ROLE OF ORGANIC MATTER IN MOBILIZATION OF PHOSPHORUS IN TALLGRASS PRAIRIE SOIL

Understanding the mobility of phosphorus (P) in soils is important for better management of agricultural practices. Prior studies have shown that soil organic matter (SOM) plays a vital role in controlling mobility of essential macro- and micro-nutrients in soils including P. In this study, we quantified and characterized the SOM in soil from Konza Prairie long term ecological research (LTER) site in north central Kansas, which enables measurements under controlled conditions of grazing and burning. Two soil samples (15-20 cm below surface) were collected in November 2020, for this analysis. The samples were dried at 60°C in air for two hours and then crushed to less than 0.2 mm particle size. Soil pH was measured in 1:1 slurry with ultrapure water (18.2 MΩ-cm resistivity) instantly using a calibrated pH probe. To quantify and characterize various fractions of SOM, the soil samples were incubated with ultrapure water, 0.1M NaOH and 0.6M HCl for 24 hours. Soil extracts were separated by centrifuging at 15,000 rpm for 15 mins. Dissolved organic carbon (DOC) and nitrogen (DON) were measured in extracts using Shimadzu TOC/TN Analyzer. Absorbance and fluorescence spectra of extractable SOM were acquired using Horiba Aqualog Fluorometer. Concentrations of major cations and anions including phosphate (PO₄^3-) were measured using Dionex Integrion / Aquion ion chromatograph. Results showed that both the Konza soils had pH of ~8 and contained 71.35 mg/kg of water-extractable organic carbon. Water-extractable SOM was found to be highly aromatic (SUVA254 ~ 2.10 L/mg.m), humified (humification index, ~ 6) and relatively decomposed (freshness index ~ 0.7). Konza soils also leached 2.5-5 mg/kg of PO₄^3-, 200 mg/kg of Ca²⁺, and 50 mg/kg of Mg²⁺ during the experiment with water which is attributed to calcareous nature of soils in this region. Results also showed that OH^- leached much more PO₄^3- (19.4 mg/kg) than water. This may be due to exchange of PO₄^3- (and also organic matter) by OH^-. These results indicate that the presence of SOM would influence the mobility of PO₄^3- in soils depending upon pH conditions. The quantity and quality of SOM may play an important role in controlling mobility of soil P through competitive sorption, complexation (organo-P), and promoting microbial activities that may consume or release P to soils.