In Asia, about 24 million hectares (Mha) of land is devoted to the rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system (RWCS) (Nawaz et al., 2019). This system is most prevalent in South Asia’s Indo-Gangetic Plains (IGP), where India alone holds 10.3 Mha land under this particular cropping system (Timsina and Connor, 2001). Rice and wheat together contribute about 45% of digestible calories, 30% of protein, and a considerable percentage of animal feed worldwide (Evans, 1996). In India, RWCS contributes 31% of food grain output (Prasad, 2005) demonstrating its significance in ensuring national food security. India produces 20% of the world’s rice, making it the second-largest rice producer. To meet the needs of its expanding population, India plans to increase rice production to the tune of 130 million tonnes by 2030 (Kumar et al., 2021). India relies extensively on RWCS to provide not just food security, but also nutritional security. However, the sustainability of the rice under RWCS has emerged as a major emerging challenge during the last decade. Some of the predominant reasons include stagnating yields, decreasing crop responses to external mineral fertilizer, multi-nutrient deficiencies, dwindling factor productivity (land, water, labor, etc.), and the use of repeated wet tillage, which oxidizes buried organic matter and breaks down macro-aggregates negatively impacting soil health (Bhatt et al., 2016). Furthermore, keeping the land idle during the intervening period of two crops does not provide any financial benefits to producers. All these factors contribute to the poor financial situations of farmers leading to agriculture becoming a non-profitable livelihood. The Global Hunger Index (2022) depicts that India ranks 107th out of 121 nations, with a score of 29.1 which is an indicator of a serious hunger problem in the country. Anthropogenic climate change will further negatively impact the grain quality of rice (Lin et al., 2010). Since soil, plant, and human health are interconnected, India’s agri-food system, especially rice in the RWCS, must be transformed to provide food and nutritional security, increasing profits without compromising soil quality.

Increasing the profitability of rice under RWCS may need adopting a vertical crop diversification approach, such as switching from ordinary rice genotypes to high-yielding basmati genotypes that have a high value in international trade. Due to its unique characteristics (such as a pleasant aroma and an extended grain length after cooking), basmati rice has a lot of potential for export. In the fiscal year 2021–22, India earned US $3,540.40 million from the export of basmati rice, making it the world’s largest exporter (APEDA, n.d.). Improving the nutritional quality of the basmati rice to combat mineral shortages and malnutrition would be a win-win situation for everyone involved in RWCS, in addition to bringing in much-needed foreign currency.

The manifestation of any trait in a plant depends on genotype, environment and the dynamic interplay between genotype and environment. Since soil is one of the most significant environmental factors affecting yield and nutritional content in grains, it must be carefully attended to break open the abeyant high yield potential and nutrient-dense grains of basmati rice genotypes. In this perspective, sustainable intensification of the fallow period in RWCS by integrating summer green manuring crops (SGM) might be one of the feasible solutions for sustainable land use and most crucially recovering soil health affected by soil harmful intense tillage operations performed under the present system. Numerous studies have demonstrated the efficacy of green manure in boosting agricultural productivity, mitigating soil erosion, lowering runoff and nitrate leaching during the fallow period, decreasing soil bulk density, raising the levels of organic matter, N, P, K, and other nutrients in the soil to lessen the need for synthetic fertilizers, maintenance of soil biodiversity and enzyme activity, etc. (Mandal et al., 2003; Salahin et al., 2013; Ma et al., 2021). Additionally, it is well established that leguminous green manures have high fertilizer value on the succeeding crops due to their ability to fix atmospheric nitrogen. Green manuring coupled with chemical fertilizer can help ensure the long-term viability of RWCS (Yadav et al., 2000). An earlier study reported that Sesbania green manure could partially replace chemical fertilizer, increase rice yield, and reverse the declining trend of partial factor productivity (PFP) in wheat. Leguminous green manures mineralize more quickly and release both macro- and micronutrients factored by their lower C/N ratio. Additionally, the incorporation of green manure can improve the bioavailability of micronutrients by chelation through the production of organic acids (Singh and Shivay, 2019).

Among the essential micronutrients, zinc (Zn) is one of the most crucial micronutrients for crop production because it is a key structural component or regulatory co-factor of numerous enzymes and proteins in many crucial biochemical pathways, including protein metabolism, auxin synthesis, pollen formation, the preservation of the integrity of biological membranes, and carbohydrate metabolism (Alloway, 2008). However, its deficiency is more evident in high pH calcareous soils in the North Western Indo-Gangetic Plains (IGP), which is linked to greater Zn sorption on the soil as hydroxides and carbonates (Prasad, 2006). Further, the prolonged submerged condition of the rice is one of the causes of the Zn deficiency in rice in particular. In this context, green manuring with Sesbania (Sesbania aculeata Pers.) has shown promising results for improving soil zinc bioavailability and producing Fe, Mn, Cu, and Zn-dense grains in basmati rice (Pooniya and Shivay, 2013). Limited scientific research has also indicated the benefits of SGM in rice under RWCS (Thind et al., 2019; Sharma et al., 2021), but it is important to consider how its integration and adoption in the present system can increase productivity, nutritional security, and profitability. Organic manures, such as green manuring, can improve soil fertility and health, but their slower mineralization rate is out of sync with plant needs, as they cannot be relied on alone to provide the plants’ nutritional demands during the crop season. Thus, the present focus on enhancing agricultural productivity and crop quality is preferred to the use of chemical fertilizers, however it is linked to a number of unfavorable environmental outcomes, such as soil hardening, decreased fertility, reduced activity of native soil microorganisms (and thus a decrease in soil biodiversity), alteration in soil pH, and the formation of a crust on the soil’s surface (Pahalvi et al., 2021). Rice being nutrient intensive, there is a pressing need to pay crucial attention to make optimal use of fertilizers, which are becoming more expensive with time.

As a result, the co-application of organic and inorganic nutrient sources needs to be appropriately maneuvered in order to capitalize on the synergism that exists between them. Green manure increases nitrogenous fertilizer efficiency for two main reasons: (a) green manure N is as efficient as urea N in rice (Ladha and Kundu, 1997), and (b) higher root density of subsequent wheat due to improved soil physical conditions enhances nutrient acquisition capacity and biological yield at a given fertilizer level (Boparai et al., 1992). In this context, the use of slow-release fertilizer has been an effective method of nutrient administration because it releases nutrients gradually, at the rate preferred by the plant, and it keeps those nutrients in the soil for an extended length of time. A study by Cole et al. (2016) suggested the benefits of slow-release fertilizer and reported it as an efficient way of administering nutrients as they are made available to the target at the desired rate or concentration level, hence sustaining the nutrients in the soil for a longer period. Enhanced nutrient use efficiency, especially for nitrogen (N) has been achieved through the use of conventional coated urea category of fertilizers such as neem-coated urea in tropical lowland rice (Mohanty et al., 2021), sulfur-coated urea in spring wheat (Shivay et al., 2016), and zinc-coated urea in RWCS (Shivay et al., 2008), etc.

The use of innovative nano-fertilizers like nano zinc oxide (ZnO) has recently reshaped the definition of enhanced fertilizer use efficiency in the agriculture sector. In comparison to traditional fertilizers, it is fundamentally different due to its unique physical, chemical, and biological properties. These fertilizers have a nutrient release rate that corresponds with crop needs, which helps in maximizing nutrient recovery and reducing nutrient losses. Also, because plants only need small amounts of these fertilizers, the greenhouse gas emissions during their production and transportation are very low. According to Pavithra et al. (2017), ZnO nanoparticles increased rice grain Zn content and grain production. The use of zinc nano fertilizer (ZnNFs) resulted in increased plant growth in pearl millet (Tarafdar et al., 2014). Applying ZnNFs (<100 nm) in maize crop has reportedly increased the plant height, root length, root volume, and dry weight of the root (Adhikari et al., 2015). In wheat, seed priming with 75 and 100 mg/l of ZnNFs has reported enhanced shoot Zn concentration (12–24%) and root Zn concentration (13–19%), compared to the control (Rizwan et al., 2019). Further, zinc oxide-coated urea has a favorable influence on grain yield and Zn content in a rice-wheat system (Shivay et al., 2008). Another study by Dimkpa et al. (2020) suggested that coating urea with nano-ZnO improved plant performance and Zn accumulation in wheat. However, the majority of the investigations have concentrated on the effect of the sole application of ZnO nanoparticles on plants. There is a dearth of research on the combining effect of summer green manuring (SGM) with the use of nano-ZnO-coated urea (NZCU). In addition, the relative efficacies of NZCU vis-à-vis bulk ZnO-coated urea with respect to grain quality of rice under RWCS have not been tested.

Therefore, the present study was contemplated with two primary objectives: (1) To elucidate the comparative efficacies of different green manure crops and diverse zinc sources on the yield response of basmati rice under RWCS, and (2) To decipher the nutritional composition particularly micronutrient and crude protein content of basmati rice as influenced by the integrated use of summer green manuring and zinc sources.

Leguminous green manure owing to a lower C/N ratio, decomposes quickly and results in net N mineralization (Khind, 1992). The incorporation of green manures adds a considerable amount of easily decomposable organic matter resulting buildup of organic carbon (Mandal et al., 2003), thereby promoting higher available N in the soil. Apart from the addition of nitrogen by green manuring, it contributed a significant amount of micronutrients thereby increasing the density of micronutrients in edible portions of food crops (Supplementary Table S1). Increased addition of micro-nutrient by summer green manuring compared to fallow promoted higher micronutrient and nitrogen-dense grain as reflected in our study. Sesbania-incorporated basmati rice outperformed cowpea in terms of increasing protein content and micronutrient density (except Mn) in rice grain factored by higher nitrogen and micronutrient addition associated with enhanced biomass production over both years. Our findings are consistent with Pooniya and Shivay (2013) who found that green manuring with Sesbania aculeata to basmati rice led to a higher accumulation of Zn, Fe, Cu and Mn and in the grain. Prior studies (Meena and Shivay, 2010) reported a positive effect of Sesbania on the agronomic profile of rice. The highest Mn content of cowpea green manured basmati rice can be attributed to significantly higher total Mn bioaccumulation in both root and shoot. Application of organic matter in the form of green manures leads to the release of decomposition products such as water-soluble organic ligands, e.g., citrate, oxalate, and malate (Jones et al., 2003) which have two-fold impacts (1) lowering of rhizosphere pH resulting in higher micronutrient bio-availability (2) complexing of these soil absorbed/fixed micronutrient thereby modulating their solubility (Evans, 1991). The incorporation of green manures in-situ may induce alterations in paddy rhizosphere like a more reducing environment driven by the addition of external carbon source, e.g., glucose and organic acids from the decomposed substrate which could accelerate the process of Fe⁺³ reduction, hence more bio-availability of Fe (Liu et al., 2013). A similar process might have promoted Mn availability and Mn-rich basmati rice grain. Organic matter also affects the activity and richness of soil microorganisms (Dhaliwal et al., 2019) consequently enhancing plant micronutrient uptake. Furthermore, organic matter improves soil structure, allowing plant roots to explore a wider soil volume (Mäder et al., 2002) and higher soil moisture retention facilitating nutrient transport to the plant roots. The deeper root penetration by summer green manuring than basmati rice might be another element that aided in the recycling of micronutrients from the subsurface layer to topsoil (Jat, 2010; Pooniya and Shivay, 2012).

The present report of increment in grain yield by application of nano-zinc oxide aligns with previous studies with rice (Bala et al., 2019; Jangid et al., 2019; Kheyri et al., 2019; Singh et al., 2019; Elshayb et al., 2021; Somaratne et al., 2021; Yang et al., 2021; Adhikary et al., 2022; Akmal et al., 2022; Sheykhzadeh et al., 2022; Waqas Mazhar et al., 2022) and with other crops like wheat (Babaei et al., 2017; Adrees et al., 2021; Sheoran et al., 2021; Adil et al., 2022). Conjoint application of nano zinc and urea has been shown to promote grain yield in rice (Kumar et al., 2022) and in wheat (Dimkpa et al., 2020; Asim et al., 2022; Beig et al., 2022). Previous agronomical studies have clearly demonstrated the beneficial effect of ZnO NPs in promoting enriched micronutrients, especially Zn, and improved quality parameters in grain of rice (Kheyri et al., 2019; Elshayb et al., 2021; Yang et al., 2021; Akmal et al., 2022) and wheat (Zhang et al., 2017; Munir et al., 2018; Zhu et al., 2020; Sheoran et al., 2021; Asim et al., 2022). This might be due to enhanced Zn uptake from ZnO NPs than bulk ZnO counterparts linked to the higher solubility of Zn in soil followed by preferential uptake of ZnO NPs by rice root. The Zn sufficiency in plant tissues from ZnO NPs can promote seed development and hence grain size and grain weight because Zn plays key roles in DNA polymerase activity as well as in cell division (Marschner, 2012). The higher Zn content in straw, hull, and bran compared to the rice kernel reflects low mobilization of Zn from different parts to the kernel, producing the lowest Zn content in the kernel indicating that when hull and bran are removed during hulling and milling, the grains lose a considerable proportion of their nutritional values. Interestingly, the dilution effect may account for the considerably reduced Zn concentration in several parts of rice grain and straw in lone urea-treated basmati rice (Table 6) compared to soil application of 5 kg Zn ha⁻¹ using bulk ZnO (Jarrell and Beverly, 1981).

The improvement in nutrient content especially N (hence crude protein) in the grain of basmati rice by the application of ZnO NPs coated urea could be ascribed to enhanced nutrient mobilization in the soil by influencing the soil microbial population and extracellular enzymes secretion like urease in the soil (Raliya and Tarafdar, 2013), hence modulating available nitrogen in the soil. The application of 2% zinc-enriched urea outperformed other materials in raising N content which could be linked to synchronized released from the zinc-enriched urea (Shivay et al., 2008). In this context, it is worth noting that zinc content in edible parts of a grain of basmati rice was found to be highly correlated to N content in the grain. Since protein represents a sink for Zn due to the involvement of Zn in protein synthesis in the grain (Cakmak et al., 2010), an increase in crude protein content with Zn application is evident from our study. A possible explanation for the increased Zn concentration is the enhanced sink strength for Zn in grain caused by the increased N supply from summer green manure crops. Our findings corroborate with the findings of Hao et al. (2007) who found that N application promoted protein and accumulation of Zn content in grains. Our investigation has established that agronomic management using an appropriate combination of organic sources like green manuring and inorganic source like Zn-coated urea in basmati rice crop has the potential in modulating zinc levels in milled rice driven by synergistic nitrogen and zinc interaction can mitigate zinc malnutrition.

Our findings on basmati rice clearly demonstrated the effectiveness of these nano ZnO-urea assemblies in positively influencing and promoting the transportation of micronutrients and nitrogen into edible tissue. Notably, the yield response of the basmati rice treated with the 0.2% NZCU was comparable to those treated with a traditional fertilization strategy, i.e., 1% BZCU, despite the zinc rate being reduced by 20%. Since the aleurone layer is removed during milling, milled rice always has a lower mineral concentration. Thus, brown rice intake might be an alternative to milled rice for boosting micronutrient status in Asian diets. The delivery of micro-nutrients into plants using agronomic manipulations like summer green manuring and zinc-coated urea and the effective translocation of these nutrients into to edible portions of grain as demonstrated in the present study holds great promise for addressing the widespread micronutrient deficiency in plants and alleviating micronutrient malnutrition in human. Further, studies focusing on synergistic response factored by the integrated usage of green manure and nano ZnO coated urea for a variety of crop species, diverse cropping systems, and soil can be explored to fully understand the versatility and cost-effectiveness of these plant fertilization strategies. Our interventions could be advantageous for improving agri-food systems of rural households of India in combating protein-micronutrient malnutrition where the vast majority of the population relies largely on plant-based food like rice.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

KB, YS, and RP led the research work, planned, supervised, and conducted field experiments, and read and edited the manuscript. KB, SM, SN, and KR collected soil, plant samples and performed chemical analysis, also wrote the initial draft of the manuscript, and prepared figures and tables. RP, YS, DK, MS, DC, RK, and CS project supervision, reviewed, read, and edited the manuscript with significant contributions. BY performed the statistical analysis. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.