Phosphorus Dynamics Under Different Management Practices.

Perennial forage crops (grasslands) occupy nearly 50% of the cultivated land in Quebec, Canada, and may receive repeated applications of manure, resulting in P accumulation in surface soils. However, we have little information about the effects of occasional tillage and long-term manure application on the release and storage of added P. Two long-term forage fields were studied. One of the two fields had received 100 m³ ha^-1 of liquid swine manure annually since 1978 and the other was an unmanured grassland. In the fall of 2007, four treatments were randomly designated to the subplots and were: (i) left with vegetation (Ctrl); (ii) chemically treated with herbicide and either left untilled (chemical fallow, CF), (iii) ploughed by full inversion tillage (FIT) in the autumn (autumn-FIT), or (iv) ploughed in the spring (spring-FIT).  Soil samples were taken regularly to monitor water soluble-P (P_w) and other forms of P using a fractionation method (P-resin, P-NaHCO₃, P-NaOH, P-HCl and total-P). Tension lysimeters were installed in all subplots (15, 30 and 45 cm) to monitor P forms in the soil solution. In the top 5 cm of soil, the most labile P forms (P_w, M3P, P-resin and Pi-NaHCO₃) and total-P were respectively 95 and 61% higher in the manured field than in the unmanured field. In the manured field, the most labile P forms of the top 5 cm were 39 to 85% higher under untilled treatments (CF and Ctrl) than under tilled treatments (autumn-FIT and spring-FIT). In the untilled treatments of the manured field, maximum concentrations of labile P forms and total P (up to 1900 mg kg^-1) were found in the top 5 cm, whereas under tilled treatments the highest values were in the 5-10 and 10-20 cm soil layers. In the unmanured soil, a different pattern was observed than in the manured soil. Soil solution P was variable (from 0 to >1.0 mg L^-1), often exceeding the standard of 0.03 mg L^-1 for surface water quality in Québec, and tended to be higher in the untilled plots. In long-term manured grasslands, occasional tillage appeared to reduce P stratification, whereas tillage and long-term manuring affected the solubility of P.