Meeting bulk density sampling requirements efficiently to estimate soil carbon stocks

Abstract

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.