Use of a fractionation procedure to assess the potential for P movement in a soil profile after 14 years of liquid pig manure fertilization

Abstract

The accumulation of P originating from animal wastes in the soil profile is of agronomic and environmental concern. Sequential extraction procedure provides relevant information to mobility, bioavailability and distribution of P in animal manure-amended soils. Such information is needed to assess the potential for biological P leaching, under organic amendments, in some Quebec agriculture ecosystems. This study was conducted using soil samples from the long-term experimental plots of the Soil Research Station of the Quebec Ministry of Agriculture, Fisheries and Food at St-Lambert de Lévis, Quebec, Canada. The objective of this work was to examine the distribution of P fractions in the profile of a Le Bras silt loam soil (Gleysol) growing corn ( Zea mays L.) which had received liquid pig manure (LPM) for 14 years. The surface soil was fertilized annually with various rates of LPM (0, 30, 60, 90 and 120 cm 3 ha −1) in a randomized complete block design with four replicates since 1979. In the fall of 1992, soil samples were collected after the corn harvest from 20 soil plots at 20-cm intervals to a depth of 100 cm. Soil samples were air-dried, crushed, sieved through a 2-mm sieve, and analyzed for soil P fractions using a sequential extraction technique. All treatments resulted in an increase in the labile P pool contents of surface or profile soil compared with the control. Without exception, the amounts of P pools decreased with soil depth and were highly correlated with the LPM rates. The proportion of total P as labile (resin-P i + NaHCO 3-P + NaOH-P) in the soil profile increased from 61 (control) to 79% (highest LPM rate). The amounts of P generally increased in the following order: moderately labile NaOH-P i and -P o (254 mg kg −1) > labile P extracted by resin and NaHCO 3 (209 mg kg −1) > stable P (172 mg kg −1). On average, the amount of labile P i forms (304 mg kg −1) was larger than the amount of labile P o forms (159 mg kg −1) and represented an important fraction of the total labile P pools. Compared to the control treatment, the mean concentration of geochemical P (resin-P i + NaHCO 3-P i + NaOH-P i + HCl-P i + residual P) increased in the soil profile by 16, 26, 33 and 50% for 30, 60, 90 and 120 m 3 ha −1 LPM, respectively. On a profile basis, the soil receiving the highest rate of LPM contained 1.8-fold the biological P (NaHCO 3-P o + NaOH-P o) of the unamended soil. Amounts of labile P o pools in the soil profile were highly correlated with the organic carbon content. Results from this study suggest that heavy application of LPM over the 14-year period promotes P movement to lower portions of the soil profile. The leaching or translocation of P compounds would be responsible for the distribution of P in the various soil layers. Management of LPM rates is necessary after a long-term fertilization of corn.