Phosphorus Transfer from Cover Crop Residues to Soil Pools and a Subsequent Wheat Crop.

Cover crops provide many benefits to nutrient management in intensive agriculture, including  reducing losses of phosphorus (P) via erosion and leaching. The rate at which P taken up by cover crops is released back into soils and made available to subsequent crops affects the effectiveness of P recycling within these systems. We quantified P transfer from cover crop residues to soils and a subsequent wheat crop in a greenhouse experiment conducted with two California P-rich soils (Davis and Salinas). We grew wheat on soils labelled with carrier-free ³³P, fertilized with N, K and micronutrients, and amended at a rate of 15 mg P kg^-1 with cover crop residues (rye, oat, fava bean, rye, mustard, or a rye-legumes mixture), P mineral fertilizer, or not fertilized with P. We analyzed plant tissues and several soil pools – resin, microbial (via hexanol fumigation), and organic (via NaOH-EDTA extraction) – for P and ³³P specific activity. In both soils, cover crops had a similar effect on resin P than mineral fertilizer, and amendment with cover crops generally increased microbial and organic P content vs. mineral fertilizer and unfertilized controls. In the Davis soil, cover crops increased wheat biomass, %P and P uptake, but less so than mineral fertilizer. Cover crops increased only wheat %P in the Salinas soil, with no consistent effect on wheat biomass and P uptake vs. unfertilized controls. P release from residue mineralization reduced the ³³P specific activity of soil resin and wheat P, demonstrating a direct effect of cover crop residues on these P pools. We found no consistent effect of the different cover crop species on P dynamics across soils. Overall, our results suggest that P taken up by cover crops can be recycled relatively fast and directly benefit soil P availability and wheat P nutrition in intensive agriculture.