Estimation of soil organic carbon stocks requires bulk density (BD) measurements. Variability in BD contributes to carbon stock uncertainty, in turn affecting how large a change in stock can be observed over time or space. However, BD is difficult and time-consuming to measure, and sample collection is further complicated by extremely dry field conditions, coarse-textured soils, and high coarse-fragment content, which are common in southern Australia and other semi-arid and Mediterranean-type climates. Two alternatives to reduce BD sampling effort are to take fewer BD samples at a site (i.e. volumetric rings or clod), and to use more time-efficient methods (i.e. gamma–neutron density meter, NDM). We evaluate these options in the context of a soil carbon stock survey in agricultural land in the south-west of Australia. The BD values within a monitoring site measured with conventional and NDM methods were statistically different when assessed using large sample sizes; the measurements diverged where the coarse fraction volume was >20%. However, carbon stocks were equivalent, reflecting the much larger relative variability in carbon percentage, which contributed 84–99% of the uncertainty in carbon stocks compared with <5% from BD. Given the maximum variability measured, soil carbon stock changes in southern Australia should be monitored on a decadal scale.
Soil carbon stock in olive groves agroforestry systems under different management and soil characteristics
