Changes in Soil Phosphorus Forms and Concentration Under Long-Term (1994-2012) Alternative Cropping Systems in Saskatchewan, Canada.

Improved phosphorus (P) use efficiency in agriculture is essential to conserve P fertilizers and minimize P losses from land to water. Long-term studies are ideal to assess the effects of crop rotations and inputs. This study investigated changes in soil P concentrations and forms after three 6-year cycles of long-term alternative cropping systems in Saskatchewan, Canada, using three input levels [High with recommended chemicals and tillage, Reduced (RED) with minimum use of chemicals and reduced tillage and Organic (ORG) with tillage and no chemicals] and three cropping rotation diversities [annual grains with fallow (LOW), diversified annual grains (DAG) and diversified annuals and perennials (DAP)] with four replicates per treatment in a split-split-plot design. External input was a significant factor with respect to P availability, despite changing total P contents with crop rotation. Total P was comparable between High and RED, and was lowest in ORG soils. For the 0-15 cm depth, input, diversity and their interaction significantly influenced total P and organic P (P<0.05), but only input affected Olsen and Mehlich-3 P (P<0.05). The RED treatments showed larger Olsen and Mehlich-3 P contents than High and ORG. This reflected fertilizer accumulation on the surface, because the RED plots had less tillage than the High and ORG plots. This would facilitate plant nutrient acquisition, but could increase the risk of P loss in runoff. For the 60-90 cm depth, the distribution of total P changed and was lowest in the RED, which demonstrated that tillage in the High and ORG plots over the 18-year period had moved P deeper in the soil profile. Preliminary results from ³¹P-NMR spectroscopy show that soils from treatments with lower inputs contained more organic P forms, while treatments with high inputs had higher concentrations of orthophosphate, consistent with the Olsen and Mehlich P results.