Soil specific surface area can be measured cost effectively by water adsorption

Soils can store large amounts of soil organic carbon (SOC) because carbon can be shielded in organo-mineral complexes that form on mineral surfaces and protect organic molecules from microbial decomposition.

Some soils have very large mineral soil surface areas, in the order of hundreds of square meters per gram of soil, that make them biochemically reactive and can lead to the protection of large amounts of SOC. Sandy soils, on the other hand, have much smaller mineral surface areas, in the order of tens of square meters per gram of soil. This gives sandy soils little capacity to protect and store significant amounts of SOC.

The simplest way to measure specific surface area is by relating it to water adsorption of air-dry samples. It is simply the weight difference between samples measured at standard room temperature (30°C and 30% relative humidity) and that obtained after heating samples in an oven at 105°C. If non-standard conditions are used, it is possible to adjust for different room temperatures and air humidities, but this introduces additional uncertainty.

However, SOC itself can also adsorb water so that measurements without correction for water adsorption by SOC are referred to as ‘apparent’ specific surface area. To estimate the functionally relevant mineral specific surface area, it is, therefore, necessary to remove the water adsorption due to SOC. To do this, we analysed data from different soil depths from the Tuapaka Farm site near Palmerston North to estimate the contribution of SOC to water adsorption. Both apparent specific surface area, A_a, and SOC contents decreased with soil depth (Fig. 1a), with changes in A_a highly correlated (r² = 0.983) with changes in SOC (Fig. 1b). From this relationship, we estimated the water adsorption by SOC, essentially by the slope of the line in Fig. 1b. A more sophisticated analysis based on the same principles can be conducted from the individual data points rather than the averages by soil depth and resulted in our best estimate of water adsorption by SOC as 0.43 ± 0.02 m² mgC^–1.

Figure 1. (a) Averaged apparent specific surface area (Aa) and soil organic carbon (SOC) with depth, and (b) Aa plotted against SOC. Error bars show 95% confidence intervals. Reproduced from Kirschbaum et al. (2020).

Water adsorption by SOC can then be subtracted from measured apparent specific surface area to calculate unconfounded mineral specific surface area, A_m, which is illustrated here for measurements at Troughton Farm in the Waikato region (Fig. 2b). The corrected data had a lower correlation coefficient (r² = 0.75) than the uncorrected data that included the confounded water adsorption by SOC (r² = 0.83) shown in Fig. 2a. But even after removal of the autocorrelation, SOC content was still strongly correlated with specific surface area. This suggests that mineral specific surface area is a functionally highly relevant measure of the soil’s protective capacity.

Figure 2: SOC and specific surface area measured for different soils at Troughton Farm (Waikato region). SOC is plotted against apparent specific surface area (a) and against mineral specific surface area (b) after subtracting the SOC based water adsorption from measurements of apparent specific surface area. Data from G.Y.K. Moinet (unpublished).

We conclude that the ease of measuring specific surface area by water adsorption, adjusted for SOC content, makes it operationally attractive as a measure to compare soils with different capacities to protect SOC.