NITROGEN BENEFITS FROM 4 GREEN-MANURE LEGUMES IN DRYLAND CROPPING SYSTEMS

Partial replacement of fallow with legume green manures has the potent ial to improve cereal production and agricultural sustainability in th e northern Great Plains. This is possible if N gains by annual legumes and enhancement of soil N availability are optimized. The objectives of the study were to (i) determine the N distribution in different veg etative components of four annual legumes; (ii) estimate their ability to accumulate N through fixation; and (iii) compare the N uptake of t he cereal crop that follows legume green manure with that of cereal gr own on fallow or of cereal receiving N fertilizer. Black lentil (Lens culinaris Medik.), Tangier flatpea (Lathyrus tingitanus L.), chickling vetch (Lathyrus sativus L.), and feedpea (Pisum sativum L.) were grow n in rotation with spring wheat (Triticum aestivum L.). Nitrogen conce ntration in legume nodules was several times greater than in any other plant part. However, N concentration in legume shoots was, on average , 27% greater than in legume roots. Total legume N content (% x mass) ranged from 41 to 126 kg ha(-1) in years of low weediness. In those ye ars, below-ground legume N content ranged between 2 and 29 kg ha(-1) a nd averaged 7, 8, 17 and 6 kg ha(-1) for black lentil, Tangier flatpea , chickling vetch and feedpea, respectively. Estimates of N-2 fixation varied between 6 and 69 kg ha(-1) and averaged 18 kg ha(-1) for black lentil, 16 for Tangier flatpea, 49 for chickling vetch, and 40 for fe edpea. Within 3 mo of green-manure incorporation, average net N minera lization across years was greatest after black lentil and chickling ve tch (38 kg N ha(-1)). The average 49 kg N ha(-1) lost through cereal g rain harvest was balanced by gains through symbiotic N-2 fixation when chickling vetch and feedpea were used as green manure, but black lent il and Tangier flatpea replaced only about 35% of the N removed in the grain.