Phosphorus release during decomposition of crop residues under conventional and zero tillage

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Field experiments were conducted at Fort Vermilion (58°23′N 116°02′W), Alberta, to determine phosphorus (P) release patterns from red clover ( Trifolium pratense) green manure (GM), field pea ( Pisum sativum), canola ( Brassica rapa) and monoculture wheat ( Triticum aestivum) residues in the 7th and 8th years of conventional and zero tillage. Phosphorus contained in crop residues ranged from 1.5 kg ha −1 in pea to 9.2 kg ha −1 in clover GM, both under zero tillage. The patterns of P release over a 52-week period sometimes varied with tillage, i.e., a greater percentage of GM residue P was released under conventional tillage than under zero tillage in the first 2–10 weeks of residue placement. Wheat residues resulted in net P immobilization under zero tillage, but the amounts immobilized were less than 1 kg ha −1. When net P mineralization occurred, the percentage of P released ranged from 24% of wheat P under conventional tillage to 74% of GM P under conventional tillage. The amounts of P released were 0.4 kg ha −1 from wheat, 0.8 kg ha −1 from canola, 0.4 kg ha −1 from pea and 5.1–5.6 kg ha −1 from clover GM residues. Therefore, only GM residues recycled agronomically significant amounts of P for use by subsequent crops in rotation. Phosphorus release was positively correlated with residue P concentration and negatively correlated with C/ P and lignin/ P ratios.