Climatically driven change in soil carbon across a basalt landscape is restricted to non-agricultural land use systems

Abstract

Soils represent a significant component of the global terrestrial carbon cycle. Historical soil carbon depletion resulting from soil and land management offers an opportunity to store additional carbon to offset greenhouse gas emissions as part of our international response to climate change. However, our ability to reliably measure, estimate and predict soil carbon storage is hindered by a range of sources of variability, not least of which is change through time. In the present study, we assessed temporal changes in soil organic carbon (SOC) and its component fractions in response to climate alone and in the absence of land use change at any given site by examining a series of soil monitoring sites across a basalt landscape in north-west New South Wales under a range of land use types over a 3-year period (March-April 2008 and March-April 2011), where a significant rainfall event had occurred in the intervening time (2010). Across the dataset, woodland soils contained the largest carbon concentration (SOC%) and total organic carbon stock (TOCs) compared with other non-wooded land use systems, which themselves were statistically similar. However, larger carbon quantities were restricted largely to the surface (0-10 cm) soil layers. Between 2008 and 2011, significant increases in SOC% and TOCs were detected, but again these were restricted to the woodland sites. No change in particulate organic carbon (POC) was detected between the two sampling times, but both humic organic carbon (HOC) and resistant organic carbon (ROC) increased in woodland soils between the two sampling times. Increased HOC we attribute to microbial processing of soil carbon following the 2010-11 rainfall event. However, we suggest that increased ROC results from limitations in mid-infrared calibration datasets and estimations. We conclude that the quantity of soil carbon and its component fractions is, indeed, driven by climatic factors, but that these effects are moderated by aboveground land use and SOC inputs.