Nitrogen benefits from four green-manure legumes in dryland cropping systems

Partial replacement of fallow with legume green manures has the potential to improve cereal production and agricultural sustainability in the 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 vegetative components of four annual legumes; (ii) estimate their ability to accumulate N through fixation; and (iii) compare the N uptake of the cereal crop that follows legume green manure with that of cereal grown 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 grown in rotation with spring wheat (Triticum aestivum L.). Nitrogen concentration 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 years, below-ground legume N content ranged between 2 and 29 kg ha(-1) and 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 feedpea. Within 3 mo of green-manure incorporation, average net N mineralization across years was greatest after black lentil and chickling vetch (38 kg N ha(-1)). The average 49 kg N ha(-1) lost through cereal grain harvest was balanced by gains through symbiotic N-2 fixation when chickling vetch and feedpea were used as green manure, but black lentil and Tangier flatpea replaced only about 35% of the N removed in the grain.