Abstract
Study was carried out to evaluate the supplementary nitrogen requirement of tef to enhance nitrogen use efficiency of tef grown under chickpea-tef rotation cropping. On-farm, experiment was conducted during the 2015 main cropping season at Tahtay Koraro District of the Tigray regional State, Ethiopia on tef after preceding chickpea. The experiment was set in a randomized complete block design with three replications. Seven treatments: Six N levels (0, 11.5, 23, 34.5, 46, and 69 kg N ha-1) under the chickpea-tef rotation and the seventh one negative control (0 kg N ha-1) under the continuous tef cropping were tested. Surface soil samples were collected before tef sowing and after harvesting. They were analyzed for selected soil properties. Clay sized particles dominate the soil of the experimental site and the textural class of the soil is clayey. There was a difference in the bulk density of the same soil between the chickpea-tef and tef-tef sequence. Nitrogen and organic carbon were higher in soil under chickpea-tef cropping sequence than in soil under continuous tef cropping. Application of different N rates under chickpea-tef rotation statistically significantly affected grain (GNU), straw (SNU) and total nitrogen uptake (TNU) (kg ha-1). The highest total tef nitrogen uptake (59 kg N ha-1) was obtained in response to application of 34.5 kg N ha-1. The highest apparent nitrogen recovery (81%), agronomic efficiency (10.48 kg kg-1) and physiological N use efficiency were obtained in response to the lower N rate (11.5 kg N ha-1), 23 kg N ha-1 rate and 34.5 kg N ha-1 respectively. The highest grain protein content (7.78%) was recorded for grain harvested from plots fertilized with 23 kg N ha-1. Hence, it could be concluded that, under chickpea-tef rotation cropping system some supplementary nitrogen is needed to fulfill the nitrogen requirement and nitrogen use efficiency of tef crop at the study area. Key words: Chickpea, tef, nitrogen uptake, nitrogen use efficiency, protein content.  
Highlights
The beneficial effects of rotating leguminous crops with cereals were well known in the study area, there was an information gap on contribution of preceding legumes on the significant nitrogen addition to the soil and its effect on nitrogen use efficiency for succeeding cereal crops
Study was carried out to determine the nitrogen use efficiency of teff as influenced by supplementary nitrogen fertilizer rates after chickpea
The highest total nitrogen uptake was recorded from the 46 kg N ha-1 rate
Summary
Nitrogen fertilizer is one of the main inputs for cereals production; as it is often the most limiting nutrient for crop yield in many parts of the world (Giller, 2004). The accommodation of the needs of the escalating world population by developing a highly productive agriculture, whilst at the same time preserving the quality of the environment (Hirel et al, 2007) is believed to be challenging for the decades. Excessive addition of this nutrient can contribute to the combined effects of denitrification, volatilization and leaching watercourse pollution (Semenov et al, 2007). Reduction of applied N fertilizer rate to an optimized level can reduce soil nitrate leaching (Power et al, 2000)
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