Organic and inorganic soil carbon in a semi-arid rangeland is primarily related to abiotic factors and not livestock grazing

Soil has the ability to sequester carbon (C) and mitigate anthropogenic enrichment of carbon dioxide, however numerous variables influence its C storage potential. Climate, soil properties, plant species composition, and livestock management practices, may all influence C storage in rangeland soils. The purpose of this study was to examine the relative importance of these factors in predicting organic and inorganic soil C in a semi-arid rangeland in northern Arizona USA. Sampling sites at 60 locations within the 40,469 ha Diablo Trust Ranches were selected using a stratified random method to encompass five distinct soil series across a precipitation gradient (230 mm–623 mm mean annual precipitation) with fence-lines that separate actively grazed areas with adjacent areas that have excluded livestock for at least 20 years. A total of 240 soil samples were collected from two depths (0–5 cm and 20–25 cm) in grazed and adjacent ungrazed sides of fences. Soil texture, bulk density, plant community composition, root biomass, soil organic carbon (SOC) and soil inorganic carbon (SIC) were measured. Across the precipitation and soil gradient, SOC was highest (up to 28.6 Mg ha⁻¹) in the surface layer of clay-rich soil derived from basalt and SIC was highest (up to 74.7 Mg ha⁻¹) in subsurface soil derived from limestone. General linear models showed that grazing had no significant impact on total soil C, instead, soil depth and soil texture were the main predictors of SOC and SIC. Precipitation was positively correlated with SOC and negatively correlated with SIC. Soil texture and precipitation were highly autocorrelated, so only soil texture was included in structural equation models (SEMs). Separate SEMs for surface and subsurface soils showed that soil texture (and autocorrelated precipitation) was the strongest predictor of SOC, SIC and plant community composition at both soil depths. The relative abundance of C₄ grasses was higher in grazed plots and was positively correlated with root biomass at the soil surface. Also, surface root biomass was weakly positively related to SOC while both surface and subsurface roots were negatively related to SIC. This study shows the importance of interactions among multiple factors in predicting soil C stocks and that SIC can be a substantial C reservoir in dryland soils, especially those derived from sedimentary parent material. We could not detect an influence of livestock grazing on either SOC or SIC in this landscape scale study. This suggests that in this semi-arid rangeland it may be difficult to increase total soil C storage through livestock management.