Soil phosphorus dynamics in a sprinkler irrigation system for land application of municipal wastewater effluent

Land treatment of municipal wastewater effluents (MWE) is a proven method for augmenting freshwater resources and avoiding direct nutrient discharge from wastewater treatment plants (WWTP) to surface waters. The crop fields and forested areas of the Penn State Living Filter (LF) site have been irrigated with MWE from the Penn State University WWTP since 1983. The dynamics of soil test phosphorus (STP) of crop fields irrigated with MWE were studied to: (1) evaluate agronomic availability of P measured by Mehlich-3 extractable soil P (M3P), and (2) to assess the potential for environmental P losses from the LF site. Depth profiles of M3P were evaluated to estimate the time required to saturate a given depth of the surface soil with respect to P.;The M3P data of the plow-layer of irrigated crop fields from 1983 to 2009 were fitted to a two-phase regression model in order to describe the long-term trends, which were subsequently interpreted on the basis of P application with MWE irrigation and soil properties. Soil chemical changes induced by long-term irrigation of MWE have altered the P sorbing properties. The surface soil equilibrium P concentration at zero sorption (EPC0) increased markedly from <1 to ∼5.5 mg L-1 after a total P application of 2528 kg ha-1 from 1983 to 2008. The long-term trends exhibit a biphasic behavior of M3P over 26 yr of system operation. After a ~9 yr initial buildup, a marked decrease in the rate of M3P change has occurred. During initial buildup phase, a total 12.4 kg P ha-1 were needed to increase M3P by 1 mg kg-1. After the buildup phase, the estimated quantity of P applied via MWE irrigation was slightly lower than the amount removed with harvested crops and leached below the plow layer so that a quasi steady-state condition exists with respect to M3P. Speciation modeling using Visual MINTEQ indicates complexation of Al by dissolved organic carbon (DOC) at soil pH conditions of the site. There appears to be a gradual loss in the P sorbing capacity of the surface horizon due to elution of Al, thereby resulting in enhanced displacement of applied P below the plow layer rather than accumulating in the surface horizon. Despite continuous P application with MWE irrigation in excess of fertility recommendations, the plow layer (0-20 cm) average M3P has remained ~110 mg kg-1 for the last 17 yr.;The depth profiles (0-7.5, 7.5-15, 15-30, and 30-45 cm) of M3P from different topographic positions (summit, side-slope, and depression) of crop fields, which received 14, 24, and 44 yr of MWE irrigation were studied to evaluate the spatial-temporal variation of M3P. Archived soil samples from the three topographic positions of the unirrigated crop field were compared with the soil samples from irrigated crop fields. Significant differences (P<0.1) in the M3P depth profiles were observed for the irrigated (14 yr, 24 yr, 44 yr) crop fields compared to the unirrigated (0 yr) crop field at all topographic positions. However, no significant differences were observed between irrigated crop fields. The non-significant (P>0.1) differences amongst the irrigated crop fields were consistent with the steady-state of M3P after ~9 yr of MWE irrigation, as concluded from the analysis of long-term data of plow-layer M3P.;Soil sampling depth was found to be a significant factor (P<0.1) and typical trends of decreasing M3P with increasing depth from the soil surface were observed at all the topographic positions of the irrigated crop fields. The effect of topography was significant (P<0.1) only at the subsurface sampling depths in the irrigated fields (below 15 cm for 14 yr and 44 yr crop fields and below 30 cm for 24 yr crop field). The M3P of depressional soils below 15 cm was found to be greater (P<0.1) than M3P of summit soils at the same depth in the crop field irrigated for 14 yr. For crop fields irrigated for 24 yr, higher (P<0.1) M3P of the depressional soils in comparison to summit soils was observed only below 30 cm. The M3P of depression soils (below 15 cm) was greater (P<0.1) than both summit and side-slope soils in the 44 yr crop field. The significant effect of topography on M3P in the subsoils was consistent with the translocation of applied P by subsurface flow pathways. Lateral movement of sprinkler applied effluent was expected to cause relatively greater P loading at the depressional areas. Greater hydraulic conductivity and high organic contents of the surface soils at the depressions provides additional thrust to the downward movement of P through the soil profiles located at the depressional areas. (Abstract shortened by UMI.).