Pulse Crop Residues Release N When It Matters.

Pulse crops are believed to contribute little N to subsequent crops because most of the N is exported with the harvested grain.  However, most such data are collected only in one subsequent crop.  We quantified N released from residues of field pea (Pisum sativa L.) varieties Camry (green pea) and 4010 (forage pea), faba bean (Vicia faba L.) grown for seed, faba bean green manure (GM), and chickling vetch (Lathyrus sativus L.) GM in three subsequent crops at Beaverlodge, Alberta.  The legumes were grown in 2007, and wheat (Triticum aestivum L.), canola (Brassica napus L.), and hulless barley (Hordeum valgare L.) were grown in 2008, 2009, and 2010, in that order.  Green manure residues had already released 77 to 79% (76-102 kg ha^-1) of their N by the time wheat was seeded in 2008.  Faba bean grown for seed had released about 40% (61 kg ha^-1) of its N, and pea residues 22 to 25% (16-28 kg ha^-1), in the same period.  When wheat and canola were grown in 2008 and 2009, respectively, pulse crop residues released more N than GM residues: 33 and 33 kg ha^-1, respectively, from faba bean residues, 30 and 29 kg ha^-1 from forage pea residues, and 30 and 22 kg ha^-1 from green pea residues, compared with 12 and 5 kg ha^-1 from vetch GM residues and 11 and 8 kg ha^-1 from faba bean GM residues.  Little N (1-9 kg ha^-1) was released from all residues to barley, the third crop grown after legumes, in 2010.  Therefore, although pulse crop residues released less N than GM residues initially, they released more N than GM residues when it mattered, i.e., during growth of two subsequent crops.  Synchronization between N release and N demand was better for pulse crop residues than GM residues.