Nitrogen management in rainfed castor: a comprehensive analysis of productivity, nutrient use efficiency, economics, energy flow, and carbon footprint

Introduction Nitrogen (N), an essential plant nutrient characterized by the lowest use efficiency and multiple loss pathways, challenges its management in rainfed castor. Therefore, a field experiment was conducted over three consecutive Kharif seasons (2022-2024) to develop an adoptable N management strategy for rainfed castor cultivation. Methodology The study investigated the impact of eight N management modules viz. , Recommended rate of fertilizer (RRF) (40-40–25 kg N-P 2 O 5 -K 2 O ha -1 ), 50% recommended rate of basal N (RRBN) +2 ml l -1 Nano Urea at 30 and 60 days after sowing (DAS), 50% RRBN + 2% Urea spray at 30 and 60 DAS, 75% RRBN + 2 ml l -1 Nano Urea at 30 and 60 DAS, 75% RRBN + 2% Urea spray at 30 and 60 DAS, 100% RRBN + 2 ml l -1 Nano Urea at 30 and 60 DAS, 100% RRBN + 2% Urea spray at 30 and 60 DAS, and recommended rate of phosphorous and potassium (RRPK) (00-40–25 kg N-P 2 O 5 -K 2 O ha -1 ) and were arranged in randomized complete block design with three replications. Results and discussions The study found that the combined soil and foliar application of N had a promising effect on crop productivity than soil-only N application. Aqueous solution of urea (2%) proved effective than Nano-Urea (2 ml L -1 ) for foliar fertilization of N. Specifically, application of 100% RRBN through soil (20 kg ha -1 ), followed by foliar sprays of 2% Urea at 30 and 60 DAS, improved growth and yield traits of castor. Consequently, it produced significantly higher seed yield (1546 kg ha −1 ), oil content (42.7%), and N uptake in castor (34.9 kg ha −1 ). Due to inverse relationship, N substituted via foliar fertilization enhanced the partial factor productivity (67.8–106 kg kg -1 ) compared to soil-only N application (32.5 kg kg -1 ). Foliar fertilization depletes the soil N pool as observed through +ve N balance index (10.7-12.1). Furthermore, 100% RRBN + foliar sprays of 2% Urea at 30 and 60 DAS found to be remunerative, besides enhancing energy (58,726 MJ ha −1 ), carbon (C) output (1,947 kg-CE ha −1 ), net energy and C-gain. These findings highlight that partial N substitution through foliar applications optimizes castor productivity, economics, energy, and environmental security in rainfed castor.