Methodological influences on bulk density estimation in tropical peat soils

Tropical peatlands are ecosystems recognised as important reservoirs of soil carbon (C), with an estimated 152 to 288 Gt of C (S. E. Page et al., 2011; Ribeiro et al., 2020). Bulk density (BD) is a key soil property for estimating carbon density and, subsequently, soil carbon stocks. Furthermore, it is also an important parameter for indicating soil compaction and porosity, including water fill pore space (WFPS), hydraulic conductivity (K), biological activity, and cation exchange capacity related to nutrient availability (USDA and NRCS, 2019).

However, there is no standardised method for collecting peat soil in the field. In studies reporting bulk density values, it is uncommon to find detailed descriptions of field sampling procedures. Instead, most studies state that bulk density is determined by measuring the mass of an oven-dried soil sample per unit volume (g cm⁻³ or kg m⁻³) (see SM-Qi et al 2025)

Several approaches are used to determine BD in peat soils, and these methods may produce different outcomes, with implications for carbon density estimates. This work aims to compare two common methods for taking bulk density samples in peat soil. The first is the core method, which is the most commonly reported approach for peat soils in the literature, and the second is the ring method, a general soil bulk density method adapted for use in peat soils. By assessing the differences in BD estimates between these two methods, we want to test and describe a more standardised and reliable protocol for determining BD in peat soils. This assessment considers all stages of the process, including sampling, transportation, and laboratory procedures.

We collected tropical peat soil samples in the field using a ring sampler of known ring volume and a Russian peat core. Our results reveal that BD estimated using the core method was significantly higher than that obtained using the ring method (paired t-test, p < 0.001). These differences may have substantial implications for tropical soil-carbon stock estimations and highlight the importance of standardising bulk density procedures across all stages, from field sampling to final laboratory analysis.