Soil organic carbon in eastern Australia

Hobley, E; Baldock, JA; Hua, Q; Wilson, B

Soil organic carbon in eastern Australia

Date

2016-12-14

Authors

Publisher

American Geophysical Union (AGU)

Abstract

We investigated the drivers of SOC dynamics and depth distribution across eastern Australia using laboratory analyses (CN, fractionation, radiocarbon) coupled with modelling and machine learning. At over 1400 sites, surface SOC storage was driven by precipitation, whereas SOC depth distribution (0-30 cm) was influenced by land-use. Based upon these findings, 100 sites were selected for profile analysis (up to 1 m) of SOC and its component fractions - particulate (POC), humus (HOC) and resistant (ROC) organic carbon. Profile SOC content was modelled using an exponential model describing surface SOC content, SOC depth distribution and residual SOC at depth and the drivers of these parameters investigated via machine learning. Corroborating previous findings, surface SOC content was highly influenced by rainfall, whereas SOC depth distribution was influenced by land-use. At depth, site properties were the most important predictors of SOC. Cropped sites had significantly lower SOC content than native and grazed sites at depth, indicating that land-use influences SOC content throughout the profile. The machine learning algorithms identified depth as the key control on the proportion of all three fractions down the profile: POC decreased whereas HOC increased with increasing depth. POC was strongly linked with total SOC but HOC and ROC were driven more by climate and soil physico-chemical properties. Human influences (land-use and management) were not important to the fractions, implying that the controls humans can exert on SOC stability may be limited. A subset of 12 soil profiles was analysed for 14C. Radiocarbon content was affected strongly by land-use, with effects most pronounced at depth. Native systems had the youngest carbon down the profile, cropped systems had the oldest SOC. All fractions reacted to land-use change down the soil profile, indicating a lack of stability when the whole profile is viewed. These results indicate that natural systems act as a carbon pump into the soil, injecting young, fresh organic carbon down the entire profile. In contrast, managed systems are deprived of this input and are depleted in SOC at all depths. Our results strongly suggest that SOC storage in the region is input driven. © AGU 2016

Keywords

Soils, Land use, Crops, Grazing, Ecosystems, Rain, Carbon cycle, Carbon 14, Australia

Citation

Hobley, E., Baldock, J., Hua, Q & Wilson, B. (2016) Soil organic carbon in eastern Australia. Paper presented at the AGU Fall Meeting, San Francisco, California, 12 to 16 December 2016. Retrieved from: https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/138214