Soils that receive large applications of animal wastes and sewage sludge are vulnerable to releasing environmentally significant concentrations of dissolved P available to subsurface flow owing to the gradual saturation of the soil's P sorption capacity. This study evaluated P sorption (calculated from Langmuir isotherms) and availability of P (as CaCl2-P and resin P) in soils incubated for 20 d with poultry litter, poultry manure, cattle slurry, municipal sewage sludge, or KH2PO4, added on a P-equivalent basis (100 mg P kg(-1)). All the P sources had a marked negative effect on P sorption and a positive effect on P availability in all soils. In the cattle slurry- and KH2PO4-treated soils, the decreases in P sorption maximum (19-66%) and binding energy (25-89%) were consistently larger than the corresponding decreases (7-41% and 11-30%) in poultry litter-, poultry manure-, and sewage sludge-treated soils. The effects of cattle slurry and KH2PO4 on P availability were, in most cases, larger than those of the other P sources. In the poultry litter, poultry manure, and sewage sludge treatments, the increase in soil solution P was inversely related (R2 = 0.75) to the input of Ca from these relatively high Ca (13.5-42 g kg(-1)) sources. Correlation analyses implied that the magnitude of the changes in P sorption and availability was not related to the water-extractable P content of the P sources. Future research on the sustainable application of organic wastes to agricultural soils needs to consider the non-P- as well as P-containing components of the waste.
The objective of this study was to evaluate the leaching potential of P from a range of sewage sludge‐ or P fertilizer (monocalcium phosphate)‐amended topsoils that had different P contents. Leaching trials (70 d) were conducted in 30‐cm‐long PVC columns; P‐related parameters were then determined at different depths in the leached soils. In most cases, more (P < 0.01) P was released from the P‐fertilized than sludge‐treated soils. Increasing native Olsen P from 15 to 62 mg kg−1 had negligible effect on P release; however, there was a sharp increase in P leached between 92 and 134 (maximum) mg Olsen P kg−1. This increase was much greater in the P‐fertilized than sludge‐treated soils. At all depths in all leached soils, resin P decreased in the order: P fertilized > sludge > control. At the lowest depth (16–20 cm) of the leached soil, the degree of P sorption saturation decreased in the same order and was related curvilinearly (P < 0.001) to cumulative P leached (at 70 d). This relationship demonstrates the importance of P sorption saturation in controlling P leaching in sludge‐ and fertilizer‐treated soils. The more rapid rate of P saturation and P leaching in P‐fertilized than sludge‐treated soils emphasizes the role of the P source in adsorption‐desorption processes.
Excessive levels of P in agricultural soils pose a threat to local water quality. This study evaluated (i) time‐dependent changes in soil P sorption (expressed as a phosphorus sorption index, PSI) and P availability (as resin P) during incubation (100 d) with poultry litter, cattle slurry, sewage sludge, or KH2PO4, added on a P‐equivalent basis (100 mg P kg−1), and (ii) the subsequent kinetics of P release, measured by repeated extractions with a mixed cation‐anion exchange resin. Soil exchangeable Ca and ammonium oxalate‐extractable Fe and Al were also determined at 100 d of incubation. The small decrease in P sorption in the litter and sludge treatments (25%), compared with that in the slurry and KH2PO4 treatments (52%) between 20 and 100 d of incubation was attributed partly to the formation of new adsorption sites for P. Subsequent P release was described by a power equation: Resin P = a(extraction number)b, where the constants a and b represent resin P obtained with a single extraction and the rate of P release per resin extraction, respectively. On average, the rate of P release decreased in the order: KH2PO4 and slurry > litter > sludge, and was inversely related to exchangeable Ca content of the incubated soils (R2 = 0.57). The slower rates of P release in the litter and sludge treatments (P < 0.001) are attributed to the large values for exchangeable Ca (1050–2640 and 1070–2710 mg kg−1, respectively) in these amended soils. Future research concerned with short‐term declines in environmentally harmful levels of P in recently amended soils should consider the differential effects of the amendments on soil P dynamics.
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