Changes in soil phosphorus lability promoted by phosphate sources and cover crops

Crop rotation and soil management can promote expressive changes in accumulated (legacy) soil phosphorus (P) lability since cover crops can cycle more P into plant tissue, and have a beneficial effect on the subsequent cash crop. This study aimed to understand the P dynamics in soil under different P sources and cover crops over six consecutive cropped years, and also to track how changes over time can achieve more efficient use of soil P in a high P-fixing soil from south Brazil. It was used five cover crops (common vetch, white lupin, fodder radish, ryegrass, and black oat) plus fallow in winter, meanwhile the summer crops were treated with soluble P fertilizer (SSP- single superphosphate) or rock phosphate (RP) every year from 2009 to 2014, under a no-tillage system. Soil samples were taken after six years of cultivation (2014) and analyzed for P fractionation by the Hedley procedure. Next the results were compared to the results previously obtained at the beginning of this period (2009), and after the third summer cycle (2011). Cover crops affected P cycling under phosphate fertilizer when SSP was used and all cover crops were able to utilize more moderately labile (mod-labile) P and enhance the proportion of labile P fractions in the soil. In general, white lupin was the cover crop most effective in retaining the most P available for the subsequent crop in the soil and may be considered a P-mobilizing species, regardless of the source of the applied P. Rock phosphate promoted the highest proportion of inorganic P accumulated in the soil while the lowest one was recorded under SSP. Organic P fractions were depleted over the period, either with or without fertilizer, being the main source of plant extractable P in non-fertilized conditions over the period.