5 Soil Erosion (Non Test)

Global Impact and Significance of Soil Erosion

• Soil erosion is classified as one of the most critical threats facing agricultural land globally.

• In Sub-Saharan Africa, it represents the single largest threat to agricultural productivity.

• The importance of the issue is highlighted by the combined pressure of rapid population growth and accelerating erosion rates.

• The documentary film "Kiss the Ground" popularizes the argument that there are only 60 harvests remaining globally due to current soil erosion rates, a topic addressed through scientific modeling in this lecture.

• While soil erosion is a naturally occurring process, human activities have dramatically increased its scale. Current estimates suggest human influence is $10$ times greater than all natural erosion processes combined.

• Primary drivers of human-accelerated erosion include the removal of vegetation, changes in vegetation types, and intensive industrial farming practices.

Comparative Global Statistics and Land Change Dynamics

• Research by Borelli et al. (published in Nature Communications) quantifies how land-use transitions impact soil loss:

  • Transitioning from semi-natural vegetation to cropland creates massive net soil losses, estimated at approximately $0.7$ picograms per year ($pg/year$).

  • Maintaining or shifting to semi-natural vegetation results in significantly lower soil loss, approximately $0.2\,pg/year$.

  • Transitioning land to forest can result in either net increases or decreases in soil, as soil formation rates in forested environments often exceed loss rates.

• Global Erosion Estimates (2012 data):

  • Total global soil erosion is estimated at $36\,pg/year$.

  • Note: In this context, a picogram is defined as $10^9\,Mg$ (Megagrams).

• Regional Erosion Rates (measured in $Mg/ha/year$ or tons per hectare per year):

  • South America, Asia, and Africa: Approximately $3.5\,Mg/ha/year$.

  • North America: Approximately $2.2\,Mg/ha/year$.

  • Europe: Approximately $0.9\,Mg/ha/year$. Differences in these rates are largely attributed to variations in climate and management practices.

Soil Erosion and the Global Carbon Cycle

• Erosion is a major contributor to the loss of Soil Organic Carbon (SOC).

• The current estimated loss of carbon due to erosion ranges between $0.3$ and $1\,pg/year$.

• Scientific debate exists regarding the "burial" of carbon, where eroded material is deposited and covered, potentially sequestering it; however, the net loss remains a significant environmental concern.

• Comparison to other sectors:

  • Global aviation is cited as producing $190\,mg$ of carbon per year.

  • Given the definition of a picogram as $10^9\,mg$ in this lecture, the scale of carbon loss from soil erosion vastly exceeds that of the aviation industry.

Regional Trends in Land Management and Erosion Rates

• Predictions of soil erosion changes between $2001$ and $2012$ show divergent regional trends:

  • Improvement Areas: North America, much of Europe, India, and China have seen decreases in soil erosion due to changes in land-use management.

  • Worsening Areas: Sub-Saharan Africa and South America exhibit devastating increases in erosion rates.

• Impact of Mitigation: The adoption of "conservation tillage" or "reduced tillage" significantly improves the outlook for erosion control. While South America shows some improvement through these practices, many African nations continue to face severe erosion challenges despite mitigation efforts.

Policy Frameworks and Climate Change Mitigation

• The IPCC (Intergovernmental Panel on Climate Change) released a report titled "Climate Change and Land" in 2019.

• Pete Smith from the University of Aberdeen led the chapter on mitigation through soil management.

• Key findings regarding soil erosion include:

  • High potential for soil erosion management to assist in climate change mitigation and adaptation.

  • Significant role in combating desertification and land degradation.

  • Critical importance for global food security.

  • Synergistic importance alongside soil compaction management and organic carbon preservation for maintaining soil function.

Foundational Concepts: Erosion, Transport, and Deposition

• Erosion: The physical removal of material from one specialized location and its subsequent transport to another place.

• Agents of Erosion: Primarily driven by wind or water.

• Deposition: The arrival and settling of eroded material at a new location.

• Colluviation: A natural process where material moves and builds up to form fertile soils, though human mismanagement accentuates this to harmful levels.

• Human Accentuators:

  • Excessive plowing and over-cultivation.

  • Farming when soil conditions are not optimal.

  • Livestock overgrazing, which removes protective roots.

  • Deforestation (removal of roots and surface litter).

  • Urbanization and forest fires.

Specific Types and Mechanisms of Water Erosion

• Splash Erosion: Occurs when raindrops hit the soil with kinetic energy, dislodging particles.

• Sheet Erosion: Water flows over the soil surface in a thin, uniform layer.

• Rill Erosion: Faster-moving water creates small, well-defined channels.

• Gully Erosion: The most severe form, where large, deep channels are carved into the landscape by turbulent water flow.

Soil Physical Properties and Erodibility

• Soil Texture:

  • Clays: Very small (< 2\,\mu m). They are cohesive and bind together, making them difficult to dislodge (low erodibility) unless dispersed.

  • Sands: Large particles ($63\,\mu m$ to $2\,mm$). While loosely bound, they are heavy and require high velocities to be transported.

  • Silts: Medium particles ($2\,\mu m$ to $50/63\,\mu m$). Silts are highly erodible because they easily enter suspension in overflowing water.

• Soil Structure: The aggregation of small particles into larger units. This is stabilized by organic matter and clay content.

• Soil Depth: Deeper soils can absorb more water, whereas shallow soils reach saturation quickly, leading to increased runoff.

The Dynamics of Aggregate Stability and Water Infiltration

• Aggregate Explosion: If dry soil is wetted rapidly, water enters pores and builds up internal air pressure. Differential swelling of clays causes the aggregates to explode or disperse, creating highly erodible fine particles.

• The Role of Pore Structure: Slow wetting allows air to escape, keeping the soil structure intact.

• Biological Influence: The presence of fungi and nitrogen-assisted plant growth improves soil structure. Soils with less fungal growth (e.g., those without added nitrogen) often exhibit faster but less stable infiltration, making them prone to erosion.

Primary Predictive Factors: Precipitation and Runoff

• Erosion potential is dictated by rainfall intensity, duration, and the total land area affected.

• Runoff: Excessive water that cannot infiltrate the soil flows over the surface, carrying sediment.

• Rainfall Events: Prediciton models often use extreme events, such as a "100-year storm event." In Central Scotland, such an event might involve sharp peaks of intensity reaching $60\,mm$ of rainfall per hour.

Biotic Factors: The Role of Vegetation and Root Architecture

• Above-Ground Surface Cover:

  • Stems increase surface roughness, slowing water flow.

  • Leaves provide "interception," absorbing the kinetic energy of raindrops so they drip slowly to the ground.

  • Increasing surface vegetation coverage correlates directly with a lower relative erosion rate.

• Below-Ground Root Systems:

  • Roots reinforce and bind the soil physically.

  • Roots contribute organic matter, restructuring the soil and promoting aggregate stability.

  • Laboratory simulations show that as plants grow, the "soil detachment rate" drops significantly as the root area ratio increases.

Topographic Characteristics and Slope Dynamics

• Slope Steepness: The ability of water to carry sediment increases at approximately the cube of the slope angle ($\text{Slope}^3$).

• Slope Length: Longer slopes allow for the development of greater erosive power. As water flows down an impervious layer, the depth and velocity of the water increase toward the bottom of the slope.

• Curvature:

  • Planar Slopes: Result in uniform sheet flow.

  • Convergent Slopes: Concentrated flow in the center increases water volume and accentuates erosion.

• Aspect: The direction a slope faces (e.g., south-facing vs. north-facing). South-facing slopes often have different vegetation patterns, which in turn influence soil formation and stability over millennia.

Modelling Soil Loss: The RUSLE Equation

• The Revised Universal Soil Loss Equation (RUSLE) is the dominant model for erosion prediction:

• Equation: $A = R \times K \times L \times S \times C \times P$

  • $A$: Predicted soil loss (tons per hectare per year).

  • $R$: Rainfall factor.

  • $K$: Soil erodibility factor.

  • $L$: Slope length.

  • $S$: Slope steepness.

  • $C$: Cover management factor.

  • $P$: Support practice factor (erosion control measures).

Experimental Case Study: Compost Application and Runoff Mitigation

• An experiment on engineered slopes compared different compost application rates:

  • Control (no compost/plants): High soil loss and high runoff.

  • $35\,Mg/ha$ (surface or incorporated): Moderate improvement.

  • $300\,Mg/ha$ (surface or incorporated): Soil loss was almost non-existent; runoff was drastically reduced.

• Hydraulic Conductivity: Soil with added compost showed significantly higher hydraulic conductivity after 12 weeks of growth compared to soil without compost. This allows water to enter the soil rather than flowing off as erosive runoff.

Technical Methodologies for Measuring Soil Erosion

• Runoff Collection:

  • Fancy chambers and loggers for long-term monitoring.

  • Bounded plots to capture sediment from specific areas.

  • Low-cost open systems using buckets to capture flow.

• Rainfall Simulators: Used for controlled experiments. These apply precise water drop sizes and rates from a fixed height to measure infiltration, runoff, and sediment detachment.

• Erosion Pins: A low-tech method involving driving a pin deep into the soil. Erosion is measured by the amount of the pin length exposed over time.

Radioactive Tracers and Cosmogenic Radionuclides in Erosion Modeling

• Cesium-137 ($^{137}Cs$):

  • This isotope was deposited globally due to nuclear testing (peaking in the $1960s$) and disasters like Chernobyl.

  • $^{137}Cs$ has a small ionic radius and binds very strongly to the top few millimeters of soil particles.

  • Mapping $^{137}Cs$ allows researchers to identify areas of erosion (low concentration) versus areas of deposition (high concentration).

  • Example: In the Loess Plateau of China, eroding mid-slopes under non-native grass show low $^{137}Cs$, while deposition zones at the bottom show high concentrations.

• Beryllium-10 ($^{10}Be$):

  • A cosmogenic radionuclide deposited by meteorites.

  • Bedrock contains mostly "old" Beryllium-9 ($^{9}Be$).

  • The ratio of $^{10}Be$ to $^{9}Be$ is used to measure long-term rock weathering and surface soil erosion.

Soil Formation Rates and the "60 Harvests" Debate

• Cosmogenic radionuclide studies allow for the comparison of soil formation vs. loss.

• Comparative Rates:

  • Arable Soil: Formation rate of $0.048\,mm/year$.

  • Forest Soil: Formation rate of $0.07\,mm/year$.

• Findings suggest that while the "60 harvests" claim targets a real issue, the lifespan of the A horizon (topsoil) in arable land is actually modeled between $260$ and $270$ years. While longer than 60 years, this is still considered a concerningly short timeframe for sustainability.

Mechanics and Measurement of Wind Erosion

• Transport Processes:

  • Creep: Large minerals rolling along the surface.

  • Saltation: Particles between $0.1$ and $0.5\,mm$ "leapfrogging" or bouncing. This can act like a sandblaster (abrasion).

  • Suspension: Particles smaller than $0.1\,mm$ kept aloft as aerosols. These can travel thousands of miles (e.g., from China to the US West Coast).

• Protective Factors: Organic matter, surface residues, and microbiology (e.g., mycorrhizal fungi produced exudates and hyphae that bind particles).

• Measurement: Wind tunnels equipped with lasers or optics to track particle size and movement rate.

Case Study: The Great Dust Bowl of the United States

• Described as the greatest social disaster in American history, affecting Oklahoma and Midwest states in the $1930s$.

• Prompted President Franklin Delano Roosevelt’s quote: "The nation that destroys its soil destroys itself."

• Cause: Transition from native prairie grasses (diverse roots/organic matter) to a monoculture of maize, followed by a severe drought.

• Result: Loss of soil structure led to catastrophic wind erosion. This historical event is the subject of John Steinbeck’s novel The Grapes of Wrath.