LONG-TERM CROP-ROTATION AND FERTILIZER EFFECTS ON PHOSPHORUS TRANSFORMATIONS .1. IN A CHERNOZEMIC SOIL

The objective of this study was to determine effects of different cropping systems and nitrogen (N) and phosphorus (P) fertilizers on soil P fractions and transformations in long-term rotation plots on a Chernozemic soil at Lethbridge, Alberta. Continuous wheat, wheat-wheat-fallow, and wheat-fallow crop rotations and fertilizer application significantly affected the amount and distribution of phosphorus among labile and stable inorganic (Pi) and organic (Po) forms as determined by a sequential fractionation procedure. Without N and P fertilizer, the proportion of total soil P in more labile forms that were sequentially extractable with an anion exchange resin (resin-Pi), sodium bicarbonate (bicarb-Pi and Po) and sodium hydroxide (NaOH-Pi and Po) were reduced to a greater extent in continuous wheat than in wheat-wheat-fallow and wheat-fallow rotations. The addition of P fertilizer significantly increased total P and all Pi fractions, with the greatest change in more labile Pi forms (resin, bicarb and NaOH), but had no effect on labile Po forms. In contrast, N fertilizer addition increased the proportion of P in labile Po fractions in all rotations, and decreased the proportion of P in labile Pi forms. The combination of fertilizer N and P generally increased both Pi and Po labile forms. The continuous wheat rotation, coupled with N and P fertilizer inputs, had the most positive effect on P cycling and transformations.