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

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.