000–250 μm) than non-planted soils. Mean rates of SOC accumulation in whole soils were 0.8 to 1.6 Mg C ha − 1 year − 1 greater under the N-fixing A. mearnsii than the non-N-fixing E. melliodora . At two of the sites, greater contribution of A. mearnsii than other tree species to new SOC was indicated by δ 13 C soil and litter data. δ 13 C data also pointed to greater responsiveness to tree establishment of SOC pools in mega- (8000–2000 μm) and macroaggregates than finer-sized fractions (microaggregate 250–53 μm; silt and clay Continued absence of disturbance from these and similar biodiverse plantings should ensure slow macroaggregate turnover and more secure contributions to soil C sequestration as the trees age. Given that greatest SOC effects in this study were under a species of relatively short life span ( A. mearnsii , ~ 10–20 years), soil C sequestration outcomes from biodiverse plantings might also be enhanced by the inclusion of a suite of N-fixing trees that encompass a range of life expectancies to ensure sustained SOC accumulation over many years.

Land-use history – the number, type, and duration of previous land uses – is relevant to many questions regarding land-use effects on soil carbon, but is infrequently reported. We examine the importance of land-use history variables, as well as topographic and edaphic variables, on soil C in a range of forest types – native forest, pine plantations, secondary forest and rehabilitated forest – at three contrasting locations in south eastern Australia. Our comparisons include a novel forest conversion of exotic pine plantations to native, broadleaf forest. Using nested ANOVAs, we detected few differences in soil C concentration indices (total C, microbial biomass C, K 2 SO 4 –C) and C content among eucalypt-dominated vegetation and pine plantations within each location (0–10 cm depth). However, planned contrasts indicated a 30% decrease in soil C content with conversion of native forest to pine plantation of age 37 years. The reverse land-use change – pine plantation to native, broadleaf forest – was associated with a decrease in soil C concentration and content at one location (40%; age 12–13 years) and no detectable changes at another (to age 7 years). Variable effect between locations of this novel land-use change on soil C could be due to differences in potential productivity, conifer species, and plantation age. We used correlation coefficients and general linear models to identify widely applicable variables for predicting soil C concentration and content at local scales (≤ 20 km 2 ). Within-location relationships with topographic variables were weak and infrequent relative to those with edaphic and land-use history variables. Soil texture was strongly correlated with soil C at each location, although the relative significance of different particle size fractions differed among locations. Electrical conductivity appeared more widely applicable since it was included in C models at two locations. Combining land-use history and edaphic variables produced strong predictive models for soil C concentrations and content at two locations (total r 2 0.83 to 0.95). Positive relationships were indicated between soil C and ‘age of current vegetation’ at one location, and negative relationships were indicated with ‘number of land uses’ at another. These data highlight a potential predictive role for land-use history variables in local-scale assessments of soil C in forested landscapes.