Species-specific effects of native trees on soil organic carbon in biodiverse plantings across north-central Victoria, Australia

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 δ13C 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 δ13C soil and litter data. δ13C 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 < 53 μm). Negligible changes in the more stable SOC pools presumably contained within these finer-sized fractions suggested little impact of tree establishment on long-term SOC sequestration to ages 9 to 17 years.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.

Research Highlights
► No indication of appreciable decrease in SOC 9 to 17 years after tree establishment.
► Acacia mearnsii establishment increased SOC in whole soil and macroaggregates.
► SOC accumulation rates under N-fixing Acacia mearnsii 0.8 to 1.6 Mg C ha− 1 yr− 1 greater than under non-N-fixing Eucalyptus melliodora.
► δ13C data indicated greater contributions of Acacia mearnsii than other tree species to new SOC.
► δ13C data also suggested greater responsiveness to tree establishment of C pools in mega- and macroaggregates than finer fractions.