Structural equation modeling of the interactions between trace elements and soil organic matter in semiarid soils

The bioavailability of trace elements is strongly influenced by the physical and chemical properties of the soil system. This study sought to establish the relationships between the trace elements (barium, strontium and rubidium), soil organic matter and soil separates in semiarid soils from Central Botswana in a bid to improve our understanding of trace elements biogeochemistry. Trace element contents were obtained using portable X-ray fluorescence spectroscopy, while routine soil analysis methods were used for soil organic matter analyses and particle size distribution. Both multivariate statistical analysis and structural equation model were applied to the data. Results for correlation matrix and principal component analysis showed trace elements affinity for silt, clay and soil organic matter: barium and silt (r = 0.54, p < 0.01), rubidium with clay (r = 0.69, p < 0.001) as well as strontium with stronger affinity for clay (r = 0.83, p < 0.001) and soil organic matter (r = 0.88, p < 0.001). Principal component analysis showed that trace elements interactions with silt, clay and soil organic matter were observed more in profiles 3 and 7. Three structural equation models were obtained and in two of them, soil organic matter showed statistically highly significant direct effect on strontium which is further supported by all the multivariate statistical analysis results. Further research may be necessary to understand detailed aspects (e.g., complexations) between strontium and soil organic matter in semiarid soils.