Impact of nitrogen fertilization and cropping systems on aggregate-bound soil organic carbon fractions in Southern India – Nature

 

Report on the Impact of Agricultural Management on Soil Organic Carbon and Sustainable Development Goals in Southern India

Executive Summary

This report details a study conducted in southern India to assess the impact of nitrogen fertilization, cropping systems, and irrigation on soil organic carbon (SOC) and soil aggregate stability. The findings are critical for advancing several United Nations Sustainable Development Goals (SDGs), particularly SDG 2 (Zero Hunger), SDG 13 (Climate Action), and SDG 15 (Life on Land). The study demonstrates that optimized nitrogen application and specific cropping systems, especially under irrigated conditions, significantly enhance total organic carbon (TOC) and its fractions. This enhancement improves soil health and productivity while promoting carbon sequestration, directly contributing to climate change mitigation. The maize cropping system and higher nitrogen application (200% of the recommended dose) showed the greatest potential for increasing SOC. These results provide a scientific basis for developing land management strategies that support food security and environmental sustainability in semi-arid regions.

1. Introduction: Aligning Agricultural Practices with Sustainable Development Goals

Soil organic carbon (SOC) is a cornerstone of sustainable agriculture, directly influencing soil fertility, nutrient cycling, and crop productivity. Its effective management is integral to achieving key Sustainable Development Goals:

• SDG 2 (Zero Hunger): Healthy soils with optimal SOC levels are more productive, supporting sustainable food production and enhanced food security.
• SDG 13 (Climate Action): Soils are a major carbon reservoir. Agricultural practices that increase SOC contribute directly to carbon sequestration, mitigating climate change.
• SDG 15 (Life on Land): Enhancing SOC improves soil structure, water retention, and biodiversity, combating land degradation and preserving terrestrial ecosystems.

This study investigates how nitrogen levels, cropping systems (fieldbean, finger millet, maize), and irrigation management (rainfed vs. irrigated) affect aggregate-associated SOC in the semi-arid climate of southern India. The objective is to identify practices that maximize carbon accumulation and soil stability, thereby providing actionable insights for sustainable land management aligned with global development targets.

2. Methodology for Sustainable Agricultural Assessment

The study was conducted at two experimental sites in Bengaluru, India, representing irrigated and rainfed conditions.

2.1 Experimental Design

A split-plot design was used to assess the following factors:

1. Cropping Systems (Main Treatment):
   • Fieldbean (Lablab purpureus)
   • Finger Millet (Eleusine coracana)
   • Maize (Zea mays)
2. Nitrogen Levels (Subplot Treatment):
   • N₁: No nitrogen (Control)
   • N₂: 100% Recommended Dose of Nitrogen (RDN)
   • N₃: 200% RDN (Reflecting common farmer over-application)
3. Moisture Conditions:
   • Irrigated
   • Rainfed

2.2 Analysis

Soil samples were collected from the 0-15 cm depth. Analyses included the separation of soil aggregates (macroaggregates >250 μm and microaggregates <250 μm) and the measurement of:

• Total Organic Carbon (TOC)
• Labile Carbon Fractions (Microbial Biomass Carbon, Dissolved Organic Carbon, Potassium Permanganate Oxidizable Carbon)
• Non-labile Organic Carbon
• Aggregate Stability (Mean Weight Diameter and Tensile Strength)

3. Key Findings and SDG Implications

3.1 Impact of Nitrogen Fertilization on Soil Health and Climate Action (SDG 13, SDG 15)

Optimized nitrogen application is crucial for enhancing soil carbon sequestration, a key strategy for SDG 13 (Climate Action).

• Increased Carbon Storage: Applying 200% RDN (N₃) significantly increased Total Organic Carbon (TOC) and its fractions compared to the control (N₁). Under irrigated conditions, TOC increased by 36.8%, and under rainfed conditions by 26.9%. This demonstrates that adequate nitrogen enhances biomass production, leading to greater carbon input into the soil.
• Enhanced Labile and Non-Labile Carbon: All carbon fractions, including microbial biomass carbon (MBC) and non-labile organic carbon (NLOC), increased with higher nitrogen levels. This improves both short-term nutrient availability and long-term carbon stabilization, supporting SDG 15 (Life on Land) by building soil resilience.
• Aggregate Stability: Nitrogen levels did not significantly influence aggregate stability as measured by mean weight diameter (MWD) or tensile strength, suggesting that while N boosts carbon content, other factors primarily govern soil structural integrity.

3.2 Influence of Cropping Systems on Food Security and Land Health (SDG 2, SDG 15)

The choice of crop significantly impacts SOC storage, which is fundamental to soil fertility and achieving SDG 2 (Zero Hunger).

• Maize System Superiority: The maize cropping system consistently resulted in the highest content of TOC and its fractions, particularly under irrigated conditions. This is attributed to higher root biomass and C:N ratio, making it a promising system for building soil carbon.
• Legume Benefits: The fieldbean (legume) system showed high microbial biomass carbon, highlighting its role in enhancing soil biological activity, which is vital for nutrient cycling and soil health.
• Varied Performance: The finger millet system demonstrated the highest MWD in rainfed soils, indicating superior aggregate stability under water-stressed conditions. This resilience is critical for sustainable agriculture in arid regions.

3.3 Role of Irrigation in Enhancing Carbon Sequestration (SDG 6, SDG 13)

Water management is a key enabler for maximizing the benefits of other agricultural inputs, linking sustainable water use (SDG 6) with climate mitigation (SDG 13).

• Amplified Carbon Gains: The positive effects of nitrogen and cropping systems on SOC were significantly more pronounced under irrigated conditions than rainfed conditions. Irrigation alleviates water stress, boosting plant growth and subsequent carbon inputs to the soil.
• Higher Carbon Fractions: Irrigated soils contained higher absolute amounts of TOC, MBC, and other fractions. This underscores the potential for irrigated agriculture, when managed responsibly, to be a powerful tool for carbon sequestration.

3.4 Soil Aggregation and Carbon Protection

Soil structure plays a vital role in protecting organic carbon from decomposition, contributing to long-term sequestration efforts under SDG 13.

• Macroaggregates as Carbon Hotspots: Macroaggregates (>250 μm) consistently held higher concentrations of TOC and all its fractions compared to microaggregates (<250 μm). This physical protection within larger aggregates is a key mechanism for carbon stabilization.
• Stability of Microaggregates: Tensile strength was highest in smaller aggregates (1-2 mm), indicating that while macroaggregates store more carbon, microaggregates form a more stable, foundational component of soil structure.

4. Conclusion and Recommendations for Sustainable Land Management

This study confirms that agricultural management practices have a profound impact on soil organic carbon dynamics and, by extension, on achieving global Sustainable Development Goals. To promote sustainable agriculture in southern India and similar semi-arid regions, the following recommendations are made:

1. Adopt Integrated Nutrient Management: Utilize optimized nitrogen fertilization to enhance crop biomass and soil carbon sequestration, contributing to SDG 13 (Climate Action) and SDG 15 (Life on Land). While higher N levels increased SOC, further research should identify the economic and environmental optimum to avoid issues like nutrient leaching, aligning with SDG 12 (Responsible Consumption and Production).
2. Promote High-Residue Cropping Systems: Encourage the cultivation of crops like maize that produce significant biomass, thereby increasing carbon inputs into the soil and supporting SDG 2 (Zero Hunger) through improved soil fertility.
3. Leverage Irrigation for Carbon Farming: Where feasible, employ efficient irrigation to amplify the carbon sequestration potential of soils. This strategy links responsible water management (SDG 6) directly with climate change mitigation.
4. Focus on Building Soil Structure: Implement practices that promote the formation of stable macroaggregates, as they are crucial for protecting and accumulating soil organic carbon.

By integrating these findings into agricultural policy and practice, it is possible to create farming systems that are not only productive but also environmentally restorative, helping to achieve a sustainable and food-secure future.

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 2: Zero Hunger
   • The article directly connects its research to agricultural sustainability by stating that understanding soil organic carbon is “crucial for enhancing soil health, crop productivity, and sustainable land management.” It investigates how different agricultural practices like nitrogen fertilization and cropping systems impact soil, which is fundamental to achieving food security and promoting sustainable agriculture.
2. SDG 6: Clean Water and Sanitation
   • The study’s comparison between “rainfed and irrigated conditions” highlights the importance of water management in agriculture. Furthermore, the investigation into different nitrogen fertilizer levels is relevant, as excessive nitrogen application can lead to leaching and water pollution, an issue implicitly linked to sustainable water management.
3. SDG 12: Responsible Consumption and Production
   • The research examines the effects of varying nitrogen doses, including “No nitrogen,” “100% of recommended dose of N (RDN),” and “200% RDN.” This analysis directly addresses the efficient use of resources (fertilizers) and promotes sustainable management practices to avoid overuse, aligning with the goal of achieving sustainable production patterns.
4. SDG 13: Climate Action
   • The article explicitly focuses on carbon dynamics in soil, mentioning “carbon sequestration” multiple times. It states that optimizing nitrogen levels and irrigation can “potentially increase the soil organic carbon content and its sequestration.” This directly relates to climate action by exploring methods to mitigate climate change through improved land management that enhances carbon storage in soils.
5. SDG 15: Life on Land
   • The core theme of the article is the health and quality of terrestrial ecosystems, specifically agricultural soils. By studying “soil organic carbon,” “soil fertility,” “aggregate stability,” and “overall soil quality,” the research aims to find ways to improve land management, combat soil degradation, and maintain the health of the land, which is the central focus of SDG 15.

2. What specific targets under those SDGs can be identified based on the article’s content?

1. Under SDG 2 (Zero Hunger)
   • Target 2.4: “By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production, that help maintain ecosystems, that strengthen capacity for adaptation to climate change… and that progressively improve land and soil quality.” The article’s investigation into how cropping systems, nitrogen levels, and irrigation affect soil organic carbon and aggregate stability is directly aimed at identifying resilient and sustainable agricultural practices to improve soil quality and support crop production.
2. Under SDG 6 (Clean Water and Sanitation)
   • Target 6.4: “By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity…” The study’s comparison of “irrigated versus rainfed” systems provides insights into the impact of water management on soil health and carbon sequestration, which is relevant to understanding water-use efficiency in agriculture.
3. Under SDG 12 (Responsible Consumption and Production)
   • Target 12.2: “By 2030, achieve the sustainable management and efficient use of natural resources.” The experiment’s design, which tests different levels of nitrogen fertilizer, seeks to determine the “optimum level of nitrogen” for both crop production and soil health, directly contributing to the goal of efficient resource use.
4. Under SDG 13 (Climate Action)
   • Target 13.1: “Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.” By enhancing soil health, soil organic carbon, and aggregate stability, the agricultural practices studied in the article contribute to making farming systems more resilient to climate stressors like drought, which is particularly relevant in the “semi-arid climatic conditions” mentioned.
5. Under SDG 15 (Life on Land)
   • Target 15.3: “By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought and floods, and strive to achieve a land degradation-neutral world.” The article’s focus on improving soil structure (“aggregate stability”), soil fertility, and soil organic carbon content is a direct contribution to restoring and improving soil health, thereby combating land degradation.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. For Target 2.4 and 15.3 (Sustainable Agriculture and Land Degradation)
   • Soil Organic Carbon (SOC) / Total Organic Carbon (TOC): The article extensively measures TOC as a primary indicator of soil health and fertility. It states, “Organic carbon, an essential element for life on Earth, is an indispensable component of soil fertility… and sustaining crop production.”
   • SOC Fractions: The study measures specific fractions like “dissolved organic carbon (DOC), microbial biomass carbon (MBC), potassium permanganate oxidizable carbon (PPOC), and non-labile organic carbon (NLOC).” These are used as sensitive indicators of changes in soil quality due to management practices.
   • Soil Aggregate Stability: This is measured using two distinct indicators:
     • Mean Weight Diameter (MWD): The article calculates MWD to determine aggregate stability against water, noting it “can serve as an indicator of the tendency of soil slaking and crusting.”
     • Tensile Strength: This is measured to assess “soil strength and structural stability against external mechanical forces.”
2. For Target 12.2 (Efficient Resource Use)
   • Nitrogen Application Levels: The different treatments (N₁: No nitrogen, N₂: 100% RDN, N₃: 200% RDN) serve as an indicator to evaluate the impact of varying levels of resource input on soil health and carbon storage, helping to identify more sustainable application rates.
3. For Target 13.1 (Climate Action)
   • Carbon Sequestration Potential: While not quantified as a single number, the entire study implies this indicator. The conclusion that “optimum N supply and inclusion of irrigation practices favour the accumulation of organic carbon” points directly to practices that enhance carbon sequestration in agricultural soils. The measurement of TOC and NLOC serves as a proxy for this.

4. Summary Table of SDGs, Targets, and Indicators

SDGs	Targets	Indicators Identified in the Article
SDG 2: Zero Hunger	Target 2.4: Ensure sustainable food production systems and implement resilient agricultural practices to improve land and soil quality.	Crop productivity (implied as a goal) Soil fertility (mentioned as a key benefit of SOC) Soil quality (measured via carbon fractions and aggregation)
SDG 6: Clean Water and Sanitation	Target 6.4: Increase water-use efficiency and ensure sustainable withdrawals.	Comparison of irrigated vs. rainfed conditions (as a proxy for water management strategies)
SDG 12: Responsible Consumption and Production	Target 12.2: Achieve the sustainable management and efficient use of natural resources.	Nitrogen fertilizer application rates (No N, 100% RDN, 200% RDN)
SDG 13: Climate Action	Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards.	Carbon sequestration potential (inferred from changes in SOC) Total Organic Carbon (TOC) and its fractions (MBC, DOC, PPOC, NLOC)
SDG 15: Life on Land	Target 15.3: Combat desertification and restore degraded land and soil.	Soil Organic Carbon (SOC) content Aggregate stability (measured by Mean Weight Diameter and Tensile Strength)

Source: nature.com