Impact of mineral nutrition and biopriming on crop performance, energetics, and the carbon footprint in rainfed castor bean (Ricinus communis L.)

Suboptimal nutrition augmented by soil moisture stress adversely impacts crop performance in rainfed agriculture. At the same situation, conjunctive use of bioinoculants with mineral nutrients alleviates moisture stress and amplifies nutrient availability that improves the crop performance. With this background, a four-year fixed plot field experiment was conducted to determine the impact of mineral nutrition and biopriming on the productivity, economics, energetics, and carbon (C) footprint in rainfed castor. Three fertilizer rates and five bioinoculants were evaluated in a split-plot design with three replications. The result revealed that, application of recommended rate of fertilizer (RRF) recorded ∼22.13 % and 82.49 % higher yields than 75 % and 50 % RRF, respectively. The highest gross returns (₹ 80,412 ha^-1), net returns (₹ 50,511 ha^-1) and benefit-cost ratios (2.68) were also found higher with RRF. Likewise, plots applied with RRF even increased the seed oil content (50.9 %), oil productivity (722 kg ha^-1), and rain-water use efficiency (1.60 kg ha^-1 mm^-1). The plots treated with RRF required in 8.12–18.64 % higher energy input (12,275 MJ ha^-1) over 75 % and 50 % RRF, respectively. Furthermore, RRF appeared to be convincing by recording significantly highest C-outputs (1,582 kg CE ha⁻¹) and also other C quotients over its counterparts. Among the binoculants, seed priming with Bacillus megaterium and Pseudomonas fluorescens surpassed the productivity potential of other bioinoculants. Bioinoculants being energy diffident inputs they did not showed significant variation in energy input, while they showed significant variation in energy output. The energy output of castor bioprimed with B. megaterium (47,011 MJ ha^-1) surpassed the remaining bioinoculants. Again, B. megaterium recorded significantly lower C-footprint (0.22 kg CE kg castor seed yield⁻¹) while it displayed highest C-outputs (1,556 kg CE ha⁻¹), C-efficiencies (4.53 kg kg⁻¹ CE), net C-gains (1,278 kg CE ha⁻¹), and C-sustainability index (4.60). Consequently, the application of RRF, i.e., 40–40–20 kg N-P₂O₅-K₂O ha^-1, along with the biopriming of B.megaterium appeared to be promising in enhancing productivity, economic returns, and resource use efficiency besides optimizing energy flow and C footprint in rainfed castor bean cultivation.