Rice straw management options impact soil phosphorus adsorption-desorption, kinetics and thermodynamics in rice-wheat system of north-western India

Fluctuations in soil management practices, temperature and moisture conditions can impact adsorption-desorption and bioavailability of phosphorus (P) in agricultural soils. Therefore, this study investigates P dynamics in straw-managed soils of Punjab collected from five treatments namely (1) conventional tillage (CT) after removal of rice straw (CT-R), (2) Treatment 1 plus biochar amendment at 2 Mg ha⁻¹ (CT+biochar), (3) zero tillage with straw retention as mulch (ZT+RM), (4) CT with straw incorporation (CT+RI) and (5) CT after rice residue burned (CT+RB) after three years from an ongoing experiment in rice-wheat cropping system. The adsorption-desorption of P followed pseudo second order kinetics (R²> 0.99) and Freundlich isotherm (R²> 0.95) for all the treatments and temperatures. Freundlich adsorption capacity (K_Fads) varied with the physico-chemical soil properties and ranged from 10.9 to 28.5, 14.3–32.2, 18.3–40.2, and 22.5–56.5 μg¹⁻ⁿg⁻¹mLⁿ at 15, 25, 35, and 45 ± 1°C, respectively. The sequential order of P adsorption was as follows: CT+ biochar > CT+RB > ZT+RM > CT+RI > CT-R, irrespective of temperature. Thermodynamic parameters revealed feasible, spontaneous and endothermic process indicative of physio-sorption via. hydrogen bonding as the dominant mechanism in in-situ straw managed soils. The Freundlich desorption coefficient (K_Fdes) ranged from 54.8 to 85.2, 39.9–60.8, 23.4–37.0, 29.6–45.7 and 19.4–36.7 μg¹⁻ⁿg⁻¹mLⁿ in CT+ biochar, CT+ RB, ZT+RM, CT+RI, CT-R, respectively at studied temperatures and was greater than adsorption in all treatments indicating hysteresis. The desorption sequence was observed as: CT-R > CT+RI > ZT+RM > CT+ RB> CT+ biochar. The greater adsorption and slower desorption of P under in-situ straw managed treatments (CT+biochar, CT+RB and ZT+RM) than CT-R and CT +RI, particularly CT+ biochar compared to CT-R will lead to more P retention in soil matrix thereby preventing eutrophication and deterioration of surface waters.