PHOSPHORUS STATUS OF FOREST AND AGRICULTURAL SOILS FROM A WATERSHED OF HIGH ANIMAL DENSITY

The objective of this work was to study the impact of animal density o n P status in soils from the Beaurivage River Watershed, Quebec, Canad a. Twenty-Eve samples from A, B, and C horizons from the five most rep resentative soil series were taken from sites under forest, hay fields of dairy farms, and farms with a known surplus of on-farm manure N (S N). Profiles were characterized for water-soluble P (P-w), Mehlich-3 e xtractable P (M3P), and P fractions. The P-w was 3.6 times larger in a gricultural than in forest soils in the A horizon. Excessive amounts o f 66.5 mg kg(-1) of M3P and of 95 mg kg(-1) Resin-P were found in the plow layer of agricultural soils and M3P was increased, on average, by 10 mg kg(-1) in the C horizon compared with forest soils. The accumul ation of added P has lead to an increase in labile (Resin + NaHCO3 + N aOH-P-o) P pools in the A horizon, whereas the resilient (total labile P) pools were the major sinks in B and C horizons. For SN farms, tota l P was increased by 435 mg kg(-1) in the A, 86 mg kg(-1) in B, and by 141 mg kg(-1) in the C horizon compared with forest soils. Dairy farm s soils had labile P contents similar to those from SN farms but were less enriched in resilient-P-i forms. The observed large P loads in su rface horizons and P-w increases in the subsoil of the agricultural so ils probably contribute to the contamination of surface waters in this watershed.