Lithology and coarse fraction adjusted bulk density estimates for determining total organic carbon stocks in dryland soils

Pedotransfer functions (PTFs) have been developed to estimate soil fine fraction bulk density (BDFF) using the relationships with soil organic carbon content (SOC) and particle size distribution. Current PTF's overlook the influence of lithology on BDFF estimations. In this study, we examine this influence by developing two PTF's for total bulk density (BDT), which includes both fine (FF) and coarse fragments (CF), using measured percent SOC in soils derived from felsic and mafic lithologies (90 felsic and 52 mafic, 142 total). Our results show that SOC is highly correlated with BDT in soils derived from felsic (R-2 value of 0.79, p < 0.0001) and mafic (R-2 value of 0.84, p < 0.0001) material and that the relationships for these two lithologies are highly distinctive. In addition, we adjust estimated BDT with soil pedon CF content (> 2 mm) to determine estimated BDFF. We use an independently collected validation subset of 70 samples to compare our model against 23 published PTFs. When accounting for lithological differences in BDT and adjusting for site-specific CF content, which is highly variable vertically and horizontally within the watershed, we observe substantial improvements (average of 10.05 +/- 4.89%) in BDFF estimation and associated errors compared to other PTFs. Findings from our study demonstrate that incorporating both lithology and CF content into site-specific empirical relationships can provide reliable BDFF and consequently soil carbon stock estimates.