Emissions From Soils - Greenhouse Gases | Agriculture Optional Notes for UPSC PDF Download

Introduction

A significant amount of research has been conducted to estimate the emissions of nitrogen oxides (NOx) from soils. However, these emissions exhibit variability due to various factors. Differences in soil type, moisture levels, temperature, seasonal changes, crop type, fertilization practices, and other agricultural methods all influence the amount of NOx emitted from soils. 

• Soils release NOx through both biological and abiological processes, and the emission rates can be classified based on fertilizer application or land use. Agricultural lands and grasslands are the primary sources of NOx emissions within this category. The amount of NOx emitted from agricultural land depends on factors such as fertilizer application and the subsequent microbial denitrification processes in the soil. Microbial denitrification occurs naturally in soil, but it is Enhanced when chemical fertilizers are applied. Both nitrous oxide (N₂O) and nitric oxide (NO) are produced from this source. 
• Emissions of NOx from soils are estimated to contribute up to 16 percent of the global NOx budget in the troposphere and about 8 percent of the NOx in North America. This section focuses specifically on emissions of N₂O from soils. For information on estimating total NOx emissions from soils using the EPA’s Biogenic Emissions Inventory System (BEIS), refer to the relevant reference. 

Agricultural Soils 

Source and Methodology Overview

• The information on estimating emissions and emission factors comes from the State Workbook and the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994 by the U.S. Environmental Protection Agency (EPA).
• For a detailed understanding of N₂O generation processes and variables, refer to the original volumes.

Agricultural Soil Management Practices

• Agricultural soil management practices, including the use of synthetic and organic fertilizers, contribute to greenhouse gas emissions.
• The application of these fertilizers adds nitrogen to soils, leading to increased natural emissions of N2O.

Other Practices and Uncertainty

• Practices such as irrigation, tillage, and fallowing can also influence trace gas fluxes to and from the soil.
• However, there is significant uncertainty regarding the impact and magnitude of these practices on emissions.

Nitrous Oxide Emissions from Fertilizer Use

• Emissions from fertilizer use are the primary focus due to the uncertainty of other practices.
• Nitrous oxide emissions from commercial fertilizer use can be estimated using the formula:

N₂O Emissions = (FC * EC * 44/28)

• Where:
• FC. Fertilizer Consumption (tons of nitrogen applied)
• EC. Emission Coefficient (0.0117 tons N₂O-N/ton N applied)
• 44/28. Molecular weight ratio of N₂O to N₂O as nitrogen (N₂O/N₂O-N)

Emission Coefficient Explanation

• The emission coefficient of 0.0117 tons N₂O-N/ton N-applied indicates the percentage of nitrogen applied as fertilizer that is released into the atmosphere as nitrous oxide.
• This coefficient was derived from USDA estimates, which suggest that 1.84 kg of N₂O is emitted per 100 kg of nitrogen applied as fertilizer.

Total Fertilizer Consumption

• Total fertilizer consumption in a state or region is the sum of all synthetic nitrogen, multiple-nutrient, and organic fertilizers applied, measured in nitrogen content.
• Data on fertilizer consumption by type and state can be obtained from the Tennessee Valley Authority’s National Fertilizer and Environmental Research Center.

Organic Fertilizer Data

• Data on organic fertilizers, such as livestock manure, may be available from state or local Agricultural Extension offices.

Conversion of Fertilizer Data

• If fertilizer consumption is reported as total mass rather than nitrogen content, conversion can be done using specific percentages.

Average Fertilizer Consumption Recommendation

• To account for variations in agricultural activities due to economic and climatic factors, it is advisable to use an average of 3 years of fertilizer consumption data.

Other Soils

• The amount of N2O emitted from non-agricultural soils depends on the soil's nutrient level and moisture content.
• It is believed that soils in tropical regions emit much more N₂O than soils in other terrestrial ecosystems.
• However, due to the variability of soil types and moisture levels, some tropical soils emit less N₂O than others.
• Global measurements of soil N₂O flux have been compiled from various sources and used to define soil N₂O emission factors.
• These emission factors are based on test data from primarily undisturbed soils and have been developed for six ecological regions.

Uncertainty

Scientific understanding of nitrous oxide (N₂O) production and emissions from fertilized soils is still developing, and there are significant uncertainties involved. These uncertainties arise from various factors, including agricultural practices, soil properties, climatic conditions, and biogenic processes. These factors influence how much nitrogen from fertilizers crops absorb, how much remains in the soil after application, and how the remaining nitrogen is transformed into N₂O or other nitrogen gases.

• One major challenge in estimating N₂O emissions is the lack of comprehensive measurement data across a wide range of controlled conditions. This makes it difficult to establish statistically valid emission factors.
• Previous efforts to develop emission factors for different fertilizer and crop types have faced scrutiny regarding their accuracy. For instance, while some studies suggest that N₂O emissions are higher with ammonium-based fertilizers compared to nitrate fertilizers, others find no consistent trend related to fertilizer types. This indicates that factors other than fertilizer type may play a more significant role in determining emissions.
• Research also indicates that N₂O emissions from the nitrification of fertilizers may be more closely linked to soil properties rather than the specific type of fertilizer used. There is a general agreement that various natural and management factors influence the biological processes of soil microorganisms responsible for N₂O emissions from nitrogen fertilizer applications.
• However, the interplay between these factors and their collective impact on N₂O emissions is not well understood. Individual factors such as soil pH, temperature, moisture, organic carbon content, and oxygen supply have been shown to enhance N₂O emissions when increased individually.
• However, the combined effects of soil moisture, organic carbon content, and microbial population on N₂O emissions are not easily predictable. Management practices also have the potential to affect N₂O emissions, although these relationships have not been thoroughly quantified.
• The type of fertilizer used may influence N₂O emissions, but the extent of this effect varies, as evidenced by the wide range of emission coefficients observed for different fertilizer types in experiments. While higher fertilizer application rates may lead to increased N2O emissions, the relationship between application rate and emissions is not well defined.
• Application techniques also matter; deep placement of fertilizer results in lower N₂O emissions compared to broadcasting or hand placement. Timing of fertilizer application can impact emissions as well, with some studies suggesting that fall applications may lead to higher emissions than spring applications.
• Tillage practices influence N₂O emissions too, with tilling generally reducing emissions, while no-till and herbicide use may increase emissions. However, limited research in specific contexts has not fully captured the complex interactions among these factors, making it challenging to predict the effects of their combinations.
• Emissions may also arise from the contamination of surface and groundwater due to nutrient leaching and runoff from agricultural systems. However, methods for estimating N₂O emissions from these sources are not currently included due to a lack of data and emission coefficients for each contributing activity.
• Given the potential significance of these N₂O emissions, they should be considered in the future as data becomes available and scientific understanding improves.