Depth Stratification of M3P After Long-Term Effluent Irrigation In Cropland.

Long-term sustainability of land application of wastewater can be restricted by progressive downward P saturation of soils. Soil samples were collected from Penn State Living Filter (LF) site to assess the impact of over four decades of effluent irrigation on soil P saturation. Effect of years of effluent irrigation (0, 14, 24, and 44 yr), topography (summit, side-slope, and depression), and depth (0-7.5, 7.5-15, 15-30, and 30-45 cm) on Mehlich3 extractable P (M3P) were analyzed. Overall, all the three factors were significant (P <0.1). Depth profile data were used to determine a threshold value of Mehlich3 extractable P saturation ratio (M3PSR). A semi-empirical relationship, based on Mehlich3 extractable Al and Fe (M3Al and M3Fe in mg kg^-1), amount of total P need to increase plow-layer M3P by 1 mg kg^-1 (P_T in kg ha^-1)   and M3PSR_threshold, was used to predict the time required (Y, yr) to saturate a given depth (d, cm) of soil:

Y=[0.711M3A��+0.341M3FeM3PSRthreshold-0.62 M3P]dPT

For the LF site (M3Al=1150, M3Fe=150, M3PSR_threshold = 0.0646, P_T = 12.4), It was estimated that top 30 cm of surface soils will become critically saturated after  ~16 yr of effluent irrigation. Results of this study indicate that time required to saturate top 30 cm of effluent-irrigated site is much longer than the previously expected value of 10 yr.