Dynamics of biosolids-derived phosphorus in a Mollisol
Land application of biosolids (treated municipal sewage sludge) is becoming more popular because it is an agronomically, environmentally, and economically acceptable means for biosolids disposal. When the amount of biosolids is applied to agricultural land according to N-based on agronomic rate, however, the amount of P applied is generally more than plants need. The subsequent accumulation of P is of environmental concern, because it could lead to eutrophication of water bodies if P is carried off-site in runoff water and eroded soils. Thus, a completely randomized block experiment with three biosolids treatments (control, low, and high biosolids application rates) and four blocks was designed for studying the fate of biosolids-derived P applied to a Mollisols. The research was focused on the redistribution of biosolids-derived P, the effect of biosolids on P in mobile soil solution and in groundwater, the changes in forms of soil P after biosolids application, and the effect of biosolids amendment on the P sorption, desorption, and buffering capacities of the soil. The results showed that (1) biosolids application at both low and high rates significantly increased total P concentration at the 0--25 cm depth of soils, (2) biosolids application at both rates significantly increased the P concentration in mobile soil solution collected at the 50 cm depth below soil surface, but the amount of P in mobile soil solution only accounted for 2% or less of P added in biosolids to soil surface, (3) biosolids application did not influence the P concentration in groundwater, (4) biosolids application at both rates significantly affected both absolute and relative concentration of P fractions at the 0--5 cm depth but not at the 20--35 cm depth of soils; at the 5--20 cm depth of soils, only some fractions of P were influenced, (5) P sorption, desorption, and buffering capacities were significantly influenced by biosolids application; P-binding energies and equilibrium P concentration were also significantly affected by the biosolids application, (6) the transformation of biosolids-derived P applied to soil has been observed from HCl-extracted P into H2O-extracted P and NaHCO3-extracted inorganic P, and probably into NaOH-extracted inorganic P, too.