INFLUENCE OF ORGANIC AND INORGANIC CHEMISTRY ON SOIL PHOSPHORUS MOBILIZATION

Understanding mobility of soil phosphorus (P) is important in agriculture. Soil organic matter (SOM) plays a vital role in controlling P mobility. In this study we characterized inorganic and organic properties of soil samples from Konza Prairie Long Term Ecological Research (Konza LTER) region. Soils were collected from ~20 cm depth from the surface after clearing debris. The soils were dried at 60˚C for 2 hours and crushed to <0.2 mm particle size. The soil pH was measured in 1:1 slurry with ultrapure water (18.2 Mohm) instantly. Soil pH ranged between 7.5 to 8.3 and had specific conductance of 221 ± 32 µS/cm. Water-leaching experiment was done by mixing soil in deionized water for 24 hours in 1:10 w/w ratio, and presence of ions and water extractable SOM were measured in the extracts. Results indicated that these soils contained 24-hour water-leachable concentrations of K⁺, Ca²⁺, Mg²⁺, NO₃^-, SO₄^2- and PO₄^3- as 32±21, 182±94, 19±13, 12±9, 2±4, and 3±3 mg/kg respectively. These soils contained water-extractable soil organic carbon (SOC) of 125±52 mg/kg, and spectroscopic analyses revealed presence of highly humic-like, aromatic, and plant-derived SOM as indicated by humification index (HIX) of 7.5±0.3. The concentrations of total soil phosphorus measured by Bray, Olsen and Mehlich-III methods ranged between 2.96-79.65, 1.11-21.92 and 0.15-10.66 mg/kg respectively, and were correlated with the humic-like fluorescence Peak C intensities (R=0.96, p < 0.05). Strong correlations were found between water-extractable K⁺ ions and Bray P (R = 0.72), and between Mg²⁺ and Bray P (R = -0.78). These results confirm that calcareous soils in Konza would leach higher amounts of Ca²⁺ and Mg²⁺ upon reaction with water. The dissolution of P-bearing minerals like apatite are the primary source of water-leachable PO₄^3- in these calcareous soils. Presence of humic-like SOM also interferes with PO₄^3- adsorption on iron (Fe) and aluminum (Al) oxides via competitive sorption mobilizing PO₄^3- to soil water. These findings will support an effort towards development of a graphene-based real-time soil P sensor that will be deployed to measure the P concentrations in soil porewaters. This work was partially supported by NSF Award # 2203517 “Real-time and continuous monitoring of phosphates in the Soil with Graphene-based Printed Sensor Arrays”.