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Current IssueSpecial IssueEditorial BoardOnline First ArticlesArchive

ABSTRACT

Background: Continuous adoption of the rice-wheat system in the Indo-Gangetic plains has led to soil health depletion, escalating production costs and rising carbon footprints. In this regard, a field experiment was conducted in the research farms of ICAR-IARI.

Methods: The experiment was conducted in a split-plot design and had three crop establishment practices (Zero tillage with residue retention (ZT + R), Zero tillage (ZT) and conventional tillage (CT) and four nitrogen placement methods, viz. control, recommended dose of nitrogen (RDN), improved RDN and improved 80% RDN with varying application timings and doses. The study optimised nitrogen use and crop yields under different tillage and nitrogen management practices.

Result: For both maize and wheat, the ZT had lower energy inputs than CT. Total output energy was higher in the ZT + R by 8.46% and ZT by 4.98% compared to CT for maize, while in wheat it was 9.48% and 2.06% compared to CT. energy use efficiency (EUE) was higher in ZT than CT by 26.28% for maize while in wheat, the ZT + R had higher energy use efficiency than CT by 22.6%. Carbon inputs were lower in ZT, with higher carbon efficiency and carbon sustainability index (CSI) in the ZT and ZT + R. Control plots had higher EUE and energy productivity. Improved RDN among NPMs had significantly higher total output energy, energy use efficiency than the RDN due to higher yield obtained in the improved RDN. The research highlights the benefits of ZT and the ZT + R in maize and wheat, reducing energy inputs, increasing efficiency and productivity and lowering carbon emissions. Along with it, the improved nitrogen application techniques like the subsurface placement of nitrogen improves carbon and energy parameters of maize and wheat in maize-wheat system.

INTRODUCTION

The rice-wheat cropping system (RWCS) currently occupies approximately 14 million hectares across the indo-gangetic plains (IGP) and is responsible for roughly 75% of the total food grain output in India (Alam et al., 2016). Nevertheless, the prolonged utilization of this sequence has precipitated multiple challenges, including the depletion of groundwater resources, mounting operational costs, a shortage of farm labour, the widespread burning of crop residues and an escalation in greenhouse gas (GHG) emissions (Jat et al., 2018; Mondal et al., 2020). Consequently, the maize-wheat cropping system has surfaced as a viable substitute, presenting benefits such as better soil preservation, decreased water requirements and increased economic returns for farmers (Jat et al., 2018). To accurately evaluate the long-term viability of shifting to this new system, the implementation of comprehensive carbon and energy auditing frameworks is essential.
       
Evaluating system efficiency through energy auditing involves comparing the total invested inputs-such as seeds, irrigation, fertilizers and mechanical operations-against the final energy value of the crop output. In parallel, calculating the carbon footprint involves tracking GHG emissions, primarily N2O and CO2, that originate from both field activities and the manufacturing of agricultural inputs (Chakraborty et al., 2023; Bhattacharyya et al., 2025). Previous energy assessments within the IGP have successfully highlighted substantial efficiency gaps between conventional farming and conservation agriculture (CA) frameworks (Jat et al., 2019; Saad et al., 2016; Meena et al., 2024). Likewise, carbon footprinting of cereal rotations dependent on nitrogen fertilization indicates that fertilizer management contributes disproportionately to the overall GHG emissions of the system, underscoring the necessity of optimizing nitrogen rates and placement to mitigate the carbon footprint (Bhatia et al., 2023; Bhattacharyya et al., 2025).
       
Even with the expanding uptake of conservation agriculture in the IGP, exhaustive energy and carbon evaluations targeting the maize-wheat system are still scarce. This data deficit complicates the task of precisely measuring the environmental compromises associated with adopting this alternative cropping system (Bhatia et al., 2023; Parihar et al., 2018). Consequently, this investigation was initiated to address the following primary goals: (i) to assess and contrast the energy inputs, output energy and energy use efficiency (EUE) of both maize and wheat when subjected to zero tillage with residue retention (ZT+R), zero tillage (ZT) and conventional tillage (CT); (ii) to evaluate how different nitrogen placement methods-specifically the RDN, Improved RDN and improved 80% RDN-influence the carbon footprint and carbon sustainability index (CSI) of the cultivated crops and (iii) to determine the optimal integration of nitrogen management and tillage that delivers the highest energy efficiency and carbon sustainability for the maize-wheat system in the indo-gangetic plains.

MATERIALS AND METHODS

Study area and experimental design
 
The current field research took place at the ICAR-Indian Agricultural Research Institute (ICAR-IARI) located in New Delhi across two consecutive growing cycles: 2022-23 and 2023-24. This specific geographical location features a subtropical semi-arid climate. Over the two respective years, the site recorded average rainfall amounts of 780 mm and 580 mm. The soil profile of the testing ground was classified as sandy clay loam. Researchers utilized a split-plot structural design, applying treatments to permanent conservation agriculture plots that had been initially established and maintained since 2012.
       
The main plots were dedicated to three distinct crop establishment practices (CEP): Zero tillage with residue retention (ZT+R), Zero tillage (ZT) and conventional tillage (CT). The secondary sub-plots incorporated four separate nitrogen placement methods (NPMs)(NPMs): (i) Control: Fertilised with only phosphorus (P) and potassium (K) at sowing time; (ii) Recommended dose of nitrogen (RDN): 1/3rd N applied as basal, followed by two 1/3rd N applications as surface placement at the V6 stage and tasselling stage in maize and at the 1st and 3rd irrigations in wheat; (iii) Improved RDN: 1/3rd N applied as basal, followed by 1/3rd N as subsurface placement at V6/1st irrigation and 1/3rd N as surface placement at the tasselling/3rd irrigation in maize and wheat, respectively; and (iv) Improved 80% RDN: 30% N applied as basal, followed by 30% N as subsurface placement at V6/1st irrigation and 20% N surface broadcast at the tasselling/3rd irrigation in maize and wheat, respectively. The mung bean crop was also planted and harvested after wheat harvest and before maize sowing in both years, as legume inclusion in cereal-based cropping systems has been demonstrated to enhance soil organic carbon inputs and improve subsequent maize productivity through residue-mediated nitrogen enrichment (Shukla et al., 2025).

Energy and carbon auditing calculations
 
To evaluate energy dynamics, the straw and grain outputs from the wheat and maize harvests, alongside their equivalent yields, were transformed into terms of energy (MJ/ha) utilizing specific coefficients established by Mittal and Dhawan (1988). The precise energy equivalent coefficients implemented for these conversions are detailed in Table S1.

Table S1: Energy equivalent of various inputs used in study for energy dynamics (Mittal and Dhawan, 1988).


       
Field emissions of N2O and CO2 were harmonized into a standardized CO2 (carbon dioxide equivalent) metric by multiplying them by their respective global warming potentials (GWP)-which are 298 for N2O and 1 for CO2 -in accordance with the IPCC Fourth Assessment Report (IPCC, 2007). To estimate direct soil emissions, the Tier 1 protocol from the IPCC (2006) was utilized, which applies an emission factor of 0.01 kg N2O-N per kg of N applied. This emission factor (0.01) was multiplied by the total N introduced into the system via crop residue and synthetic fertilizer to compute the emitted kg N2O-N per kg N input. Standardized emission coefficients were further utilized to translate the scope of field operations and applied inputs into their corresponding equivalent emissions.
 
N2O emissions = Quantity of external N applied (kg/ha) × 0.01 × 1.571 (kg N2O/year)
 
Factor 1.571 was the conversion of N to N2O (44/28). GWP of emitted CO2 was computed as follows:
 
GWP = (Emitted N2O × 298) + Emitted CO2
 
The carbon indices i.e., C-output, c-efficiency (CE) and C-sustainability index (CSI) were calculated according to Jat et al., (2019); Choudhary et al., (2017) and Lal (2004). Carbon efficiency (CE) measures the ratio of carbon assimilated in crop biomass to the carbon emitted through all inputs, indicating the carbon return per unit of carbon cost.

 
The carbon sustainability index (CSI), proposed by Lal (2004), represents the net carbon gain relative to inputs; a positive CSI indicates carbon accumulation in the system.

RESULTS AND DISCUSSION

Energy auditing
 
Maize
 
The implementation of conservation tillage substantially minimized the required energy inputs while concurrently boosting the total output energy, thereby demonstrating a distinct efficiency advantage over CT. ZT reduced total energy inputs by 13.50% relative to CT, yet the ZT and ZT + R achieved 4.98% and 8.46% higher output energy, respectively (Table 1; Fig 1). When evaluating the NPMs, the Improved RDN strategy consumed the highest amount of energy input-remaining statistically comparable to both the Improved 80% RDN and RDN-yet it generated the greatest output energy, exceeding RDN by 3.28%. EUE followed a consistent tillage hierarchy: ZT exceeded CT by 26.28%, while ZT+R, despite superior output energy, registered lower EUE owing to its elevated input burden from residue incorporation. A significant CEP × NPM interaction regarding EUE confirmed that conservation tillage significantly amplifies the benefit of optimized nitrogen management.

Table 1: Effect of conservation tillage and nitrogen placement methods on the energy auditing of maize and wheat in maize-wheat system (pooled mean over two years).



Fig 1: Effect of crop establishment practices and nitrogen placement methods on the energy auditing of maize-wheat system (pooled mean over 2 years).



Wheat
 
In wheat, the energy efficiency advantage shifted from ZT (as observed in maize) to ZT+R, which recorded the highest EUE-exceeding ZT and CT by 7.17% and 22.68%, respectively. Output energy ranked ZT+R > ZT > CT, with margins of 9.48% and 2.06% over CT, while Improved RDN consistently outperformed RDN and Improved 80% RDN in output energy by 7.57% and 6.71%, respectively. No significant CEP × NPM interaction was detected for total output energy (Table 1; Fig 1). Energy productivity under ZT+R and ZT exceeded CT by 22.58% and 13.97%, respectively.
 
Carbon auditing
 
Maize
 
The CT registered the maximum carbon input (608.05 kg ha-1), although it significantly lagged behind ZT+R and ZT in terms of carbon output by 9.04% and 4.52%, respectively. Evaluating the NPMs, the Improved 80% RDN strategy established the most advantageous balance between carbon inputs and outputs, recording significantly lower carbon inputs than Improved RDN and RDN due to its 20% cutback in applied nitrogen. Across the crop establishment practices, both CE and CSI maintained a consistent ranking order of ZT > ZT+R > CT (Table 2; Fig 2).

Table 2: Effect of crop establishment practices and nitrogen placement methods on the carbon auditing of the maize and wheat crop in maize-wheat system (pooled mean over two years).



Fig 2: Effect of crop establishment practices and nitrogen placement methods on the carbon auditing of maize-wheat system (pooled mean over two years).


 
Wheat
 
Wheat carbon trends closely paralleled those observed in maize. ZT+R and ZT surpassed CT in carbon output by 9.61% and 2.11%, respectively and exceeded CT in CE by 13.38% and 9.83% and in CSI by 14.95% and 10.98% (Table 2; Fig 2). Among NPMs, Improved RDN delivered the highest carbon output, exceeding RDN and Improved 80% RDN by 7.46% and 6.72%.
 
Energy efficiency under conservation tillage: Mechanisms beyond input reduction
 
The notable dominance of ZT compared to CT regarding energy use efficiency across both crops highlights an underlying mechanistic reality that goes far beyond merely subtracting the arithmetic cost of reduced tillage passes. Following a decade of conservation protocols on the experimental plots (established 2012), the minimized soil disruption under ZT fosters the formation of stable macroaggregates and continuous biopores, which enhance deep root proliferation, nutrient interception efficiency and water infiltration. This directly translates into an improved ratio of yield-per-unit-input-the fundamental agronomic basis for elevated EUE-which functions independently of and additively to, the immediate energy conserved by skipping tillage operations (Parihar et al., 2018; Jat et al., 2019).
       
Our investigation provides novel insights regarding how this efficiency advantage differs crop-specifically within a continuous maize-wheat cycle, demonstrating that the leadership in EUE shifts from ZT during the maize phase to ZT+R during the wheat phase-a pattern that has not been previously documented for this rotation in the region. Mechanistically, this transition is attributable to the carry-over benefits of maize residues left on the soil surface throughout the rabi season. Notably, maize residue retained on the soil surface under ZT+R has a high C:N ratio and decomposes slowly, enabling a gradual and sustained release of carbon and nitrogen that supports long-term soil organic carbon accrual (Kumar et al., 2024).
       
The significant interaction between CEP and NPM regarding EUE, which was uniquely observed in maize, highlights a functional synergy uniting precision nitrogen placement with conservation tillage. Within a ZT framework, the structural integrity of the soil profile is maintained, preserving the spatial geometry of subsurface-banded nitrogen strictly within the active zone of root absorption. Conversely, the repeated soil inversion inherent in CT destabilizes this carefully arranged placement, largely dissipating the spatial benefits provided by the Improved RDN strategy.
 
Carbon footprint dynamics: Dual-pathway advantage of conservation agriculture
 
The sustained superiority of the ZT+R and ZT paradigms over CT in relation to the carbon efficiency (CE) and carbon sustainability index (CSI) is driven by two independent, compounding mechanisms. The initial pathway is emission suppression: by completely eliminating tillage procedures and thereby slashing diesel consumption, the heightened direct CO2 emissions characteristic of CT are successfully avoided. The secondary pathway involves biomass carbon accumulation: the robust biological activity, superior moisture retention and upgraded soil architecture fostered under long-term ZT+R stimulate greater crop biomass generation, subsequently elevating carbon output. The resulting CSI measurements confirm that the ZT+R system sustains a net positive carbon balance, which serves as a fundamental prerequisite for the long-term accrual of soil organic carbon (SOC).
       
The near-parity in carbon input between CT and ZT+R observed specifically in wheat (0.87% difference), in contrast to the substantial divergence recorded in maize, reflects the lower tillage intensity of conventional wheat establishment relative to post-kharif maize land preparation. This crop-specific asymmetry demonstrates that system-level carbon footprint assessments cannot be extrapolated from single-crop data.
 
Nitrogen management and carbon footprint: The non-linear emission pathway
 
The markedly reduced carbon inputs observed in the Improved 80% RDN configuration relative to the full-rate NPMs-and its consequent dominance in CSI-is dictated by the highly non-linear relationship linking applied synthetic nitrogen with N2O emissions. Whenever the concentration of mineral nitrogen in the soil surpasses the immediate demand of the crop, the processes of nitrification and denitrification accelerate disproportionately, generating N‚ O flux rates that climb much more rapidly than the linear addition of fertilizer. Consequently, decreasing the nitrogen load by 20% under the Improved 80% RDN protocol yields an amplified, greater-than-proportional drop in the overall carbon loading driven by N2O.
 
System-level synthesis: Toward sustainable intensification in the IGP
 
Across all energy and carbon parameters, the ZT+R × Improved RDN combination consistently emerged as the most resource-efficient and environmentally sustainable management strategy. To the best of our knowledge, this is the first study to simultaneously quantify the full energy audit and carbon footprint profile across both crops of a maize-wheat rotation under long-term CA plots integrated with subsurface nitrogen banding in the IGP.
       
The policy implications of these results are directly actionable. The Government of India’s stringent enforcement against crop residue burning creates an urgent demand for alternatives. ZT+R directly addresses this by incorporating residue into a productive system. Simultaneously, the carbon footprint benefits of Improved RDN and Improved 80% RDN align with the objectives of India’s Soil Health Card scheme and the PM-PRANAM programme.
 
Limitations and future scope
 
Although this investigation supplies a rigorous, full-rotation assessment spanning two years and two crops, it remains constrained by certain factors. Primarily, the trial was exclusively executed on the sandy clay loam soils of ICAR-IARI, New Delhi; consequently, the outcomes might differ if replicated across diverse pedological profiles. Therefore, multi-location validation is highly recommended. Second, the specific energy equivalent coefficients applied were derived from Mittal and Dhawan (1988), which may not perfectly capture the energy demands of contemporary agricultural operations. Subsequent investigations should update these multipliers using the latest life cycle inventory data. Finally, this current research did not directly record belowground biomass carbon, methane (CH4) flux, or definitive changes in soil organic carbon (SOC) stocks. Future scholarly work should prioritize complete LCA-driven carbon footprinting that includes these crucial dynamics, alongside comprehensive economic analysis to gauge the practical adoption feasibility for smallholder farmers.

CONCLUSION

The present study demonstrates that the adoption of zero tillage-based crop establishment practices -particularly ZT+R-in conjunction with precision nitrogen placement through Improved RDN, constitutes the most energy-efficient and carbon-sustainable management combination for the maize-wheat cropping system in the indo-gangetic plains. ZT consistently reduced total energy inputs relative to CT while simultaneously delivering higher output energy, energy use efficiency and energy productivity. The carbon auditing data reinforced this conclusion, with ZT+R achieving superior carbon efficiency and CSI values. Among nitrogen placement methods, Improved RDN delivered the highest output energy, while Improved 80% RDN achieved the most favourable carbon input profile through its non-linear suppression of N2O emissions. Collectively, these findings provide robust, rotation-level quantitative evidence that the integration of conservation tillage with subsurface nitrogen banding represents an agronomically superior and environmentally sound alternative to conventional management.

ACKNOWLEDGEMENT

The present study was supported by the ICAR-IARI.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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    ABSTRACT

    Background: Continuous adoption of the rice-wheat system in the Indo-Gangetic plains has led to soil health depletion, escalating production costs and rising carbon footprints. In this regard, a field experiment was conducted in the research farms of ICAR-IARI.

    Methods: The experiment was conducted in a split-plot design and had three crop establishment practices (Zero tillage with residue retention (ZT + R), Zero tillage (ZT) and conventional tillage (CT) and four nitrogen placement methods, viz. control, recommended dose of nitrogen (RDN), improved RDN and improved 80% RDN with varying application timings and doses. The study optimised nitrogen use and crop yields under different tillage and nitrogen management practices.

    Result: For both maize and wheat, the ZT had lower energy inputs than CT. Total output energy was higher in the ZT + R by 8.46% and ZT by 4.98% compared to CT for maize, while in wheat it was 9.48% and 2.06% compared to CT. energy use efficiency (EUE) was higher in ZT than CT by 26.28% for maize while in wheat, the ZT + R had higher energy use efficiency than CT by 22.6%. Carbon inputs were lower in ZT, with higher carbon efficiency and carbon sustainability index (CSI) in the ZT and ZT + R. Control plots had higher EUE and energy productivity. Improved RDN among NPMs had significantly higher total output energy, energy use efficiency than the RDN due to higher yield obtained in the improved RDN. The research highlights the benefits of ZT and the ZT + R in maize and wheat, reducing energy inputs, increasing efficiency and productivity and lowering carbon emissions. Along with it, the improved nitrogen application techniques like the subsurface placement of nitrogen improves carbon and energy parameters of maize and wheat in maize-wheat system.

    INTRODUCTION

    The rice-wheat cropping system (RWCS) currently occupies approximately 14 million hectares across the indo-gangetic plains (IGP) and is responsible for roughly 75% of the total food grain output in India (Alam et al., 2016). Nevertheless, the prolonged utilization of this sequence has precipitated multiple challenges, including the depletion of groundwater resources, mounting operational costs, a shortage of farm labour, the widespread burning of crop residues and an escalation in greenhouse gas (GHG) emissions (Jat et al., 2018; Mondal et al., 2020). Consequently, the maize-wheat cropping system has surfaced as a viable substitute, presenting benefits such as better soil preservation, decreased water requirements and increased economic returns for farmers (Jat et al., 2018). To accurately evaluate the long-term viability of shifting to this new system, the implementation of comprehensive carbon and energy auditing frameworks is essential.
           
    Evaluating system efficiency through energy auditing involves comparing the total invested inputs-such as seeds, irrigation, fertilizers and mechanical operations-against the final energy value of the crop output. In parallel, calculating the carbon footprint involves tracking GHG emissions, primarily N2O and CO2, that originate from both field activities and the manufacturing of agricultural inputs (Chakraborty et al., 2023; Bhattacharyya et al., 2025). Previous energy assessments within the IGP have successfully highlighted substantial efficiency gaps between conventional farming and conservation agriculture (CA) frameworks (Jat et al., 2019; Saad et al., 2016; Meena et al., 2024). Likewise, carbon footprinting of cereal rotations dependent on nitrogen fertilization indicates that fertilizer management contributes disproportionately to the overall GHG emissions of the system, underscoring the necessity of optimizing nitrogen rates and placement to mitigate the carbon footprint (Bhatia et al., 2023; Bhattacharyya et al., 2025).
           
    Even with the expanding uptake of conservation agriculture in the IGP, exhaustive energy and carbon evaluations targeting the maize-wheat system are still scarce. This data deficit complicates the task of precisely measuring the environmental compromises associated with adopting this alternative cropping system (Bhatia et al., 2023; Parihar et al., 2018). Consequently, this investigation was initiated to address the following primary goals: (i) to assess and contrast the energy inputs, output energy and energy use efficiency (EUE) of both maize and wheat when subjected to zero tillage with residue retention (ZT+R), zero tillage (ZT) and conventional tillage (CT); (ii) to evaluate how different nitrogen placement methods-specifically the RDN, Improved RDN and improved 80% RDN-influence the carbon footprint and carbon sustainability index (CSI) of the cultivated crops and (iii) to determine the optimal integration of nitrogen management and tillage that delivers the highest energy efficiency and carbon sustainability for the maize-wheat system in the indo-gangetic plains.

    MATERIALS AND METHODS

    Study area and experimental design
     
    The current field research took place at the ICAR-Indian Agricultural Research Institute (ICAR-IARI) located in New Delhi across two consecutive growing cycles: 2022-23 and 2023-24. This specific geographical location features a subtropical semi-arid climate. Over the two respective years, the site recorded average rainfall amounts of 780 mm and 580 mm. The soil profile of the testing ground was classified as sandy clay loam. Researchers utilized a split-plot structural design, applying treatments to permanent conservation agriculture plots that had been initially established and maintained since 2012.
           
    The main plots were dedicated to three distinct crop establishment practices (CEP): Zero tillage with residue retention (ZT+R), Zero tillage (ZT) and conventional tillage (CT). The secondary sub-plots incorporated four separate nitrogen placement methods (NPMs)(NPMs): (i) Control: Fertilised with only phosphorus (P) and potassium (K) at sowing time; (ii) Recommended dose of nitrogen (RDN): 1/3rd N applied as basal, followed by two 1/3rd N applications as surface placement at the V6 stage and tasselling stage in maize and at the 1st and 3rd irrigations in wheat; (iii) Improved RDN: 1/3rd N applied as basal, followed by 1/3rd N as subsurface placement at V6/1st irrigation and 1/3rd N as surface placement at the tasselling/3rd irrigation in maize and wheat, respectively; and (iv) Improved 80% RDN: 30% N applied as basal, followed by 30% N as subsurface placement at V6/1st irrigation and 20% N surface broadcast at the tasselling/3rd irrigation in maize and wheat, respectively. The mung bean crop was also planted and harvested after wheat harvest and before maize sowing in both years, as legume inclusion in cereal-based cropping systems has been demonstrated to enhance soil organic carbon inputs and improve subsequent maize productivity through residue-mediated nitrogen enrichment (Shukla et al., 2025).

    Energy and carbon auditing calculations
     
    To evaluate energy dynamics, the straw and grain outputs from the wheat and maize harvests, alongside their equivalent yields, were transformed into terms of energy (MJ/ha) utilizing specific coefficients established by Mittal and Dhawan (1988). The precise energy equivalent coefficients implemented for these conversions are detailed in Table S1.

    Table S1: Energy equivalent of various inputs used in study for energy dynamics (Mittal and Dhawan, 1988).


           
    Field emissions of N2O and CO2 were harmonized into a standardized CO2 (carbon dioxide equivalent) metric by multiplying them by their respective global warming potentials (GWP)-which are 298 for N2O and 1 for CO2 -in accordance with the IPCC Fourth Assessment Report (IPCC, 2007). To estimate direct soil emissions, the Tier 1 protocol from the IPCC (2006) was utilized, which applies an emission factor of 0.01 kg N2O-N per kg of N applied. This emission factor (0.01) was multiplied by the total N introduced into the system via crop residue and synthetic fertilizer to compute the emitted kg N2O-N per kg N input. Standardized emission coefficients were further utilized to translate the scope of field operations and applied inputs into their corresponding equivalent emissions.
     
    N2O emissions = Quantity of external N applied (kg/ha) × 0.01 × 1.571 (kg N2O/year)
     
    Factor 1.571 was the conversion of N to N2O (44/28). GWP of emitted CO2 was computed as follows:
     
    GWP = (Emitted N2O × 298) + Emitted CO2
     
    The carbon indices i.e., C-output, c-efficiency (CE) and C-sustainability index (CSI) were calculated according to Jat et al., (2019); Choudhary et al., (2017) and Lal (2004). Carbon efficiency (CE) measures the ratio of carbon assimilated in crop biomass to the carbon emitted through all inputs, indicating the carbon return per unit of carbon cost.

     
    The carbon sustainability index (CSI), proposed by Lal (2004), represents the net carbon gain relative to inputs; a positive CSI indicates carbon accumulation in the system.

    RESULTS AND DISCUSSION

    Energy auditing
     
    Maize
     
    The implementation of conservation tillage substantially minimized the required energy inputs while concurrently boosting the total output energy, thereby demonstrating a distinct efficiency advantage over CT. ZT reduced total energy inputs by 13.50% relative to CT, yet the ZT and ZT + R achieved 4.98% and 8.46% higher output energy, respectively (Table 1; Fig 1). When evaluating the NPMs, the Improved RDN strategy consumed the highest amount of energy input-remaining statistically comparable to both the Improved 80% RDN and RDN-yet it generated the greatest output energy, exceeding RDN by 3.28%. EUE followed a consistent tillage hierarchy: ZT exceeded CT by 26.28%, while ZT+R, despite superior output energy, registered lower EUE owing to its elevated input burden from residue incorporation. A significant CEP × NPM interaction regarding EUE confirmed that conservation tillage significantly amplifies the benefit of optimized nitrogen management.

    Table 1: Effect of conservation tillage and nitrogen placement methods on the energy auditing of maize and wheat in maize-wheat system (pooled mean over two years).



    Fig 1: Effect of crop establishment practices and nitrogen placement methods on the energy auditing of maize-wheat system (pooled mean over 2 years).



    Wheat
     
    In wheat, the energy efficiency advantage shifted from ZT (as observed in maize) to ZT+R, which recorded the highest EUE-exceeding ZT and CT by 7.17% and 22.68%, respectively. Output energy ranked ZT+R > ZT > CT, with margins of 9.48% and 2.06% over CT, while Improved RDN consistently outperformed RDN and Improved 80% RDN in output energy by 7.57% and 6.71%, respectively. No significant CEP × NPM interaction was detected for total output energy (Table 1; Fig 1). Energy productivity under ZT+R and ZT exceeded CT by 22.58% and 13.97%, respectively.
     
    Carbon auditing
     
    Maize
     
    The CT registered the maximum carbon input (608.05 kg ha-1), although it significantly lagged behind ZT+R and ZT in terms of carbon output by 9.04% and 4.52%, respectively. Evaluating the NPMs, the Improved 80% RDN strategy established the most advantageous balance between carbon inputs and outputs, recording significantly lower carbon inputs than Improved RDN and RDN due to its 20% cutback in applied nitrogen. Across the crop establishment practices, both CE and CSI maintained a consistent ranking order of ZT > ZT+R > CT (Table 2; Fig 2).

    Table 2: Effect of crop establishment practices and nitrogen placement methods on the carbon auditing of the maize and wheat crop in maize-wheat system (pooled mean over two years).



    Fig 2: Effect of crop establishment practices and nitrogen placement methods on the carbon auditing of maize-wheat system (pooled mean over two years).


     
    Wheat
     
    Wheat carbon trends closely paralleled those observed in maize. ZT+R and ZT surpassed CT in carbon output by 9.61% and 2.11%, respectively and exceeded CT in CE by 13.38% and 9.83% and in CSI by 14.95% and 10.98% (Table 2; Fig 2). Among NPMs, Improved RDN delivered the highest carbon output, exceeding RDN and Improved 80% RDN by 7.46% and 6.72%.
     
    Energy efficiency under conservation tillage: Mechanisms beyond input reduction
     
    The notable dominance of ZT compared to CT regarding energy use efficiency across both crops highlights an underlying mechanistic reality that goes far beyond merely subtracting the arithmetic cost of reduced tillage passes. Following a decade of conservation protocols on the experimental plots (established 2012), the minimized soil disruption under ZT fosters the formation of stable macroaggregates and continuous biopores, which enhance deep root proliferation, nutrient interception efficiency and water infiltration. This directly translates into an improved ratio of yield-per-unit-input-the fundamental agronomic basis for elevated EUE-which functions independently of and additively to, the immediate energy conserved by skipping tillage operations (Parihar et al., 2018; Jat et al., 2019).
           
    Our investigation provides novel insights regarding how this efficiency advantage differs crop-specifically within a continuous maize-wheat cycle, demonstrating that the leadership in EUE shifts from ZT during the maize phase to ZT+R during the wheat phase-a pattern that has not been previously documented for this rotation in the region. Mechanistically, this transition is attributable to the carry-over benefits of maize residues left on the soil surface throughout the rabi season. Notably, maize residue retained on the soil surface under ZT+R has a high C:N ratio and decomposes slowly, enabling a gradual and sustained release of carbon and nitrogen that supports long-term soil organic carbon accrual (Kumar et al., 2024).
           
    The significant interaction between CEP and NPM regarding EUE, which was uniquely observed in maize, highlights a functional synergy uniting precision nitrogen placement with conservation tillage. Within a ZT framework, the structural integrity of the soil profile is maintained, preserving the spatial geometry of subsurface-banded nitrogen strictly within the active zone of root absorption. Conversely, the repeated soil inversion inherent in CT destabilizes this carefully arranged placement, largely dissipating the spatial benefits provided by the Improved RDN strategy.
     
    Carbon footprint dynamics: Dual-pathway advantage of conservation agriculture
     
    The sustained superiority of the ZT+R and ZT paradigms over CT in relation to the carbon efficiency (CE) and carbon sustainability index (CSI) is driven by two independent, compounding mechanisms. The initial pathway is emission suppression: by completely eliminating tillage procedures and thereby slashing diesel consumption, the heightened direct CO2 emissions characteristic of CT are successfully avoided. The secondary pathway involves biomass carbon accumulation: the robust biological activity, superior moisture retention and upgraded soil architecture fostered under long-term ZT+R stimulate greater crop biomass generation, subsequently elevating carbon output. The resulting CSI measurements confirm that the ZT+R system sustains a net positive carbon balance, which serves as a fundamental prerequisite for the long-term accrual of soil organic carbon (SOC).
           
    The near-parity in carbon input between CT and ZT+R observed specifically in wheat (0.87% difference), in contrast to the substantial divergence recorded in maize, reflects the lower tillage intensity of conventional wheat establishment relative to post-kharif maize land preparation. This crop-specific asymmetry demonstrates that system-level carbon footprint assessments cannot be extrapolated from single-crop data.
     
    Nitrogen management and carbon footprint: The non-linear emission pathway
     
    The markedly reduced carbon inputs observed in the Improved 80% RDN configuration relative to the full-rate NPMs-and its consequent dominance in CSI-is dictated by the highly non-linear relationship linking applied synthetic nitrogen with N2O emissions. Whenever the concentration of mineral nitrogen in the soil surpasses the immediate demand of the crop, the processes of nitrification and denitrification accelerate disproportionately, generating N‚ O flux rates that climb much more rapidly than the linear addition of fertilizer. Consequently, decreasing the nitrogen load by 20% under the Improved 80% RDN protocol yields an amplified, greater-than-proportional drop in the overall carbon loading driven by N2O.
     
    System-level synthesis: Toward sustainable intensification in the IGP
     
    Across all energy and carbon parameters, the ZT+R × Improved RDN combination consistently emerged as the most resource-efficient and environmentally sustainable management strategy. To the best of our knowledge, this is the first study to simultaneously quantify the full energy audit and carbon footprint profile across both crops of a maize-wheat rotation under long-term CA plots integrated with subsurface nitrogen banding in the IGP.
           
    The policy implications of these results are directly actionable. The Government of India’s stringent enforcement against crop residue burning creates an urgent demand for alternatives. ZT+R directly addresses this by incorporating residue into a productive system. Simultaneously, the carbon footprint benefits of Improved RDN and Improved 80% RDN align with the objectives of India’s Soil Health Card scheme and the PM-PRANAM programme.
     
    Limitations and future scope
     
    Although this investigation supplies a rigorous, full-rotation assessment spanning two years and two crops, it remains constrained by certain factors. Primarily, the trial was exclusively executed on the sandy clay loam soils of ICAR-IARI, New Delhi; consequently, the outcomes might differ if replicated across diverse pedological profiles. Therefore, multi-location validation is highly recommended. Second, the specific energy equivalent coefficients applied were derived from Mittal and Dhawan (1988), which may not perfectly capture the energy demands of contemporary agricultural operations. Subsequent investigations should update these multipliers using the latest life cycle inventory data. Finally, this current research did not directly record belowground biomass carbon, methane (CH4) flux, or definitive changes in soil organic carbon (SOC) stocks. Future scholarly work should prioritize complete LCA-driven carbon footprinting that includes these crucial dynamics, alongside comprehensive economic analysis to gauge the practical adoption feasibility for smallholder farmers.

    CONCLUSION

    The present study demonstrates that the adoption of zero tillage-based crop establishment practices -particularly ZT+R-in conjunction with precision nitrogen placement through Improved RDN, constitutes the most energy-efficient and carbon-sustainable management combination for the maize-wheat cropping system in the indo-gangetic plains. ZT consistently reduced total energy inputs relative to CT while simultaneously delivering higher output energy, energy use efficiency and energy productivity. The carbon auditing data reinforced this conclusion, with ZT+R achieving superior carbon efficiency and CSI values. Among nitrogen placement methods, Improved RDN delivered the highest output energy, while Improved 80% RDN achieved the most favourable carbon input profile through its non-linear suppression of N2O emissions. Collectively, these findings provide robust, rotation-level quantitative evidence that the integration of conservation tillage with subsurface nitrogen banding represents an agronomically superior and environmentally sound alternative to conventional management.

    ACKNOWLEDGEMENT

    The present study was supported by the ICAR-IARI.
     
    Disclaimers
     
    The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

    CONFLICT OF INTEREST

    The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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