PLANT-AVAILABLE NITROGEN FROM LENTIL AND WHEAT RESIDUES DURING A SUBSEQUENT GROWING-SEASON

Lentil (Lens Culinaris Medikus) is being grown increasingly on the Canadian prairies as a pulse or green manure crop, and may increase N availability to a succeeding crop. This study was designed to compare the effects of lentil green manure, lentil straw, and wheat (Triticum aestivum L.) straw on plant-available N during the growing season after application. The fate of N-15 from fall-applied (1988) lentil green namure, lentil straw, and wheat straw and spring-applied (1989) fertilizer (NH4)2 SO4 was determined four times during the 1989 growing season at a field site located at Outlook, Saskatchewan, Canada, on a Bradwell sandy loam (Typic Boroll). Dentrification and leaching losses of N-15 from added lentil and wheat straw were negligible, but 24 and 30% of the N-15 in lentil green manure and fertilizer, respectively, were lost in the 6-wk period after planting (8 May 1989). By wheat harvest (8 Aug. 1989), 7% of the N-15 in lentil and wheat straw and 37% of the N-15 in lentil green manure were mineralized. Addition of green manure increased net mineralization of indigenous soil N at the time of planting by 0.4 g m-2, equivalent to 10% of added green manure N. Immobilization of soil and fertilizer N was similar for lentil and wheat straw. The smaller fraction of N-15 assimilated from green manure (19%) than from fertilizer (34%) by wheat was due solely to less net mineralization of green-manure N rather than net immobilization of fertilizer N. Of the N-15 added in lentil and wheat straw, 5.5% was assimilated by wheat. Thus, lentil straw was not a significant source of N in this study, while almost-equal-to 40% of the N in lentil green manure was potentially availabel for plant uptake.