Changes in phosphorus fractions of a Humic Gleysol as influenced by cropping systems and nutrient sources

Abstract

Information about the dynamics of soil P fractions is useful to predict their bioavailability and risk of P transfer from soils to surface waters. The objective of this study was to assess the effects of cropping systems and nutrient sources on P fractions in a Labarre silty clay (Humic Gleysol). Soil samples (0-15 cm) were collected in 1989, 1994 and 1997 from a field with four cropping systems, combining two crop rotations, barley (Hordeum vulgare L.) monoculture and 3-yr barley-forage rotation, with two tillage operations (chisel and moldboard plowing) as main plots, and two nutrient sources (mineral fertilizer and liquid dairy manure) as subplots. A modified Hedley sequential fractionation scheme was used. The inorganic P (Pi) fractions (resin-P, NaHCO3-Pi, and NaOH-Pi) increase with time in all cropping system and nutrient source combinations. Organic (Po) fraction (NaHCO3-Po and NaOH-Po) changes were related to C inputs and total soil C contents. The barley monoculture combined with mineral fertilizer slightly reduced NaHCO3-Po and NaOH-Po. The barley-forage rotation increased labile Po fractions. Rotation, chisel plowing and liquid dairy manure addition result in a buildup of labile P. A larger labile P increment per unit of P added, in surplus to plant exports, was observed with dairy manure than for mineral fertilizer, suggesting a higher risk of surface water contamination by P. Cropping systems and nutrient sources have a large influence on the changes in P fractions in this fine-textured Gleysolic soil. Key words: P fractions, crop rotations, liquid dairy manure, chisel plowing, moldboard plowing