Using light fraction and macroaggregate associated organic matters as early indicators for management-induced changes in soil chemical and biological properties in adjacent native and plantation forests of subtropical Australia

Abstract

Soil physical structure causes differential accessibility of soil organic matter (SOM), including carbon (C) and nitrogen (N) pools, to decomposer organisms and is an important determinant of SOM storage. Physical fractionation method of SOM in conjunction with elemental as well as isotopic analyses (C, N, δ 13C, δ 15N) of those soil fractions are used in this study to determine the land use and management-induced changes of SOM and associated processes in three adjacent sites of native forest (NF), first (1R) and second rotation (2R, including tree planting row (2R-T) and windrow of harvest residues (2R-W)) of hoop pine plantations in southeast Queensland, Australia. The results suggest that there is a greater accumulation of C and N in the light fraction (LF) of NF site than that of plantation sites (1R and 2R), and the C and N losses following conversion from mixed species NF to the single-species plantation are attributed to the reduction in C and N stocks for all physical fractions separated by wet sieving. In contrast, the C and N losses induced by the rotation practices (e.g., between 1R and 2R-T) are attributed to the C and N decreases in the LF and macroaggregates (250–2000 µm) only. The C and N increases upon the residue management (e.g., between 2R-W and 2R-T) are primarily attributed to the C and N increases in the LF and macroaggregates as well. Coupled with 30 soil chemical and biological parameters obtained in our previous studies, further principal component and multivariable regression analyses were conducted and the results showed that both the LF and macroaggregates were highly related to the status of C and N pools, the processes of N transformation and soil respiration, and the diversity of microbial communities, and thus could serve as diagnostic SOM fractions responsible for the changes of soil properties and processes within the tested ecosystem induced by the land uses and management practices. Knowledge of the interactive relationships between aggregate classes within SOM and soil chemical and biological processes in this study represents a further step towards diagnostically measuring and understanding important soil C and N processes in response to the land use and management changes in the soil ecosystems such as forests in subtropical Australia.